In Vacuum Science text books, training courses and the wisdom passed down the generations etc. we are instructed that ‘all vacuum connections should be short and wide as possible’. But, what happens when we do not do this? What is the consequence of non-compliance?
What is conductance in vacuum technology?
In vacuum terminology the conductance C between two points is defined as the gas throughput Q (through a component) divided by the pressure drop (ΔP) across it, where Pup is the upstream pressure of the system and Pdown is the downstream pressure:
here S is the pumping speed at any point in the vacuum system.
Gas flow mechanisms can be divided into different regimes: continuum (where molecule-molecule collisions dominate behaviour), molecular (molecule-wall collisions dominate) and a transitional flow regime between these two regimes.
This is illustrated below (for air at 293K) where the conductance of a 1 metre length pipe is plotted for different diameters and pressures conductance varies as 1/length for long pipes.
For molecular flow, conductance is independent of pressure (here <~ 0.01 mbar), for continuum flow conductance is a linear function of pressure (here >~ 1 mbar) and the transitional flow and is an ‘admixture’ of the extreme pressure dependencies.
What does this mean in the real vacuum world?
We can illustrate by looking at a few examples.
We can see that at higher pressures (where the pipe conductance is highest) there is no impact on the net speed. The percentage difference though becomes more pronounced at < 10 mbar (50% loss) and then only becomes negligible at the ultimate pressure of the system (with zero net speed).
Consider a system with a turbo-molecular pump (TMP) connected directly to a chamber via an ISO100 gate valve (which has a stated relatively large molecular conductance of ~ 1,700 l/s). The graph below shows the net System speed (SUp) with a range of TMP speeds (SDown); a small loss of conductance in molecular flow conditions.
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