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[Pressure] Can you explain pressure measurement?
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Tacoma Droner Offline

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Post: #1
Can you explain pressure measurement?
(Note: I searched this topic, found several threads, but none of them answered my specific question.)

I want to understand CPAP pressure in terms I can relate to. What I really want to know is how much pressure is my machine delivering, as compared to not using any CPAP at all. Before you jump in with a trigger reply, read the rest of this post.

I started my research by finding out what normal atmospheric pressure is on the planet Earth. Atmospheric pressure varies a bit due to weather and altitude, but I found out that the average atmospheric pressure at sea level is generally somewhere around 100kPa (a meteorologist would call this about 30 inches of mercury.) What's a kPa? --It's a kilopascal, or 1000Pa. So what's a Pa? It's a "Pascal Unit" which is a scientifically standardized measure of pressure.

I don't really need to know what a Pascal Unit is, or how it's measured, all I want to know is what's the pressure level of the air in my back yard. And the answer is--around 100kPa (which by the way is around 14-1/2 PSI). Good enough. But our CPAP machines deliver IPAP pressures in the neighborhood of 5 to 20 centimeters of water (cmH2O). So how many centimeters of water equals 100kPa? --Easy enough to Google that.--

Here's where it gets weird. 100kPa is around 1000cmH2O!! So if my CPAP is delivering 15cmH2O, then that's like breathing whisper-thin air in a low Earth orbit, which means I would suffocate and die within minutes. (15cmH2O is about 5% of the air pressure on the top of Mt. Everest.) So my original question remains, what are CPAP manufacturers talking about when they talk about centimeters of pressure?

Then I thought, maybe they are talking about centimeters of pressure "on top of" or "in addition to" ambient atmospheric pressure. But that doesn't make sense either because, let's say, my IPAP pressure is set to 15cm. 15cm is equal to about 1-1/2kPa. One and one-half kPa is equal to (approximately) the difference in normal atmospheric pressure between getting into an elevator on the 10th floor and taking it down to the lobby. Which is to say, negligible. So that can't be it.

Which brings me back to my original question. What I really want to know is, how many cmH2O do I set my CPAP machine to so that it delivers NO pressure, or basically SEA LEVEL pressure? In other words, how many cmH2O is the equivalent of 100kPa?

Anybody know? Huh
11-12-2015 12:45 PM
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justMongo Offline

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Post: #2
RE: Can you explain pressure measurement?
The machine delivers X cm-H2O above ambient pressure. It's not delivering absolute pressure relative to vacuum.
You can estimate the delta pressure using 30 ft of water as 1 atmosphere.

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11-12-2015 12:59 PM
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Tacoma Droner Offline

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Post: #3
RE: Can you explain pressure measurement?
(11-12-2015 12:59 PM)justMongo Wrote:  The machine delivers X cm-H2O above ambient pressure. It's not delivering absolute pressure relative to vacuum.
You can estimate the delta pressure using 30 ft of water as 1 atmosphere.

Actually one atmosphere (1ATM) is about 34 feet of water, which is about 1036 centimeters of water, which is what I said above.

And "delta pressure" for those of you who don't know what he's talking about, is a shorthand way that scientists and engineers say "the difference in pressure." There's actually a symbol for this -- ΔP --if you're using it in a mathematical formula.
11-12-2015 02:04 PM
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Jimsp1 Offline

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Post: #4
RE: Can you explain pressure measurement?
(11-12-2015 12:45 PM)Tacoma Droner Wrote:  ...Then I thought, maybe they are talking about centimeters of pressure "on top of" or "in addition to" ambient atmospheric pressure. But that doesn't make sense either because, let's say, my IPAP pressure is set to 15cm. 15cm is equal to about 1-1/2kPa. One and one-half kPa is equal to (approximately) the difference in normal atmospheric pressure between getting into an elevator on the 10th floor and taking it down to the lobby. Which is to say, negligible. So that can't be it.

...

When you ride down in an elevator your whole body is exposed to the pressure change and as long as the airway is open there won't be any difference in pressure between the airway and the outside of your body. Hooked to the CPAP only your airway has the increased pressure which is what can open the airway if it is obstructed.
11-12-2015 02:35 PM
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Terry Offline

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Post: #5
RE: Can you explain pressure measurement?
The machine creates a pressure difference between the room air (ambient) and the air in your lungs.

If your machine is set for 10cm (~4 inches), this means that the pressure in your lungs is the current ambient pressure + 10cm.

If you had a glass of water and a straw, this much pressure wouldn't be enough to blow bubbles.
11-12-2015 02:48 PM
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justMongo Offline

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Post: #6
RE: Can you explain pressure measurement?
(11-12-2015 02:04 PM)Tacoma Droner Wrote:  
(11-12-2015 12:59 PM)justMongo Wrote:  The machine delivers X cm-H2O above ambient pressure. It's not delivering absolute pressure relative to vacuum.
You can estimate the delta pressure using 30 ft of water as 1 atmosphere.

Actually one atmosphere (1ATM) is about 34 feet of water, which is about 1036 centimeters of water, which is what I said above.

And "delta pressure" for those of you who don't know what he's talking about, is a shorthand way that scientists and engineers say "the difference in pressure." There's actually a symbol for this -- ΔP --if you're using it in a mathematical formula.

Ah, we have another engineer/scientist on the forum.
BSEE here.

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.
11-12-2015 04:26 PM
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PaytonA Offline
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Post: #7
RE: Can you explain pressure measurement?
Most measures of air pressure like the pressure in your car tires is the amount of pressure above atmospheric pressure just like the pressure delivered by CPAP.
11-12-2015 05:01 PM
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eseedhouse Offline

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Post: #8
RE: Can you explain pressure measurement?
To a pretty fair approximation 10cm of water is about 1% of normal sea level ambient atmospheric pressure. Even the max most machines can deliver is about 2% of standard pressure. It's the difference between what the machine provides and the outside pressure that causes your breathing passages to open wider

So if the ambient pressure at your bedside is 1000 millibars then the machine at 10 cm of water will deliver about 1010 millibars to your nose throat and lungs. This is only an approximation, but it's a pretty good one. A really bad storm at sea level may drop the ambient pressure to around 980 millibars, and of course if you live on the hights it will be lower too.

Ed Seedhouse
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11-12-2015 05:19 PM
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Sleepster Offline
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Post: #9
RE: Can you explain pressure measurement?
(11-12-2015 12:45 PM)Tacoma Droner Wrote:  Then I thought, maybe they are talking about centimeters of pressure "on top of" or "in addition to" ambient atmospheric pressure. But that doesn't make sense either because, let's say, my IPAP pressure is set to 15cm. 15cm is equal to about 1-1/2kPa. One and one-half kPa is equal to (approximately) the difference in normal atmospheric pressure between getting into an elevator on the 10th floor and taking it down to the lobby. Which is to say, negligible. So that can't be it.

But, it is it. It's not negligible. It's enough to make the difference between suffering from obstructive sleep apnea and not!

Quote:Which brings me back to my original question. What I really want to know is, how many cmH2O do I set my CPAP machine to so that it delivers NO pressure, or basically SEA LEVEL pressure? In other words, how many cmH2O is the equivalent of 100kPa?

Zero. But you can't set it to that because it wouldn't be safe to strap on a mask under those conditions. You need at least 4 cm of pressure to keep the hose purged of exhaled air, so that is the minimum setting.

When you have your machine set at a pressure of 15 cm, it keeps the pressure on the inside of your airway 15 cm above the pressure on the outside of your airway.

You do the same thing when you inflate your car tire to a pressure of 30 psi. If the pressure outside your tire is 15 psi, then the pressure inside is 45 psi. The gauge reads 30 psi, which is the so-called gauge pressure.

CPAP pressures are gauge pressures.

By the way, one pascal of pressure is a force of one newton spread out over an area of one square meter. A newton of force is about the weight of one apple.

Sleepster
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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.
11-12-2015 08:27 PM
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justMongo Offline

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Post: #10
RE: Can you explain pressure measurement?
(11-12-2015 08:27 PM)Sleepster Wrote:  You do the same thing when you inflate your car tire to a pressure of 30 psi. If the absolute pressure outside your tire is 15 psi, then the If the absolute pressure inside is 45 psi. The gauge reads 30 psi, which is the so-called gauge pressure.

You must differentiate between absolute and relative pressure.

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.
(This post was last modified: 11-12-2015 08:31 PM by justMongo.)
11-12-2015 08:31 PM
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