Obstruction always associated with increase in leak graph

[mesenteria]

Sheepless, the machine is set to a certain fixed pressure or to a range of pressures, depending on the treatment required.  When a leak occurs, the machine won't attempt to increase pressure because it is not designed to change pressure, except within the range if there is one.  If a leak takes place when the machine thinks it's doing a good job at pressure X, it won't raise pressure to compensate for the leak it detects because that will only exacerbate the leak.  The result would be even more leakage than before, and noisier. 

Instead, the machine attempts to maintain the pressure it thinks is working, but because there is a leak that it must contend with, it speeds up the fan...NOT to increase pressure, but to maintain the volume flowing through the hose at the correct pressure.  This works on the 'back pressure' principle which gasoline internal combustion engines and even steam locomotives need to get the dynamic flow of gases and vapour they need inside of chambers and out of ports.

I didn't understand this myself, about the CPAP machines, until a person on another site 'splained it to me.  If you place a manometer inside the tube of a PAP machine and take a pressure reading while the machine works as it is designed to do, and then cause a leak, the manometer will not show a substantial or sustained difference in reading when the machine compensates for the leak.  IOW, no pressure change.