Vacuum conductance

Vacuum conductance relates gas throughput to a pressure difference across a connection: $Q=C(p_1-p_2)$. For throughput in $\mathrm{Pa}{\mathrm{m}}^3{\mathrm{s}}^{-1}$, conductance $C$ has units ${\mathrm{m}}^3{\mathrm{s}}^{-1}$, the same units as pumping speed.

For a pump of speed $S$ connected through a line of conductance $C$, a simple steady balance gives $1/S_{\mathrm{eff}}=1/S+1/C$. Thus a $100\mathrm{L}{\mathrm{s}}^{-1}$ pump behind a $10\mathrm{L}{\mathrm{s}}^{-1}$ connection supplies only about $9.1\mathrm{L}{\mathrm{s}}^{-1}$ at the chamber. Conductance depends on geometry, gas and flow regime; the pressure-independent molecular-flow value should not be used indiscriminately in viscous flow.^[ref]