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[Pressure] Can you explain pressure measurement?
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trish6hundred Offline

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Post: #11
RE: Can you explain pressure measurement?
Hi Tacoma Droner,
WELCOME! to the forum! Much success to you with your CPAP therapy.

trish6hundred
11-12-2015 10:26 PM
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spottymaldoon Offline

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Post: #12
RE: Can you explain pressure measurement?
(11-12-2015 02:04 PM)Tacoma Droner Wrote:  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."

You can appreciate just what this small extra pressure does if you have your nose mask running and switch over to mouth-breathing - you can feel all the soft tissues in your upper airway collapsing back to their normal (and apnea-prone) state as the pressure there is released (via the mouth): it doesn't take a lot of pressure to 'balloon' them out and so assist in freeing up the airway.
11-12-2015 10:43 PM
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cpetku Offline

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Post: #13
RE: Can you explain pressure measurement?
(11-12-2015 02:48 PM)Terry Wrote:  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.

Hmm,

I placed the climateline hose in a glass of water with the machine set to 8cm H20 and it blew some pretty big bubbles.

Wish I could find an adapter to try to inflate a dinghy next. Since it only requires a couple of PSI I'm sure I can inflate it but it may not get really firm...

(sorry, I couldn't resist)
(This post was last modified: 11-12-2015 11:06 PM by cpetku.)
11-12-2015 10:50 PM
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Mark Douglas Offline

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Post: #14
RE: Can you explain pressure measurement?
1 cmH2O ≈ 0.980665 mbar so with that knowledge when a sleep doc told me my my pressure settings were going to harm my alveoli I know he is full of crap and doesn't want me independent of the High Priests of sleep med.

I use my PAP machine nightly and I feel great!
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11-13-2015 09:38 AM
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spottymaldoon Offline

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Post: #15
RE: Can you explain pressure measurement?
(11-12-2015 02:48 PM)Terry Wrote:  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.

Responding with my physicist's hat on, if it is set to 10 cm of water pressure then it will blow bubbles until the tip of the straw is 10 cm below the water surface - then it will stop!
11-13-2015 09:43 AM
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Sleeprider Online
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Post: #16
RE: Can you explain pressure measurement?
(11-12-2015 08:31 PM)justMongo Wrote:  
(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.

I believe the correct term is differential pressure. CPAP pressure is calibrated by a manometer in cmH2O against ambient atmospheric pressure. Some fancy pressure sensors and algorithms in the machines are supposed to compensate for altitude differences up to 5000 feet MSL. I also found mine worked just fine one night at Furnace Creek in Death Valley well below sea level. Laugh-a-lot

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11-13-2015 09:59 AM
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Terry Offline

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Post: #17
RE: Can you explain pressure measurement?
(11-13-2015 09:43 AM)spottymaldoon Wrote:  
(11-12-2015 02:48 PM)Terry Wrote:  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.

Responding with my physicist's hat on, if it is set to 10 cm of water pressure then it will blow bubbles until the tip of the straw is 10 cm below the water surface - then it will stop!

A straw is about 7" long, I wasn't taking "cheating" and holding the straw out of the water into account. 8-)
11-13-2015 09:59 AM
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spottymaldoon Offline

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Post: #18
RE: Can you explain pressure measurement?
A fair length .... ;o)
11-13-2015 10:51 AM
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Sleepster Offline
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Post: #19
RE: Can you explain pressure measurement?
(11-13-2015 09:59 AM)Sleeprider Wrote:  I believe the correct term is differential pressure.

Depends on who you ask. Usually, in the physics books I read, pressure means absolute pressure, and gauge pressure means a difference in pressure between two locations.

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11-13-2015 05:54 PM
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archangle Offline
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Post: #20
RE: Can you explain pressure measurement?
Some of this has been said above, but I'd summarize it as:

"Gauge" pressure is the common technical term for the pressure measurement used in CPAP. If you think of it as being like the pressure in a car tire, gauge pressure is the difference in pressure inside a car tire vs. the outside of the tire. In common speech, "pressure" is "gauge" pressure, unless you specify otherwise.

10 cmH2O is about:

0.14 PSI
1% of atmospheric pressure
300 feet difference in altitude
10 mmHg, the common barometric pressure reading.
10 mb (millibar).

Even 20 cmH2O, the max pressure used for CPAP is a really low pressure in terms of how we usually think about it.

Gauge pressure is what matters for CPAP, because it's the difference in pressure between the inside of your airway and the outside of your body that matters. That's what inflates your throat and keeps your airway open.

If you put your entire body in a pressure changer, and applied 10 cmH2O pressure to the whole room, it would do basically nothing to help your apnea.

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11-14-2015 02:36 PM
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