Torr

The torr is a non-SI unit of pressure, named after Evangelista Torricelli. Its symbol is Torr.

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One way of defining 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 to give atmospheric 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.

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Because the standard atmosphere has been precisely defined (10th CGPM, 1954), the torr is hence defined as exactly 101325 / 760 ≈ 133.3223684 pascals. Although the torr is still in common use in low-pressure engineering, the pascal is now the recommended unit of pressure.

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This unit, usually under the millimetre of mercury name, remains the most common unit for the measurement of blood pressure in much of the world.

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Although they are synonyms in practice, the torr and millimetre of mercury are very slightly different by virtue of their definitions (http://www.sizes.com/units/mmHg.htm, http://www.npl.co.uk/pressure/punits.html). 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 http://stommel.tamu.edu/~baum/paleo/paleogloss-old/node38.html 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/cm?, at a location where the acceleration due to gravity is exactly 980.665 cm/s?" http://home.att.net/~numericana/answer/constants.htm. The "conventional density of mercury" used makes 760 mmHg equal a pressure of exactly 101,325.0144354 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.

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