Convert between millimeters of mercury and other major units of pressure.
A unit of pressure, obsolete since 1960, about 133.322 pascals. Symbol, mmHg. (Notice that there is no space between “mm” and “Hg”). One mmHg is the pressure exerted at the bottom of a vertical column exactly 1 millimeter deep of a fluid whose density is exactly 13.5951 grams per cubic centimeter, at a location where the acceleration due to gravity is exactly 980.665 centimeters per second per second. The second exact value is, of course, a value for the density of mercury at 0°C, and the third the conventional value for the acceleration due to gravity at the Earth's surface. 1 mmHg = 13.5951 × 9.806 65 newtons per square meter.
The mmHg is often described as synonymous with the torr, but it differs from it, although “by less than 2 × 10⁻⁷ Torr.”¹
According to the current national standard in the United States², the millimeter of mercury is not to be used. The pascal should be used instead.
In Europe, Council Directive 80/181/EEC of 20 December 1979 allowed the continued use of the millimetre of mercury (thus derogating Directive 71/354/EEC). Revisions of the directive continue to sanction the unit's use, but that use is restricted to measurement of “blood pressure and pressure of other body fluids.”³
1. Pure and Applied Chemistry, volume 21 (1970).
Page 24, footnote 1.
2. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.
See Section 3.3.3.
3. Council Directive 80/181/EEC as amended by February 2000. See Annex, Chapter 1, Section 4, Units and Names of Units Permitted in Specialized Fields Only
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Blood pressure is conventionally measured in millimeters of mercury, because for many years doctors measured blood pressure with a mercury sphygmomanometer. This instrument contained a tube, one end of which was submerged in a pool of mercury in an airtight chamber. The cuff was connected to this chamber, and air pressure in the cuff also pushed on the pool of mercury, forcing the mercury up the tube. A scale marked in millimeters placed next to the column readily indicated the pressure in millimeters of mercury.
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For 30 years NIST (the former U.S. Bureau of Standards) has kept two columns of mercury 3 meters high, each containing 225 kilograms of mercury, for precise measurements of high pressures. One was finally dismantled in 2020.¹ The other manometer remains the basis for realizing the U.S. pressure standard, but it is scheduled to be replaced by a new, light-based technology.
Workers removing the casing protecting one of NIST's giant manometers
1. Press release, accessed 29 September 2020.
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