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Measuring Pump Throughput

Two methods of measuring Pump Throughput are used. The first works only for chambers pumped with gas transfer pumps but is pretty fundamental, no frills measurement. The second method can be applied to any system. However, it demands more knowledge of the system than the first method.

Method I

Since it measures the gas exiting the mechanical pump's exhaust, it is not applicable while pumping gases that are condensible, adsorbed in the foreline trap, or soluble in the pump oil. In addition the flow rate must be reasonably large.

Close the mechanical pump's gas ballast valve. Plug the pump's exhaust with a rubber stopper that has a metal tube through it. Connect the metal tube to a bubble flow-meter with a length of rubber tube and measure the flow rate in cc/minute at atmospheric pressure. Convert the value to units of choice (torr.liter/sec or cfm).

If you need a precise number, measure the exhaust gas temperature and the barometric pressure and re-calculate the quantity of gas at STP with Boyle's and Charles' law combined:

P1V1/T1 = P2V2/T2

Example: The measured flow of gas at the pump's exhaust is 2.3 atm cc/minute. The exhaust gas temperature is 32°C and the barometric pressure is 750 mm Hg. What is the throughput at standard temperature and pressure (STP) from the chamber?

2.3 atm cc/min = 2.3/60 atm cc/sec
2.3/60 atm cc/sec = 2.3/60 x 760/1000 torr.liter/sec
2.3/60 x 760/1000 T.L/sec at 750 torr and 32°C = 2.3/60 x 760/1000 x 750/760 x 273/305 T.L/sec at STP

Throughput = 0.0257 torr.liter/sec

Method 2

First measure or calculate, the effective pumping speed (EPS) from the chamber by a convenient technique (see here.) The throughput, at any particular pressure, is then calculated by multiplying the EPS by the chamber pressure.

Example: The measured EPS is 83 L/sec. The chamber pressure is 5 x 10- 6 torr. What is the pump throughput?

83 L/sec x 5 x 10-6 torr

Throughput = 4.15 x 10-4 torr.L/sec

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