Quote:I think the reason is because of the way CPAP machines measure flow rate. They pass air first through one pressure sensor and then a second pressure sensor. The drop in pressure between the two sensors can tell you how fast the air is moving, but that determination is a function of the air density. The air in Denver is less dense than the air in Houston, so this has to be compensated for if the machine is to be able to accurately determine flow rates. Knowing the flow rate is necessary because the machine needs to know how fast to spin the impeller to maintain the pressure. This the real engineering feat of the CPAP machine: it maintains the same pressure regardless of how fast the patient is inhaling (or exhaling).