(07-31-2015, 03:59 PM)justMongo Wrote: Since the blower is an impeller type, it does indeed speed up when one inhales. Has to speed up to keep constant pressure with the higher outflow. I can hear it speed and slow on my S9.
I am not sure it would matter whether it is an impeller or not. Also, we are talking about a small volume of air in an otherwise partly-closed (vented) system. If it were speeding up and slowing down to maintain precise total
pressure (a function of static pressure combined with velocity pressure), a change in fan speed would probably be imperceptible in that small volume at these relatively weak pressures, because the change in static (and therefore in the total) pressure as you inhale and exhale is very minimal. But it probably is not maintaining precise total
pressure anyway, because there is no therapeutic reason for it to do that.
The pressure is based on cm of H20 (how far a column of water will rise when static pressure combined with velocity pressure is applied to a pressure tap in the airflow), so these are tiny differential pressures compared to atmospheric pressure or an increase of a couple of PSI in a tire.
The CPAP bases pressure settings on velocity
pressure, which is set to be constant, even though the total
pressure in the system might vary slightly through the respiration cycle due to slight changes in static
pressure when you breathe in and out. Compare the airflow of a normal exhale to the airflow of a xPAP putting out 10-15 cm, and the difference is pretty great.
pressure varies slightly as you breathe in and out, so the total
pressure does vary slightly, but not enough that the velocity
pressure must be modulated by the xPAP to keep the total
pressure constant. There is no real therapeutic advantage to having the tiny change in static
pressure variance compensated for by modulating the velocity
Also, the pressure of CPAP is therapeutic only during inhalation; there is no need to have that pressure maintained all that precisely during exhalation, and for the few patients where significant EPAP control is necessary, a CPAP is not prescribed anyway.
OSA is a problem with airway collapse on inhale. The airway does not generally collapse on exhale. So whether the total pressure in the system goes up during exhale is not relevant to the therapy, and compensating for that change in total pressure during exhalation is not needed.
IOW, the total
pressure does not need to be maintained that precisely, only the velocity
pressure to assist inhalation. The therapeutic difference between a pressure of 10 and a pressure of 10.1 is probably not even discernable medically, which is probably why the granularity is not any greater than that.
That applies to CPAP fixed-brick pressure. If you have an auto machine, or an ASV, VPAP, or a Bi-PAP, or if you are using pressure relief for inhale, certainly velocity
pressure and therefore total
pressure will vary, and you might be able to notice that. But a properly functioning S9 AutoSet should be absolutely silent regardless what velocity
pressure it is putting out or how much it may be varying.