Inch of mercury
Encyclopedia
Inches of mercury, 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.
It is defined as the pressure exerted by a column of mercury
of 1 inch
in height
at 32 °F (0 °C
) at the standard acceleration of gravity
.
Aircraft altimiters
measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. Within the U.S. and Canada, these readings are provided in inches of mercury. 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 altimeter
s 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. The resulting altimeter readings are known as flight level
s.
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)
In English units: 1 inHg = 0.491098 psi
, or 2.036254 inHg = 1 psi.
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
. It is still widely used for barometric pressure in weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
reports, refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
and aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, but is seldom used elsewhere.
It is defined as the pressure exerted by a column of mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
of 1 inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
in height
Height
Height is the measurement of vertical distance, but has two meanings in common use. It can either indicate how "tall" something is, or how "high up" it is. For example "The height of the building is 50 m" or "The height of the airplane is 10,000 m"...
at 32 °F (0 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
) at the standard acceleration of gravity
Standard gravity
Standard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...
.
- 1 inHg = 3,386.389 pascalPascal (unit)The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
s at 0 °C.
Aircraft altimiters
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...
measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. Within the U.S. and Canada, these readings are provided in inches of mercury. 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 altimeter
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...
s to a standard pressure of 29.92 inHg (1 atm = 29.92 inHg) or 1,013.25 hPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(1 hPa = 1 mbar) regardless of the actual sea level pressure. The resulting altimeter readings are known as flight level
Flight level
A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...
s.
Piston engine aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
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
Champ Car
Champ Car was the name for a class and specification of open wheel cars used in American Championship Car Racing for many decades, primarily for use in the Indianapolis 500 auto race...
Indy car racing
American Championship Car Racing
Since 1916 there has been a recognized United States national automobile racing National Championship for drivers of professional-level, single-seat open wheel race cars. The championship has been under the auspices of several different sanctioning bodies since 1909. Since 1911, the Indianapolis...
, inches of mercury was the unit used to measure turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
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
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
, or 2.036254 inHg = 1 psi.
See also
- TorrTorrThe torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
(millimeters of mercury) - Bar (unit)Bar (unit)The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
- Mercury-in-glass thermometerMercury-in-glass thermometerA mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German physicist Daniel Gabriel Fahrenheit in 1724 and is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the...
, a thermometer having the element of mercury in a glass tube