Torr
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The torr (symbol: Torr) or millimetre of mercury (mmHg) is a non-SI unit of pressure. It is the atmospheric pressure that supports a column of mercury 1 millimetre high. The unit is named after Evangelista Torricelli, Italian physicist and mathematician, for his discovery of the principle of the barometer.
One way to define pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base. Although a manometer may use any fluid in principle, common fluids like water give heights that cannot be contained in a normal room. A water column needs to be of the order of 10 metres high to exert 1 atmosphere of pressure. Therefore a very dense fluid is required—mercury. Normal atmospheric pressure can support around 760 mm of mercury; hence 1/760 of an atmosphere, or 1 mm of mercury (mmHg), has been a convenient measure of pressure for a long time, and is sometimes also called a torr.
Because the standard atmosphere has been precisely defined (10th CGPM, 1954), and the standard atmosphere had previously been defined as 760 mmHg exactly, those two definitions are now combined to define the torr as exactly 101325/760 ≈ 133.3223684 pascals. Although the pascal is now the more commonly used unit of pressure, the torr is still used in high vacuum engineering, particularly where pressures are low enough that viscosity is absent.
The torr, usually under the millimetre of mercury name, remains a common unit for the measurement of blood pressure in much of the world.
mmHg is also used in the United Kingdom as a measure of Intraocular Pressure (IOP) within the eye, especially in patients who suffer from glaucoma. Anything between 12mmHg and 22mmHg is considered normal.
Although they are synonyms in practice, the torr and millimetre of mercury are very slightly different by virtue of their definitions in British Standard BS 2520 ([1], [2]). While the torr is defined as given above, the millimetre of mercury (called the "conventional millimetre of mercury") is defined by the World Meteorological Organization [3] as "the pressure exerted at the bottom of a vertical column exactly 1 mm deep of a fluid whose density is exactly 13.5951 g/cm3, at a location where the acceleration due to gravity is exactly 980.665 cm/s2" [4]. The "conventional density of mercury" used makes 760 mmHg equal a pressure of exactly 101325.0144354 Pa, a percentage difference from the standard atmosphere of about 0.14 μPa/Pa (i.e., 0.000014 %). Such a small difference is utterly negligible in most practical applications.
See also
- inHg (Inches of mercury)
External links
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