Guest wrote:Pesser wrote:Guest wrote:Guest wrote:
If it were 1000 cmH2O there would be flow "into" the cpap machines instead of "out", no?
When your cpap pressure is 15cmH20 there would be flow into the machine IF Atmos were 1000 cmH2O. With a pressure diff of 985 cmH2O which way would the flow be?
No! Your room is at 1,000cmh20. When your machine is set at 15cmh20 it ramps up enough pressure to deliver the additional 15 cmh20, giving you 1,015 cmh20. If your room pressure was 0; you would explode. Your CPAP would not be able to get the air pressure needed because at 0cmh20 there is no air! That would mean there is no air pressure. You would need to be in outer space.
One of us is not understanding this correctly, it could be me but your explanation is not the way I understand it. It it were 1000 + 15 cmH2O why wouldn't they say 1015 cmH2O? If that is what it is.
ooops I just exploded and I am outta here.
that one would be you.
pesser is talking absolute atmospheric pressure. air around is under pressure. he's spot on converting the normal 14.7 psi air pressure at sea level to 1033.5122800951199 cm h2o.
where you're screwing up is not realizing that both sides of the cpap, and indeed you, are at this same 1033cmh2o pressure, so you don't notice anything,
the cpap operates at a *RELATIVE* pressure difference of between 4-20cmh2o (or 25-30 for bilevel/niv machines) compared to ambient (the existing 1033cmh2o. this all works because the outside of your chest/throat has 1033 (at sea level) cm pressure on it, and the inside has 1037-1057cm pressure.
anybody that's ever used a vacuum chamber knows that air has pressure, (or, indeed a regular household vacuum... they don't actually suck, they create a lower air pressure inside and the external pressure forces air in thorugh the nozzle..) someone asked me once what would happen if I opened the lid to my chamber vacuum sealer while it was operating. I had to laugh, and explain that when it had been running more than a few seconds, there was over a TON (2000lbs) of pressure holding that lid closed. (appx 13x13 lid and around 14psi air pressure outside=2366lbs) I like my chamber sealer, and it's amusing to watch cold tap water boil.
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