The torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.
Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressure. Scientists at the time were familiar with small fluctuations in height that occurred in barometers. When these fluctuations were explained as a manifestation of changes in atmospheric pressure, the science of meteorology was born.
Over time, 760 millimeters 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.
In 1954, the definition of 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. The torr was then re-defined as 1/760 of one atmosphere. This change in the definition of “torr” has been a source of confusion ever since.
These four SI-related pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures. The torr, a more convenient unit for low pressures, is used in high-vacuum physics and engineering.
Manometric units are units such as millimeters of mercury or centimeters of water that depend on an assumed density of a fluid and an assumed acceleration of gravity. These units are now regarded as obsolete, and their use is discouraged. Nevertheless, manometric units are used routinely in medicine and physiology, and they continue to be used in areas as diverse as weather reporting and scuba diving.
The millimeter of mercury (symbol: mmHg) is defined as the pressure exerted at the base of a column of fluid exactly 1 mm high, when the density of the fluid is exactly 13.5951 g/cm3, at a place where the acceleration of gravity is exactly 9.80665 m/s2.
There are several things to notice about this definition:
These assumptions limit both the validity and the precision of the mmHg as a unit of pressure. No metrology laboratory measures or calibrates pressure directly in these terms. It would be extremely difficult to find a fluid with exactly this density, and a place where g was exactly 9.80665 m/s². According to the UK’s National Physical Laboratory (NPL):
The need to assume fixed and exact - but ultimately incorrect - values of liquid density and acceleration due to gravity will inherently limit knowledge of the relationship between [the millimeter of mercury] and the pascal. By contrast, the magnitude of pressure values expressed in the SI pressure unit, the pascal, can flex (albeit not by much) to take account of technological improvements in the underlying definitions of mass, length and time – the SI base quantities from which pressure is derived.
The performance of modern transducers approaches the precision required to distinguish between the torr and the millimeter of mercury. The NPL concludes
Thus, in the near future, the accuracy claims being made for otherwise state-of-the-art instruments scaled in manometric units will become inherently inferior. Even now, confusion and large errors abound through the use of differing definitions, including alternative values of ‘standard’ gravity and varying assumptions about the density and temperature of the fluid. Misunderstandings about temperature assumptions alone can lead to errors of several tenths of a percent and there are many stories of this leading to major mistakes in pressure measurement.
In medicine, the mmHg (measured with a sphygmomanometer) is the gold standard for blood pressure measurement. In physiology, manometric units are used to measure Starling forces. Other applications include:
Manometric results in medicine are sometimes given in torr. This is usually incorrect, since the Torr and the mmHg are not the same thing. Pressures obtained with a manometer (or its transducer equivalent) should be reported in mmHg.
The mmHg is defined as 13.5951 x 9.80665 = 133.322387415 Pa. This is an exact number, although it is too long to be of any practical use.
The torr is defined as 1/760 of one atmosphere, while the atmosphere is defined as 101,325 Pa. Therefore, one Torr is equal to 101325/760 of one Pa. The decimal form of this fraction (133.322368421...) is, unfortunately for practical use, an infinitely long, periodically repeating decimal, as is its reciprocal.
The relationship between the Torr and the mmHg is:
Torr = 0.999 999 857 533 699... mmHg
1 mmHg = 1.000 000 142 466 321... Torr
The mmHg and the Torr differ from one another by less than 2 x 10-7 Torr. The difference between one atmosphere (101325 Pa) and 760 mmHg (101325.0144354 Pa) less than 0.2 μPa/Pa (less than 0.00002%). This small difference is negligible for most applications outside metrology.