It is an instrument used to measure vacuum.
This value can not be measured with ordinary pressure gauges. At pressures below 1Torr (1 mmHg), electronic vacuum meters are used, which indicate pressures below 50mTorr.
These electronic meters are very sensitive to variations in pressure, as a result of this, avoid subjecting them to positive pressures, that is, to carry out measurements of pressures above 1 atm (0 psig).
