Anti-asphyxiation ports on ffm

[yankees123]

Just wondering why only ffm have anti-asphyxiation ports?  Is it due to the large surface area?  Thanks

[BoxcarPete]

It's because you can just breathe through your mouth if the machine shuts off unexpectedly and you're wearing a nasal mask.

[yankees123]

Boxcare Pete, can you explain a little further what you mean, sorry don’t follow you.

[BoxcarPete]

Direct from ResMed's website:

The AAV is a safety feature in all ResMed full face masks. If the therapy device stops delivering air for any reason (e.g. in a power failure), the AAV allows you to breathe in fresh air from the room rather than re-breathe exhaled air built up in your mask.

If you are wearing a mask that is not a FFM, the only thing you need to do to get fresh air is breathe through your mouth which is not covered by a nasal mask.

[yankees123]

Got it, thanks for the explanation.

[Valtor]

...until you start mouth taping.  

[G. Szabo]

...until you start mouth taping.  

Indeed, there is suffocation danger when you use a nasal mask, which has no anti-asphyxiation port, with mouth taping when the pressure support ceases for some reason. To deal with it, I have set up a power outage alarm. However, what happens when there is no outage when the CPAP instrument malfunctions and stops delivering pressure? 
We would need a pressure gauge with an alarm. Unfortunately, I could only find such items in the high-pressure range, which are expensive. 

Do you know some low-budget pressure gauge with an alarm? 

Of course, one can build one with a U-tube water manometer and some cheap water leak detector, but I want to have somewhat more solid and professional. 
Any suggestion?