Pneumatic Vacuum Gauges
Conventional pressure gauges of the capsule or pressure type may be suitable for measurements down to 25 torr. This range is referred to as low vacuum. For high vacuum (lower absolute pressure), special vacuum gauges are used.
The mercury manometer can be used down to 1 torr. The McLeod gauge is a special form of mercury manometer with three limbs, one of which is sealed at the top. Pressure is determined by measurement of the compression of the gas in the sealed limb. This takes the range of the mercury manometer down to 10 -6. It is intended for laboratory use but more robust forms have been developed for general use.
Medium vacuum can be measured by thermocouple and Pirani gauges. These operate by measuring the thermal conductivity of the gas, which is proportional to the number of molecules present (and thus the pressure). The range is from 10 -3 to 1 torr.
For very high vacuums, down to 10 -1° torr, special thermionic gauges are available and for extremely high vacuum down to 10 ~2, mass spectrometer analyser techniques are used.