About low pressure measurement
Adaptations of most of the common types of pressure transducer can be used for absolute pressure measurement in the vacuum range (less than atmospheric pressure). Special forms of Bourdon tubes measure pressures down to 10 mbar, manometers and bellows-type instruments measure pressures down to 0.1 mbar, and diaphragms can be designed to measure pressures down to 0.001 mbar. Other more specialized instruments are also used to measure vacuum pressures. These instruments include the thermocouple gauge, the Pirani gauge, the thermistor gauge, the McLeod gauge and the ionization gauge.
Below pressures of 0.001 mbar, only the more specialized instruments are applicable. Thermocouple/thermistor types measure down to 10-4 mbar, Pirani gauges to 10-5 mbar and ionization gauges down to 10-13 mbar.