MikeFerrara:
Just look at it based on fluid flow principals.
The flow out of the tank is dependant on the pressure differenntial and the resistance to flow inbetween. All a reg can do is add resistance to flow from the inside of the tank to the outside world.
Whew!
Alright class, listen up...
There are a couple of factors here, but y'all are all trying to make it too complicated, and you have one bad data point and some misplaced ideas about fluid dynamics from which the rest is flowing. Regardless of what someone remembers, you cannot empty a full 72 through a standard "K" valve in 30 seconds. Not at the surface, not at any recreational diving depth.
As Mike says, flow rate is dependent on pressure differential and flow restriction.
Forget the regulator... a "perfect" regulator would provide
zero resistance to the gas flow through it and therefore allow flow to proceed at its maximum possible rate as though it weren't there at all. So the two pressures that create the ultimate differential are tank pressure and ambient pressure, and maximum possible flow rate (with an extremely efficient, high performance regulator) actually decreases with depth because the pressure differential between the tank pressure and the ambient pressure decreases. (Before you get your brain in a knot on that one, just think 200 ATM ambient pressure - about 2000 M or 6600 feet deep - at that depth a standard aluminum 80 would never empty (or already be empty, depending on your point of view) because there would be no pressure differential across which the gas could flow; indeed, if you went deeper then opening the tank valve would allow water to flow into the tank, not gas to flow out)
So if you want to see what the worst case could be, just open a tank valve wide open and see how long it takes to empty. At depth it'll take a
little longer for the same amount of gas to escape as it does on the surface. But at normal diving depths, the dropoff in pressure differential is very small compared to the flow restriction provided by the valve; at the surface a tank filled to 3000 psig has a 3000 psi "fall" for the gas to drop through the K valve, and taking that tank to 100 feet only reduces the "height" of the fall by 45 psi, and the pressure differential is still 2955 psi initially. It is the restriction of the "K" valve that for all intents and purposes is the limiting factor here, and opening a valve wide open on the surface will give you a very close approximation of how much time you'll have at any recreational diving depth.
Rick
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