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Rebreather enthusiasts and those waiting for their CCR 500's, 
After much experimenting with, and witnessing problems with, full face
masks used in conjunction with a rebreather, I believe I have found the
solution, at least to the problems I was having with the setup.  
 
Hydrostatic pressure from the counterlung when in a supine position (as if
shooting a silhouette of the surface or just looking up) would force the
mask to be pushed away from my face, causing gas to leak out, screwing up
the mix, as well as wasting gas.  
 
Since the oral nasal cavity is not completely sealed from the rest of the
airspace in the mask, the gas escapes the same way it does as if you exhale
through  your nose when using a half mask.  The tighter I pulled the straps
the more jaw fatigue I had.   
 
Going through an old box of diving gear I came across a Cressi Sub full
face mask that my father had used, that was it. This mask has an oriface
that a regulator mouth piece or a rebreather mouthpiece nipple can fit
through and then the rubber mouth piece that goes in your mouth can be
attached inside the mask, therefor when you put the mask on your lips seal
around it and the hydrostatic pressure no longer goes into the mask but
into your mouth, just as if you had a half mask on with a standard
rebreather mouthpiece.  The CS FFM also seals way under your chin. If it
leaks a little water in you clear it the same way you do a half mask.  
 
One cool thing about the CS FFM is its price, 50 bucks. The only draw back
is com gear, there is no port for com gear. I understand there are a few
more masks on the market with the same configuration, Scuba Pro and US
Divers makes one, maybe they're mask has a com port.  
 
At 200 feet I let my jaw go slack just as if I had passed out and I could
still breathe. I closed the surface valve on my mouth piece took off the
mask underwater, put it back on, cleared it, opened the valve and I was
good to go. Simple and cheap. 
 
Why wear a FFM? Insurance against drowning. Keeping your face warm in cold
water.  
 
 
John

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