I usually agree with you Wayne, but this is a post which I personally cannot agree with. You are claiming that breathing off a spare air at 250' FFW or FSW will guarantee a safe ascent? Have you done the math? Is this a 3 cu.ft or 6 cu.ft spare air? You are guaranteeing that a spare air will have sufficient gas in it to get any diver to the surface safely. While I agree that a 100' CESA might save "your behind" (not your's specifically
) in an emergency, I was not aware that any agency was teaching it (if you say yours does then I believe you).
To be clear Bob, I'm not really trying to claim anything. I simply stated that I would personally be comfortable doing a 250' ascent with a Spare Air. Considering that I have done 180 foot free ascents quite regularly in the past (I make it a point to practice them on most deep ascents), I didn't think that a 250' ascent with a breathing source would be overly difficult. Some of the worst criticism that I've heard on SB towards Spare Air has been from those who don't dive with redundancy. My point is that if the diver is diving beyond his ability to CESA, that a suitable redundant system is a worthwhile consideration and any amount of air is better than not having any.
But you're right, lets look at the numbers. Lets assume that the diver can only ascend from 50 feet on one breath of air. He starts a CESA from say 100', so from 50' onwards they would need to use the Spare Air as the sole source of air.
The average adult lung capacity is about 5000 cu. cm (5 liters), or about .18 cubic feet (dependent on sex, age and fitness). The average tidal volume is approximately 500 cubic cm or (.018 cu feet). At 50', a 3 cu.ft Spare Air would have just over one cu ft of air, or about 60 breaths, approximately 4 minutes of breathing time at this depth. A 6 cu ft unit would have twice this amount.
I don't personally suggest that a diver breath off a Spare Air like they would a SCUBA tank. I recommend that they take a couple of breaths, relax and do another CESA, repeat as required. If the diver can't make the surface in an emergency with this much air, they shouldn't be diving imo.
I have been teaching emergency ascent since I became a recreational instructor and also for the Navy and military helicopter aircrew. There is nothing that I'm aware of that prohibits such training by any agency. Students have functional regulators in their mouth and must hold a proper ascent rate beside the instructor. I've never had any problems.
I think that posts like yours here lead way too many new divers to believe that carrying a tiny bottle such as a 3 cu.ft spare air will make them a safe diver. It does nothing to make them any safer. Your post leads the reader to believe that there is more than enough air to get them to the surface which you cannot support IMO because every person and situation is different.
Any redundant air is better than no redundant air. I teach gas planning and insist that my students dive a comprehensive dive plan. Any secondary source of air, whether it be Spare Air or a Pony bottle is not considered as an air source for planning purposes. I teach the diver to stay within touching distance of his buddy and that s/he is the first source of needed air. Accidents happen and I would rather a diver carry a Spare Air than nothing at all. Its use is reserved for emergencies only. If it's required, they have a better chance of survival with it than without it. Why would any instructor dissuade such a piece of safety equipment? As I've said, many pieces of diving equipment have an operational envelope. You use the equipment within its envelope.
I do NOT advocate the use of spare air for scuba diving however I do agree that every additional breath (1-3-80-500) that I can have will get me that much farther in the direction that I want to go.
That's your prerogative. As to the rest, I totally agree.
