From: gd@gehenna.sw.stratus.com (Greg Dawe)
Newsgroups: rec.scuba
Subject: Re: Nitrox Diving
Date: 15 Dec 1992 18:34:48 GMT
Organization: Stratus Computer, Software Engineering
NNTP-Posting-Host: gehenna.sw.stratus.com

In article <1992Dec15.135928.11277@newssun.med.miami.edu> dbrown@newssun.med.miami.edu (Daniel Brown) writes:
>In article <1992Dec14.151657.4167@cattnts.canada.ncr.com> craig@cattnts.canada.ncr.com (Craig Campbell) writes:
>
>>	    he unexpectedly experienced a complete air failure.  (suck..no
>>	    air forthcoming)  He thought he was finished.  He reached back
>>	    and turned his (dual tank) isolation valve.  Thwump!! 2000 PSI
>>	    now available.  Someone on the boat had closed his valve (by 
>
>While this is a commendable reaction to a stressful, my first reaction was
>to ask myself, "shouldn't this guy have checked his air before he did his
>giant stride?"  The last thing I do before I go underwater before EVERY 
>dive is check my air while watching my SPG -- to make sure the air hasn't been
>accidentally turned off!

All of my double tank rigs have dual valve isolation manifolds.  One of the
things I've learned the hard way is that "Nobody is allowed to touch the
valves."  When I bring these tanks to a shop for air, I personally open all the
valves to ensure that both tanks get filled.  When I'm gearing up I check the
valves myself and then tell the DM or mate NOT to touch them.  I've had
instances where they've spun the valves on me just prior to getting into the
water, and I've backed-up, taken the set off, checked the valves, and then
regeared.  I have no problem getting to the valves underwater, but they can
be confusing to use (try it while reaching back over your shoulders) and a
well-meaning DM not intimately familiar with their operation can easily turn
them OFF - particularly the valve on the diver's LEFT shoulder which opens in
the opposite direction from what one might expect.

Unfortunately, because of all the gear many technical divers wear, particularly
in cold water it's very easy for the diver NOT to feel a well-meaning DM step 
up behind him and gently spin a valve.  In the case of a closed isolation valve
the only clue you have is a higher-than-expected air consumption rate, since
you're only breathing off of one tank.

For those interested, a dual-valve isolation manifold is used to join the
air supplies of two tanks.  It provides redundancy in the case of both first
stage and tank O-ring failures while underwater, and is commonly used while 
diving in overhead environments (caves, wrecks, etc).  The manifold has 
fittings for the attachment of two first stages, one over each tank.  Between 
the two first stages is an isolation valve, which when OPEN, allows both 
regulators to access the air in BOTH tanks.  If the isolation valve is closed 
then each first stage can only access the air in the tank located below the 
its first stage fitting.  
-- 
Greg Dawe                        | Disclaimer: I don't speak for my employer,
Voice : (508)-490-6666           |             nor they for me...
e-mail: Greg_Dawe@vos.stratus.com|            
From: burton@misty.boeing.com (Brandy Burton-Tarantino)
Date: Tue, 15 Dec 1992 16:32:45 GMT
Subject: Re: ***I need equipment***
Organization: Boeing Commercial Airplane BCS Support