Meters of water column

Physical unit
Unit name	Meters of water column
Unit symbol	${\ displaystyle \ mathrm {mH_ {2} O, \, mWS}}$
Physical quantity (s)	pressure
dimension	${\ displaystyle {\ mathsf {K \; L ^ {- 2}}}}$
system	Technical measurement system
In SI units	${\ displaystyle \ mathrm {1 \, mH_ {2} O = 9 {,} 806 \, 65 \; kPa = 9 {,} 806 \, 65 \ cdot 10 ^ {3} \; {\ frac {kg} {m \, s ^ {2}}}}}$
See also: Torr , Pascal , Inch of water

The liquid pressure at the bottom is identical in all three vessels with the same liquid level.

The water column meter (abbreviation mH ₂ O or mWS) is a non- SI -conform unit for measuring pressure . One meter of water column at 4 ° C corresponds to one megapond per square meter and thus under standard acceleration of fall 9.806 65 kPa (around 0.1 bar ). Since January 1, 1978, the unit is no longer a legal unit in the Federal Republic of Germany . It is mainly used in the sanitary area, in organ building , in industry, for leakage information (e.g. for tent skins ) and in medicine for mechanical ventilation . Clearly, the hydrostatic pressure on the bottom of a straight liquid cylinder , the base of which is horizontal, corresponds exactly to the contact pressure due to the weight of the liquid on the bottom.

In addition to the specification in mWS or mH ₂ O, specifications in mmWS (or mmH ₂ O) or cmWS (or cmH ₂ O) are also common , depending on the measured variable . In the Anglo-American system of measurement , the unit inch of water is used analogously , abbreviation wc or inH ₂ O.

definition

A pressure of 1 meter water column was originally defined as the pressure that corresponds to the hydrostatic pressure in 1 meter water depth. At 4 ° C the water has its maximum density of 999.972 kg / m ³ (with two decimal places that is 1000.0 kg / m ³ ). Since the density of the water is temperature-dependent and has already dropped to 995.02 kg / m ³ at 32 ° C , measurements of the pressure over the water depth would only be suitable if a measurement error of 0.5% can be tolerated. The deviation increases at higher temperatures.

In order to be independent of the density of the water, it is specified:

1 mWS = 98.0665 mbar

Conversions

1 mmH ₂ O = 9.806 65 Pa

10 mmH ₂ O = 1 p / cm ² = 98.0665 Pa

1 mH ₂ O = 1000 mmHg ≈ 73.556 mmHg = 73.556 Torr ${\ displaystyle \ mathrm {\ frac {\ rho _ {H2O}} {\ rho _ {Hg}}} \ cdot}$
1 mH ₂ O = 9.806 65 kPa = 98.0665 hPa = 98.0665 mbar
10 mH ₂ O = 1 at = 1 kp / cm ² = 98.0665 kPa ≈ 0.967 84 atm

Watertightness

The water column is also a unit of measurement to determine the water resistance e.g. B. of technical fabrics ( tents , functional and rainwear ).

DIN EN ISO 811: 2018 regulates the method for determining the resistance to the penetration of water. The following “hydrostatic water pressure test” must be carried out: The outside of the material is exposed to the water. The water pressure starts at zero, the water column increases by 100 mm water column or 600 mm water column per minute, depending on the standard. The time is measured until the third drop can be seen on the inside. The pressure at this point is then given in millimeters of water.

According to the European standard EN 343: 2003 ("protective clothing against rain"), a product with a water column from 800 mm is "waterproof (class 2)" and from 1300 mm "waterproof (class 3)". The Eidgenössische Materialprüfungs- und Forschungsanstalt (EMPA) in St. Gallen, Switzerland assumes that a functional material is waterproof from a water column of 4000 mm. When sitting on a damp surface, a pressure is built up that corresponds to a water column of approx. 2000 mm. When kneeling in a crouch, a water column of approx. 4800 mm presses on the clothing. Special clothing, for example for sailors , can reach values of up to 20,000 mm and still remain breathable .

Upper tents apply mm from 1500 and tent floors from 2000 mm to DIN as waterproof .

For watches, according to DIN 8310 (DIN 8306 for diving watches ), an equivalent to the height of a water column (30 minutes in 1 m water depth and 90 seconds in 20 m water depth) is specified that all seals have to withstand in order to be designated as water-tight. Since these meters are information about the pressure, they cannot be understood one-to-one as practicable water depths (diving depths) in which the watch can be used: Even violent swimming movements or a hit on the water multiply the seals the pressure corresponding to the water depth.

Organ building

In organ building the wind pressure is given in mmWS with which the various organ registers are blown.

literature

Dieter Meschede (Ed.): Gerthsen Physik . 24th revised edition Springer, Berlin 2010, ISBN 978-3-642-12894-3 (EA Berlin 1956).

Individual evidence

^ ^A ^b Alfred Böge, Walter Schlemmer: Collection of tasks for technical mechanics . Springer DE, January 1, 2011, ISBN 978-3-8348-8115-1 , p. 216 (accessed August 5, 2013).
↑ Bernd Hein: Elderly care specifically health and disease theory . Elsevier, Urban & FischerVerlag, 2007, ISBN 978-3-437-27710-8 , p. 8 (accessed on August 5, 2013).
↑ Density of pure water
↑ DIN 1301, Part 3: Units - Conversion of non-SI units , 2018

[BögeSchlemmer2011-1] A ^b Alfred Böge, Walter Schlemmer: Collection of tasks for technical mechanics . Springer DE, January 1, 2011, ISBN 978-3-8348-8115-1 , p. 216 (accessed August 5, 2013).

[Hein2007-2] Bernd Hein: Elderly care specifically health and disease theory . Elsevier, Urban & FischerVerlag, 2007, ISBN 978-3-437-27710-8 , p. 8 (accessed on August 5, 2013).

[3] Density of pure water

[4] DIN 1301, Part 3: Units - Conversion of non-SI units , 2018