Pfeiffer Vacuum

1.2.7 pV throughput

Dividing the general gas equation (Formula 1-6) by time t obtains the gas flow

\[q_{pV}=\frac{p\cdot V}t = \frac{m\cdot R\cdot T}{M\cdot t}\]

Formula 1-15: pV throughput

$q_{pV}$ pV throughput [Pa m3 s-1]

As can be seen from the right-hand side of the equation, a constant mass flow is displaced at constant temperature T. This is also referred to as pV flow or gas throughput. Throughput is the gas flow rate transported by a vacuum pump.

\[q_{pV} = S\cdot p = \frac{dV}{dt}\cdot p\]

Formula 1-16: Throughput of a vacuum pump

Dividing the throughput by the inlet pressure obtains a volume flow rate, the pumping speed of a vacuum pump:

\[S = \frac{dV}{dt}\]

Formula 1-17: Volume flow rate, or pumping speed, of a vacuum pump

A conversion table for various units of throughput is given in Table 1.8.

Pa m3 / s = W mbar l / s Torr l / s atm cm3 / s lusec sccm slm Mol / s
Pa m3 / s 1 10 7.5 9.87 7.5·103 592 0.592 4.41·10-4
mbar l / s 0.1 1 0.75 0.987 750 59.2 5.92·10-2 4.41·10-5
Torr l / s 0.133 1.33 1 1.32 1,000 78.9 7.89·10-2 5.85·10-5
atm cm3 / s 0.101 1.01 0.76 1 760 59.8 5.98·10-2 4.45·10-5
lusec 1.33·10-4 1.33·10-3 10-3 1.32·10-3 1 7.89·10-2 7.89·10-5 5.86·10-8
sccm 1.69·10-3 1.69·10-2 1.27·10-2 1.67·10-2 12.7 1 10-3 7.45·10-7
slm 1.69 16.9 12.7 16.7 1.27·104 1,000 1 7.45·10-4
Mol / s 2.27·103 2.27·104 1.7·104 2.24·104 1.7·107 1.34·106 1.34·103 1

Table 1.8: Conversion table for units of throughput

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