cm H2O is Displacement, Not Pressure

[KrisasMan]

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.

The physics teacher in me loves your response... we just have to throw in one variable, what temperature is the water, or Hg? Must account for that expansion. The WolframAlpha link I posted above give you the option to specify the water temperature if you want to get really accurate.

[RationalEntropy]

Perhaps I've over and under evaluated this. It is perhaps as much of a measure of pressure as mmHg, but it drives me crazy that it seems like a deliberately esoteric unit. It could also be that I was previously referring to 1-d displacement, a corresponding 1-d force, and a cross-sectional area, to convert from a cmH2O measure to Pascals, torr or whatever. A liquid material being displaced a certain distance (1-d displacement, not volume displacement) by pressure being the same thing as pressure? No, just a functional way to measure it. Throw stat. mech/thermodynamics into the mix... and see what changes.

How much do these machines compensate for certain altitudes?

If possible I would like to see this in yards of mead, but that is just another spergzilla post. I really did not expect to be thought of as a retard for posting this. It was something interesting/amusing/irritating to me. Had I known the responses received I would not have wasted time. Perhaps had I thought that people look at mmHg, and cmH2O as a DIRECT measure of pressure. Expansion, contraction, and linear displacement are measured and correlated with pressure changes, perhaps out of necessity to make numbers more convenient, understandable or easily measured.

[idamtnboy]

A liquid material being displaced a certain distance (1-d displacement, not volume displacement) by pressure being the same thing as pressure? No, just a functional way to measure it.

As is any instrument that measures pressure, like a pressure transducer whether it be in a jet engine or a home blood pressure monitor.

How much do these machines compensate for certain altitudes?

Pressure of 10 cm H2O water column is the same at sea level as on top of the Rockies, EXCEPT for the slight difference caused by air density differences! (That is of course assuming the water in the column is the same temp at both altitudes! )

I really doubt an S9 takes into account altitude differences since that aspect would have limited effect on the therapy pressure. What we don't know is how accurate the S9 readout is, in absolute numbers, in relation to the actual pressures in the hose. When mine is set for 8.4 minimum pressure Resscan shows it at something like 8.38, or some such number. What I find really intriguing is the ability of the S9 sensors to measure pressure differences, and at 25 Hz at that. As I see it, the S9 is an extremely sensitive machine with some incredibly capable pressure and temp sensors and flow sensors. Internally it's like a Swiss watch, but its readout has hands so wide you can't tell if it's showing 5 or 5.5 minutes past the hour!

If possible I would like to see this in yards of mead, but that is just another spergzilla post. I really did not expect to be thought of as a retard for posting this. It was something interesting/amusing/irritating to me. Had I known the responses received I would not have wasted time. Perhaps had I thought that people look at mmHg, and cmH2O as a DIRECT measure of pressure. Expansion, contraction, and linear displacement are measured and correlated with pressure changes, perhaps out of necessity to make numbers more convenient, understandable or easily measured.

Sorry if I, or others, convey a retard reaction. You certainly are not a retard. I do/did kind of find your thoughts a little bit confusing! That's probably because of a clash of paradigms. I have trouble figuring out why you have seem to have a problem with seeing cm H2O as a direct measure of pressure. They are just as direct as is psi or any other usual units of measure. If you want to be absolutely precise then characteristics such as atmospheric pressure, temperature, etc., need to acknowledged and factored in. But often that kind of precision does not add to the end level of understanding.

This, to me, is not a waste of time and forum space. Sometimes we need to engage in discussions of the obvious because we tend to get locked into a certain way of thinking. It's good to get jolted once in awhile.

[RationalEntropy]

Thank you for your input, all. I was probably overreacting about being incorrect/poorly stating my observations. It is all about learning, and at times it is more difficult to discern the tone that a person is using through a computer.

[sickwithapnea17]

I wonder how H2O relates to Ti, Isn't Ti the time you inhale so does that effect how deeply the air gets into your lungs and your O2 saturation? also BPM can be adjusted
there seems to be a volume of air/time here which can also be related to the variables like O2 saturation
so there must be a physics reason for how the IPAP/EPAP increase or decrease O2 saturation

[SleepyToo2]

As a scientist, this thread has me intrigued. However, isn't the displacement of water actually a measure of the relative difference in pressure between atmospheric pressure and the pressure needed to keep the airway open? As such, it is very difficult to measure/request/set the real pressure accurately because of the variations that have already been mentioned. For this reason, I am quite happy to accept 10 cm of water as the measure of what is needed. If I have my "pressure" increased by 1 cm of water, that is much easier to understand than having it increased by 0.0017 (or whatever the value actually is) psi. For sleep-deprived brains like I had, KISS certainly applied - and I am not sure that as my treatment worked that situation changed significantly!

[RationalEntropy]

I'm still early in my therapy, and am still prone to moments of absent mindedness. It was a very obvious error on my part, as one can easily draw a relationship between a linear displacement and pressure. See mmHg, for example.

I was wanting to bust out the fluid mechanics, determine flux, etc. and derive more "intuitive units" such as those explicitly written as force per area, while giving others a greater understanding of fluids through a model (numerical solutions of PDEs), but I've learned a valuable lesson: IT IS NOT WISE FOR ME TO POST ANYTHING JUST AFTER I'VE AWAKENED! I'm far dumber after waking up. So, now I don't really want to post as frequently, since being wrong can shrink the ego a bit, especially if it is about something I should be semi-well versed in. I could always use the "I work with condensed matter on the nanoscale" excuse... or I could just be truthful, and say, "only post when semi-aware".

[sickwithapnea17]

it would be nice if someone could post graphs of variables like IPAP/EPAP/Ti/BPM vs O2 saturation/AHI/sleep continuity
my sleep tech was increasing pressure in response to central apnea and flow limitation
it seems that many studies I've seen only analyze in terms of AHI in simplistic sense

http://www.ncbi.nlm.nih.gov/pubmed?term=bipap%20
some of these articles are free

[archangle]

The real problem with "cmH2O" is that when I go back to visit my home planet, since gravity is stronger, the same cmH2O reading is a different pressure.

My incompetent DME doesn't seem to know how to change the value of g from 9.80665 m/s**2 to 13.47 m/s**2.

We also have a much higher concentration of deuterium in the water. Do I need to recalibrate for that as well?

It's hard enough getting hold of the clinician's manual. Does anyone know where to download the space alien's manual for a PRS1 machine?

By the way, if you think it's hard to get a CPAP machine through security for an airplane flight, you ain't seen nothing yet. ETSA agents are REALLY fond of probes.

Strangely enough, the electrical outlets back home are 60 Hz 120VAC with a standard NEMA 5-15 connector.

[deltadave]

Perhaps I've over and under evaluated this. It is perhaps as much of a measure of pressure as mmHg, but it drives me crazy that it seems like a deliberately esoteric unit. It could also be that I was previously referring to 1-d displacement, a corresponding 1-d force, and a cross-sectional area, to convert from a cmH2O measure to Pascals, torr or whatever. A liquid material being displaced a certain distance (1-d displacement, not volume displacement) by pressure being the same thing as pressure? No, just a functional way to measure it. Throw stat. mech/thermodynamics into the mix... and see what changes.

How much do these machines compensate for certain altitudes?

If possible I would like to see this in yards of mead, but that is just another spergzilla post. I really did not expect to be thought of as a retard for posting this. It was something interesting/amusing/irritating to me. Had I known the responses received I would not have wasted time. Perhaps had I thought that people look at mmHg, and cmH2O as a DIRECT measure of pressure. Expansion, contraction, and linear displacement are measured and correlated with pressure changes, perhaps out of necessity to make numbers more convenient, understandable or easily measured.

The Total Collective Effect (TCE) of these influences (water, pressure, displacement, altitude, sushi, etc.) was clearly demonstrated in this poster's conundrum:

[rocklin]

When I go to the gas station, and place the tire-inflator thingie in my ear, I have a very specific goal, and I want the reading in PSI.