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Do Hypoallergenic filters lower the output pressure in a ResMed S9
#21
(04-03-2016, 11:21 PM)archangle Wrote:
(04-02-2016, 04:21 PM)omega2498 Wrote: I did a flow test without the mask in place - there is a marked reduction in flow when the filter was placed on the delivery port on the S9. As the machine presumably senses the pressure at the machine, rather than at the mask, I presume this type of filter does reduce the pressure at the mask, as there is no feedback loop from the actual mask pressure back to the blower :-( ?? So basically I think my actual pressure at the mask will be less than my Clinician recommended 8 cm H2O if I use the filter ...... :-(
(Of course - flow with mask fitted will be much lower than the free air delivery)
Has anyone used a manometer to actually measure the pressure drop across this type of filter at typical breathing flow rates ?)

First, let me point out that we're discussing the antibacterial filter that goes on the output of the blower, not the hypoallergenic filters this thread is about.

Testing without the mask is not that meaningful. The machine will probably be running the fan at maximum speed because it can't keep the pressure up. The pressure will not be constant at the output of the CPAP. With the mask in place, the flow rate will be much lower, and the machine will regulate the output pressure to a fixed value. Pressure drop through the filter goes up with flow rate. It's not exact, but I think pressure drop is roughly proportional to the square of the flow rate, so the difference between mask and no mask will be larger than you expect.

The question is more complicated than it appears at first. When you inhale, the flow rate will be higher through the bacteria filter, so the pressure drop will be higher. When you exhale, the flow rate will drop, and may actually go below zero. In that case, the pressure at the mask will be higher than the output of the blower. When you're in apnea, flow through the filter will be equal to the leak rate of your mask, probably 20 l/m or less.

Note that this is sort of a negative Flex/EPR/exhale relief function.

The effect of the varying pressure at the mask on your apnea is hard to predict, but the average pressure will be lower.

I think I've heard numbers about pressure drop being around 1 to 2 cmH2O. Yet another reason that it's good to have an APAP machine. It would try to adjust pressure to compensate and keep your AHI down.

I suspect that the antibacterial filter will affect the ability of the machine to distinguish central apnea, and maybe to do EPR/Flex/bilevel/BiPAP/ASV/etc. Snore detection might also suffer, which would affect APAP adjustment. Flow limitations might also be affected.

In practice, I suspect it's not too big a problem, but if you have a fully data capable machine, it shouldn't be too hard to change your pressure settings to adapt. You might also need to use or increase your EPR/Flex/bilevel pressure support a bit.

For the hypoallergenic filters that go on the air input to the machine, the machine should sense the pressure at the output of the machine to compensate unless you have really dirty filters.

From Omega2498 - thanks - I'm new to this game, but something to filter out tree pollen would be great at this time of year ! My background is in engineering design of systems with very powerful centrifugal compressors, which are a fair way removed from this topic, but Fluid Mechanics is still Fluid Mechanics !
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