Viscosity Molecular Gauges

Authored by: Igor Bello

Vacuum and Ultravacuum

Print publication date:  November  2017
Online publication date:  November  2017

Print ISBN: 9781498782043
eBook ISBN: 9781315155364
Adobe ISBN:


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Viscosity molecular gauges operate on the principle of transferring molecular momenta between gaseous molecules and mechanical elements that are in motion. The motion of the mechanical element is affected by a friction, drag, and forces of residual gases in vacuum. Thus, the frictional force, tangential momentum transfer between molecules, and a mechanical element are the measure of pressure. The internal friction of gases f A = − η d d v d z and the frictional forces of gases with surfaces f A  = εv are discussed in Chapters 5.2 and 5.6 under viscous (low-vacuum) and free molecular (high-vacuum) conditions, where the symbols η d and ε stand for the coefficients of gas viscosity at these respective conditions. Under low-vacuum conditions, the coefficient of gas viscosity and friction force are independent of pressures. However, the magnitude of these two parameters varies when changing the pressure in free molecular conditions. The transition between the two regions also depends on the geometrical configuration, and it can be deduced from Figure 5.9.

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