Low-pressure (vacuum) measurement uses mechanisms distinct from medium/high-pressure gauges: the McLeod gauge compresses a fixed low-pressure gas sample to a pressure high enough to read with a simple mercury manometer, giving a gas-composition-independent standard; the Pirani and thermocouple gauges infer pressure from the pressure-dependent thermal conductivity of a gas (heat loss from a heated filament falls as pressure/molecule density falls, raising filament temperature and resistance); the ionization gauge derives pressure from the ratio of ion current to (essentially constant) electron current produced when accelerated electrons ionize gas molecules; and the Knudsen gauge measures pressure via differential momentum transfer between a heated fixed plate and a cooler movable vane spaced less than a mean free path apart. This belongs to vacuum/low-pressure transduction theory in industrial instrumentation, extending pressure-measurement theory into a regime where gas kinetic-theory effects, not bulk hydrostatic or elastic-deformation effects, dominate.
Low-pressure (vacuum) measurement uses mechanisms distinct from medium/high-pressure gauges: the McLeod gauge compresses a fixed low-pressure gas sample to a pressure high enough to read with a simpl…