Inch of mercury

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.

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]

Conditions Pressure
conventional 3386.389 pascals
32 °F 3386.38 pascals
60 °F 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 = 3376.85 Pa

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

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


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.

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.

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


  1. ^ a b Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811, Appendix B [1]
  2. ^ From the Ground Up - 29th Edition
Acronyms and abbreviations in avionics

This is a list of the acronyms and abbreviations used in avionics.


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.


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, pressure systems and frontal boundaries.

Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument, but used for different purposes. An altimeter is intended to be used at different levels matching the corresponding atmospheric pressure to the altitude, while a barometer is kept at the same level and measures subtle pressure changes caused by weather.

Conversion of units

Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors.

Eastern Air Lines Flight 21

Eastern Air Lines Flight 21, registration NC28394, was a Douglas DC-3 aircraft that crashed while preparing to land at Candler Field (now Hartsfield-Jackson Atlanta International Airport) in Atlanta, Georgia, on February 26, 1941. Eight of the 16 on board were killed including Maryland Congressman William D. Byron. Among the injured was Eastern Air Lines president and World War I hero Eddie Rickenbacker.

Hurricane Patricia

Hurricane Patricia was the second-most intense tropical cyclone on record worldwide, behind Typhoon Tip in 1979, with a minimum atmospheric pressure of 872 mbar (hPa; 25.75 inHg). Originating from a sprawling disturbance near the Gulf of Tehuantepec, south of Mexico, in mid-October 2015, Patricia was first classified a tropical depression on October 20. Initial development was slow, with only modest strengthening within the first day of its classification. The system later became a tropical storm and was named Patricia, the twenty-fourth named storm of the annual hurricane season. Exceptionally favorable environmental conditions fueled explosive intensification on October 22. A well-defined eye developed within an intense central dense overcast and Patricia grew from a tropical storm to a Category 5 hurricane in just 24 hours—a near-record pace. On October 23, the hurricane achieved its record peak intensity with maximum sustained winds of 215 mph (345 km/h). This made it the most intense tropical cyclone on record in the Western Hemisphere, and the strongest globally in terms of 1-minute maximum sustained winds.

Late on October 23, dramatic weakening ensued and Patricia made landfall near Cuixmala, Jalisco, with winds of 150 mph (240 km/h). This made it the strongest landfalling hurricane on record along the Pacific coast of Mexico. Patricia continued to weaken extremely quickly, faster than it had intensified, as it interacted with the mountainous terrain of Mexico. Within 24 hours of moving ashore, Patricia weakened into a tropical depression and dissipated soon thereafter, late on October 24.

The precursor to Patricia produced widespread flooding rains in Central America. Hundreds of thousands of people were directly affected by the storm, mostly in Guatemala. At least six fatalities were attributed to the event: four in El Salvador, one in Guatemala, and one in Nicaragua. Torrential rains extended into southeastern Mexico, with areas of Quintana Roo and Veracruz reporting accumulations in excess of 19.7 in (500 mm). Damage in Chetumal reached MX$1.4 billion (US$85.3 million).As a tropical cyclone, Patricia's effects in Mexico were tremendous; however, the affected areas were predominantly rural, mitigating a potential large-scale disaster. Violent winds tore roofs from structures and stripped coastal areas of their vegetation. Preliminary assessments indicated hundreds of homes to be destroyed; seven fatalities were linked to the hurricane directly or indirectly, including one during evacuations. Total damage from Patricia was estimated to be at least $462.8 million (2015 USD); the damage in Mexico alone was estimated to be in excess of MX$5.4 billion (US$325 million), with agriculture and infrastructure comprising the majority of losses. Flooding partially associated with remnant moisture from Patricia inflicted US$52.5 million in damage across Southern Texas.

Inch (disambiguation)

An inch is a unit of measurement.

Inch or inches may also refer to

Inflow (meteorology)

Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.

List of atmospheric pressure records in Europe

The following is a List of atmospheric pressure records in Europe and the extratropical Northern Atlantic (it does not include localised events, such as those that occur in tornados).

Extreme pressure values in Europe show both seasonal and geographical differentiation. the greatest pressure extremes occur in winter (January) with the deepest lows occurring to the northwest of the continent with a diminishing influence of low pressure to the southeast towards Central Europe and Southeast Europe. This is related to the main cyclonic centre of the Icelandic low, and the North Atlantic extratropical storm track, close to which have been observed some of the lowest atmospheric pressures of the Northern Hemisphere outside the tropics. Extreme high values are favoured over the north east of Europe where intense cold and long winter nights lead to cooling of the air column by radiative cooling causing sinking air reinforcing the development of the highest pressures. Other influences include the semi-permanent Azores high, and Siberian highs.

Millimetre of mercury

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.Although not an SI unit, the millimetre of mercury is still routinely used in medicine, meteorology, aviation, and many other scientific fields.

One millimetre of mercury is approximately 1 Torr, which is 1/760 of standard atmospheric pressure (101325/760 ≈ 133.322368421053 pascals). The two units are not exactly equal; however, the relative difference (less than 0.000015%) is negligible for most practical uses.

Mooney M20

The Mooney M20 is a family of piston-powered, four-seat, propeller-driven, general aviation aircraft, all featuring low wings and tricycle gear, manufactured by the Mooney International Corporation.The M20 was the 20th design from Al Mooney, and his most successful. The series has been produced in many variations over the last 60 years, from the wooden-wing M20 and M20A models of 1955, to the M20V Acclaim Ultra that debuted in 2016. More than 11,000 aircraft in total have been produced.

In November 2008, the company announced that it was halting all production as a result of the late-2000s recession, but would still provide parts and support for the existing fleet. With the injection of Chinese capital after the company's purchase, production of the M20 resumed in February 2014. Since then, the company has released two more M20 models.


A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. When released, the restoring force acting on the pendulum's mass causes it to oscillate about the equilibrium position, swinging back and forth. The time for one complete cycle, a left swing and a right swing, is called the period. The period depends on the length of the pendulum and also to a slight degree on the amplitude, the width of the pendulum's swing.

From the first scientific investigations of the pendulum around 1602 by Galileo Galilei, the regular motion of pendulums was used for timekeeping, and was the world's most accurate timekeeping technology until the 1930s. The pendulum clock invented by Christian Huygens in 1658 became the world's standard timekeeper, used in homes and offices for 270 years, and achieved accuracy of about one second per year before it was superseded as a time standard by the quartz clock in the 1930s. Pendulums are also used in scientific instruments such as accelerometers and seismometers. Historically they were used as gravimeters to measure the acceleration of gravity in geophysical surveys, and even as a standard of length. The word "pendulum" is new Latin, from the Latin pendulus, meaning 'hanging'.

Perm (unit)

A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane.


Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.

Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre; similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure, and the torr is defined as ​1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.

Pressure measurement

Pressure measurement is the analysis of an applied force by a fluid (liquid or gas) 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 meters or pressure gauges or vacuum gauges. A manometer (not to be confused with nanometer) 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. There are also digital pressure Meters available, that can measure positive and negative gauge pressure as well as differential pressure.

A vacuum gauge is a pressure gauge used to measure pressures lower than the ambient atmospheric pressure, which is set as the zero point, in negative values (e.g.: −15 psig or −760 mmHg equals total vacuum). Most gauges measure pressure relative to atmospheric pressure as the zero point, so this form of reading is simply referred to as "gauge pressure". However, anything greater than total vacuum is technically a form of pressure. For very accurate readings, especially at very low pressures, a gauge that uses total vacuum as the zero point may be used, giving pressure readings in an absolute scale.

Other methods of pressure measurement involve sensors that can transmit the pressure reading to a remote indicator or control system (telemetry).


The torr (symbol: Torr) is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere (101325 Pa). Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).

Historically, one torr was intended to be the same as one "millimeter of mercury". However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.

The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.

Vegetable oil

Vegetable oils, or vegetable fats, are fats extracted from seeds, or less often, from other parts of fruits. Like animal fats, vegetable fats are mixtures of triglycerides. Soybean oil, rapeseed oil, and cocoa butter are examples of fats from seeds. Olive oil, palm oil, and rice bran oil are example of fats from other parts of fruits. In common usage, vegetable oil may refer exclusively to vegetable fats which are liquid at room temperature.

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