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CPAP pressure comparison
#1
CPAP pressure comparison
I've been thinking about the pressures we use in our therapy and how they compare with atmospheric pressure at different elevations. The pressures that feel so high to us are really quite low.

I have used the following references to obtain the necessary data for my calculations.

https://www.convertunits.com/from/cmH2O/to/kPa
1 cm H20 = 0.0980665 kPa (or 0.014223343 psi)

https://www.engineeringtoolbox.com/air-a...d_462.html
atmospheric pressure at           0 ft elevation =   101 kPa
atmospheric pressure at   10,000 ft elevation =  69.7 kPa

so:
31.3 kPa atmospheric pressure change per 10,000 ft elevation change
or 0.00313 kPa atmospheric pressure change per 1 ft elevation change

Since 1 ft change in elevation changes the atmospheric pressure by 0.00313 kPA,
and 1 cm H2O equals 0.0980665 kPa,
then a change of 1 cm H2O changes the elevation by 31.3 ft
here's the calculation: 0.0980665/0.00313=31.3

Table of equivalent elevation changes at CPAP pressures
cm H20     kPa          elevation change (ft)
1          0.0980665               31.3
2          0.196133                 62.7
3          0.2941995               94.0
4          0.392266               125.3
5          0.4903325             156.7
6          0.588399               188.0
7          0.6864655             219.3
8          0.784532               250.6
9          0.8825985             282.0
10        0.980665               313.3
11        1.0787315             344.6
12        1.176798               376.0
13        1.2748645             407.3
14        1.372931               438.6
15        1.4709975             470.0
16        1.569064               501.3
17        1.6671305             532.6
18        1.765197               564.0
19        1.8632635             595.3
20        1.96133                 626.6

As an example, using a CPAP pressure of 10cm H20 would be similar to descending about 313 ft in elevation.

I hope all my figures, calculations and conclusions are correct but would appreciate peer review.
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#2
RE: CPAP pressure comparison
I don,t get the point of this. my cpap pressure is what it is at whatever pressure at least until i get close to the clouds . you're surely not suggesting raising it or lowering it if you spend the night 1000 ft higher or lower?
First Diagnosed July 1990

MSgt (E-7) USAF (Medic)
Retired 1968-1990
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#3
RE: CPAP pressure comparison
I don't think the chart is very practical, and atmospheric pressure increases inversely to altitude, so the altitude column needs a minus sign in front of all of those pressure. As pressure increases, it is equivalent to descending in altitude, not increasing.
Sleeprider
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#4
RE: CPAP pressure comparison
(11-29-2020, 10:59 PM)Sleeprider Wrote: the altitude column needs a minus sign in front of all of those pressure

Good find! Thank you for noticing. I tried to edit and add the negative signs but I cannot find a way to edit the post.
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#5
RE: CPAP pressure comparison
You set out to show how low CPAP pressure is relative to atmospheric pressure, and the conclusion that the 1-20 cm range of CPAP is equivalent to a change in altitude of plus or minus 627 vertical feet is impressively low. Most of us don't sense altitude changes less than 1000 feet (300 M), with early indicators of rapid altitude change being pressure in the ears. CPAP works by creating a differential pressure to atmospheric pressure, rather than an absolute pressure to maintain the airway, and assist in respiratory effort. Speaking for myself, the comparison of cm-H20 to psi, has always shown just how low those pressures are.
Sleeprider
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INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
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#6
RE: CPAP pressure comparison
4-20 cmH2O = 0.0568934-0.284467 PSI
Dave

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INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEBSITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
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#7
RE: CPAP pressure comparison
it's interesting even if kind of impractical for use. I'll bet it was a fun exercise.

it did get me to thinking that cpap's low pressure expressed in centimeters of water is pretty easy to understand by itself. the straw in a glass of water analogy often described on this site is descriptive enough. in addition, asking from a non scientific laymen's point of view, wouldn't 20 cmw 'feel' the same as swimming in 20 cmw? as an old hobby level scuba diver, or just as a swimmer & occasional bathtub user, I know the pressure at 20 cmw would barely be noticeable, if at all, and wouldn't come close to pressure requiring equalizing pressure in my ears.

otoh, talking while masked will sometimes increase pressure in my ears, so maybe it's not entirely unnoticeable.

I wonder how much elevation / altitude differential is required to make one's ears 'pop' and whether it varies by individual and/or elevation / altitude.
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#8
RE: CPAP pressure comparison
My wife's ears "pops" at different altitudes than mine. Her ears are much more sensitive. For my ears to "pop", I need to go up in elevation a lot. And, I do mean a lot. 

Where we live, we are over 5K feet in elevation. I'm going to just assume that my DME supplier set my machine up accordingly.  Bigwink
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