Inch of mercury 2319766 224856855 2008-07-10T18:48:43Z 129.139.1.68 {{for|the suction device|mercury vacuum}} '''Inches of mercury''', '''inHg''' or '''"Hg''' is a measuring unit for [[pressure]]. It is still widely used for [[barometric pressure]] in [[weather]] reports and [[aviation]] in the [[United States]], but is considered somewhat outdated elsewhere. It is defined as the pressure exerted by a column of [[mercury (element)|mercury]] of 1 [[inch]] in [[height]] at 32&nbsp;°F (0 [[celsius|°C]]) at the [[standard gravity|standard acceleration of gravity]]. :1 inHg = 3,386.389 [[pascal (unit)|pascal]]s at 0&nbsp;°C. Aircraft operating at higher altitudes (at or above what is called the [[Flight level#Transition altitude|Transition Altitude]], which varies by country) set their barometric [[altimeter]]s to a standard pressure of 29.92 inHg or 1,013.2 [[pascal (unit)|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 level]]s. [[Piston engine]] [[aircraft]] with [[constant-speed propeller]]s also use inches of mercury to measure [[manifold pressure]], which is indicative of engine power produced. In older literature, an inch of mercury based on the height of a column at 60 °F (15.6 °C) was common.<ref name="NIST811">Barry N. Taylor, ''Guide for the Use of the International System of Units (SI),'' 1995, [[NIST]] Special Publication 811, Appendix B[http://physics.nist.gov/Pubs/SP811/appenB8.html#I]</ref> :1 inHg<sub>60 °F</sub> = 3,376.85 Pa In English units: 1 inHg = .491098 psi, or 2.036254 inHg = 1 psi. ==See also== *[[Torr]] (millimeters of mercury) *[[Bar (unit)]] *[[Mercury-in-glass thermometer]], a thermometer having the element of mercury in a glass tube ==References== {{reflist}} [[Category:Units of pressure]] {{measurement-stub}} [[it:InHg]] [[ja:水銀柱インチ]] [[simple:Inch of mercury]]