In an oil-sealed rotary vane vacuum pump, oil is not simply there to reduce friction.

It lubricates moving components, carries heat away from the pumping mechanism, helps seal clearances between internal parts, and contributes directly to the pressure the pump can achieve. Edwards describes oil in an oil-sealed rotary vane pump as both a sealant and coolant, while Leybold notes that oil-sealed pumps rely on oil to achieve higher compression ratios and better internal cooling. (Edwards Vacuum)

That makes the oil specification part of the vacuum system.

Put the wrong lubricant into a gearbox and you may shorten bearing life.

Put the wrong lubricant into an oil-sealed vacuum pump and you can also change pump-down performance, ultimate pressure, starting behavior, sealing, oil consumption, and service life.

One of the most important specifications is viscosity.

But viscosity is only the beginning.

What Does cSt Mean on Vacuum Pump Oil?

Vacuum pump oil data sheets commonly give kinematic viscosity in centistokes, abbreviated cSt.

You may also see the same measurement written as:

mm²/s

One centistoke is equivalent to one square millimeter per second, so an oil listed as 68 cSt at 40°C is also 68 mm²/s at 40°C.

The temperature is critical.

Oil becomes thinner as temperature increases, which is why technical data commonly specifies viscosity at both 40°C and 100°C.

For example, Busch publishes its VM 068 mineral vacuum pump oil at 68 mm²/s at 40°C but only 8.5 mm²/s at 100°C. Its VM 100 is approximately 100 mm²/s at 40°C and 10.7 mm²/s at 100°C. (Busch Vacuum Solutions)

The oil did not change products.

The temperature changed.

And that changed the viscosity dramatically.

This is one reason operating temperature matters when selecting a vacuum pump lubricant.

What Does ISO VG Mean?

ISO VG stands for ISO Viscosity Grade.

The number identifies the oil’s nominal kinematic viscosity range at 40°C.

So an oil designated:

ISO VG 32 is centered around approximately 32 cSt at 40°C.

ISO VG 68 is centered around approximately 68 cSt at 40°C.

ISO VG 100 is centered around approximately 100 cSt at 40°C.

Those numbers are viscosity classes—not product names.

Busch, for example, currently sells mineral oils in ISO VG 32, 68, 100 and 150 classes. (Busch Vacuum Solutions)

Vacuum Pump Supply’s own mineral-oil line follows the same general convention. Current published specifications include:

These are real measured or supplier-published viscosity values, which illustrates an important point: ISO VG 68 does not mean every ISO VG 68 oil will measure exactly 68.000 cSt. It identifies a viscosity class. (Vacuum Pump Supply)

Why Does Vacuum Pump Oil Viscosity Matter?

Inside an oil-sealed rotary vane pump there are very small clearances between moving and stationary components.

Oil has to move through the lubrication system while also maintaining an effective lubricating and sealing film.

If the lubricant is significantly different from what the pump was designed to use, those functions can change.

Edwards specifically warns against using low-viscosity oils that are not appropriate for its rotary vane pumps, noting that incorrect low-viscosity oil can negatively affect pumping speed, ultimate vacuum, and pump life. (Edwards)

That is important because the symptom may not immediately look like an oil problem.

A technician may see:

longer pump-down time,

poor ultimate vacuum,

higher oil consumption,

abnormal temperature,

or shortened component life,

and start looking for worn vanes, seals, or leaks.

Sometimes the first question should be much simpler:

What oil is actually in the pump?

What Happens if Vacuum Pump Oil Is Too Thin?

An oil that is too thin for the pump and operating conditions may not maintain the sealing film the pump was designed around.

In an oil-sealed vacuum pump, that can matter both mechanically and pneumatically.

Reduced oil-film effectiveness can contribute to poorer internal sealing, which can affect attainable pressure and pumping performance.

It can also reduce the lubricant’s ability to maintain the intended film between loaded surfaces.

This does not mean that “thicker is always better.”

It means the pump was engineered around a particular lubricant behavior.

Edwards’ rotary-vane service guidance is a useful real-world example: the company specifically connects inappropriate low-viscosity oil with deterioration in pump speed, ultimate vacuum, and lifetime. (Edwards)

What Happens if Vacuum Pump Oil Is Too Thick?

Going the other direction creates a different set of problems.

A much heavier oil resists flow more strongly, particularly when the pump is cold.

That can increase starting load, slow oil circulation, and change lubrication behavior until operating temperature is reached.

This is one reason manufacturers publish both the viscosity grade and temperature-related properties rather than simply labeling a product “vacuum oil.”

Modern synthetic lubricants may be selected specifically because their viscosity changes less dramatically across a wide temperature range. Busch, for example, describes its PAO synthetic VSI 100 as having high viscosity-temperature stability for lubrication at elevated operating temperatures; it publishes the oil at 100.8 mm²/s at 40°C and 14.0 mm²/s at 100°C. (Busch Vacuum Solutions)

This also explains why two oils that both say ISO VG 100 may behave differently as the operating temperature changes.

What Is Viscosity Index?

You may occasionally see another number called VI, or viscosity index.

Viscosity index describes how strongly an oil’s viscosity changes with temperature.

A lubricant with better viscosity-temperature stability maintains its viscosity characteristics over a wider temperature range.

This becomes important in equipment exposed to cold starts, high operating temperatures, seasonal ambient changes, or continuous heavy-duty operation.

For example, Busch publishes a viscosity index of 141 for its PAO synthetic VSI 100. Its mineral VM 022 is published with a VI of 90. These are different oils designed for different applications, but the comparison illustrates why viscosity grade alone does not tell the complete temperature-performance story. (Busch Vacuum Solutions)

Viscosity Is Not the Same as Vapor Pressure

This distinction is particularly important in vacuum service.

An oil can have the correct viscosity and still be unsuitable for the vacuum level you are trying to achieve.

Why?

Because the oil itself has a vapor pressure.

Every liquid has some tendency to evaporate.

Inside a vacuum pump operating at very low pressure, an oil with excessive vapor pressure can become part of the gas load and ultimately limit the pressure the system can achieve.

This is why vacuum oils are formulated differently from ordinary machine lubricants.

Busch specifically identifies low vapor pressure as an important characteristic of its VMA 055/A119 vacuum oil because lower vapor pressure allows a pump to reach lower ultimate pressures. Leybold similarly describes its vacuum oils as formulated for low vapor pressure in addition to thermal stability and oxidation resistance. (Busch Vacuum Solutions)

So:

Viscosity tells you how the oil flows.

Vapor pressure tells you how readily the oil itself contributes vapor under vacuum.

Those are different properties.

Both can matter.

Can I Use Hydraulic Oil or Compressor Oil With the Same ISO VG Number?

This is where ISO grade can become dangerous if it is treated as the entire specification.

An ISO VG 68 hydraulic oil and an ISO VG 68 vacuum pump oil may have similar viscosity at 40°C.

That does not make them equivalent fluids.

A vacuum pump lubricant may be selected or formulated around vapor pressure, volatility, oxidation stability, additive chemistry, gas separation, water handling, elastomer compatibility, and process-gas exposure in addition to viscosity.

Leybold describes its vacuum oils as specifically developed around characteristics including thermal stability, low vapor pressure, oxidation resistance, and application-specific chemistry. (Leybold Global Webshop)

Vacuum Pump Supply’s own SDS and oil-reference resource therefore states explicitly that oil viscosity alone does not determine compatibility. Pump model, lubricant type, process conditions, operating temperature, and chemical compatibility all need to be considered.

That is the correct way to evaluate a substitute.

Mineral vs. Synthetic Vacuum Pump Oil

Two oils can have nearly identical viscosity values but completely different base chemistry.

Consider an ISO VG 100 mineral oil and an ISO VG 100 PAO synthetic oil.

Their room-temperature viscosity may be quite close.

Their performance under high temperature, oxidation exposure, water contamination, or long service intervals may not be.

VPL M100 is currently specified as a hydrocracked and isodewaxed mineral vacuum pump oil with a published viscosity of 103 cSt at 40°C. VPL S100 is a PAO synthetic oil with a published viscosity of 100 cSt at 40°C. (Vacuum Pump Supply)

From a viscosity-number standpoint, they look extremely similar.

From a chemistry standpoint, they are different lubricants.

That is exactly why choosing oil by “100 weight” or “close enough viscosity” alone is not a reliable method.

What About PFPE Vacuum Pump Oil?

PFPE is an even clearer example.

Perfluoropolyether fluids are used where ordinary hydrocarbon oils may be inappropriate, including certain chemically aggressive or oxidizing applications.

Busch describes PFPE fluids as chemically inert and suitable for specific corrosive or elevated-oxygen applications. Leybold similarly specifies PFPE-based LVO 400 for particular reactive and oxidizing service. (Busch Vacuum Solutions)

A PFPE oil may happen to have a viscosity numerically similar to a mineral oil.

That does not make those oils interchangeable.

Changing a pump between hydrocarbon and PFPE service can require manufacturer-specific preparation, cleaning, compatible seals, and application review.

For oxygen-rich, oxidizing, reactive, or chemically aggressive processes, lubricant selection becomes a process-safety decision, not simply a maintenance choice. Use only a fluid and conversion procedure approved for the exact pump and application.

What Does Oil Vapor Pressure Have to Do With Ultimate Vacuum?

Imagine a perfectly tight pump with fresh seals and no process contamination.

As pressure falls, eventually the gases released by surfaces, seals, process materials—and potentially the pump oil itself—become increasingly important.

If the lubricant has significant vapor pressure at operating temperature, oil molecules entering the gas phase can establish a practical pressure floor.

This is one reason oils designed for deeper-vacuum service emphasize low volatility and low vapor pressure.

It is also one reason a lubricant intended for a rough-vacuum industrial pump should not automatically be assumed suitable for a two-stage pump being used as a backing pump in a high-vacuum system.

Edwards describes its Ultragrade Performance 19 oil as intended for small and medium two-stage rotary vane pumps where both good lubrication and high ultimate vacuum are required. (Edwards Vacuum)

The pump, operating pressure, and lubricant have to be considered together.

Why Does Viscosity Change When Oil Gets Hot?

Oil molecules move more freely as temperature increases.

The practical result is lower viscosity.

This is why a pump may behave differently immediately after a cold start than it does after reaching normal operating temperature.

It is also why manufacturers specify viscosity at standardized temperatures rather than simply describing an oil as “thin” or “heavy.”

Take Busch VM 100 again:

at 40°C, its published viscosity is approximately 100 mm²/s.

At 100°C, that falls to approximately 10.7 mm²/s. (Busch Vacuum Solutions)

That is nearly an order-of-magnitude change.

Operating temperature therefore affects how the oil flows through the pump and how the lubricating film behaves.

Why Does Old Vacuum Pump Oil Change Performance?

Oil does not remain chemically and physically unchanged forever.

Heat, oxygen, process vapor, water, solvents, particulates, and reactive gases can gradually alter it.

Oxidation can generate heavier degradation products and deposits.

Process contamination can change viscosity.

Water can create cloudy or emulsified oil.

Light solvents can thin the lubricant.

Other contamination can make it thicker or produce sludge.

So even if the pump originally contained exactly the correct oil, the oil may no longer behave like the original product after extended contaminated service.

Leybold recommends more frequent oil changes when aggressive vapors consume the protective properties of specialty oils and notes that water contamination can prevent some oil-filled pumps from reaching their ultimate pressure. (Leybold)

Edwards likewise emphasizes oil condition as a major part of rotary vane pump maintenance. (Edwards)

Does Dark Oil Always Mean the Viscosity Is Wrong?

No.

Color is a useful maintenance indicator, but it does not identify viscosity by itself.

Dark oil can indicate oxidation, overheating, process contamination, suspended material, or chemical degradation.

Milky oil commonly suggests water or other condensable contamination.

An oil can also appear relatively clean while its chemical or physical properties have changed.

If oil condition is questionable, the safest approach is to determine why it changed, correct the process problem, and replace it with the correct fluid.

For severe contamination, a flushing procedure may be appropriate before adding the new operating oil.

Can Two Different Brands of ISO VG 68 Vacuum Oil Be Mixed?

Not automatically.

Even if both fluids are labeled ISO VG 68, they may use different base stocks and additive systems.

Mixing can change viscosity, foaming behavior, demulsibility, oxidation resistance, material compatibility, and vacuum performance.

Vacuum Pump Supply’s current M-series product pages therefore advise against casually mixing oils and recommend draining the existing fluid and following the pump manufacturer’s change or conversion procedure whenever possible. (Vacuum Pump Supply)

That becomes even more important when changing between mineral oil, ester, PAO synthetic, silicone, or PFPE chemistry.

Why Manufacturer Oil Numbers Can Be Confusing

Manufacturers rarely put the complete technical specification into the product name.

A customer may see names such as:

Edwards Ultragrade Performance 19,

Leybold LVO 130,

Busch VM 068,

Welch DuoSeal oil,

or Agilent DS19.

Those names do not tell you enough by themselves to safely substitute one lubricant for another.

You need the underlying specifications.

That is why an oil cross reference should compare more than branding.

Vacuum Pump Supply’s current oil cross-reference resource includes oil chemistry, viscosity information, typical applications, and replacement direction across Edwards, Leybold, Busch, Welch, Agilent/Varian, Becker, Kinney, and other vacuum pump families.

The correct sequence is:

identify the pump → identify the specified oil → verify chemistry and viscosity → consider the process → then evaluate the replacement.

A Practical Example: ISO VG 32 vs. 68 vs. 100

The three VPS mineral-oil grades provide an easy way to see how significant the viscosity differences are.

VPL M32 is approximately 33 cSt at 40°C.

VPL M68 is approximately 67 cSt at 40°C.

VPL M100 is approximately 103 cSt at 40°C. (Vacuum Pump Supply)

M100 is therefore more than three times as viscous as M32 at the published 40°C test point.

These are not trivial differences.

They are different lubricant grades intended for pumps and applications requiring different oil characteristics.

This is why pouring “whatever vacuum oil is on the shelf” into an oil-sealed pump can produce disappointing results even though the bottle says vacuum pump oil.

Frequently Asked Questions

Is ISO VG 68 the same as 68 cSt?

ISO VG 68 identifies a viscosity grade centered around approximately 68 cSt at 40°C. The actual published viscosity of a specific oil can vary within the grade. For example, VPS M68 is published at 67 cSt at 40°C, while Busch VM 068 is published at 68 mm²/s. (Vacuum Pump Supply)

Is cSt the same as mm²/s?

Yes. For kinematic viscosity, one centistoke equals one square millimeter per second.

Is ISO VG 100 thicker than ISO VG 68?

Yes, at the same reference temperature. An ISO VG 100 lubricant has a higher nominal kinematic viscosity than an ISO VG 68 lubricant.

Does thicker vacuum pump oil create a deeper vacuum?

Not necessarily.

Ultimate pressure depends on pump design, internal condition, oil sealing behavior, lubricant vapor pressure, temperature, gas load, contamination, and other factors.

Using a heavier oil than specified is not a reliable way to improve vacuum.

Can I substitute an oil if the cSt value matches?

Not based on cSt alone.

You also need to verify base chemistry, vapor pressure requirements, pump design, additives, operating temperature, process gas compatibility, seal compatibility, and manufacturer requirements. VPS’s current SDS resource specifically warns that viscosity alone does not establish compatibility.

Why does vacuum pump oil need low vapor pressure?

The oil is exposed directly to vacuum. An oil with excessive vapor pressure can itself contribute gas to the system and interfere with the pump’s ability to reach lower pressure. Busch specifically identifies low vapor pressure as an important characteristic of oil intended for lower ultimate-pressure operation. (Busch Vacuum Solutions)

Is synthetic vacuum pump oil always better than mineral oil?

No.

Synthetic oil can provide advantages in specific applications, including thermal stability or wider-temperature performance, but the correct fluid is the one specified or approved for the pump and process.

A mineral oil is not automatically inferior, and a synthetic oil is not automatically compatible.

Can I use automotive oil in a vacuum pump?

Do not select oil for an oil-sealed vacuum pump simply because an automotive lubricant appears to have a similar viscosity. Vacuum pump oils are chosen around vacuum-specific requirements including volatility, vapor pressure, sealing, oxidation behavior, contamination exposure, and pump-material compatibility.

The Bottom Line

The viscosity number on a bottle of vacuum pump oil matters.

But it does not tell the whole story.

ISO VG tells you the viscosity class.

cSt tells you the measured kinematic viscosity at a stated temperature.

Viscosity index tells you something about how viscosity changes as temperature changes.

Vapor pressure affects how the oil behaves at low pressure.

Base chemistry determines how the lubricant interacts with heat, process gases, contamination, seals, and the application.

That is why the correct question is not:

“Do you have a 68-weight vacuum oil?”

The better question is:

“What oil does this exact pump require in this application?”

Vacuum Pump Supply’s oil pages now include viscosity and oil-type specifications for many VPS, Edwards, Leybold, and specialty fluids, along with an expanding OEM oil cross-reference and SDS library. (Vacuum Pump Supply)

For oil selection, send the pump manufacturer, complete model number, current oil name or part number, and process information if the pump handles unusual gases, solvents, moisture, oxygen-rich service, or aggressive chemistry.

That allows the oil to be matched to the pump instead of matching a bottle by number alone.

Useful internal links for this article are the Vacuum Pump Oil & Fluids category, the Vacuum Pump Oil Cross Reference, and the Vacuum Pump Oil Safety Data Sheets library. Product-level links can also go directly to M32, M68, M100, and the relevant OEM oil when the article discusses a real viscosity example.

A Vacuum Pump Can Be Working Correctly While the System Still Will Not Reach Vacuum

A vacuum system that refuses to reach its expected pressure does not automatically have a bad vacuum pump.

The pump may be operating normally while outside air enters through a seal, fitting, valve, hose, feedthrough, flange, shaft seal, weld, or damaged sealing surface.

At lower pressures, there is another complication: gas can also be released from surfaces and materials inside the chamber. This is called outgassing.

That means poor vacuum performance can come from at least three different places:

The pump itself.

A real leak into the system.

Gas being released from inside the system.

Correctly identifying which one you have can save hours of unnecessary pump repair.

Leybold identifies leaks and gas liberation from vessel walls and seals as two of the primary reasons a vacuum system may fail to reach its desired ultimate pressure. (Leybold)

What Is a Vacuum Leak?

In everyday language, a vacuum leak is simply a path that allows gas to enter an evacuated system.

Vacuum engineering usually expresses the size of that leak as a leak rate, often in:

mbar·L/s

The basic idea is gas throughput: how much gas enters the system over time.

Leybold defines a leak rate of 1 mbar·L/s as the amount of gas flow that would cause the pressure in an isolated 1-liter vessel to rise by 1 mbar in one second. (Leybold)

That definition becomes extremely useful because it means you can estimate the leak rate of a chamber without immediately using a helium leak detector.

You need the chamber volume, a vacuum gauge, and a stopwatch.

First Determine Whether the Pump or the System Is the Problem

Before chasing every O-ring on the machine, separate the pump from the rest of the vacuum system.

If practical for the equipment, isolate or blank off the pump inlet and determine whether the pump can approach the pressure expected for that pump under appropriate test conditions.

If the pump performs normally when isolated but performs poorly when connected to the chamber, the problem is likely somewhere in the system, process, or connecting plumbing rather than inside the pump.

If the pump still cannot reach an acceptable pressure when properly isolated, investigate the pump itself: oil condition, gas-ballast position, filters, internal seals, vanes, valves, or other maintenance issues.

This is an important distinction because replacing a repair kit will not fix a leaking chamber flange.

Likewise, replacing every chamber O-ring will not fix a worn vacuum pump.

Our earlier article on Vacuum Pump CFM vs. Ultimate Vacuum explains why the pressure measured in a complete system can differ significantly from the pump’s published ultimate-pressure specification. (Vacuum Pump Supply)

Vacuum Pump CFM vs. Ultimate Vacuum

The Pressure-Rise Test: One of the Most Useful Vacuum Troubleshooting Tools

A pressure-rise test is one of the simplest ways to determine how well an evacuated chamber holds vacuum.

Pump the chamber down as far as reasonably possible.

Then isolate the chamber from the vacuum pump.

Record the pressure immediately after isolation.

Continue recording pressure versus time.

If gas is entering or being released inside the chamber, the pressure will begin to rise.

The rate and shape of that pressure rise provide useful information about what is happening.

Leybold describes this method as closing the valve between the evacuated vessel and pump, measuring the pressure increase over a known period, repumping the system, and repeating the test. (Leybold)

How to Calculate Vacuum Leak Rate From a Pressure-Rise Test

Once chamber volume is known, the basic leak-rate equation is:

qL = V × Δp / Δt

where:

qL = leak rate
V = chamber volume
Δp = change in pressure
Δt = elapsed time

If volume is expressed in liters, pressure in mbar, and time in seconds, the resulting leak rate is expressed in mbar·L/s. (Leybold)

For example, suppose you have a 50-liter vacuum chamber.

After isolating it from the pump, pressure rises from:

0.010 mbar

to:

0.025 mbar

in:

60 seconds

The pressure change is:

0.015 mbar

So:

qL = 50 × 0.015 / 60

The calculated gas load is approximately:

0.0125 mbar·L/s

That number is far more useful than simply saying, “The vacuum comes up pretty fast.”

It gives you a measurable baseline.

You can repair a suspected leak, repeat the test, and see whether the system actually improved.

A Pressure Rise Does Not Automatically Mean You Have a Leak

This is one of the most important points in vacuum troubleshooting.

An isolated chamber can rise in pressure even when there is no conventional hole allowing room air into the vessel.

Materials inside the system may release trapped or adsorbed gas.

Elastomers can release gas.

Moisture can desorb from chamber surfaces.

Porous materials can contain trapped gas.

Process residue can continue evaporating.

This is outgassing.

Leybold notes that a true leak tends to produce a more constant gas load, while gas liberation from surfaces normally decreases with time. That difference can often be seen in the shape of the pressure-rise curve. (Leybold)

Leak or Outgassing? Look at the Shape of the Curve

Imagine plotting pressure vertically and time horizontally.

A real leak with relatively constant gas flow tends to produce a pressure-rise curve that remains more nearly linear.

Outgassing usually behaves differently.

As surfaces release gas and gradually become depleted, the rate of pressure increase slows. The curve begins to flatten.

Real systems often contain both effects simultaneously, so the distinction is not always perfect. But repeating the pressure-rise test after additional pump-down time can help.

Leybold recommends repeating the measurement after repumping the system. If the time required for the same pressure increase remains roughly constant, that supports the presence of a real leak. If the pressure rise becomes progressively slower, declining outgassing is likely contributing significantly. (Leybold)

This distinction matters because the solutions are completely different.

A damaged O-ring needs repair.

A wet chamber may simply need more pump-down time, warming, cleaning, or process correction.

What Is a “Virtual Leak”?

Technicians sometimes describe trapped gas or slowly released gas as a virtual leak.

There may be no actual passage from atmosphere into the chamber.

Instead, gas is trapped somewhere inside the vacuum system and slowly escapes into the evacuated volume.

Examples can include blind threaded holes, trapped volumes behind seals, porous materials, contaminated surfaces, or spaces inside poorly designed assemblies.

To the vacuum gauge, this gas load can look remarkably similar to a real leak.

That is another reason simply watching the vacuum gauge during pump-down does not always tell you what is wrong.

Where Do Vacuum Leaks Usually Occur?

Start with anything that was recently disturbed.

Common locations include flange O-rings, KF centering rings, clamps, valve seals, threaded fittings, hose connections, flexible vacuum tubing, feedthroughs, gauge connections, viewports, chamber doors, shaft seals, welds, and scratched sealing surfaces.

An O-ring does not need to be visibly destroyed to leak.

A hair, small particle, twist, nick, or scratch across the sealing surface can create a significant gas path at vacuum.

Before using more sophisticated equipment, visually inspect accessible sealing surfaces and confirm that components are assembled correctly.

Do not automatically tighten every clamp harder.

Vacuum seals work through correct geometry and sealing-surface contact, not unlimited clamping force.

Why Gauge Location Matters

The pressure displayed by a gauge is the pressure where the gauge is installed.

It is not necessarily identical to the pressure at the pump inlet or elsewhere in the chamber.

Restrictions between the gauge, chamber, and pump can create pressure differences while gas is flowing.

This is the same conductance effect discussed in last week’s pump-sizing article.

During a pressure-rise test, however, the system is isolated from the pump and there is no continuous pumped flow, making the gauge especially useful for tracking the chamber’s gas load over time.

Make sure the gauge itself and its connection are not creating the leak you are trying to find.

Also remember that different gauge technologies cover different pressure ranges and may respond differently to various gases.

VPS maintains a conversion resource for comparing Torr, micron, mbar, Pa, kPa, inHg, and mmHg when equipment uses different pressure units. (Vacuum Pump Supply)

Vacuum Pressure Conversion Chart

When a Pressure-Rise Test Is Not Enough

A pressure-rise test tells you that gas is entering or being released inside the system.

It does not necessarily tell you where.

For that, a tracer-gas leak detector is often the better tool.

Helium is widely used because it is inert, readily detectable, and can pass through very small leak paths. Mass-spectrometer helium leak detectors can locate leaks far smaller than those that could be identified using simple mechanical methods. (Leybold)

How Helium Vacuum Leak Detection Works

In vacuum-mode helium leak testing, the test object is evacuated.

A helium-sensitive leak detector is connected to the vacuum system.

A small amount of helium is then applied externally around suspected leak locations.

If helium enters through a leak, the detector senses the helium and displays a response.

This allows a technician to move methodically around flanges, welds, seals, feedthroughs, valves, and other possible leak points until the location is identified.

Leybold recommends applying helium slowly and methodically rather than flooding the entire test piece. Too much helium can make it difficult to determine where the gas entered the system. (Leybold)

Vacuum Mode vs. Sniffer Mode

Helium leak detectors can often operate in two fundamentally different configurations.

Vacuum mode places the component under vacuum and looks for tracer gas entering from outside.

Sniffer mode usually involves a component containing tracer gas at higher pressure while a handheld probe searches externally for escaping gas.

Edwards describes its ELD500 as capable of vacuum mode for precise leak-rate measurement and sniffer mode for locating leaks. (Edwards)

The correct method depends on the component and how it operates in service.

Vacuum equipment should generally be tested in a configuration representative of actual vacuum operation whenever possible. Leybold similarly distinguishes the vacuum method—gas moving from outside into an evacuated component—from positive-pressure methods where gas escapes outward. (Leybold)

How Sensitive Is a Helium Leak Detector?

Very sensitive.

The current Edwards ELD500 specification lists minimum detectable helium leak rates down to 5 × 10⁻¹² mbar·L/s for certain versions in vacuum operation, while sniffer operation is specified down to 7 × 10⁻⁹ mbar·L/s. (Edwards)

Vacuum Pump Supply currently carries Edwards ELD500 and Leybold PHOENIX helium leak-detection equipment for applications requiring quantitative leak testing and precise localization. (Vacuum Pump Supply)

Edwards ELD500 Wet Helium Leak Detector

Leybold PHOENIX Quadro Leak Detector

Do You Always Need a Helium Leak Detector?

No.

Many routine vacuum problems can be diagnosed by systematically isolating sections of the system, inspecting seals, verifying the pump independently, and performing pressure-rise testing.

Helium becomes especially valuable when the required leak tightness is much more stringent, the leak cannot be found visually, the chamber contains many possible leak points, quantitative leak-rate documentation is required, or downtime makes faster localization economically important.

A $10 O-ring can stop an expensive machine.

Finding which $10 O-ring is leaking is often the difficult part.

Why Installing a Bigger Pump Can Hide a Leak

There is an interesting connection between leak rate and pumping speed.

At steady state, additional pumping speed can sometimes lower the observed operating pressure even though the leak itself has not changed.

Leybold notes that, in principle, increasing effective pumping speed can compensate for a fixed leak rate enough to reach a specified operating pressure. In practice, that can become impractical or expensive. (Leybold)

This is important because installing a larger pump can sometimes make a leaking system appear to perform better.

But the leak is still there.

If the process requires genuine leak integrity—not merely a lower gauge reading—more pump is not the same thing as repairing the system.

What If the Pump Itself Is Leaking?

Vacuum pumps contain their own sealing components.

Shaft seals can wear.

O-rings and gaskets can harden.

Valve components can become damaged.

Oil leaks can sometimes accompany air leakage depending on the pump design and location.

If isolation testing points toward the pump, use the manufacturer’s service documentation and exploded parts diagrams to identify the correct seals, gasket sets, repair kits, filters, vanes, and other components.

The Vacuum Pump Supply technical library currently includes manuals, exploded diagrams, schematics, and parts references for Edwards RV and E2M, Busch R5, Leybold TRIVAC and SOGEVAC, Welch DuoSeal and CRVPro, Alcatel/Adixen, and other common pump families. (Vacuum Pump Supply)

Vacuum Pump Schematics, Manuals & Parts Diagrams

Quick Diagnostic Guide

Frequently Asked Questions About Vacuum Leak Detection

How do I know if my vacuum chamber has a leak?

Pump the chamber down, isolate it from the pump, and observe how the pressure changes over time. A repeatable pressure rise can indicate gas entering or being released inside the chamber. Repeating the test after additional pump-down time helps distinguish a constant leak from declining outgassing. (Leybold)

What is a good vacuum leak rate?

There is no universal “good” leak rate.

The acceptable leak rate depends on the operating pressure, chamber volume, process gas load, required stability, and application. A system operating at modest rough vacuum can tolerate a leak that would be completely unacceptable in a high- or ultra-high-vacuum instrument. Leybold specifically notes that leak-tightness requirements become more stringent as the required pressure becomes lower. (Leybold)

Can a vacuum pump overcome a leak?

A pump can remove gas entering through a leak, and a sufficiently high pumping speed may maintain an acceptable operating pressure despite some leakage. That does not make the system leak-tight, and increasing pump size is not a substitute for correcting a leak when system integrity matters. (Leybold)

Why does my chamber lose vacuum after I shut off the pump?

Possible causes include a real leak, outgassing, vapor evaporation, permeation through materials, or a leaking isolation valve. A pressure-rise test combined with repeated measurements can help distinguish the dominant cause.

Why does the pressure-rise test improve each time I repeat it?

If the pressure rises more slowly after additional evacuation, declining outgassing may be responsible for part of the original gas load. A true fixed leak tends to remain more consistent from test to test. (Leybold)

Why is helium used for vacuum leak testing?

Helium is an effective tracer gas because it is inert, readily detectable with mass-spectrometer equipment, and can pass through extremely small leak paths. This allows helium systems to locate very small leaks rapidly and quantitatively. (Leybold)

The Bottom Line

When a vacuum system cannot reach or hold the expected pressure, do not immediately blame the pump.

First determine whether the pump can perform normally when separated from the system.

Then determine whether the chamber has a measurable gas load.

Use a pressure-rise test to quantify that load.

Watch how the pressure-rise curve changes over time to help distinguish a constant leak from outgassing.

Inspect recently disturbed seals and connections.

And when the leak is too small or too difficult to locate mechanically, helium leak detection provides a far more sensitive method of finding the actual problem.

The goal is not simply to get the gauge reading lower.

It is to understand where the gas is coming from.

That is the difference between repairing the vacuum system and merely treating the symptom.

If troubleshooting points back to the pump, Vacuum Pump Supply’s technical library can help identify the correct manual, schematic, seal, filter, vane set, or repair kit for the exact pump model. If the system requires quantitative leak testing, VPS also carries Edwards and Leybold helium leak-detection equipment.

For assistance, send the pump manufacturer, complete model number, chamber type, target vacuum, current measured pressure, and what happens during an isolated pressure-rise test. That information can narrow the problem considerably before parts are replaced.

Contact Vacuum Pump Supply

Rebuild, Replace, or Upgrade? Start With the Problem You Are Actually Trying to Solve

A vacuum pump that no longer meets your process requirements does not automatically need to be replaced.

And a larger pump is not automatically an upgrade.

Sometimes a worn pump needs a rebuild.

Sometimes the existing pump is perfectly healthy, but the application has changed.

Sometimes the system is limited by piping, leaks or process load rather than the pump itself.

And sometimes an older pump should genuinely be replaced with a different size or technology.

The expensive mistake is making that decision based on one specification—usually CFM—without looking at the complete system.

Earlier this week we covered why vacuum pump CFM and ultimate vacuum are two different specifications. A larger CFM number generally means greater pumping speed, not necessarily a deeper vacuum. (Vacuum Pump Supply)

That leads to the next question:

If your current pump is no longer doing the job, should you rebuild it, replace it, or upgrade it?

Here is how to decide.

Option 1: Rebuild the Vacuum Pump You Already Have

If a pump performed correctly for years and has gradually lost performance, the first question should usually be:

What changed inside the pump?

Wear items such as vanes, seals, gaskets, O-rings, bearings, shaft seals and filters do not last forever.

Oil contamination and improper maintenance can accelerate that wear.

A pump that once reached the required vacuum but now has longer pump-down times, oil leakage, increased noise or difficulty reaching its previous pressure may simply be due for service.

In that situation, replacing the entire pump can be unnecessary.

Vacuum Pump Supply carries rotary vane repair kits and parts for Edwards, Busch, Leybold, Welch, Alcatel/Adixen, Becker, Rietschle and other common pump families. The current VPS rotary-vane section includes overhaul kits, minor kits, seals, vanes, filters, gaskets and model-specific service components. (Vacuum Pump Supply)

Shop Rotary Vane Vacuum Pump Repair Kits & Parts

When Rebuilding Usually Makes Sense

A rebuild is often worth considering when the pump previously met the application requirements, the major castings and pumping mechanism remain serviceable, the motor is good, and replacement parts are readily available.

It is especially attractive when the pump is already installed into equipment with established plumbing, electrical connections and controls.

Replacing the pump may require more than purchasing the pump itself.

A different model can mean new fittings, base plates, electrical changes, exhaust plumbing, inlet adapters or modifications to the machine.

Keeping the original pump can avoid many of those secondary costs.

For established pump families such as Edwards RV and E2M, Leybold TRIVAC, Busch R5, Welch and Alcatel/Adixen, VPS also maintains model-specific resources and service pathways rather than forcing buyers to start from scratch. (Vacuum Pump Supply)

Option 2: Replace the Pump With a Comparable Model

Eventually, replacing a pump becomes the better decision.

That may happen because of severe internal damage, obsolete components, repeated failures, motor problems, unacceptable downtime or simply because the cost of rebuilding no longer makes economic sense.

But this is where another common mistake occurs:

Finding another pump with roughly the same CFM and assuming it is a direct replacement.

Vacuum Pump Supply's current Vacuum Pump Replacement Guide specifically cautions against doing that. A proper replacement should also consider vacuum level, voltage, motor specifications, footprint, inlet and exhaust connections, chemistry compatibility and operating duty.

Vacuum Pump Replacement Guide

For example, the VPS cross-reference system compares industrial pump families such as Busch R5, Becker U Series, Leybold SOGEVAC, Rietschle VC/VGD and Edwards industrial rotary vane pumps separately from deeper-vacuum laboratory families such as Edwards RV/E2M, Leybold TRIVAC, Welch and Alcatel/Adixen.

That distinction matters.

A 20 CFM industrial single-stage pump and a 20 CFM two-stage laboratory pump may have similar headline flow numbers while being intended for very different operating pressures.

Do Not Pay for CFM You Cannot Use

This is where Monday's technical discussion becomes especially important.

Suppose your current pump is rated around 10 CFM and your pump-down time feels too slow.

The natural reaction is:

“Let's buy a 20 CFM pump.”

That may work.

Or it may accomplish surprisingly little.

The actual pumping speed available at the vacuum chamber can be restricted by narrow hoses, long piping runs, valves, elbows, traps, filters and other components between the chamber and pump.

If that conductance is the limiting factor, doubling the pump size does not necessarily double the pumping speed at the process. (Vacuum Pump Supply)

Before spending money on a larger pump, look at the entire vacuum path.

A plumbing improvement can sometimes be a better investment than adding horsepower.

When a Larger Pump Really Is the Right Answer

There are situations where increasing pumping speed is absolutely justified.

Perhaps production has expanded.

A larger vacuum chamber was installed.

Multiple chambers are now connected to one vacuum system.

The process generates more gas than it did originally.

Or management wants to reduce pump-down time to increase throughput.

Those are genuine reasons to evaluate a higher-capacity pump.

Vacuum Pump Supply currently carries new rotary vane pumps from Edwards, Leybold, Welch and other manufacturers for laboratory and industrial service. The rotary-vane category includes both oil-sealed and dry-vane technologies across a wide range of application classes. (Vacuum Pump Supply)

Shop New Rotary Vane Vacuum Pumps

But the correct replacement should still be selected around required operating pressure and application, not simply the highest CFM number the budget allows.

Option 3: Upgrade to a Different Vacuum Pump Technology

Sometimes the problem is not that the current pump is worn out.

The problem is that the pump technology is no longer ideal for the application.

An oil-sealed rotary vane pump may be extremely reliable in one process and a constant maintenance headache in another.

If oil contamination, oil vapor, process contamination or frequent oil changes are creating operational problems, moving to a dry pump may be worth evaluating.

Vacuum Pump Supply's dry scroll section currently includes new Edwards scroll pumps, Leybold scroll pumps, OEM repair components and OEM-equivalent tip-seal and overhaul kits. VPS describes scroll pumps as oil-free options commonly used in laboratories, analytical instruments, clean processes, vacuum ovens, freeze-drying support and backing applications. (Vacuum Pump Supply)

Shop Dry Scroll Vacuum Pumps & Parts

Moving from an oil-sealed pump to a dry scroll pump is not automatically the right solution for every application.

But if eliminating oil from the pumping mechanism solves a recurring process or maintenance problem, the higher initial pump cost may be justified by lower oil consumption, less contamination risk and simpler routine maintenance.

That is an application upgrade, not merely a pump replacement.

Repair Cost vs. Replacement Cost Is Only Part of the Equation

Maintenance departments often compare two numbers:

Cost of the repair kit

versus

Cost of the new pump

That is a useful starting point, but it is not the entire economic calculation.

A better comparison considers the total cost of getting the system operational again.

A $1,000 repair may appear expensive until it is compared with purchasing and installing a $5,000 or $10,000 replacement.

On the other hand, repeatedly rebuilding a pump that no longer suits the application can be false economy.

The right answer depends on why the pump is failing or underperforming.

Three Questions That Usually Reveal the Right Path

Before ordering anything, answer these three questions.

Did the pump ever perform correctly in this exact application?

If yes, and performance gradually deteriorated, investigate maintenance and rebuilding first.

Has the process changed?

If chamber volume, required pressure, gas load, production rate or operating conditions have changed, the existing pump may simply be undersized for the current job.

Is the pump technology creating the problem?

If oil contamination, solvent exposure, corrosive vapors or process cleanliness are the recurring issue, replacing an oil-sealed pump with another oil-sealed pump may not solve anything.

Those answers usually narrow the choice considerably.

A Vacuum Pump Cross Reference Is a Starting Point, Not a Purchase Order

Cross-reference charts are extremely useful when an existing pump has been discontinued or when a buyer wants to see equivalent pump families.

Vacuum Pump Supply's current cross-reference compares Busch, Becker, Leybold, Edwards, Welch, Alcatel/Adixen, Varian/Agilent, Rietschle and other commonly used pump families by flow class, vacuum level, horsepower and service direction.

Vacuum Pump Cross Reference Chart

But a cross reference should narrow the search—not replace engineering review.

Two pumps in the same general class may still have different motors, connections, dimensions, gas-ballast capabilities, accessories or process limitations.

Before substituting one model for another, verify the complete application.

Do Not Replace a Pump Because of a $50 Problem

This sounds obvious, but it happens.

A vacuum pump may suddenly lose performance because of contaminated oil, an inlet leak, clogged exhaust filter, damaged seal, open gas ballast valve or another service issue.

Those problems can make a healthy pump appear worn out.

Before condemning the pump, perform basic troubleshooting.

Check the oil.

Check the filters.

Check the gas ballast.

Check the inlet system for leaks.

Check whether the pump can reach acceptable pressure when isolated from the process.

Check the maintenance history.

VPS maintains manuals, exploded diagrams and technical resources for many common pump families to help maintenance teams identify components before ordering. (Vacuum Pump Supply)

Vacuum Pump Schematics, Manuals & Parts Diagrams

When a Repair Kit Is the Best Upgrade

There is another possibility that often gets overlooked:

The best upgrade may be returning the pump you already own to original performance.

A worn vacuum pump gradually becomes the new normal.

Pump-down takes a little longer.

Ultimate pressure is not quite as good.

Oil consumption increases.

The pump runs hotter.

Eventually, someone decides the pump is too small.

But if the pump originally met the process requirement, restoring worn vanes, seals, bearings, gaskets and filters may bring back the performance that was lost over years of operation.

That is one reason repair kits can provide such strong value compared with automatically purchasing a larger pump.

Vacuum Pump Supply supports both genuine OEM and OEM-equivalent service paths, allowing buyers to choose based on application requirements, maintenance philosophy and budget.

When Replacement Is the Better Investment

There are also times when rebuilding no longer makes sense.

Replacement becomes more attractive when the pump has major mechanical damage, repeated rebuild history, badly damaged internal surfaces, an obsolete motor or electrical configuration, persistent parts-availability problems, or when the process has permanently outgrown the pump.

Replacement can also make sense when downtime is more costly than the equipment.

A production line waiting several days for a rebuild may justify keeping a replacement pump available even if the original pump could theoretically be repaired.

This is where purchasing strategy matters as much as pump engineering.

The Best Answer May Be “Repair This One and Buy a Spare”

For mission-critical systems, the decision does not always have to be rebuild or replace.

Sometimes it should be both.

Rebuild the existing pump and return it to service.

Then purchase a compatible spare.

The next time maintenance is required, the spare pump can be installed while the original is serviced.

This approach can dramatically reduce emergency downtime for facilities operating critical vacuum equipment.

It also creates flexibility: maintenance can be scheduled rather than performed under pressure because production is stopped.

Let the Application Choose the Pump

The best vacuum pump is not the one with the highest CFM.

It is not necessarily the newest pump.

It is not automatically the most expensive pump.

And it is not always the pump already installed on the machine.

The best choice is the pump that provides the required pumping speed, operating pressure, process compatibility and reliability at a reasonable total cost.

Sometimes that means rebuilding what you have.

Sometimes it means replacing it with a modern equivalent.

Sometimes it means moving to a larger pump.

And sometimes it means changing pump technologies entirely.

Need Help Deciding? Send Us the Pump and the Application

If you are trying to decide whether to rebuild, replace or upgrade a vacuum pump, start with five pieces of information:

Manufacturer and complete model number, nameplate photo, current application, required vacuum level, and what problem you are trying to solve.

Vacuum Pump Supply can then help identify the practical paths available—repair kits and service parts, comparable replacement families, oils and filters, or new-pump options.

Start with the VPS Replacement Guide if you already know the model. Find Your Vacuum Pump Replacement

Or use the main pump cross reference to compare pump families. Compare Vacuum Pump Models

Do not buy more pump than you need. Do not replace a pump that only needs service. And do not keep rebuilding a pump that no longer fits the application.

Match the solution to the problem—and spend the maintenance budget where it actually improves the system.

Vacuum Pump CFM and Ultimate Vacuum Are Not the Same Thing

When comparing vacuum pumps, two specifications usually appear near the top of the data sheet:

Pumping speed, often expressed in CFM, m³/h or L/s.

And:

Ultimate vacuum, usually expressed in Torr, micron, mbar or Pascal.

It is easy to assume that the pump with the larger CFM number must also be the “stronger” vacuum pump.

That is not necessarily true.

A useful way to think about the difference is:

CFM tells you approximately how fast the pump can remove gas. Ultimate vacuum tells you how low a pressure the pump can ultimately reach.

Those are related characteristics, but they are not interchangeable.

A 20 CFM pump does not automatically pull twice as deep a vacuum as a 10 CFM pump.

In fact, two pumps can have dramatically different pumping speeds and almost identical ultimate-pressure specifications.

Or the opposite can happen: a relatively small pump may be capable of reaching a much lower ultimate pressure than a larger pump designed primarily for high gas throughput.

Understanding this distinction can prevent one of the most common mistakes in vacuum pump selection.

What Does CFM Mean on a Vacuum Pump?

CFM stands for cubic feet per minute.

It is a measure of volumetric pumping speed: how much gas volume the pump can move through its inlet per unit of time under specified conditions.

Manufacturers may instead express pumping speed in:

CFM, cubic meters per hour (m³/h), or liters per second (L/s).

These units describe essentially the same type of performance and can be converted from one to another.

Vacuum Pump Supply maintains a Vacuum Pressure Conversion Chart for pressure units such as Torr, micron, mbar and Pa, which is useful when comparing manufacturers that publish specifications differently. (Vacuum Pump Supply)

One important detail is that the pumping speed shown on a data sheet is not necessarily constant throughout the entire operating-pressure range.

As inlet pressure falls, the actual pumping-speed curve of many pumps changes.

Leybold's vacuum-technology reference notes that the pumping speed of single-stage rotary vane and rotary piston pumps can decrease significantly as they approach their lower pressure range.

That means a pump described as “20 CFM” should not be assumed to move exactly 20 cubic feet of gas every minute at every pressure.

What Is Ultimate Vacuum?

Ultimate vacuum—or ultimate pressure—is the lowest pressure a pump can approach under specified test conditions.

This is the specification that tells you how deep a vacuum the pump is capable of producing.

And this is where vacuum terminology can become confusing.

With vacuum, a lower absolute pressure means a deeper vacuum.

So:

0.1 Torr is a deeper vacuum than 1 Torr.

0.001 Torr is deeper than 0.1 Torr.

And 1 micron, which equals 0.001 Torr, is deeper than 100 microns. The VPS conversion reference lists 1 Torr as 1,000 microns and 1 mbar as approximately 0.750 Torr. (Vacuum Pump Supply)

This is why it is dangerous to compare vacuum pumps using a pressure number without first checking the units.

A Real Example: Edwards E2M28 and E2M30

An actual manufacturer's specification sheet demonstrates the difference nicely.

The Edwards E2M28/E2M30 manual lists a maximum displacement of 23 CFM at 60 Hz, while its maximum Pneurop pumping speed is 19.4 CFM. The same specification table separately lists ultimate total pressure without gas ballast at approximately 7 × 10⁻⁴ Torr.

Three different numbers are therefore describing three different things:

This is why copying only the biggest CFM number from a catalog does not provide enough information to properly compare vacuum pumps.

Higher CFM Usually Means Faster Pump-Down—Not Deeper Vacuum

Imagine two vacuum pumps.

Pump A is rated at 5 CFM and can reach an ultimate pressure of 0.001 Torr.

Pump B is rated at 20 CFM and can also reach an ultimate pressure of 0.001 Torr.

Assuming the pumps have appropriate operating curves and are connected to the same chamber under similar conditions, Pump B should evacuate the chamber more quickly through much of the usable pressure range.

But it does not necessarily create a deeper final vacuum.

Both pumps have the same stated ultimate-pressure capability.

Now imagine a third pump rated at only 5 CFM but capable of reaching 0.0001 Torr.

That smaller pump could potentially achieve a deeper vacuum than the 20 CFM pump even though it removes gas more slowly.

That is the fundamental distinction.

Pumping speed affects time. Ultimate pressure affects depth.

How Long Should It Take to Pump Down a Vacuum Chamber?

For a simplified rough-vacuum system, pump-down time can be approximated using:

t = (V / S) × ln(P₀ / P)

where:

t = pump-down time
V = chamber volume
S = effective pumping speed at the chamber
P₀ = starting absolute pressure
P = desired absolute pressure

Leybold gives this relationship for rough-vacuum chamber evacuation when effective pumping speed is approximately constant and the pump's ultimate pressure is substantially below the desired target pressure.

For example, consider a 100-liter chamber connected to a pump providing an effective pumping speed of about 10 CFM, or approximately 4.72 L/s.

Starting near atmospheric pressure at 760 Torr and targeting 1 Torr gives an idealized pump-down time of roughly 2.3 minutes.

That sounds wonderfully simple.

Real vacuum systems rarely behave that perfectly.

Why the Real Pump-Down Time Is Usually Longer

The CFM printed on the pump nameplate is not necessarily the pumping speed actually available inside the chamber.

The tubing between the chamber and pump creates flow resistance.

So do valves.

So do filters, traps, elbows, small fittings and other restrictions.

In vacuum engineering, this flow limitation is described as conductance.

Leybold defines the effective pumping speed of a vacuum system as the pumping speed that actually prevails at the vessel and specifically notes that baffles, cold traps, filters, valves and connecting tubing between the vessel and pump affect that value.

That distinction becomes extremely important when someone installs a larger vacuum pump but sees surprisingly little improvement in pump-down time.

The pump may not be the limitation.

The plumbing may be the limitation.

A large pump connected through a long, narrow vacuum hose cannot necessarily deliver its full rated pumping speed to the chamber.

Bigger Vacuum Lines Can Matter More Than a Bigger Pump

Suppose you replace a 10 CFM pump with a 20 CFM pump but leave the same restrictive hose, valves and fittings in place.

You might expect pump-down time to be cut in half.

It may not happen.

If conductance through the inlet plumbing is limiting the gas flow, much of the extra pump capacity never reaches the chamber.

This is why vacuum-system design should consider the entire path between the process and the pump.

Shorter lines generally help.

Larger-diameter lines generally help.

Unnecessary restrictions generally hurt.

And the effect becomes increasingly important as pressure decreases.

A vacuum pump should therefore be sized as part of a system, not as an isolated machine.

Why Ultimate Vacuum on the Data Sheet May Not Match Your Gauge

Another common question is:

“The manual says my pump reaches 1 × 10⁻³ mbar. Why doesn't my system reach that?”

Because a manufacturer's ultimate-pressure specification is normally measured under controlled conditions.

Your actual system may include chamber outgassing, tiny leaks, contaminated process gas, long vacuum lines, valves, traps, worn seals and other sources of additional gas load.

At lower pressures, outgassing from chamber surfaces becomes increasingly important. Leybold notes that in high-vacuum systems, gas released from chamber walls and system leakage can become dominant factors in the pressure that can ultimately be maintained. (Vacuum Pump Supply)

So there are really two different questions:

How low can this pump go under test conditions?

And:

How low can my complete vacuum system go during my actual process?

They are not always the same number.

Pumping Speed Also Changes With Pressure

Another reason CFM comparisons can be misleading is that pumps have pumping-speed curves.

A vacuum pump may deliver close to its rated pumping speed over one part of its operating range and considerably less as it approaches ultimate pressure.

The Edwards RV-series manual, for example, provides pumping-speed-versus-inlet-pressure curves rather than treating pumping speed as one constant number across the full pressure range. (Vacuum Pump Supply)

This matters when selecting a pump for a process that must operate continuously at a specific pressure.

Do not only ask:

“What is the maximum CFM?”

Also ask:

“What pumping speed does the pump provide at my actual operating pressure?”

That is a much better engineering question.

CFM vs. Ultimate Vacuum When Choosing a Pump

The correct priority depends on the application.

If you are evacuating a large chamber and the target pressure is relatively modest, pumping speed may be the dominant concern.

If you are operating an analytical instrument that requires much lower pressure, ultimate pressure may matter more.

If the process continuously generates gas or vapor, then throughput at the actual operating pressure becomes critical.

And if the application is sensitive to oil contamination, the pump technology itself may matter just as much as either number.

Vacuum Pump Supply's current Rotary Vane Vacuum Pump section covers oil-sealed pumps used across laboratory and industrial applications, while its Dry Scroll Pump section focuses on oil-free pumping for clean processes, instrumentation and backing applications. (Vacuum Pump Supply)

What Size Vacuum Pump Do I Need?

Start with the process rather than the pump catalog.

You need to know the chamber volume, starting pressure, target pressure, desired pump-down time, expected continuous gas load, process vapors, inlet-line size and length, and whether the application can tolerate oil.

Only then does the CFM number become useful.

For a basic rough-vacuum chamber with little additional gas load, the pump-down equation can provide a useful starting estimate.

For a production system, vacuum oven, freeze dryer, coating system, degassing process or high-vacuum backing application, real gas load and conductance should also be considered.

And the pump's ultimate pressure should have enough margin below the required operating pressure that the system is not trying to operate at the absolute limit of the pump.

Frequently Asked Questions

Does higher CFM mean a stronger vacuum?

No. Higher CFM generally means higher pumping speed. It does not automatically mean the pump can reach a lower ultimate pressure.

Does a bigger vacuum pump pull a deeper vacuum?

Only if the larger pump also has a lower ultimate-pressure specification. A larger pump with the same ultimate pressure will generally evacuate the system faster, not necessarily deeper.

Why didn't my pump-down time improve much after installing a larger pump?

Check the effective pumping speed at the chamber. Small hoses, long lines, valves, filters, traps and fittings can restrict conductance and prevent the chamber from seeing the pump's full rated capacity.

Is displacement the same as pumping speed?

Not necessarily. Manufacturer data may list displacement and measured pumping speed separately. The Edwards E2M28/E2M30 specifications are a good example: maximum displacement at 60 Hz is listed as 23 CFM while Pneurop pumping speed is listed as 19.4 CFM.

How do I compare CFM with m³/h?

They are different units for volumetric flow. When comparing pumps from different manufacturers, convert both to the same unit before making the comparison. Pressure specifications should likewise be converted into the same units before comparing ultimate vacuum.

Is ultimate vacuum the same as operating vacuum?

No. Ultimate vacuum is the lowest pressure the pump can approach under specified conditions. Your normal operating pressure may be considerably higher because of process gas load, leaks, outgassing and system conductance.

The Bottom Line

When comparing vacuum pumps, do not ask only:

“How many CFM is it?”

Ask two questions:

How quickly does the pump need to remove gas?

And:

How low does the pressure need to go?

Pumping speed answers the first question.

Ultimate pressure answers the second.

Then consider chamber volume, actual operating pressure, gas load and the conductance of everything between the pump and the process.

That combination tells you far more about whether a pump will work than the largest number printed on the specification sheet.

If you are comparing replacement pumps or trying to determine why a vacuum system is not meeting its expected pump-down time, Vacuum Pump Supply can help identify the correct pump family and supporting service products.

Send the pump manufacturer, complete model number, chamber size, target pressure and application.

That gives us the information needed to make a meaningful comparison instead of simply comparing CFM numbers.

Recommended Internal Links

Use Vacuum Pressure Conversion Chart when the article first discusses Torr, micron, mbar and Pa. Vacuum Pressure Conversion Chart

Use Vacuum Pump Cross Reference near the pump-comparison section; the current page already compares many pump families by approximate CFM and ultimate vacuum. Vacuum Pump Cross Reference

Use Rotary Vane Vacuum Pumps in the pump-selection section. Rotary Vane Vacuum Pumps

Use Dry Scroll Pumps and Parts where oil-free pumping is discussed. Dry Scroll Pumps and Parts

Use Vacuum Pump Schematics & Manuals Library near the section recommending that readers check the complete pumping-speed curve rather than just the headline CFM number. Vacuum Pump Schematics & Manuals Library

Gas Ballast Is Only Step One: How to Protect Your Vacuum Pump From Water, Solvents & Oil Contamination

Open the sight glass on an oil-sealed vacuum pump and the oil should tell you a lot.

If it has become cloudy, milky, unusually dark, or contaminated with process material, the problem may not be the pump itself.

The problem may be what the pump is being asked to swallow.

Water vapor, solvents and other condensable process vapors can enter an oil-sealed rotary vane pump, condense internally and contaminate the lubricating oil. Once that happens, vacuum performance can suffer and internal pump components may be exposed to accelerated wear or corrosion. Edwards identifies condensable-vapor contamination as a reason to use gas ballast and recommends allowing the pump to clear residual vapor after processing. (Edwards)

Earlier this week we explained what a gas ballast valve does and when to use it.

Gas ballast is an important part of the solution.

But for many applications, it should not be the entire solution.

A better approach is to control contamination at several points in the vacuum system: before it reaches the pump, while the pump is operating, inside the oil system, and at the exhaust.

Here is how.

Stop the Contamination Before It Reaches the Pump

The best place for water, solvent vapor and process contamination is outside the vacuum pump.

That is where an inlet trap or cold trap can make a major difference.

A vacuum trap is installed between the process and the pump. Depending on the design, it can capture condensable vapors, liquids, particulates or other contamination before those materials enter the pumping mechanism.

Vacuum Pump Supply carries vacuum inlet traps, foreline traps and cold traps designed for this type of pump protection. VPS describes these products as a way to capture vapors, condensables, particulates and process contamination upstream of the pump. (vacuumpumpsupply.com)

For applications involving significant condensable vapor, this can be far more effective than repeatedly allowing the contamination into the pump and then trying to clean it out afterward.

When Does a Cold Trap Make Sense?

Consider a cold trap when the process produces a meaningful vapor load from materials such as water or suitable solvents and the process conditions allow a trap to be used safely.

A cold trap lowers the temperature of the vapor stream so a portion of that material condenses in the trap rather than inside the vacuum pump.

Leybold specifically discusses using condensers together with gas-ballast pumps for water-vapor applications. (Leybold)

Vacuum Pump Supply’s 1420H-14 Cold Trap, for example, is designed to provide pre-pump vapor protection using a removable dry-ice/isopropanol cooling well. (vacuumpumpsupply.com)

The exact trap should always be selected around the process chemistry, vapor load, pressure range, connection size and safety requirements.

The important concept is simple:

If you can capture the contaminant before it reaches the pump, the pump does not have to deal with it later.

Use the Gas Ballast Correctly

If condensable vapor does reach an oil-sealed rotary vane pump, the gas ballast can help prevent that vapor from condensing during the pump’s compression cycle.

Edwards describes gas ballast as a means of reducing vapor contamination in pump oil, while Leybold explains that introducing ballast gas allows condensable vapor to be discharged before it condenses internally. (Edwards)

That makes gas ballast extremely valuable for wet processes.

But there is an important limitation:

Every pump has a finite vapor-handling capability.

Gas ballast does not turn a rotary vane pump into an unlimited solvent or water separator.

If the process continually feeds more condensable vapor into the pump than the pump can safely handle, oil contamination may still occur.

That is why applications with substantial vapor loads often benefit from a combination of:

proper gas-ballast operation + upstream vapor trapping.

For the detailed operating explanation, see our complete gas ballast guide.

Watch the Oil — It Is an Early Warning System

Vacuum pump oil performs several jobs inside an oil-sealed rotary vane pump.

It lubricates moving components, helps seal internal clearances and carries heat away from the pumping mechanism.

When process contamination changes the condition of the oil, pump performance can change with it.

Cloudy or milky oil is a particularly important warning sign when water or another condensable material may be entering the pump.

Do not simply keep running the pump and hope the oil clears.

First determine why the contamination is entering the pump.

Then correct the process or operating condition that caused it.

Depending on the severity of contamination, the next step may be gas-ballast decontamination, an oil change, or a complete flushing procedure.

Vacuum Pump Supply carries a broad selection of vacuum pump oils and specialty fluids for Edwards, Leybold, Busch, Welch, Alcatel/Adixen, Becker and many other pump families. VPS also maintains a Vacuum Pump Oil Cross Reference to help match OEM fluids with compatible service options. (vacuumpumpsupply.com)

One example is Plus UG19 Oil – 4 Liter, which VPS lists for numerous Edwards RV and E2M models as well as several Leybold, Welch and Alcatel/Adixen pumps. Always verify the exact oil specification for the pump and process before changing fluid types. (vacuumpumpsupply.com)

When an Oil Change Is Not Enough

There is a major difference between draining contaminated oil and cleaning contamination out of a pump.

Deposits, degraded lubricant and process material can remain on internal surfaces after the drain plug has been opened.

If the oil was severely contaminated, simply pouring fresh oil into a dirty pump can immediately contaminate the new oil.

That is where flushing fluid becomes useful during maintenance.

Vacuum Pump Supply’s VP FF Vacuum Pump Flushing Fluid is intended to help loosen and carry away old lubricant, deposits, dirt and contamination before the pump is refilled with the correct operating oil. (vacuumpumpsupply.com)

Flushing is particularly worth considering after severe contamination events, degraded oil, or before refilling a pump after maintenance when the internal oil circuit needs to be cleaned.

It is not a substitute for fixing the contamination source.

If a process continues feeding water or solvent into the pump, the new oil will eventually suffer the same fate.

The goal should be:

Clean the pump. Correct the source. Refill with the proper oil.

Do Not Forget the Exhaust Side

Contamination control does not stop at the pump inlet.

Oil-sealed rotary vane pumps can discharge oil aerosol through the exhaust, particularly during higher gas throughput.

Edwards notes that even a pump operating normally may produce some oil mist and that significant oil mist over time can contribute to loss of the pump’s oil charge. (Edwards)

An exhaust mist filter helps capture that oil aerosol before it enters the surrounding workspace or exhaust system.

Vacuum Pump Supply carries oil mist filters, replacement elements and exhaust filtration for a wide variety of vacuum pumps.

For Edwards RV-series pumps, for example, the Edwards EMF20 Oil Mist Filter is available as a genuine OEM exhaust-filter assembly. (vacuumpumpsupply.com)

Edwards also offers oil-return arrangements for some pump and mist-filter combinations, and Vacuum Pump Supply lists these alongside its Edwards filtration products. (vacuumpumpsupply.com)

This becomes especially relevant when a pump spends significant time operating with gas ballast, because higher gas throughput can increase the amount of oil aerosol reaching the exhaust.

Think of Contamination Control as a System

The most reliable setup is rarely a single replacement part.

Instead, look at the complete path the process gas follows.

This is why troubleshooting strictly by pump model can sometimes miss the real problem.

Two identical vacuum pumps can have completely different maintenance requirements if one pumps clean dry air and the other processes water vapor or solvents.

The application matters just as much as the pump.

Has Contamination Already Damaged the Pump?

Sometimes correcting the oil and vapor problem restores performance.

Sometimes it exposes wear that has already occurred.

If the pump still has poor ultimate vacuum, long pump-down times, oil leakage or abnormal operation after the contamination source has been corrected and the oil system has been serviced, internal wear may need to be addressed.

Vacuum Pump Supply stocks rotary vane vacuum pump repair kits and parts for Edwards, Leybold, Busch, Becker, Welch, Alcatel/Adixen, Rietschle and other pump manufacturers.

For example, the current VPS OEM Edwards E2M28/E2M30 major repair kit contains vanes, shaft seals, O-rings, gaskets, filters, bearings and other wear components used during a comprehensive rebuild. (vacuumpumpsupply.com)

Replacing parts, however, should come after identifying why the contamination occurred.

A freshly rebuilt pump placed back into the same uncontrolled wet process can simply start the cycle again.

Spend Money Preventing the Failure, Not Recovering From It

The cost of vacuum pump contamination is rarely limited to a few liters of oil.

There can also be labor spent troubleshooting, repeated oil changes, filters, expedited parts, production downtime and eventually a premature pump rebuild.

That is why contamination control can be one of the better places to spend a maintenance budget.

A cold trap may prevent gallons of contaminated oil.

A correctly used gas ballast may prevent vapor from condensing inside the pump.

An exhaust mist filter may reduce oil loss and improve the working environment.

The correct oil protects the pump after the contamination source has been addressed.

And flushing fluid can help start the pump with a cleaner oil system after a serious contamination event.

The objective is not to sell more parts.

It is to stop buying the same maintenance parts because the original process problem was never corrected.

Not Sure What Your Pump Needs? Start With the Application.

If you are dealing with cloudy vacuum pump oil, water contamination, solvent vapor, excessive oil mist or repeated oil changes, send Vacuum Pump Supply:

the pump manufacturer, complete model number, process being pumped, current oil, and a photo of the pump nameplate.

That information often tells us far more than the pump model alone.

Vacuum Pump Supply can help identify the correct vacuum pump oil, trap or mist filter, repair kit or maintenance component for the application.

Protect the oil. Protect the pump. Reduce the rebuilds.

What Does a Gas Ballast Valve Do on a Vacuum Pump? When to Open It and When to Close It

What Is a Gas Ballast Valve on a Vacuum Pump?

If you operate an oil-sealed rotary vane vacuum pump, you may have noticed a small knob or valve labeled Gas Ballast.

It is one of the most useful—and frequently misunderstood—controls on the pump.

A gas ballast valve allows a controlled amount of air, dry gas, or in some systems an approved inert gas to enter the pump during the compression portion of the pumping cycle. Its purpose is to help prevent water vapor, solvent vapor, and other condensable vapors from turning back into liquid inside the pump. (Edwards)

That matters because condensed vapor can mix with the vacuum pump oil, contaminate it, reduce vacuum performance, and eventually contribute to corrosion, deposits, accelerated wear, or more frequent oil changes. (Edwards)

The simplest rule is:

Open the gas ballast when pumping condensable vapors. Close it when you need the best ultimate vacuum and are pumping dry gases.

That sounds simple, but understanding why makes troubleshooting a vacuum pump much easier.

Why Does Vapor Condense Inside a Vacuum Pump?

A rotary vane vacuum pump draws gas from the vacuum system and progressively compresses it before exhausting it to atmosphere.

If that gas contains water vapor or another condensable material, compression increases its partial pressure. Under the right combination of temperature and pressure, the vapor reaches its saturation point and condenses into liquid inside the pump.

The gas ballast changes those conditions by introducing additional non-condensable gas late in the compression cycle. This allows the exhaust valve to open before the condensable vapor reaches the pressure at which it would otherwise liquefy inside the pump. (Edwards)

Instead of remaining inside the pump and contaminating the oil, more of the vapor leaves through the exhaust.

This principle was developed specifically to improve the ability of oil-sealed pumps to handle vapor loads. (Edwards)

When Should the Gas Ballast Valve Be Open?

The gas ballast should generally be used when the process introduces significant amounts of condensable vapor into the vacuum pump.

Water vapor is one of the most common examples. Processes involving drying, evaporation, vacuum ovens, moisture removal, laboratory work, solvent processing, or other wet loads can introduce substantial vapor into the pump.

Edwards specifies gas-ballast operation for high concentrations of condensable vapor on its E2M rotary vane pumps, while Leybold similarly recommends operating the pump at temperature with the gas ballast open when pumping vapors. (Vacuum Pump Supply)

For the Edwards E2M28 and E2M30, for example, the manufacturer procedure calls for isolating the pump from the system, opening the gas ballast, warming the pump for approximately 30 minutes, and then opening the isolation valve while continuing to operate with the ballast open.

That 30-minute procedure is model-specific, so always check the operating manual for your exact pump rather than assuming every vacuum pump uses the same warm-up time.

You can find pump manuals, schematics, exploded diagrams, and service information in the Vacuum Pump Supply Schematics & Manuals Library. (Vacuum Pump Supply)

When Should the Gas Ballast Be Closed?

If you are pumping dry gases and trying to reach the pump's specified ultimate vacuum, the gas ballast is normally closed.

Opening the ballast intentionally adds gas to the pump. That helps with vapor handling, but it also raises the attainable ultimate pressure.

In other words, the pump will generally not pull as deep a vacuum with the gas ballast open.

This is normal operation—not necessarily a sign that the pump is worn out.

Edwards specifically lists an open gas-ballast control as one possible reason an E2M pump will not reach its specified ultimate vacuum. The manufacturer's operating instructions call for the ballast to be closed when pumping dry gases or when ultimate vacuum is required. (Vacuum Pump Supply)

This makes the gas ballast one of the first things worth checking when someone says:

“My vacuum pump runs normally, but it suddenly won't pull as low as it used to.”

Before tearing apart the pump, verify that the gas ballast is actually closed.

Why Does the Pump Run Warmer With the Gas Ballast Open?

A pump operating with gas ballast generally runs somewhat warmer because it is compressing and moving additional gas.

That temperature increase is intentional and can actually help with vapor handling because a warmer pump makes condensation less likely. Leybold notes that pump temperature rises during gas-ballast operation, while Edwards describes this additional heat as one factor that improves vapor handling. (Leybold)

The important distinction is between a normal temperature increase caused by gas-ballast operation and abnormal overheating caused by a blocked exhaust, low oil level, contaminated oil, poor cooling airflow, excessive process load, or another mechanical problem.

If your pump is unusually hot, do not assume the gas ballast is the only cause.

Can Gas Ballast Clean Contaminated Vacuum Pump Oil?

Sometimes.

If the oil has become contaminated primarily with condensable process vapor, operating the pump with gas ballast can help drive that vapor back out of the oil.

The Edwards E2M28/E2M30 operating manual specifically describes a decontamination procedure in which the pump is isolated from the vacuum system and operated with the gas ballast fully open until contaminated oil clears. The same manual says cloudy or discolored oil can indicate contamination by process vapor.

This does not mean every contaminated oil charge can be saved.

If the oil remains cloudy, dark, chemically degraded, dirty, or contaminated with solids or aggressive process chemicals, the correct solution may be an oil change or even a flushing procedure.

Vacuum Pump Supply carries mineral, synthetic, PFPE, flushing, and specialty vacuum fluids in the Vacuum Pump Oil & Fluids section. (Vacuum Pump Supply)

The critical point is to correct the source of the contamination, not simply keep replacing oil.

Gas Ballast vs. an Inlet Trap: They Do Different Jobs

A gas ballast helps the pump handle vapor after that vapor enters the pump.

An inlet trap attempts to stop or reduce contaminants before they enter the pump.

Those are two different functions.

For processes with large vapor loads, an appropriately selected cold trap, condensate trap, foreline trap, or other inlet protection can substantially reduce the amount of material reaching the pump in the first place. Vacuum Pump Supply's Filters, Traps & Mist Eliminators section includes inlet traps intended to capture vapors, condensables, particulates, and process contaminants before they reach the pump. (Vacuum Pump Supply)

In demanding applications, the best solution may involve both proper inlet protection and correct gas-ballast operation.

Gas ballast should not be treated as unlimited vapor-handling capacity. Every pump has application limits, and manufacturers may specify a maximum water-vapor handling capacity or other restrictions. (Edwards)

Why Is There More Oil Mist With the Gas Ballast Open?

Operators sometimes notice increased oil mist or oil consumption while running an oil-sealed pump with the gas ballast open.

That can be normal.

More gas is moving through the pump, which can carry more oil aerosol toward the exhaust. Edwards specifically notes an increased rate of oil loss during open gas-ballast operation and recommends an oil mist filter for certain applications where the pump is frequently run with ballast open. (Vacuum Pump Supply)

An exhaust filter or oil mist eliminator can capture much of that oil mist before it enters the work area.

This is another reason the exhaust side of the pump should be considered part of the overall vacuum system rather than an afterthought.

Can You Leave the Gas Ballast Open All the Time?

You can operate many pumps for extended periods with ballast when the process requires it, but that does not mean the valve should automatically stay open for every application.

Leaving it open when it is unnecessary can result in poorer ultimate vacuum, higher gas throughput, increased operating temperature, and potentially increased oil loss. (Vacuum Pump Supply)

For a dry process where the goal is maximum vacuum, closing the ballast normally makes more sense.

For a wet process where vapor contamination is the bigger concern, running with ballast may be exactly what the pump requires.

The operating mode should follow the process.

A Gas Ballast Valve Is Not the Same Thing as a Vacuum Leak

This causes a lot of confusion.

Opening the gas ballast intentionally admits gas into the pump, so the pump's ultimate pressure becomes higher.

But that does not mean the vacuum chamber itself suddenly has a conventional leak.

The gas is introduced at a controlled location in the pumping mechanism specifically to alter compression conditions and help discharge vapors.

That distinction is useful during troubleshooting because a pump that reaches acceptable vacuum with the gas ballast closed but not with it open may be operating exactly as designed.

What If the Gas Ballast Valve Is Damaged or Stuck?

A damaged, leaking, blocked, or incorrectly assembled gas-ballast mechanism can cause operating problems.

Depending on the pump, symptoms might include difficulty reaching ultimate vacuum, improper vapor handling, oil leakage around the ballast assembly, or an inability to properly control the gas-ballast flow.

The Edwards E2M manual, for example, includes inspection and cleaning of the gas-ballast filter as part of pump maintenance and identifies an open ballast control as a possible cause of poor ultimate vacuum. (Vacuum Pump Supply)

Vacuum Pump Supply stocks gas-ballast components for several pump families, including genuine OEM Edwards RV components such as the Edwards RV Gas Ballast Assembly. (Vacuum Pump Supply)

For Edwards RV3, RV5, RV8, and RV12 owners, additional repair information, parts, oil, seals, and troubleshooting resources are available through the Edwards RV Vacuum Pump Repair & Maintenance Center.

Important Safety Note About Gas Ballast

Gas ballast normally introduces air unless the system is specifically configured for another gas.

That can be a serious consideration if the process contains flammable, reactive, pyrophoric, or otherwise hazardous vapors.

Edwards specifically warns against introducing air or oxygen when process materials could become flammable and describes inert-gas purge arrangements for applications that require them. (Edwards)

Never assume that opening an atmospheric gas ballast is appropriate simply because the process contains condensable vapor.

For hazardous processes, follow the pump manufacturer's application and safety instructions and confirm that the pump, ballast gas, exhaust treatment, and entire vacuum system are suitable for the material being pumped.

Quick Gas Ballast Troubleshooting Guide

Manufacturer instructions take precedence because ballast position, warm-up time, flow rate, and vapor-handling limits vary by pump model. (Edwards)

Frequently Asked Questions

Does opening the gas ballast make vacuum better or worse?

For ultimate pressure, opening the gas ballast normally makes the measured vacuum worse because additional gas is intentionally introduced into the pump. Its benefit is improved handling of condensable vapors, not deeper ultimate vacuum. (Vacuum Pump Supply)

Should the gas ballast be open when pulling down a vacuum chamber?

If the chamber contains a substantial moisture or solvent-vapor load, ballast may be useful during that portion of the process. Once the vapor load has been removed and the goal becomes the deepest possible vacuum, the ballast is typically closed, subject to the pump manufacturer's procedure. (Leybold)

Why is my vacuum pump oil milky?

Milky or cloudy oil often indicates contamination by water or another condensed process vapor. Some pumps allow the oil to be decontaminated by isolating the system and operating with gas ballast, but oil that does not clear or has otherwise degraded should be replaced according to the manufacturer's instructions.

Does every vacuum pump have a gas ballast valve?

No. Gas-ballast design and operation vary by pump technology and model. It is common on many oil-sealed rotary vane pumps and is also used on certain dry pumps, but you should consult the manual for the specific pump. (Edwards)

Should I leave the gas ballast open overnight?

Only when the pump manufacturer and process allow it. Extended ballasted operation can help remove condensable contamination in some pumps, but it can also increase oil loss and prevent the pump from reaching its best ultimate pressure. (Vacuum Pump Supply)

The Bottom Line

The gas ballast valve is not just another knob on a vacuum pump.

Used correctly, it can help prevent condensable vapors from contaminating pump oil, improve performance in wet processes, and reduce the maintenance problems caused by internal condensation.

Used at the wrong time, it can also make a perfectly healthy vacuum pump appear to have a vacuum problem.

Remember the basic operating principle:

Condensable vapor present: consider gas ballast.
Dry process and deepest vacuum required: close the gas ballast.

And when there is any doubt, check the operating manual for the exact pump model.

Vacuum Pump Supply maintains a growing technical library of vacuum pump schematics, manuals, exploded views, cross references, and Resource Centers covering Edwards, Busch, Leybold, Welch, Rietschle, Alcatel/Adixen, and other commonly serviced vacuum pumps. (Vacuum Pump Supply)

If you need help identifying the correct oil, filter, trap, gas-ballast component, repair kit, or maintenance part, send Vacuum Pump Supply the pump manufacturer, complete model number, and part number when available.

We’ll help you identify the correct component without turning a routine maintenance question into a parts-guessing exercise.

Vacuum Pump Spare Parts Stocking Guide: 5 Maintenance Items to Keep on Hand

A vacuum pump failure is inconvenient.

A vacuum pump failure when the replacement part you need is several days away can shut down an entire process.

For maintenance departments, laboratories, manufacturing facilities and vacuum service companies, one of the easiest ways to reduce downtime is surprisingly simple: keep the most common vacuum pump maintenance items on the shelf before you need them.

This week we looked at 10 common reasons a vacuum pump may not pull full vacuum. Many of those problems eventually lead to the same maintenance items—oil, filters, seals, gaskets, vanes and repair kits.

The question is not whether every facility should stock an entire vacuum pump parts warehouse.

It is:

Which vacuum pump parts are inexpensive enough, frequently needed enough and important enough to keep on hand?

Here are five places to start.

1. Vacuum Pump Oil

For oil-sealed vacuum pumps, oil is not simply a lubricant.

It also helps seal internal clearances, carries heat away from the pumping mechanism and protects internal components.

Contaminated, degraded or incorrect oil can contribute to poor vacuum performance, higher operating temperatures and accelerated wear.

That makes fresh vacuum pump oil one of the most useful maintenance supplies to keep in inventory.

If your facility operates several pumps, consider standardizing oils where the pump manufacturer and application allow it. This can reduce the number of different fluids your maintenance department needs to stock.

Vacuum Pump Supply offers mineral, synthetic, PFPE and specialty vacuum fluids along with cross-reference information for many common pump manufacturers.

Shop: Vacuum Pump Oil & Fluids

Before changing oil types or chemistry, always verify compatibility with the exact pump and application.

2. Exhaust Filters and Filter Elements

A restricted exhaust filter can create problems that are easily mistaken for a more serious pump failure.

Possible symptoms can include increased operating temperature, oil mist, reduced performance or excessive exhaust pressure.

For facilities running oil-lubricated rotary vane pumps continuously, replacement exhaust-filter elements are often ideal maintenance-stock items.

They are relatively easy to store, commonly replaced and can help avoid turning a routine service issue into an emergency shutdown.

Facilities operating multiple similar pumps may want to keep at least one complete filter-service set available for each major pump family.

Browse: Vacuum Pump Filters, Traps & Mist Eliminators

3. Shaft Seals, O-Rings and Gaskets

A vacuum pump can be mechanically healthy and still perform poorly because of a relatively small sealing component.

Shaft seals, O-rings and gaskets are inexpensive compared with the cost of unexpected equipment downtime.

They are especially worth stocking when your facility operates several identical or similar vacuum pumps.

A leaking shaft seal may cause oil leakage.

A hardened O-ring can create a vacuum leak.

A damaged gasket can prevent a pump from sealing correctly after maintenance.

The part may cost relatively little.

The lost production time usually does not.

For pumps that are critical to production, keeping common sealing components available can make the difference between a same-day repair and waiting for parts.

4. Major or Overhaul Repair Kits

Not every vacuum pump problem requires a rebuild.

But when a high-hour rotary vane pump reaches the point where vanes, seals, gaskets and other wear components need attention, having the correct overhaul kit available can dramatically simplify the repair.

A comprehensive repair kit can also reduce the temptation to replace only the visibly failed component while leaving other worn components inside the pump.

Vacuum Pump Supply supports many popular pump families from manufacturers including Edwards, Busch, Leybold, Becker, Welch, Alcatel/Adixen, Rietschle and others.

Browse: Rotary Vane Vacuum Pump Repair Kits & Parts

For critical equipment, consider purchasing the rebuild kit before the pump reaches the end of its maintenance interval rather than after it fails.

5. Model-Specific Wear Parts

The final category depends heavily on the pumps in your facility.

Common examples include vanes, shaft sleeves, oil filters, inlet filters, exhaust filters, bearings, valve components and other wear items.

This is where maintenance inventory should become pump-specific.

A facility operating six Busch R5 pumps should not stock the same maintenance inventory as a laboratory operating two Edwards RV pumps.

Start with the equipment you cannot afford to have offline.

Record the complete pump model and serial number.

Then identify the parts most likely to be required during normal service.

Vacuum Pump Supply's Schematics, Manuals & Parts Diagram Library can help identify parts and service components for many commonly used vacuum pumps.

Don't Just Stock Parts—Stock the Correct Parts

One of the biggest mistakes in vacuum pump maintenance is ordering by pump family alone.

Model suffixes, production versions, serial numbers and configurations can change the parts used inside otherwise similar pumps.

For example, two pumps with similar model numbers may require different filters, seals or repair kits.

Before adding a part to your maintenance inventory, verify:

Pump manufacturer → complete model → version or suffix → serial number → correct part number

Taking a few minutes to establish this information now can prevent a much larger problem during an emergency repair.

Start With Your Most Critical Pumps

You do not need to purchase spare parts for every vacuum pump in your facility at once.

Start with the equipment where downtime matters most.

Identify the pumps supporting production equipment, test systems, vacuum ovens, packaging lines, research equipment or other processes that cannot easily be bypassed.

Then build a basic maintenance package around each pump:

Oil + filters + seals + routine wear parts + appropriate repair kit

That relatively small inventory can provide a substantial operational advantage when something goes wrong.

Planned Maintenance Is Usually Cheaper Than Emergency Maintenance

Emergency repairs tend to create costs beyond the repair itself.

Production may stop.

Technicians may spend hours troubleshooting.

Purchasing may need to locate parts immediately.

Expedited freight may be required.

And a minor maintenance problem can become a larger mechanical failure if a pump continues operating in poor condition.

Keeping the right maintenance items available helps shift vacuum pump service from reactive maintenance toward planned maintenance.

That is usually better for the pump—and much easier on the maintenance budget.

Build Your Vacuum Pump Maintenance Inventory

If you manage multiple vacuum pumps, Vacuum Pump Supply can help identify the repair kits, oil, filters and replacement parts associated with your equipment.

Start with the pump manufacturer and complete model number.

Then use our online resources to identify what belongs on your shelf:

Shop Vacuum Pump Repair Kits & Parts

Shop Vacuum Pump Oil & Fluids

Shop Filters, Traps & Mist Eliminators

View Vacuum Pump Schematics & Manuals

Don't know which parts belong with your pump?

Send us the manufacturer, complete model number, serial number and—when possible—a photo of the pump nameplate.

We'll help you identify the right maintenance components before you need them.

Vacuum Pump Supply
866-640-3523

Keep the parts on the shelf. Keep the pump running.

Here is the rewritten version ready for BigCommerce. I’m using clickable VPS links in the copy.

Vacuum Pump Not Pulling Full Vacuum? 10 Common Causes and What to Check First

When a vacuum pump suddenly stops reaching its normal vacuum level, it doesn't necessarily mean the pump needs to be rebuilt.

In many cases, the problem is something much simpler: contaminated oil, a leaking fitting, a clogged filter, an incorrect valve position, or another issue that can be identified before the pump ever comes apart.

Before ordering parts—or taking a pump out of service—work through these common causes.

1. Check for Vacuum Leaks First

Before blaming the pump, make sure the vacuum system itself is tight.

Leaks can occur at:

• Hose and tubing connections

• Flanges and fittings

• O-rings and gaskets

• Valves

• Vacuum chamber seals

• Feedthroughs and instrumentation connections

A relatively small leak can prevent an otherwise healthy vacuum pump from reaching its normal ultimate pressure.

Quick test: If possible, isolate the pump from the vacuum system and measure vacuum directly at the pump inlet. If the pump performs normally when isolated, the problem is likely somewhere in the system rather than inside the pump.

That simple test can save a lot of unnecessary troubleshooting.

2. Look at the Vacuum Pump Oil

For an oil-sealed rotary vane pump, the condition of the oil can have a major effect on performance.

Look through the sight glass.

Healthy vacuum pump oil is generally clear and relatively clean. Depending on the application and oil type, warning signs can include oil that is:

• Dark or heavily discolored

• Cloudy or milky

• Thick

• Contaminated with process material

• Full of visible particles

• Unusually low or high in the sight glass

Milky oil can indicate moisture contamination, while dark oil can indicate contamination, overheating, oxidation, or extended service.

If there is any doubt about the condition of the oil, an oil change is one of the simplest places to begin.

Important: Always use the correct oil for the pump and application. Oil viscosity, vapor pressure, chemistry, and process compatibility can all affect performance.

Need help identifying the correct oil? Use the Vacuum Pump Oil Cross Reference Chart to compare common OEM oils with VPS equivalents.

3. Verify the Oil Level

Even clean oil won't help if the level is incorrect.

Check the manufacturer's recommended oil level with the pump in the specified operating condition.

Too little oil can affect lubrication and sealing.

Too much oil can also create operating problems and may contribute to excessive oil carryover.

Don't simply fill the reservoir to the top of the sight glass. Follow the level specified for your particular pump.

If you don't have the manual, check the Resources section on Vacuum Pump Supply. Our site includes schematics, manuals, parts diagrams, cross-reference information and manufacturer-specific Resource Centers. (Vacuum Pump Supply)

4. Check the Inlet and Inlet Filter

A restricted inlet can dramatically reduce pumping performance.

Inspect the:

• Inlet filter

• Inlet screen

• Vacuum hose

• Piping

• Isolation valve

• Inlet connection

Filters can become loaded with dust, process material, oil, or other contaminants.

Also check flexible vacuum hose. A hose can look perfectly normal externally while collapsing internally under vacuum.

If the pump reaches good vacuum but takes significantly longer than it used to, an inlet restriction should be on your troubleshooting list.

5. Inspect the Exhaust System

People naturally look at the inlet side of a vacuum pump when performance drops, but don't forget the exhaust.

A clogged or saturated exhaust filter can increase backpressure and affect pump operation.

Common symptoms can include:

• Excessive oil mist

• Increased operating temperature

• Reduced pumping performance

• Higher motor load

• Oil leaks

• Unusual exhaust noise

If your pump uses replaceable exhaust filters or oil-mist separators, inspect them according to the manufacturer's recommended maintenance schedule.

6. Is the Pump Running Too Hot?

Vacuum pumps generate heat during normal operation, but excessive temperature can be a warning sign.

Check for:

• Blocked cooling fins

• Dirty cooling surfaces

• Failed cooling fans

• Poor ventilation around the pump

• High ambient temperature

• Incorrect oil

• Low oil level

• Exhaust restriction

Temperature and vacuum performance can be related.

A pump that consistently operates above its intended temperature range can experience accelerated oil degradation, seal deterioration and premature wear.

7. Check Pump Rotation

This one is easy to overlook—particularly after electrical work, motor replacement or moving equipment.

Some vacuum pumps can run while rotating in the wrong direction but will not pump correctly.

If the problem started immediately after electrical work, verify motor rotation against the manufacturer's direction-of-rotation indicator before doing anything more invasive.

8. Consider Worn or Sticking Vanes

Rotary vane pumps depend on the vanes maintaining contact with the pump chamber to create the compression stages required for vacuum.

Over time, vanes can:

• Wear

• Chip

• Crack

• Become contaminated

• Stick in their rotor slots

Symptoms of vane problems can include:

• Poor ultimate vacuum

• Reduced pumping speed

• Difficulty starting

• Unusual noise

• Performance that deteriorates as the pump ages

If the oil, filters, system plumbing and external components check out, internal vane condition becomes increasingly important.

This is where a minor repair kit, vane kit or major repair kit may become necessary.

9. Worn Seals, O-Rings and Gaskets

A vacuum pump contains numerous sealing points.

As seals age, they can harden, flatten, crack or lose elasticity.

A seal doesn't necessarily have to produce a visible external oil leak to affect vacuum performance. An internal air leak can reduce ultimate vacuum without leaving an obvious external clue.

This is one reason comprehensive major repair kits contain much more than just vanes.

Depending on the pump, a major repair kit may include:

• Vanes

• Shaft seals

• O-rings

• Gaskets

• Valve components

• Springs

• Washers

• Bearings

• Other common wear components

Replacing the appropriate wear components during an overhaul helps restore the sealing and mechanical condition of the pump.

Vacuum Pump Supply carries both OEM and OEM-equivalent repair solutions across many major vacuum pump manufacturers.

10. The Pump May Simply Be Due for an Overhaul

Vacuum pumps are mechanical equipment.

After enough operating hours, even a properly maintained pump will experience wear.

If you've checked:

✓ System leaks
✓ Oil condition
✓ Oil level
✓ Inlet restrictions
✓ Exhaust restrictions
✓ Operating temperature
✓ Rotation
✓ External components

…and the pump still won't reach its expected vacuum, it may be time to investigate internal wear.

The good news is that many industrial rotary vane vacuum pumps are designed to be rebuilt rather than replaced.

A properly selected repair kit can often return a serviceable pump to reliable operation at a fraction of the cost of replacing the entire pump.

A Simple Vacuum Pump Troubleshooting Order

When a pump isn't reaching vacuum, don't start by taking it apart.

Work from the easiest and least expensive possibilities toward the more involved ones:

1. Isolate the pump from the system.
Determine whether the problem is actually the pump.

2. Check the oil.
Check condition, type and level.

3. Check the inlet.
Look for restrictions and leaks.

4. Check the exhaust.
Inspect filters and look for excessive backpressure.

5. Check temperature and rotation.

6. Measure pump performance directly.

7. Only then investigate internal wear.

Following a logical troubleshooting sequence can prevent unnecessary parts replacement and significantly shorten troubleshooting time.

Find Parts and Technical Information for Your Pump

One of the biggest projects underway at Vacuum Pump Supply is expanding our online technical resources so customers can find more than just a part number.

Our growing Vacuum Pump Resource Centers provide access to repair kits, maintenance information, parts, manuals, schematics and other technical resources for many of the industry's most widely used vacuum pumps.

Current resources cover manufacturers and pump families including Edwards, Leybold, Busch, Welch, Becker, Rietschle and Alcatel/Adixen. (Vacuum Pump Supply)

These resources can help you go from:

Pump problem → pump model → technical information → correct replacement part

without having to search through multiple websites.

Don't Know Which Repair Kit or Part You Need?

That's where Vacuum Pump Supply can help.

Since 2006, we've helped laboratories, manufacturers, service companies, universities and maintenance departments identify replacement parts for industrial vacuum pumps.

We carry thousands of products for major manufacturers including:

• Edwards

• Leybold

• Busch

• Welch

• Becker

• Alcatel/Adixen

• Stokes

• Rietschle

• And others

Our selection includes repair kits, vacuum pump oil, vanes, shaft seals, O-rings and gaskets, inlet filters, exhaust filters and replacement parts.

If you know the pump manufacturer and model, start with the Resource Centers or site navigation.

If you know the part number, search it directly.

And if you're not sure what you need, contact Vacuum Pump Supply or request a quote and tell us what pump you're working on. Our team can help identify the correct parts.

Keep Your Vacuum Pumps Running—and Your Maintenance Costs Under Control

A vacuum pump that won't reach full vacuum doesn't automatically need replacement.

Start with the basics. Find the cause. Fix what actually needs fixing.

And when parts are required, compare your options before automatically paying OEM prices.

Vacuum Pump Supply
866-640-3523
Parts. Kits. Performance.

Related Vacuum Pump Resources

Vacuum Pump Oil Cross Reference Chart — Find OEM vacuum pump oil equivalents and VPS replacement options by manufacturer, viscosity, oil type and application.

Vacuum Pump Resource Centers — Access manufacturer-specific repair information, schematics, manuals, repair kits, oil recommendations and replacement parts through the Resources menu. (Vacuum Pump Supply)

Contact Us / Request a Quote — Send VPS your pump model or part number if you need help identifying the correct product.

Published: August 2026

How Much Should a Leybold Trivac Major Repair Kit Cost?

If you're maintaining Leybold Trivac vacuum pumps, you've probably noticed that repair kit pricing can vary dramatically from one supplier to another. Whether you're responsible for a single laboratory pump or an entire production facility, comparing prices before ordering can save your maintenance budget hundreds—or even thousands—of dollars each year.

To help maintenance managers, purchasing departments, and service technicians make informed decisions, Vacuum Pump Supply compared publicly available pricing for four of the most commonly purchased Leybold Trivac B-Series Major Repair Kits.

The results speak for themselves.

Leybold Trivac B-Series Major Repair Kit Price Comparison

Biggest Savings: Rebuilding a Leybold D65B with Vacuum Pump Supply can save more than $1,000 compared to OEM list pricing.

Prices were collected from publicly available sources at the time of publication. Prices are subject to change without notice.

Why the Difference?

OEM pricing often reflects the manufacturer's complete distribution network and overhead. Independent suppliers may offer lower prices, but pricing can still vary significantly.

At Vacuum Pump Supply, our mission is simple:

• Offer competitive pricing on OEM-equivalent repair kits.

• Deliver quality components you can depend on.

• Help maintenance departments reduce operating costs without sacrificing reliability.

We believe customers should be able to compare prices openly and make the decision that's best for their operation.

Why Maintenance Teams Compare Prices

For facilities maintaining multiple pumps, the savings add up quickly.

Imagine rebuilding just four pumps:

• D16B

• D25B

• D40B

• D65B

Choosing Vacuum Pump Supply instead of paying OEM list pricing could reduce maintenance costs by over $3,300 on those four rebuilds alone.

For companies operating dozens of pumps, annual savings can be substantial.

What's Included in a Major Repair Kit?

Our Leybold Trivac B-Series Major Repair Kits are designed for comprehensive pump overhauls and typically include:

• Rotary vanes

• Shaft seals

• O-rings

• Gaskets

• Valve components

• Additional wear items required for a complete major rebuild

Each kit is designed specifically for its compatible pump model to help simplify maintenance and reduce downtime.

Compatible Pump Models

This comparison includes major repair kits for:

• Leybold Trivac D16B

• Leybold Trivac D25B

• Leybold Trivac D40B

• Leybold Trivac D65B

Need help identifying the correct kit? Our team is happy to help.

The VPS Price Check Series

This article is part of our ongoing VPS Price Check series, where we compare pricing across popular vacuum pump repair kits and replacement parts.

Future comparisons will include:

• Busch R5 Series

• Edwards E2M Series

• Edwards RV Series

• Leybold Sogevac Series

• Welch CRV Series

• Becker Rotary Vane Pumps

Our goal is to provide maintenance professionals with transparent pricing information so they can make informed purchasing decisions.

Ready to Save on Your Next Rebuild?

If you're planning maintenance on a Leybold Trivac B-Series pump, compare prices before you buy.

Vacuum Pump Supply offers competitive pricing, knowledgeable technical support, fast shipping, and thousands of vacuum pump repair parts and kits for leading manufacturers.

Request a quote or shop online today to see how much you could save on your next rebuild.

For nearly 20 years, Vacuum Pump Supply has focused on one simple goal: helping customers keep their vacuum pumps running while spending less than they would buying directly from the OEM. Since opening our doors in 2006, we've grown from a small supplier into a trusted source for vacuum pump repair kits, replacement parts, filters, oils, and complete pumps serving thousands of customers across North America.

As we celebrate our 20th anniversary, we're investing more than ever into expanding both our product selection and the technical resources available to our customers.

Thousands of Products Added

One of our biggest projects this year has been dramatically expanding our online catalog. Every new product page is built with detailed compatibility information, specifications, product images, and helpful technical resources whenever available.

We're continuing to add products for many of the industry's leading manufacturers, including:

• Edwards

• Welch

• Leybold

• Busch

• Becker

• Stokes

• Alcatel/Adixen

• Rietschle

Our goal is simple: if your vacuum pump needs maintenance, we want you to find every repair kit, filter, oil, seal, vane, bearing, and replacement part you need in one place.

New Vacuum Pump Resource Centers

Buying parts is only one piece of keeping a vacuum system operating reliably.

That's why we've begun building complete Resource Centers for many of the most popular vacuum pump families. These pages are designed to become a single destination for service information, troubleshooting, manuals, exploded diagrams, maintenance recommendations, and product selection.

Our growing library includes Resource Centers for:

• Edwards rotary vane pumps

• Busch R5 vacuum pumps

• Leybold Trivac pumps

• Leybold Sogevac pumps

• Welch rotary vane pumps

• Additional manufacturers coming soon

Whether you're identifying a pump, looking for the correct oil, or trying to find the right repair kit, these Resource Centers are designed to save time and eliminate guesswork.

More Technical Information Than Ever Before

We've also expanded the educational content throughout our website.

Customers will now find:

• Vacuum pump troubleshooting guides

• Pump identification resources

• Oil cross-reference information

• Maintenance recommendations

• Repair kit contents and complete bill of materials (where available)

• Product compatibility charts

• Service documentation

We believe informed customers make better purchasing decisions, and we want our website to become one of the industry's most valuable technical resources—not simply another online catalog.

Continuing to Expand Our OEM Equivalent Product Line

Many customers know us for offering high-quality OEM equivalent replacement parts that help reduce maintenance costs without sacrificing performance.

We're continuing to expand our OEM equivalent offerings while also carrying genuine OEM products when available. This allows customers to choose the solution that best fits their maintenance strategy and budget.

Whether you're maintaining a single laboratory pump or supporting an entire manufacturing facility, we strive to provide dependable products backed by knowledgeable customer support.

Built Around Customer Service

As our catalog grows, one thing hasn't changed.

When customers contact Vacuum Pump Supply, they speak with people who understand vacuum pumps—not just order takers. We work with maintenance departments, universities, laboratories, manufacturers, research facilities, and service companies every day to help identify the correct replacement parts the first time.

Our mission remains the same as it was in 2006:

Provide quality vacuum pump parts, fast shipping, competitive pricing, and knowledgeable support that customers can depend on.

Looking Ahead

The work is far from finished.

Over the coming months we'll continue adding new product lines, expanding our Resource Centers, publishing additional troubleshooting guides, and making it even easier to identify the correct parts for your vacuum pump.

Whether you've been a customer for years or are visiting us for the first time, thank you for helping make our first 20 years possible.

Here's to the next 20.

About Vacuum Pump Supply

Vacuum Pump Supply is a leading supplier of vacuum pump repair kits, vacuum pump oil, filters, replacement parts, and new vacuum pumps. We support Edwards, Busch, Leybold, Welch, Becker, Alcatel, Stokes, Rietschle, and many other major manufacturers while helping customers reduce downtime with expert technical support and fast shipping.