CPAPtalk.com

You can measure cfm at any altitude and 10 will be ten. But a machine that is measuring, at least the non compensating ones, measure electronicly and are set at sea level. When the DME's set up my first machine they used a mechanical measuring devise that is actual. My machine was set at 8 cfm on the machine to compensate for the 7 cfm that the Doc perscribed for me. A machine that is auto compensating would have a way to measrue the altitude and have a chart implanted in it's system. Simular to the computers in late model vehicles, it automaticly adjusts for altitude. Still haven't found a way to paste the article. Also, BuffaloAl, I tried to send an email but it came back. Try emailing me twistedcherokee@aol.com.

Toby

I do not know how the auto altitude machines work. Most likely, they have a simplified altimeter and adjust the pressure accordingly, up to the limits of the altimiter. This is why some will not adjust beyond 6500ft.

Your right that 10cfm is 10cfm. And a cubic foot is a cubic foot. It's what is in the cubic foot that is changing. The reason there is lower air pressure is that there is less air in each cubic foot. This is why your ears pop when you're in a plane. At 30,000 feet there is definitely not enough air to breathe.

2 years ago I went to Cedar Breaks NP in Southern Utah. If you ever go to Zion NP and Bryce NP, don't skip this one! But at 10,000+ ft, this sea level dweller got dizzy driving.