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Inch of mercury (inHg and ″Hg) is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports , refrigeration and aviation in the United States . It is the pressure exerted by a column of mercury 1 inch (25.4 mm) in height at the standard acceleration of gravity .
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133.322 387 415 pascals [1] or approximately 133.322 pascals. [2] It is denoted mmHg [3] or mm Hg. [4] [2]
The now-conventional inch of mercury is defined to as 25.4 mmHg, the same as the relationship between an inch and a millimeter (this isn't necessarily true when comparisions include either unit of pressure in an older version, and the relationship between the units of length has also varied over time and place).
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters of mercury (mmHg). A torr was originally defined as 1 mmHg. The pressure is quoted as the level of the mercury's height in the vertical column.
In medicine, blood pressure is measured in millimeters of mercury (mmHg, very close to one Torr). The normal adult blood pressure is less than 120 mmHg systolic BP (SBP) and less than 80 mmHg diastolic BP (DBP). [16] Convert mmHg to SI units as follows: 1 mmHg = 0.133 32 kPa. Hence the normal blood pressure in SI units is less than 16.0 kPa SBP ...
millimetre of mercury: mmHg mmHg 1.0 mmHg (0.13 kPa) inch of mercury: inHg inHg 1.0 ...
When millimetres of mercury (or inches of mercury) are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. [7] One millimetre of mercury is approximately equal to one torr. The water-based units still depend on the density of water ...
So, for example, pressure head might be written "742.2 mm Hg" or "4.2 in H 2 O at 59 °F" for measurements taken with mercury or water as the manometric fluid respectively. The word "gauge" or "vacuum" may be added to such a measurement to distinguish between a pressure above or below the atmospheric pressure.