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Concept
Inch of mercury
Summary
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 in height at the standard acceleration of gravity. Conversion to metric units depends on the temperature of mercury, and hence its density; typical conversion factors are:
In older literature, an "inch of mercury" is based on the height of a column of mercury at .
1 inHg60 °F =
In Imperial units: 1 inHg60 °F = 0.489 771 psi, or 2.041 771 inHg60 °F = 1 psi.
Aircraft altimeters measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. In the United States, Canada and Japan, these altimeter readings are provided in inches of mercury, but most other nations use hectopascals. Ground readings vary with weather and along the route of the aircraft as it travels, so current readings are relayed periodically by air traffic control. Aircraft operating at higher altitudes (at or above what is called the transition altitude, which varies by country) set their barometric altimeters to a standard pressure of 29.92 inHg (1 atm = 29.92 inHg) or 1013.25 hPa (1 hPa = 1 mbar) regardless of the actual sea level pressure. The resulting altimeter readings are known as flight levels.
Piston engine aircraft with constant-speed propellers also use inches of mercury to measure manifold pressure, which is indicative of engine power produced in engines equipped with a supercharger or turbosupercharger (naturally aspirated engines measure manifold vacuum instead). In automobile racing, particularly United States Auto Club and Champ Car Indy car racing, inches of mercury was the unit used to measure turbocharger inlet pressure. However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum or pressure within the engine's intake manifold.
Official source
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