ABSTRACT

In high vacuum, rather than measuring the pressure, what is actually measured is the gas density or the number of particles per unit volume. This can then be related to pressure assuming that the temperature and gas species are known. The gases used to measure pressure in vacuum systems can be divided into two categories: those measuring total pressure, and gas analyzers that measure partial pressures of different gases in a mixture. In the simplest types of Bourdon or capsule gauges, the element is enclosed in a glass-fronted leaktight case and the movement transmitted mechanically to a pointer visible through the glass. The inside of the case and the mechanism is exposed to the vacuum system and consequently, subject to damage by process conditions. The bridge voltage is held constant. A change in the pressure in the open envelope causes a change in the temperature of the exposed filament which, in turn, changes the filament resistance and unbalances the bridge.