Originally posted by Goldminer
Your formula also does not take into account temperature. This is okay in a static situation where temp is a constant but while we are breathing down a tank the air is expanding and the temp therefore dropping. Your calculation performed during a dive could give misleading results.
Yes, I am familiar with the fact that temperature changes pressure, tho it will not change the actual volume. The example I gave was meant to be used as a guideline and the volumes are approximate. If you were to double check my numbers you would see that if you were to take the original formula (112/2640 = .042) then multiply that back by the original working pressure (.042 x 2640) you come up with 110 cft of air. Did we lose 2 cft? No. It's just a matter of the fact that when round the numbers off, it throws a slight bit of error into the equation.
I would hope that any certified diver was taught in their basic open water class that leaving a tank out in a hot car causes the pressure to increase. I would also hope (and give the readers of this board credit for having more sense) that no one would assume that just because their tank is now 110 degrees and has significantly higher pressure, that doesnt mean that the volume of the tank changed and they can get more bottom time from it.
While you do make a good point about the behavior of gases in relation to temperature, it is my belief that anyone who is needing to know to that an exact degree what the volume of air is in a tank should be taking additional training, where such laws and rules will be taught to them as well as the reasons why they are important.
Again, my reply was just to give an approximate answer and illustrate a point, not to provide a starting point for gas blending or to have them start caculating deco stop points or turn points. Since the original question didn't ask *at a given temperature* I didn't feel the need to elaborate on the effect of temperature.
Thank you however for posting the follow up information, as that may be enlightening to others who never gave much consideration to their tank temperature.
BTW, since we are on the subject, does anyone know the ACTUAL temperature that an aluminum 80 filled to 3000 psi would have to be at to be truly 'full'? Or do we just accept the standard of 3000 psi @ room temperature = a complete fill?
Food for thought...
Dive often, but dive safe
John