Torr

Torr is a pressure unit which is defined as 1 standard atmosphere divided by 760 (1 atm/760 or 101325 Pa/760). Used mostly for measuring high vacuum, the torr has largely been superseded by the hPa (mbar) pressure unit. 1 torr equals 133.322 pascals.

The torr is closely related to millimetres of mercury (mmHg) and the two values are almost exactly the same. However the torr is an exact quantity, whereas the mmHg is not due to variations in local gravity and temperature, and therefore the two pressure units should not be treated as the same.

The type of instrumentation that uses torr units are low vacuum sensors such a Pirani or Ionisation Gauges. The torr is also used to specify the lowest pressure rating for high vacuum equipment such as vacuum pumps, valves and chambers.