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Todd,
I'm sure every manifold is different but I'm equally sure that the isolation
valve will pass gas faster than any of us would like if we are loosing gas.
I have never heard that lingering hiss of gas passing from one tank to the
other just after a fast fill of air over Oxygen so there can't be any
significant restriction to flow. It will allow gas to move many many many
times faster than you can breath it, I assure you.
The tank that blew a disk was not yet doubled up which is why I used the
figure of about 1500 psi / 40 seconds. With 2 tanks you would have twice the
volume at the same pressure so that with the same rate of volume loss
pressure would drop half as fast. It's all an estimation anyway - I was
simply amazed at how fast it was all gone.
Just as an interesting sidepoint - the turbulence created when O2 passes
through these valves causes areas of higher compression and therefore heating
(not unlike the situation in a car engine where engineers strive to create
even combustion and gas expansion in the cylinders). These hot spots are
what present the danger of combustion of hydrocarbons in and around such
restrictions (called adiabatic combustion). The same situation exists
whether you are filling or emptying a tank with high O2 content so drain as
well as filling slowly where high O2 is involved.
I wouldn't count on the resistance presented by the isolation valve to
significantly slow the loss of gas from the second tank and I wouldn't close
it down part way to create more resistance. The only reasonably reliable
defenses against death by burst disk failure are to eliminate the disk
situation or have a good partner and watch your gas management.
These kinds of situations are what make solo exploits into overhead
environments more risky that those in open water where a small bailout supply
can get you home.
Sometimes there is no substitute for a competent and attentive partner for
keeping the risk at an acceptable level.
Anytime you cannot feasibly carry enough of the "right" gas to bailout on in
the event of a catastrophic failure a good partner becomes a vital part of
risk management. As much as we strive to be as independent and self
sufficient as possible there are situations where one man can not carry
enough equipment to secure his future - a limitation all wise men face.
Chuck
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