Why is pressure measured in cmH2O?

[macmermaid]

I'm just curious ---

Just wondering if anybody knows why pressure is measured in cmH2O. And what that is really. Centimeters of water? Is it centimeters of water pushed a certain distance over a particular time?

Also, leakages are reported in liters per second or minute. Is there a way to tell what the total liters of air were based on the amount of pressure? Would the level of humidity affect this number? And maybe elevation?

[Guest]

Just wondering if anybody knows why pressure is measured in cmH2O. And what that is really. Centimeters of water? Is it centimeters of water pushed a certain distance over a particular time?

It is most likely that cpap pressure is expressed in cm of water because lung pressure is measured in those units. Why those units were chosen for lung pressure I have no idea.

And yes, it is how far, in centimeters, a column of water can be lifted. I also have no idea what the standards are for that column of water (height and diameter being the important ones).

[TheDuke]

The use of cm. of water as a unit of pressure is related to the ease of making a simple manometer of glass or clear plastic charged with water and resting against a little centimeter measuring scale. This is a time honored method of measuring low pneumatic pressures. 10 cm is closely 4 inches. In the sciences, the older pressure unit was usually millimeters of mercury (as atmospheric pressure is approximately 760 mm. at sealevel. But mercury is very dense , 13.5 times as dense as water, so 10 cm of water would be only .74 cm of mercury and would be difficult to read small changes. The official pressure units now are Pascals, but we won't get into that.

Incidentally, the diameter of the water column doesn't matter. The pressure measurement is independent of the diameter.

TheDuke

[rested gal]

I'm not good at math or physics, but here's a link to some more discussions about the "cm H2O" thingy --

viewtopic.php?t=13632

[macmermaid]

Cool, thanks for the link Rested Gal, and the comments Duke and Guest.

I did a little research, still mulling it over. Will post later.

[Guest]

Just wondering if anybody knows why pressure is measured in cmH2O. And what that is really. Centimeters of water? Is it centimeters of water pushed a certain distance over a particular time?

It is most likely that cpap pressure is expressed in cm of water because lung pressure is measured in those units. Why those units were chosen for lung pressure I have no idea.

And yes, it is how far, in centimeters, a column of water can be lifted. I also have no idea what the standards are for that column of water (height and diameter being the important ones).

the standards are between 4 and 11

[JDS74]

I'm just curious ---

Just wondering if anybody knows why pressure is measured in cmH2O. And what that is really. Centimeters of water? Is it centimeters of water pushed a certain distance over a particular time?

Also, leakages are reported in liters per second or minute. Is there a way to tell what the total liters of air were based on the amount of pressure? Would the level of humidity affect this number? And maybe elevation?

The use of water in the measurement is likely because water is easily obtained in a purity appropriate to making pressure measurements. As was mentioned before, the density of water is so much less than that of mercury, the other common pressure standard, that it was deemed to be much more practical in measuring the relatively low pressures involved in CPAP therapy. On a manometer, you can easily see a change of 0.5cm H2O but such a change would be infinitesimal if viewed using Mercury.

The use of cm in the measurement likely came from the near universality of the metric sysyem by the time Dr. Colin Sullivan, an Australian phyisician, invented CPAP therapy in 1980 by which time the conversion to the metric system in Australia was nearing completion.

Reporting leaks in liters per minute is just reporting flow rate. If you want volume, you have to multiply by the time interval to get total volume. At the pressures involved in CPAP therapy, the slight differences between that calculation and the volume you would get after converting to sea level pressure would be quite small.

Elevation, on the other hand, affects air pressure significantly. Modern CPAP machines adjust their therapy pressures to compensate for the difference between sea level air pressure and the air pressure where you are. There are comments in the literature about limits to that adjustment so that above some altitude, CPAP therapy is impaired but I don't recall what those limits are.

Humidity affects air density and therefore, to some extent, oxygen concentration. More water vapor in the air means a change in the concentration of Oxygen, Nitrogen, etc. as a percent of the total gas volume. I suspect that the effect is so small as to be completely ignorable in the CPAP treatment world

[Goofproof]

XPAP pressures are in a range that using PSI would require harder calculations, and defining would be harder to set. When measuring something you need to choose a scale that's the easy to use. HVAC pressures are also in this range. Jim

[idamtnboy]

The simplest answer, I think, is the medical profession has for many years, if not always, used the metric system.

If the link above doesn't point you to it, Google manometer. You'll find a complete explanation of pressure and different ways to measure it. The US heating/air conditioning industry uses inches water to measure air pressure.

[rpiazzo]

I'm just curious ---

Just wondering if anybody knows why pressure is measured in cmH2O. And what that is really. Centimeters of water? Is it centimeters of water pushed a certain distance over a particular time?

Also, leakages are reported in liters per second or minute. Is there a way to tell what the total liters of air were based on the amount of pressure? Would the level of humidity affect this number? And maybe elevation?

Atmospheric pressure at sea level is 1034 cmH2O, 760 mmHg, or 14.7 lbs/square inch. A sneeze generates >90 cmH20. Since CPAP is a relatively low pressure, 4-20 cmH2O, converting to mmHg or lbs./square inch would be a crude and in exact metric. CPAP is measured in cmH2O out of necessity.

[chunkyfrog]

Centimeters opposed to inches; decimals vs fractions.
Science likes metric for a reason.

[manomitpete]

Most of the posts cover the topic and the answers are scattered in them. Cmh2o ? Its easy, its international, its become the standard measurement of pressure for artificial ventilation of anything that breaths or needs assistance with breathing. mm of Mercury , Pascal's etc. have been used but fall bye the way in medical devices..
cmh20 and inches h20 are measurements used in industry .air-conditioning, and all types of low pressure gas usage.
Simple testing and calibrating of gauges and pressure sensing devices is easy with a clear vertical tube, water, and a cm ruler.
CPAP and PEEP techniques were in common use in 1960s and 1970s in anaesthetics and intensive care ventilation and were not "invented " to threat sleep apnea .

[aspen]

Centimeters opposed to inches; decimals vs fractions.
Science likes metric for a reason.

Science likes metric because the metric system is used in every other country in the world, other than the U.S. Not just in science, but in day to day life.

In order for scientists in the States to communicate on the world scientific stage, they have to speak metric.

[Goofproof]

Centimeters opposed to inches; decimals vs fractions.
Science likes metric for a reason.

Science likes metric because the metric system is used in every other country in the world, other than the U.S. Not just in science, but in day to day life.

In order for scientists in the States to communicate on the world scientific stage, they have to speak metric.

Science has to keep it easy, The Professors can't do anything hard. It didn't help that failed Mars probe, You can't apply one system and have someone use the other and expect to hit your target. OPP's! failed again. Jim

[Goofproof]

I'm just curious ---

Just wondering if anybody knows why pressure is measured in cmH2O. And what that is really. Centimeters of water? Is it centimeters of water pushed a certain distance over a particular time?

Also, leakages are reported in liters per second or minute. Is there a way to tell what the total liters of air were based on the amount of pressure? Would the level of humidity affect this number? And maybe elevation?

Atmospheric pressure at sea level is 1034 cmH2O, 760 mmHg, or 14.7 lbs/square inch. A sneeze generates >90 cmH20. Since CPAP is a relatively low pressure, 4-20 cmH2O, converting to mmHg or lbs./square inch would be a crude and in exact metric. CPAP is measured in cmH2O out of necessity.

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either you or the duke lost a decimal place

N.A.S.A, we just lost another Mars Lander Jim