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

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 1/760th 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 is now a defined quantity in the SI system of units, the *torr* is hence defined as exactly 101325 / 760 (or 2533.125 / 19) ≈ 133.3223684 pascals. Although the *torr* is still in common use in low-pressure engineering, the pascal is now the recommended unit of pressure.

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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Last updated: 05-07-2005 02:23:01

Last updated: 05-13-2005 07:56:04