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On Sat, 24 Aug 1996, Chris Hellas wrote: > 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. This is not true. If you want to keep your face warm or speak to someone or desire protection from drowning if O2 seizure happens then the FFM is the ticket. In tropical waters, I like to use a half-mask, mouthpiece and separate second stage for OC bailout. I have Robinette's Widolf mask and used it with CCR and liked it for what I wanted it for. I have an Aga that is harder to slightly harder to breathe than the Widolf and I got a Cressis-Sub because of John's experience. It is by far the best because the mask volume is not part of the loop volume and there are no fluctuations in your sinuses as you change attitude in the water. Having said all of this I still believe that FFM's with rebreathers are a giant pain in the ass because the Widolf and Aga both breathe harder than a mouthpiece because the loop volume is smaller with mouthpiece; you would predict that all FFM's of their design would, too, CO2 buildup in the loop becomes less of a concern with mouthpiece. So, the safest FFM of the three mentioned is the Cressi-Sub where you are drown-proof and use a mouthpiece. But, there is no oral comm with the Cressi-Sub whereas the Aga and Widolf have it. The Aga and Widolf are fitted with a switching block for OC bailout. You carry a second stage around your neck for OC bailout with the Cressi Sub. > > 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. > One good reason for this besides the CO2 thing is that if the FFM is flexible, it is harder to breathe. With FFM not only are your lung muscles expending energy to move gas through the loop (normal gas movement from your mouth through an orifice of your mouth's diameter through the hoses (not much different diameter than mouth's) to and from the counterlung, they expend energy moving gas from smaller orifice (mouth and nose) through a cavernous (by comparison) volume (FFM) back into a small diameter (hose). If the FFM flexes at all, lung muscles must expend more effort to push gas through the system. Because of this, if you are working hard with an FFM you may begin to feel like you cannot get enough gas to breathe. So, you want the most rigid FFM available, which makes Kirby-Morgan FFM's attractive for CCR that requires comm. > (b) You must be able to bail out to OC 'EFFECTIVLEY'. > You do not want the FFM surgically attached to your face or if you do, use a Delrin zipper to detach for OC bailout. If you use the Cressi-Sub FFM, and need a half-mask to breathe an open circuit second stage, then you need to carry one with for on a dive. The Aga and Widolf can be fitted with switching blocks for OC bailout. > 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 > -- > Send mail for the `techdiver' mailing list to `techdiver@terra.net'. > Send subscription/archive requests to `techdiver-request@terra.net'. >
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