Conductance (C) is a measure of the ease of volumetric flow through an element such as a pipe. It has units of volumetric flow such as cfm, l/s or cubic meters per hour.
The flow through any desired piping element, i.e. pipe or hose, valves, nozzles, openings in a wall between two vessels, etc., is indicated with q = (p1 – p2) x C = ∆p · C.
Here ∆p = (p1– p2) is the differential between the pressures at the inlet and outlet ends of the piping element. The proportionality factor C is designated as the conductance value or simply “conductance”. It is affected by the geometry of the piping element and can even be calculated for some simpler configurations.
In the high and ultrahigh vacuum ranges, C is a constant which is independent of pressure; in the rough and medium-high regimes it is, by contrast, dependent on pressure. As a consequence, the calculation of C for the piping elements must be carried out separately for the individual pressure
ranges.
From the definition of the volumetric flow it is also possible to state that: The conductance value C is the volumetric flow through a piping element. The equation could be thought of as “Ohm’s law for vacuum technology”, in which q corresponds to current, ∆p the voltage and C the electrical conductance value. Analogous to Ohm’s law in the science of electricity, the
resistance to flow (R = 1/C) has been introduced as the reciprocal value to the conductance value. The equation (p = n·k ·T) can then be rewritten as: q = 1/R·∆p.