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A mercury vapor diffusion pump is capable of evacuating a space to a very low pressure, but will not pump against a high back pressure. The pressure to which a vessel can be reduced with this principle is of the order of a fraction of a micron. (One micron is that pressure which will support a column of mercury 0.001 millimeters, it is 1/760,000 of an atmosphere). The back pressure against which a mercury diffusion pump will exhaust is from 250 to 500 microns. In the three-stage type pump, two additional stages of the ejector nozzle type exhaust in series from the discharge of the first, or diffusion stage, to a back pressure of the order of 20 millimeters.

Progress Report.pdf (1287 kB)
Technique of High Vacuum and Vacuum Gauges



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