Torr

The torr is a non-SI unit of pressure (symbol: torr) which is equal to ¹/₇₆₀ of an atmosphere (symbol: atm). It was selected to be approximately equal to the fluid pressure exerted by 1 millimeter of liquid mercury (symbol: mmHg) and thus 1 torr ≈ 1 mmHg.

It was named after Evangelista Torricelli, an Italian physicist and mathematician who, in 1644, first explained that a barometer responded to fluctuations in atmospheric pressure.

Brief history

Prior to Torricelli, it was thought that the atmosphere was weightless, but Torricelli believed that the atmosphere did have weight and that the water barometers then in use were responding to fluctuations in the weight of the atmospheric. Water barometers required tubes of water that were about 10.5 meters (≈ 35 feet) tall. Knowing that mercury was about 14 times as heavy as water, Torricelli built a manometer that used mercury instead of water and demonstrated that it only required a tube that was about 76 centimeters (≈ 30 inches) tall which was about ¹/₁₄ of the height needed for a water barometer. That, in effect, proved that the atmosphere did indeed have weight. He is considered to have provided the first modern explanation of atmospheric pressure.

Over time, 760 millimetres of mercury (abbreviated mmHg) came to be regarded as the standard atmospheric pressure. In honor of Torricelli, the torr was defined as a unit of pressure equal to one mmHg.

Prior to 1954, the torr was defined simply as being equal to 1 mmHg. In 1954, the definition of the atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM) to the currently accepted definition: one atmosphere is equal to 101,325 pascals. Because mercury's specific gravity varies with temperature, the height of mercury equivalent to one atmosphere changes with temperature. Thus it became necessary to re-define the torr as ¹/₇₆₀ of an atmosphere, which is approximately 1 mmHg.

Various units of pressure

Pressure Units

	pascal (Pa)	bar (bar)	atmosphere (atm)	torr (torr)	pound-force per square inch (psi)	kilogram-force per square centimeter (kgf/cm2)
1 Pa	≡ 1 N/m2	10−5	9.8692×10−6	7.5006×10−3	145.04×10−6	1.01972×10−5
1 bar	100,000	≡ 106 dyn/cm2	0.98692	750.06	14.504	1.01972
1 atm	101,325	1.01325	≡ 1 atm	760	14.696	1.03323
1 torr	133.322	1.3332×10−3	1.3158×10−3	≡ 1 torr ≈ 1 mmHg	19.337×10−3	1.35951×10−3
1 psi	6,894.76	68.948×10−3	68.046×10−3	51.715	≡ 1 lbf/in2	7.03059×10−2
1 kgf/cm2	98,066.5	0.980665	0.967838	735.5576	14.22357	≡ 1 kgf/cm2

Example reading:  1 Pa = 1 N/m²  = 10⁻⁵ bar  = 9.8692×10⁻⁶ atm  = 7.5006×10⁻³ torr, etc.
Note: mmHg is an abbreviation for millimetre of mercury
About the torr: There is no consensus in the technical literature about whether the name of the torr should be "Torr" or "torr". Nor is there any consensus about whether the symbol for that unit of pressure should be "Torr" or "torr". Both the United Kingdom's National Physical Laboratory (see Pressure Units) and New Zealand's Measurement Standards Laboratory (see Barometric Pressure Units) use "torr" as the name and as the symbol. An extensive search of the website of the U.S. National Institute of Standards and Technology found no such clear-cut definitions. Therefore, this table uses "torr" as both the name and the symbol.