- Inch of mercury
Inches of mercury, (inHg) is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere.
- 1 inHg = 3,386.389 pascals at 0 °C.
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 1,013.25 hPa (1 hPa = 1 mbar) regardless of the actual sea level pressure, with inches of mercury used in the U.S. and Canada. 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 automobile racing, particularly USAC and CART Indy car racing, inches of mercury was the unit used to measure turbocharger inlet pressure. However, the inHG unit is still used today in car performance modification to measure the amount of vacuum within the engine's intake manifold. This can be seen on Boost/Vacuum gauges.
In air conditioning and refrigeration, inHg is often used to describe "inches of mercury vacuum", or pressures below 0 psig, for recovery of refrigerants from air conditioning and refrigeration systems, as well as for leak testing of systems while under a vacuum, and for dehydration of refrigeration systems. The low side gauge in a refrigeration gauge manifold indicates pressures below 0 psig in "inches of mercury vacuum" (inHg), down to a 30 inHg vacuum.
In older literature, an "inch of mercury" is based on the height of a column of mercury at 60 °F (15.6 °C) 
- 1 inHg60 °F = 3,376.85 Pa
In English units: 1 inHg = 0.491098 psi, or 2.036254 inHg = 1 psi.
- Torr (millimeters of mercury)
- Bar (unit)
- Mercury-in-glass thermometer, a thermometer having the element of mercury in a glass tube
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