Centimetre of waterEdit
|centimetre of water|
|1 cmH2O in ...||... is equal to ...|
|SI units||98.0665 Pa|
|English Engineering units||1.422334×10−2 psi|
A centimetre of water (US spelling centimeter of water) is a unit of pressure. It may be defined as the pressure exerted by a column of water of 1 cm in height at 4 °C (temperature of maximum density) at the standard acceleration of gravity, so that 1 cmH2O (4°C) = 999.9720 kg/m3 × 9.80665 m/s2 × 1 cm = 98.063754138 Pa ≈ 98.0638 Pa, but conventionally a nominal maximum water density of 1000 kg/m3 is used, giving 98.0665 Pa.
The centimetre of water unit is frequently used to measure the central venous pressure, the intracranial pressure while sampling cerebrospinal fluid, as well as determining pressures during mechanical ventilation or in water supply networks (then usually in metres water column). It is also a common unit of pressure in the speech sciences. This unit is commonly used to specify the pressure to which a CPAP machine is set after a polysomnogram.
Millimetre of waterEdit
|millimetre of water|
|1 mmH2O in ...||... is equal to ...|
|SI units||9.80665 Pa|
|English Engineering units||1.422334×10−3 psi|
Millimetre of water (US spelling millimeter of water) is a 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 = 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 ≈ 0.03937008 inH2O ≈ 9.678411×10−5 atm ≈ 0.07355592 torr ≈ 0.07355591 mmHg ≈ 0.002895902 inHg ≈ 0.001422334 psi
In limited and largely historic contexts it may vary with temperature, using the equation:
- P = ρ·g·h/1000,
- 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.
- NOTE: A centimetre of water is abbreviated as cmH2O or cm H2O.
- nist.gov – Guide for the Use of the International System of Units (SI) page 47, 2008 Edition
- NOTE: Millimetre of water is also sometimes addressed as : millimetres water column, abbreviated as mmwg, mmH2O, or mmwc.