torr

A torr is a unit of absolute pressure equal to 133.322 pascals, or about 1/760 of standard atmospheric pressure at sea level. The unit emerged from the barometer itself, since 1 torr was originally defined as the pressure needed to support a column of mercury exactly 1 millimeter high at standard gravity. It remains the standard vocabulary in vacuum technology, gas chromatography, mass spectrometry, and leak detection.

The torr is named after Evangelista Torricelli, the 17th-century Italian physicist who invented the mercury barometer. Since atmospheric pressure at sea level holds up roughly 760 millimeters of mercury, one torr equals one millimeter of mercury, or 1 mmHg. This heritage means vacuum engineers often speak of pressure in torr rather than pascals, even though SI units have officially replaced it.

Vacuum systems are classified by the torr ranges they achieve. Rough vacuum spans 760 down to 1 torr; medium vacuum drops from 1 torr to 0.001 torr (1 micron); high vacuum reaches 0.001 to 10, 7 torr; and ultra-high vacuum goes below 10, 7 torr. A rotary vane pump handles rough vacuum. A diffusion pump or turbomolecular pump is needed for high vacuum work. Process vacuum chambers for coating, drying, or degassing often operate between 0.1 and 10 torr.

Reading and specification

Pressure gauges in vacuum work read directly in torr, microns (where 1 micron equals 0.001 torr), or occasionally in millibar. A Pirani gauge works down to roughly 0.001 torr; a capacitance manometer handles the 0.01 to 1000 torr range accurately and without mercury. Thermal conductivity gauges drift with gas composition, while ionization gauges measure below 0.0001 torr but require calibration. Specifications for vacuum equipment always state target pressure in torr or pascals, so conversion between them is routine work.

The torr persists in industrial practice because it maps intuitively to barometer mercury columns and to the practical vacuum pumps that define the boundaries of useful operation. While metrologists and standards bodies have migrated to pascals, the shop floor, the vacuum chamber catalog, and the leak detector display still speak torr. Understanding both units prevents costly specification errors when procuring pumps, gauges, or checking system performance.