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:
| Specification | What It Tells You |
|---|---|
| Displacement | Theoretical or geometric gas-moving capacity of the pump |
| Pumping speed | Actual rated gas-removal performance under specified test conditions |
| Ultimate pressure | The lowest pressure the pump can approach under specified conditions |
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