Information AboutTorr |
| CATEGORIES ABOUT TORR | |
| non-si metric units | |
| units of pressure | |
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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 Metre s high to exert 1 Atmosphere of pressure. Therefore a very dense fluid is required— Mercury . Normal atmospheric pressure can support around 760 mm (29.92 in) 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 Pascal s. 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 ( [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 of about 0.14 μPa/Pa (i.e., 0.000014 %). Such a small difference is utterly negligible in most practical applications. SEE ALSO
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