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Robert:
I was involved on some of the same chamber runs as Anthony was, and the
operator's warning was second nature for anytime the chamber ascended for
any reason. This chamber was used primarily for training for comercial
diving and chamber operations, but a great many open water students also
passed through.
Jules' theory when dealing with o/w basic students was to dive them in the
chamber as soon as they covered the appropriate theory and preferably before
they had ever used tanks (even in the pool). The idea was that they could
dive in a dry controlled environment with an inside tender, and be taught
and allowed to try several methods of clearing, with valsalva being the last
resort. If at any time the student had problems clearing, he simply said
"stop" and the run stopped until the student cleared. If the problem
persisted the chamber would be brought up in 2' steps. The warning
"ascending 2 feet No Valsalva" was always given. It became so inbred that
the NO VALSALVA came with any ascent.
I personally had the opportunity to run several hundred basic students
through this program and found the experience invaluable. The stundents came
away with a clear understanding of gas laws, clearing techniques,
decompression, nitrogen narcosis etc. It made teaching the rest of the
course a snap. Profiles run were normally to 130' for about 20 minutes BT
(depending on descent times) on the DCIEM long tables with a 10'/5 minute
fudge factor. There are a great many Southern Ontario divers with a 130'
chamber dive as the first dive in their log books. Ian
Anthony wrote:
>>Years ago I went on some chamber runs (as part of a diver education
>>program), and I still remember good old Jules Fortin (one of the pillars
>>of the Canadian Navy underwater program) saying as our bottom time was up
>>and he was about to start turning valves "Going up ... and NO
>>VALSALVA!". The impression I got at the time was that this would be
>>because it wouldn't be smart to do something to increase your internal
>>pressure at a time when external pressure was decreasing, and embolisms
>>were what was feared, but would this perhaps also help avoid PFO shunts?
>>
>>I've always since then employed Valsalva strictly for descent only, FWIW.
>
Robert wrote:
>Hard to second guess people I don't know, but it sounds like this might
>at least be part of the reason (if not the complete explaination).
>
>Truth be told, it usually is *NOT necessary* to clear on the way *UP*
>anyway, since the compressed gas trapped in the middle ear will expand on
>it's own as you ascend. The eustachian tubes at their openings at the
>roof of the mouth and throat have a little flap of tissue that acts like
>a flutter valve. It can be fairly large in some people ("patulant"
>eustachians in doc-speak) so as you descend (either while diving or
>flying) and gas is trying to get *INTO* the middle ear, this flap may
>slam shut and block the tube. It may take no more than 3-5 mm of pressure
>difference before you can't generate enough force with a Valsalva to
>overcome it (unless you do what the manuals say and start to ascend a few
>feet and try again). That's why it's important to get on your ears as
>soon as you hit the water so as not to create a pressure differential
>greater than your capacity to override.
>
>But as you go *up*, the expanding gas pushes this flap out of the way
>(since it's hinged to "go with the flow"). Since a valsalva temporarily
>raises the right sided heart pressures, a patent foramen ovale *could
>theoretically* create a transient right to left shunt. (since such a
>manuever only lasts a second or less, it's hard to envision much of a
>shunt in practical terms, but I'll defer to the laboratory evidence).
>
>Now if you inflame that flap of tissue (along with a bunch of other stuff
>in your ears and throat) with a head cold, it might not budge on the
>ascent either. But, no one here would dive with a cold now...*WOULD
>THEY*!
>
>Personally, it's very comforting to listen to my ears squeeking on hangs
>since it tells me the "system" is working! (damn little else is!)
>
>;-)
>
>Robb Wolov
>--
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>Send subscription/archive requests to `techdiver-request@terra.net'.
>
>
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