Image: Vacuum Technology 2


Measurement of Vacuum



Devices used to measure vacuum conditions

Devices used to measure vacuum conditions are called gauges, and all vacuum is measured as absolute pressure, because all degrees of it are above hypothetical, unattainable zero pressure. The mercury manometer measures pressures from atmospheric (760 torr) down to 1 torr and its variant, the McLeod gauge, extends its range down to 10-6 torr. Two mechanical gauges cover the same range as the mercury manometer, one operating by Bourdon tube, the other by a diaphragm. These are sometimes calibrated as gauge vacuum instead of absolute pressure. Medium vacuum is measured by thermocouple and Pirani gauges. These operate on the principle that the thermal conductivity of gas is proportional to the number of residual gas molecules, in other words, the pressure. Their range is from 10-3 torr to 1 torr. The radioactive ionization gauge responds between 10-3 torr and 760 torr. High and very high vacuums are measured by the electrical charge carried by a gas ionized by electron bombardment. Two gauges that use this principle are the thermionic, or hot-cathode, type and the glow, or cold-cathode discharge, type. The former covers a 10-1 torr to 10-11 torr range, the latter 10-2 torr to 10-7 torr range. As a vacuum approaches the ultrahigh range, its measurement becomes increasingly difficult. Pressures of 10-12 torr and less are measured by specially designed mass spectrometer analyzer techniques. All gauges that depend on thermal conductivity or ionized gas for measurement respond differently to different gases, and hence they are usually calibrated against dry air, using the McLeod gauge as a reference.

Pumps

Pumps, valves, operating sequences, and protective interlocks for the operation occurring in the work chamber are controlled by electrical components located in a control on or near the machine. The controls can be manual or computerized. Baffles and traps are often used in the vacuum pipes to check the movement of unwanted gas molecules from the pumps to the work chamber, or the reverse. The baffles operate either at local temperature or are chilled to dry ice or liquid nitrogen temperature, condensing or adsorbing the molecules on the baffle surfaces. Other components of vacuum systems are valves, piping and pipe connections, designed to operate with no leakage from outside.

See also

"See also "Air Compressor.

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