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I've gotten a bunch of private email with the same kind of question, so I'm posting the reply once to the lists. > At 12:18 AM 6/22/2001 -0400, David Pearson wrote: > > Hello David, > > >day and turned on the tank to see what would happen. At a starting > pressure > >of 3000PSI on an AL63 I left the valve full open for 1 minute. When I > >checked the pressure after, it had only dropped 400PSI. > > At 330 feet (10 AtA more than at the surface), that same test would > drain ~4000 PSI in one minute. The small fixed orifice of the high > pressure port helps mitigate the severity of the problem, but at > 300+ feet, any blown hose is a *very* big deal. > > -Mike Rodriguez > <mikey@mi*.ne*> > http://www.mikey.net/scuba > Pn(x) = (1/(2^n)n!)[d/dx]^n(x^2 - 1)^n Someone else wrote: >It seems to me that if I took the extreme end of an example and said that >the tank was submersed to 6700 feet I'd be at about 205 AtA, or 3000 psi. >At that depth the external pressure would match the internal pressure of the >tank and I shouldn't drain at all. Therefore I'd assume that with >increasing depth a pressurized tank opened to the sea would drain at reduced >rates all the way down..... ? OK, it's a little complicated. Basically, there are two opposing physical effects at work, one dominates the experiment at great depth, the other dominates at shallow depth. At ~6700 feet, the tank *would* drain slower or even back-fill with water because the ambient pressure would be a significant proportion (~100%) of the tank's internal pressure. The molecules are squeezed closer together which would tend to make moving large numbers of molecules in or out of the tank easy, but since the ambient pressure is (nearly) equal to the tank's internal pressure, very little or no gas would actually move. At normal diving depths, however the ambient pressure is so tiny by comparison to the enormous pressure inside the tank (less than 5%) that it has practically no effect on slowing the exiting gas. Still that 10 AtA presses the molecules 10 times closer together so they can exit the tank 10 times faster (minus the insignificant slowing the ambient pressure causes at diveable depths). So, the net of all this is that at normal diving depths, the effect the ambient pressure has on impeding the flow of gas is insignificant while the effect the ambient pressure has on squeezing the molecules closer together is very significant. The relationship reverses as you take the experiment deeper. Make sense? -Mike Rodriguez <mikey@mi*.ne*> http://www.mikey.net/scuba Pn(x) = (1/(2^n)n!)[d/dx]^n(x^2 - 1)^n -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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