Inch of mercury

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See also: Inch of water
Inch of mercury
Early American barometer calibrated in inches of mercury
General information
Unit ofPressure
SymbolinHg, ″Hg
Conversions
1 inHg in ...... is equal to ...
   
SI units
   3.38639 kPa
   United States customary units   0.491154 psi

Inch of mercury (inHg and ″Hg) is a non-

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. 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]

1 inHg60 °F = 3,376.85 pascals (33.7685 hPa)

In Imperial units: 1 inHg60 °F = 0.489 771 

psi
, or 2.041 771 inHg60 °F = 1 psi.

Applications

Aircraft and automobiles

Aircraft

mbar) regardless of the actual sea level pressure. The resulting altimeter readings are known as flight levels
.

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. 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.

Cooling systems

In air 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.

Vacuum brakes

Inches of mercury was the usual unit of pressure measurement in railway vacuum brakes.

See also

References

  1. ^
    NIST Special Publication 811, Appendix B [1]
  2. ^ From the Ground Up – 29th Edition [ISBN missing] [page needed]
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