conductance of vacuum components


There are other considerations related to the construction to the vacuum system, that can severely limit the achievable pressure. Keeping in mind the situation in the molecular flow regime, it is probably a bad idea to connect a turbomolecular pump to the system via a long and narrow tube with several bends. We can quantify this by introducing the conductance C of a component in the system, such as a tube, as

where P2 - P1 is the pressure drop over the component and Q the flux. Note that this definition of conductance is analogous to that used in electricity with Q corresponding to the current and P2 - P1 corresponding to the voltage drop. The inverse of the conductance is the resistance. The conductance is measured in m3s-1. As in the case of electricity, the conductance of two combined elements C1 and C2 can be calculated as C= C1 + C2 when these are in parallel and

when they are in series.

In the molecular flow regime, the conductance of a tube with length l and diameter d is given by   

where v is the thermal speed of the molecules. Note that the conductance is independent of the pressure. This is intuitively clear because the density of molecules is so low that they ``do not know about each other''. In the viscous flow regime, in contrast, the conductance of a tube depends on the pressure and the flow through a long, narrow tube is orders of magnitude higher than in the molecular flow regime. Since the system's temperature in normal operation, and thus the thermal speed of the molecules, is nearly always room temperature, simple conductance formulas that depend only on the geometry of the connection can be derived. One rule of thumb, for example, is that a right angle bend in a tube reduces the conductance by a factor of two, compared to a straight tube with the same total length and diameter.


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