Gas flow expressed as pressure times volumetric flow, Q = p·S, in mbar·L/s, Torr·L/s or Pa·m³/s. Because it counts gas quantity rather than volume, throughput is conserved along a pumping line in steady state: the gas leaving the chamber is the gas arriving at the backing pump. Engineers use it to size backing pumps, to check whether a turbo can swallow a process flow, and to put leaks and outgassing in the same units.
Process flow is 200 sccm, that's about 3.4 mbar-litres a second of throughput the turbo has to swallow.
To size a backing pump, divide the high vacuum pump's maximum throughput by the highest foreline pressure it tolerates; that is the minimum backing speed.
Also heard as
- Q
- gas throughput
- pV flow
Related terms
Pumping speed
Volume of gas a pump removes from its inlet per unit time, measured at the inlet pressure.
Conductance
How easily a pipe, orifice or valve lets gas through, expressed in litres per second.
sccm
Standard cubic centimetres per minute: a mass flow unit for process gases, referred to 0 °C and 1 atm.
Backing pump
Pump that keeps the exhaust of a turbo, diffusion or Roots pump below its maximum allowed pressure.