|High Vacuum Equipment-Glossary of Terms 1
Orifice conductance
In the case of a øA (cm2) hole in an ultra-thin plate, conductance “C” results from “V”, the average velocity of the gas; “R”, the gas constant; “M”, the molecular weight; and “T”, the absolute temperature. From the formula C=11.6A (L/sec) at an air tempearture of 20°C.
Cylinder conductance
With length “L” (cm) and diameter “D” (cm) where L>>D, from the formula C=(2πRT/M)0.5D3/6L, the conductance C=12.1 D3/L (L/sec) at an air temperature of 20°C.
Short pipe conductance
From the Clausing’s factor “K” and hole conductance “C” in Graph 1. (Clausing’s factor drawing), the short pipe conductance CK is easily found as CK=KC.
Conductances combined
When each of the separate conductances are given as C1, C2 and Cn, the composite conductance ΣC is expressed as:
ΣC=1/(1/C1+1/C2+···+1/Cn) when in series, and
ΣC=C1+C2+···+Cn, when in parallel.