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In message <199608240424.EAA23815@pi*.la*.us*.pi*.co*>, "John E.
McKenney" <mckenney@us*.pi*.co*> writes
>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,
I spoke to Tracy Rabinette last year on this very subject. As
most folks know (or maybe they don't!), the trouble with a lot of FFM's
is that when they are used with C2 it actually becomes part of the loop
volume. Tracy said that from the work he has done he found that:-
1. To dive C2 he felt that FFM's were the way to go.
2. For a FFM to be considered suitable for use with C2 it must:-
(a) Have the smallest possible ora-nasal inner mask to
keep the increase in loop volume to a minimum.
(b) You must be able to bail out to OC 'EFFECTIVLEY'.
I guess we have already had a taste of some of the problems discussed by
Heydude/Rod Farb & Rich Pyle.
Care to comment guy's??.
Chris Hellas
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