cm H2O is Displacement, Not Pressure

[RationalEntropy]

As a man of physics I would like to say that cm H2O are rather counter-intuitive units for pressure. In fact, any physics textbook worth its weight in working cpap parts will define pressure as force per unit area. Cm H2O is a displacement of water given a particular pressure (the actual pressure can be solved for given the cross-sectional area of the cylinder (which one can measure)). This has been driving me nuts for a long time. I am finally awake and cogent enough to post this gripe.

[robysue]

Curiosity makes me ask: Do you have the same problem with barometric pressure being measured in inches of mercury?

[Robespierre]

As a man of physics I would like to say that cm H2O are rather counter-intuitive units for pressure. In fact, any physics textbook worth its weight in working cpap parts will define pressure as force per unit area. Cm H2O is a displacement of water given a particular pressure (the actual pressure can be solved for given the cross-sectional area of the cylinder (which one can measure)). This has been driving me nuts for a long time. I am finally awake and cogent enough to post this gripe.

Going by the strict definition of pressure, you are, of course, correct. But since displacement of H2O is proportional to the pressure causing the displacement, it is a convenient way to express pressure, particularly for comparative purposes. In the case of CPAP pressure, the comparison is to ambient atmospheric pressure. As long as everyone concerned uses the same units, I think it's a moot point. Everybody uses cm H2O for CPAP pressure and mm Hg for blood pressure. That's the medical field for you. The units are useful even if not in keeping with the precise definition of pressure.

[jnk]

I assume that something easy (as far as scale and significant digits) was chosen by the respiratory-medicine pioneers according to the convenient traditions and conventions of medicine as an applied science, in which further exactitude would be tedious to work with in emergency circumstances, especially.

But clinicians can always convert it to whatever they want, should they have a need to do so, as shown under "Pressure" on p. 640 of the following: http://www.rcjournal.com/guidelines_for ... ymbols.pdf

[BlackSpinner]

all of the metric system is based on water. Do you have problems with Milliliters, Milligrams and centigrade concepts too?

[tetragon]

Curiosity makes me ask: Do you have the same problem with barometric pressure being measured in inches of mercury?

I just checked my local forecast, and it states the barometric pressure in kilopascals. Perhaps RationalEntropy lives in a place that uses a unit of pressure to report on pressure.

[jnk]

"In many situations, measuring pressures in units of length of the liquid in the manometer is perfectly adequate."--David M. Harrison, Dept. of Physics, Univ. of Toronto, July 2002.

http://www.upscale.utoronto.ca/PVB/Harr ... meter.html

[KrisasMan]

I believe cm of H2O was chosen because of how early CPAP machines regulated pressure, I believe some infant systems still use this. The pressurized output of the machine is connected the patient and to a large tube that is immersed in water to the specified depth. If the pressure ever gets too high (a big issue with infants with IRDS) the air just bubbles out of the bottom of the large tube relieving the pressure and preventing damage to the fragile lungs. Because of this setup they just went by how deep they immersed the tube in the water for a standard.

It is a bit of a pain but nobody wants to dial in 0.1422334 psi for their machine, whole numbers are much nicer. Besides, WolframAlpha can do the math just fine so it is standard enough. http://www.wolframalpha.com/input/?i=10 ... H2O+in+PSI

[RationalEntropy]

No qualms with water, or the metric system, just ranting about this point. Yes, displacement is proportional to pressure, but it is also proportional to resisting pressures, that is displacement varies in different tissues. Blah blah blah Mutatis mutandis..

There was an incompetent TA during undergrad who we trolled using archaic, and unusual units to answer problems, so, I'm familiar with units, it is just that when something seems illogical it drives me insane.

I think it is normal to gripe occasionally. Sorry to 'sperg out.

[jnk]

The trick is not to let the pressure displace you.

Good post. Made me think, despite myself.

[MaxDarkside]

I prefer feet of beer myself

[handyguy]

In industrial process control instrumentation inches of water is used as a measure of the amount of pressure it takes to form a bubble at the end of a tube immersed in a container of water. For example if a tube were inserted in the bottom of a tank, one could determine how deep the water was by applying air pressure to the tubing until a bubble would leave the tube and rise to the top surface. By using a gage (or manometer) cailbrated in centimeters,meters,inches or feet of water,connected to the tube, one could read the tank's level.
The pressures used in our machines would be below 1pound per square inch of pressure and finding a gage calibrated in psi with resolution to see the slight differences between .2psi and .8 psi etc. would be difficult.

[redjoe]

I prefer feet of beer myself

Ever try to drink a yard of ale? It's kind of fun, although potentially very messy.

[idamtnboy]

As a man of physics I would like to say that cm H2O are rather counter-intuitive units for pressure. In fact, any physics textbook worth its weight in working cpap parts will define pressure as force per unit area. Cm H2O is a displacement of water given a particular pressure (the actual pressure can be solved for given the cross-sectional area of the cylinder (which one can measure)).

Uh, you are my friend a little off the mark. As soon a you factor in the cross sectional area of the cylinder or tube you are immediately converting from pressure to force. Pressure is independent of area as it is, as you say, force per unit area.

Cm of H2O, or feet of H2O, or inches of H2O, or mm Hg, or inches Hg, or psi, or newtons/sq meter, or pascals, or nanopascals, or grams-force/sq meter, or atmospheres, or ..... are all different ways of expressing the same physical characteristic, force/area. It's just that the various units of the various measurement systems lend differing visualizations of the force.

Cm of H2O is just a variation of inches of H2O which is the common system for expressing air pressure, especially in HVAC systems. You put one end of a tube into the high pressure side of a fan, the other end into the low pressure side, have water in the tube, and measure the height difference from the one side to the other. Voila, you have pressure expressed in linear measurement units of water column. The tube diameter has no bearing on the reading. The tube length has no bearing on the reading as long as it's long enough to allow the water to freely move in the tube without exiting it.

Pump output pressures are given as head in feet of water. That is solely a measure of the pressure out minus the pressure in of the fluid moving through the pump and is independent of the type of fluid.

Sorry to disagree flat out with you, but cm of H2O IS a measurement of pressure. Oh, BTW it is not a measure of displacement. Displacement is a volume measurement, such as in the displacement in cubic feet caused by a fully loaded boat vs an empty boat.

[jnk]

Idamtnboy, I believe the point of the OP may be that some methods used for measuring pressure that are plenty good enough for CPAP or HVAC are not good enough for more precise scientific endeavors, since variables beyond changes in pressure can affect the readings. In that sense, I believe, it can technically be stated from that perspective that it is not pressure alone that is being measured.

As one source states:

"Manometry measurements are functions of both density and gravity. The values of these two are not constant. Density is a function of temperature, and gravity is a function of latitude and elevation. . . . The universal acceptance of a standard for water has been slow. . . . Recognizing that manometers may be read outside standard temperature and gravity, corrections should be applied to improve the accuracy of a manometer reading at any given condition."

http://www.meriam.com/pdf/appnotes/mech ... ppnote.pdf

None of that may matter all that much in the rough measurement of pressure applied to the upper airway, as I understand it as a patient, anyway. (CPAP prescriptions don't generally come with decimal points, I don't think. ) Some docs will throw in an extra cm or two for good measure, just in case more pressure is needed for a night when a virus happens to meet a martini.

But I can see how the less-precise world of CPAP prescriptions can sound irritating to someone used to working in a field that requires more specific and precise definitions and measurements of things like "pressure."