Basic Conductance Concept
Vacuum technology novices have difficulty distinguishing conductance from pumping speed (find more info here). These terms seem to describe similar concepts and use identical flow units of volume per unit time. But they should not be confused.
Definition of Conductance
The formal definition of conductance is:
- The ratio of throughput, under steady-state conservation conditions, to the pressure differential between two specified isobaric sections inside the pumping system.
The conductance of a “passive” vacuum component (e.g. tube, nipple, elbow, tee, valve, non-colled baffle, etc.) is a measure of that component's ability to transmit gas molecules from end-to-end in some given time. High conductance is of paramount importance in achieving rapid chamber pump down times and low base pressures.
One characteristic that determines conductance is the clear diameter of the “opening” through the component. A wide opening offers a bigger target for molecules to enter during their random flights around the chamber and, obviously, until a molecule enters the component it cannot be transmitted. Another characteristic is the number of wall collisions molecules make during their transmission through the component. When molecules hit surfaces, they are not reflected like light from a mirror. Rather, they “stick”, often for a very short time, lose all information about their arrival direction, and desorb following a cosine distribution. This distribution gives the molecules an equal chance of heading in either direction along the tube and a maximum probability of heading diametrically across the tube. The more surface hits a molecule makes, the less likely it is to complete the journey quickly and the lower is that component's conductance. So the practical interpretation of conductance is that molecules pass more readily through a tube that is that wide these characteristics have high conductance.
Conductance is a volumetric flow measured in units of volume per unit time, specifically:
- liters per second: L/s
- cubic meters per hour: m3/h
- liters per minute: L/m
Expressing conductances as volumetric flows has two benefits:
- conductances can be combined by simple math (see below)
- in the molecular flow regime, a component's conductance is constant and independent of pressure.
The time to calculate conductances is before any vacuum component is purchased. The approximate operating characteristics of soon-to-be-built or about-to-be-modified system should be known while it is still a scratch-pad-idea. When the system is constructed, it is a trivial matter to reduce conductance but an expensive re-build to increase conductances that are too low.
A component's conductance in continuum or transitional flow depends on gas pressure and uses different equations than those governing molecular flow. Calculating conductances from atmospheric pressure to high vacuum requires iterative processes ideally suited to computer calculation. PEC's VacTran, is an exceptionally powerful program for vacuum technology calculations including the calculation of series and parallel conductances for any pressure range and many different cross-sectional shapes (cones, slots, ovals, annuli, and triangles).