Ok, let's talk a little about tank pressures and what you can get out of one and all that...
We need to understand that what you read on the tank pressure gauge (if it's accurate) is the difference in what's in the tank and what's outside the tank. At sea level, zero gauge pressure is 1 ATA, or about 1 bar or 15 psi; at 100 FSW, zero gauge pressure is about 4 ATA/4 bar/60 psi; at around 6000 FSW a full AL 80 will be empty
We need to understand that IP is about 150 psi above ambient, so at 100 FSW the absolute pressure in the IP hose is about 210 psi, but as the environment's giving a 60 psi "push-back," there's still 150 psi shoving it into the second stage box when you inhale.
And, at 100 FSW, the gas is four times as dense as it is on the surface, which makes it sixteen times as hard to pull (or push, depending on your point of view) through the hose between the tank and your mouth.
All this combines to make an "empty" tank at 100 FSW still have at least 45 psi in it when it comes back to the surface, and likely a bit more... but...
For emergency gas planning purposes, as the pressure gauge is also reading low at depth by the same ambient pressure as is affecting your ability to get to it, you can still plan to zero, as long as you include a little gravy
Rick
