Here’s one you’ve no doubt experienced first hand if you cave dive: It’s a hot day. Your tanks have been sitting in your vehicle all the way to the dive site and are now out in the open on a picnic table or tank rack. While attaching your regulators you note the pressure. It’s a very comfortable 248 bar/3,600 psi. After suiting up, you enter the water and begin your pre-dive checks — including checking your final starting pressure. You notice that your tanks now read 227 bar/3,300 psi. What happened?
What happened is, of course, simple physics. Your tanks still have just as many molecules inside as they did before. It’s just that, as they cooled to ambient water temperature, molecular activity slowed, and the pressure reading dropped.
Other than being a little disappointing, is this a problem? It depends on how and when you calculate your gas turnaround.
- If you wait until after your tanks have cooled to ambient water temperature, you most likely won’t have any issues.
- If you did your turnaround calculations based on what your pressure gauge(s) read when the tanks were hot, however, you could be setting yourself up for a problem.
Let’s take a look at a few examples. We’ll use pounds per square inch for this, as it’s what most of our readers use. And, for simplicity, we’ll assume backmounted doubles as opposed to sidemount.
- For our first example we’ll assume both buddies have a “hot” starting pressure of 3,600 psi. They plan to turn at 2,400 psi. Upon entering the water, their apparent starting pressure drops to 3,300 psi. If they stick with their original turn pressure of 2,400 psi, the net effect will be to make their dive plan even more conservative than is theoretically necessary.
- Now let’s turn the tables. Let’s assume it’s a cold winter day and each diver’s tanks have been sitting in his truck overnight in near-freezing temperatures. The divers base their gas plan on an initial pressure reading of 3,300 psi, giving them a turn pressure of 2,200 psi. Upon entering the water, the gas in the tanks warms by over 30°F, raising the apparent starting pressure to 3,600 psi. If they stick with their original turn pressure of 2,200 psi, they will be less conservative and less safe.
- Things get even dicier when dissimilar starting volumes are involved. Dissimilar cylinder calculations rely on knowing precisely how much gas, by volume, is in each diver’s tanks at the start of the dive. If you don’t base this on pressure values that reflect the temperature the cylinders will maintain throughout the dive, your turn pressures may not be accurate.
The bottom line is that turnaround points need to be based on tank pressures that reflect ambient water temperature. This means that you should not be calculating these values until after you get in the water and have allowed your tanks to cool or warm up accordingly.
If the method you use to calculate turn pressures is so complicated that it cannot be accomplished without the use of a calculator or a lot of multiplication or long division, you need to look for a simpler method. But that’s a topic for another article.