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| Inch of mercury | |
|---|---|
_-_DSC06342.JPG) Early American barometer calibrated in inches of mercury | |
| General information | |
| Unit of | Pressure |
| Symbol | inHg, ″Hg |
| Conversions | |
| 1 inHgin ... | ... is equal to ... |
| SI units | 3.38639 kPa |
| United States customary units | 0.491154 psi |
Inch of mercury (inHg,″Hg, orin) is a non-SIunit of measurement forpressure. It is used forbarometric pressure inweather reports,refrigeration andaviation in theUnited States.
It is the pressure exerted by a column ofmercury 1 inch (25.4 mm) in height at thestandard acceleration of gravity. Conversion to metric units depends on the density of mercury, and hence its temperature; typical conversion factors are:[1]
| Conditions | Pressure |
|---|---|
| conventional | 3386.389 pascals |
| 32 °F (0 °C) | 3386.38 pascals |
| 60 °F (16 °C) | 3376.85 pascals |
In older literature, an "inch of mercury" is based on the height of a column of mercury at 60 °F (15.6 °C).[1]
In Imperial units: 1 inHg60 °F = 0.489 771 psi, or 2.041 771 inHg60 °F = 1 psi.
Aircraftaltimeters measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. In North America[2] and Japan,[3] these altimeter readings are provided in inches of mercury, but most other nations usehectopascals. 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 thetransition altitude, which varies by country) set their barometricaltimeters 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 asflight levels.
Piston engineaircraft withconstant-speed propellers also use inches of mercury to measuremanifold pressure, which is indicative of engine power produced in engines equipped with asupercharger orturbosupercharger (naturally aspirated engines measure manifold vacuum instead). In automobile racing, particularlyUnited States Auto Club andChamp CarIndy car racing, inches of mercury was the unit used to measureturbocharger 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. This can be seen on "boost gauges (forced induction) or vacuum gauges (natural induction), which give a rough indication of the relative power being produced at any given time.
Inair conditioning and refrigeration, inHg is often used to describe "inches of mercury vacuum", or pressures below ambient atmospheric pressure, 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 ambient in "inches of mercury vacuum" (inHg), down to a 30 inHg vacuum.
Inches of mercury is also used in automotive cooling system vacuum test and fill tools. A technician will use this tool to remove air from modern automotive cooling systems, test the system's ability to hold vacuum, and subsequently refill using the vacuum as suction for the new coolant. Typical minimum vacuum values are between 22 and 27 inHg.
Inches of mercury was the usual unit of pressure measurement in railwayvacuum brakes.