Millimeters, water gauge

Millimeters, water gauge, also known as a millimetre of water (US spelling millimeter of water) or millimetres water column and abbreviated to mmwg, mmH2O or mmwc, respectively, is a less commonly used unit of pressure. It may be defined as the pressure exerted by a column of water of 1 mm in height at 4 °C (temperature of maximum density) at the standard acceleration of gravity, so that 1 mmH2O (4°C) = 999.9720 kg/m3 × 9.80665 m/s2 × 1 mm = 9.8063754138 Pa ≈ 9.80638 Pa, but conventionally a nominal maximum water density of 1000 kg/m³ is used, giving 9.80665 Pa.

1 mmH2O (conventional)= 9.80665 pascals[1]
= 0.001 metre water (mH2O), metre water column (m.wc) or metre water gauge (m wg)
= 0.1 cm wg
= 0.0980665 mbar or hPa
≈ 0.0393700787401575 inH2O
≈ 9.67841105354059E-05 atm
≈ 0.0735559240069085 torr
≈ 0.073555913527668 mm Hg
≈ 0.00289590210738851 inHg
≈ 0.00142233433071196 psi

In limited and largely historic contexts it may vary with temperature, using the equation:

P = ρ·g·h/1000
where
P: pressure in Pa
ρ: density of water (conventionally 1000 kg/m3 at 4°C)
g: acceleration due to gravity (conventionally 9.80665 m/s2 but sometimes locally determined)
h: water height in millimetres

The unit is often used to describe how much water rainwear or other outerwear can take or how much water a tent can resist without leaking.

See also

References
  1. nist.gov – Guide for the Use of the International System of Units (SI) page 47, 2008 Edition
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