Inch of mercury

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Inch of mercury
Rare American Barometer, Lyman King, Clifton Springs, New York, c. 1860 - Museum of Science and Industry (Chicago) - DSC06342.JPG
Early American barometer calibrated in inches of mercury
General information
Unit of Pressure
1 inHg in ...... is equal to ...
   SI units   3.38639 kPa

Inch of mercury (inHg and ″Hg) is a unit of measurement for pressure. It is still used for barometric pressure in weather reports, refrigeration and aviation in the United States.

Unit of measurement real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (International vocabulary of metrology)

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement.

Pressure Force distributed continuously over an area

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

Weather forecasting application of science and technology to predict the conditions of the atmosphere for a given location and time

Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th century. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere at a given place and using meteorology to project how the atmosphere will change.


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 temperature of mercury, and hence its density; typical conversion factors are: [1]

Mercury (element) Chemical element with atomic number 80

Mercury is a chemical element with symbol Hg and atomic number 80. It is commonly known as quicksilver and was formerly named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element that is liquid at standard conditions for temperature and pressure; the only other element that is liquid under these conditions is the halogen bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

The standard acceleration due to gravity, sometimes abbreviated as standard gravity, usually denoted by ɡ0 or ɡn, is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is defined by standard as 9.80665 m/s2. This value was established by the 3rd CGPM and used to define the standard weight of an object as the product of its mass and this nominal acceleration. The acceleration of a body near the surface of the Earth is due to the combined effects of gravity and centrifugal acceleration from the rotation of the Earth ; the total is about 0.5% greater at the poles than at the Equator.

conventional3386.389 pascals
32 °F3386.38 pascals
60 °F3376.85 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 = 3376.85 Pa

In English units: 1 inHg60 °F = 0.489 771  psi, or 2.041 771 inHg60 °F = 1 psi.

Pounds per square inch unit of pressure or stress

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. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.


Aircraft and automobiles

Aircraft altimeters measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground. In the United States, Canada [2] and Japan, these altimeter readings are provided in inches of mercury, the majority of nations use hectopascals. 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 altimeters 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 as flight levels.

Altimeter meteorological instrumentation

An altimeter or an altitude meter 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 under water.

Pascal (unit) SI unit of pressure

The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre. It is named after the French polymath Blaise Pascal.

In aviation and aviation meteorology, flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft's actual altitude either above sea level or above ground level.

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 United States Auto Club and Champ Car 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 within the engine's intake manifold. This can be seen on boost/vacuum gauges.

Aircraft machine that is able to fly by gaining support from the air other than the reactions of the air against the earth’s surface

An aircraft is a machine that is able to fly by gaining support from the air. It 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. Common examples of aircraft include airplanes, helicopters, airships, gliders, and hot air balloons.

Constant-speed propeller

A constant-speed propeller is a variable-pitch aircraft propeller that automatically changes its blade pitch in order to maintain a chosen rotational speed. The power delivered is proportional to the arithmetic product of rotational speed and torque, and the propeller operation places emphasis on torque. The operation better suits modern engines, particularly supercharged and gas turbine types.

United States Auto Club Auto racing sanctioning body in the United States

The United States Auto Club (USAC) is one of the sanctioning bodies of auto racing in the United States. From 1956 to 1979, USAC sanctioned the United States National Championship, and from 1956 to 1997 the organization sanctioned the Indianapolis 500. Today, USAC serves as the sanctioning body for a number of racing series, including the Silver Crown Series, National Sprint Cars, National Midgets, Speed2 Midget Series, .25 Midget Series, Stadium Super Trucks, TORC: The Off-Road Championship, and Pirelli World Challenge.

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 systems 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

Related Research Articles

Pressure measurement technique to measure pressure

Pressure measurement is the analysis of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure gauges or vacuum gauges. A manometer is a good example, as it uses a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

Vacuum Space that is empty of matter

Vacuum is space devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object that is surrounded by a vacuum.

Atmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 1013.25 mbar (101325 Pa), equivalent to 760 mmHg (torr), 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

A barometer is a scientific instrument used to measure air pressure. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems and frontal boundaries.

Bar (unit) non-SI unit of pressure

The bar is a metric unit of pressure, but is not approved as part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa, which is slightly less than the current average atmospheric pressure on Earth at sea level.

Pressure altitude within the atmosphere is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question.

Packard V-1650 Merlin piston aircraft engine, Packard-built version of the Rolls-Royce Merlin

The Packard V-1650 Merlin is a version of the Rolls-Royce Merlin aircraft engine, produced under license in the United States by the Packard Motor Car Company. The engine was licensed in order to expand production of the Rolls-Royce Merlin for British use. The engine also filled a gap in the U.S. at a time when similarly-powered American-made engines were not available.

A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133.322387415 pascals. It is denoted by the symbol mmHg or mm Hg.

Pressure sensor measurement device

A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed. For the purposes of this article, such a signal is electrical.

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

The manifold absolute pressure sensor is one of the sensors used in an internal combustion engine's electronic control system.

Boost gauge

A boost gauge is a pressure gauge that indicates manifold air pressure or turbocharger or supercharger boost pressure in an internal combustion engine. They are commonly mounted on the dashboard, on the driver's side pillar, or in a radio slot.

Armstrong limit the altitude above which water boils at human body temperature, making it absolutely impossible for humans to survive unpressurized; approx. 18–19 km above sea level

The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. On Earth, the limit is around 18–19 km above sea level, above which atmospheric air pressure drops below 0.0618 atm.

In fluid mechanics, pressure head is the height of a liquid column that corresponds to a particular pressure exerted by the liquid column on the base of its container. It may also be called static pressure head or simply static head. It is mathematically expressed as:

Supercharger air compressor for an internal combustion engine

A supercharger is an air compressor that increases the pressure or density of air supplied to an internal combustion engine. This gives each intake cycle of the engine more oxygen, letting it burn more fuel and do more work, thus increasing power.

Supermarine Spitfire variants: specifications, performance and armament specific model of the Supermarine Spitfire aircraft family

The British Supermarine Spitfire was one of the most popular fighter aircraft of the Second World War. The basic airframe proved to be extremely adaptable, capable of taking far more powerful engines and far greater loads than its original role as a short-range interceptor had allowed for. This would lead to 24 marks of Spitfire, and many sub-variants within the marks, being produced throughout the Second World War and beyond, in continuing efforts to fulfill Royal Air Force requirements and successfully combat ever-improving enemy aircraft.


  1. 1 2 Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811, Appendix B
  2. From the Ground Up - 29th Edition