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Greg-g's solution is what is being done on the new Zest-Q masks. I've considered building a deflector but it doesn't bother me that much. I personally think flannel might be a bit too much.

One way to tell if your solution is going to cause problems is turn off the cpap machine and wear the the thing for 20 minutes while someone else watches you to see if you pass out. If you feel like your suffocating, then the CO2 levels are building up. If you pass out, your breathing just exhaled air. The need for air feeling is caused by excessive CO2, not lack of O2.

Air flow testing, the hobby version.

Read the information on the Swift FX mask with regards to air flow through the exhaust vent at the pressure you are using.

http://www.resmed.com/us/assets/documen ... sa_eng.pdf

Remove the pillows.

Plug the big hole with something that is air tight.

Take a 30 gallon trash bag and secure the end of the hose in the bag. You hand holding it will work fine.

Set your machine to CPAP and your pressure, and shut the Ramp and EPR features off.

Turn on the machine and time how long it takes to fill the bag.

Refer to the specifications and see if your crude tests comes close to the advertized specifications. 30 gallons is close to 114 liters.

Now you have a baseline to compare to, and this baseline is close to what the manufacturer specifies. Note that this is not exact, but a rough comparison. Some error can be expected but the error should not be astronomically large.

Install your diffuser and run the test again.

Compare the wide open flow with the diffused flow.

For example with the Swift FX at 10 cm H2O of pressure, you would expect the exhaust flow to be 33 liters per minute. If you time how long it takes to fill a 30 gallon trash bag and it comes in around 207 seconds, you are right at that specification. If you add your diffuser and find that it now takes 218 seconds, you are at about 95% of the initial flow rate. Instead of 33 liters per minute, you have reduced that to about 31 liters per minute.

Note that most of the lowest flow rates are at 20 liters per minute. It is probably a good idea to keep your exhaust flow above this.