José J. Martínez Aug 13, 2011:

Here is a better explanation A centimetre of water (US spelling centimeter of water, abbreviated cmH2O) is a less commonly used unit of pressure derived from pressure head calculations using metrology. It 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.
It is 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.
This unit is commonly used to specify the pressure to which a CPAP machine is set after a polysomnogram.
1 cmH2O = 98.0638 pascals

≈ 0.98064 mbar or hectopascals

≈ 0.39370 inH2O

≈ 0.00096781 atm

≈ 0.73554 torr

≈ 0.73554 mmHg

≈ 0.028958 inHg

≈ 0.014223 psi

José J. Martínez Aug 13, 2011:

You may be right but it is certainlt a non kosher way to post a pressure.

Metro de columna de aguaDe Wikipedia, la enciclopedia libre
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Unidad de presión del Sistema técnico de unidades, y equivale a la presión ejercida por una columna de agua pura de un metro de altura. Es un múltiplo de la unidad columna de agua. Su símbolo es m.c.a.

También se utiliza el milímetro de columna de agua (mm.c.a.)

Su equivalencia es:

1 m.c.a. = 0,1 kg/cm² = 9,81 kPa (kilopascal)

1 m.c.a. = 1000 mm.c.a.

Taña Dalglish Aug 13, 2011:

@ Asker. Oops! You may have seen my errant posting a minute ago. The penny just dropped, i.e. mm. Good luck!

José J. Martínez Aug 12, 2011:

I have searched near and far, even in the most obscure in pressure units... and cannot find the unit you mention.