millimeter of water

Two very similar units of pressure, 19th – 20th centuries, in concept the pressure at the bottom of a column of water 1 millimeter deep at the Earth's surface. For the more precise measurements the value was corrected for temperature, that is, for the varying density of water. In the 19th century it was commonly used to measure the draft in chimneys, and later it served in measuring pressures inside living animals (or people!). Expressing a measurement of a low pressure in millimeters of water yields a handier number than expressing it in millimeters of mercury.


Symbol, mm water. A unit of pressure, = 9.806 375 41 pascals, the pressure exerted by a column of water 1 millimeter high, at water's maximum density, at 101,325 pascals (760 torr = standard atmospheric pressure) and at the standard acceleration due to gravity.


Symbol, mmH₂O. The conventional millimeter of water, a unit of pressure = 9.806 65 pascals, the pressure exerted by a column of liquid 1 mm high with a density of 1 gram per cubic centimeter, under the standard acceleration due to gravity. In practice, the unit is actually based on water at 4°C at standard pressure.

Compare conventional millimeters of mercury.

for further reading

World Meteorological Organization.
International Barometric Convention (adopted 1953, implemented by 1955)

International Organization for Standardization.
Quantities and units — Part 4: Mechanics.
ISO 80000-4.
Geneve: ISO (2006).

Defines standards used in deriving all the non-SI units of pressure.



Trial No. 2--Nos. 1 and 2 Boilers

Description of Fuel: Refuse from Screens. Duration of trial, 9½ hours. Draught, 9 millimeters of water (3½ tenths of an inch). Pressure of steam, 50 pounds per square inch.

Engineering and Mining Journal, vol 37, no 2 page 22, (January 12, 1884)

Note two interesting features of this excerpt: First,  mixture of metric and conventional measure. Second, that “3½ tenths” (instead of “0.35 inch”), though to modern eyes awkward, falsely precise, and adding an accessory and unneeded unit, perfectly reflects the realities of reading a scale marked in tenths of an inch.


However, the maximum allowable inhalation resistance of complete type BE respirators shall be 3 inches (75 millimeters) of water rather than 2 inches (50 millimeters) of water allowed for dust, fume, or mist respirators by Part 14.

1938 Supplement to the Code of Federal Regulations of the United States of America, page 2948.

Note how the rounding of the conversion from inches to metric matches the application. As a U.S. regulation, the inches come first.

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