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Dive instructors teach Charles law, and tell us that when a tank is heated the pressure goes up.
Not calling you (or any instructors) out personally, but if I had a dollar for every time I heard this...
For a fixed mass of gas:
Charles' law states that volume is directly proportional to temperature. Pressure is constant in this relationship.
Gay-Lussac's Law states that pressure is directly proportional to temperature when volume (i.e. the inside of your cylinder) is constant.
I know that PADI conflates the two, and don't get me started on those who insist a pressure cooker is a practical example of Boyle's law...
WOW! Thats the longest and most complex way to say "theire fine in the garage unless you light the barbecue theire standing on" Ive seen all dayThe thread on scuba tanks in a hot car trunk is here: http://www.scubaboard.com/forums/basic-scuba-discussions/335436-scuba-tanks-my-car-trunk.html
iztok is right - humidity outside the tank doesn't matter.
My response at http://www.scubaboard.com/forums/5244637-post14.html is repeated here:
FWIW, within the ideal gas approximation, the increase in pressure (or volume) upon heating is:
Tf/Ti
where Tf is the final temperature and Ti is the initial temperature. All temperatures are in terms of Kelvin - for a Celsius version:
(TCf+273)/(TCi+273)
where TCf is the final temperature and TCi is the initial temperature, both in Celsius. I believe that the conversion from Celsius (TC) to Fahrenheit (TF) is:
(TF-32)*5/9 = TC (could be wrong).
If a tank is filled to 3000 psi at 25C (TCi=25) (77F?) and will burst at 5000 psi (I am just making this up), then
(TCf+273)/(TCi+273) = 5000/3000
TCf = 5/3 * (TCi+273) - 273
= 5/3 * (25+273) -273
= 224 C or 435 F
So, your tank will have to heat to 224C/435F for the tank to rupture at 5000 psi.
Caveat - real gases depart from ideal gas behaviour at high temperatures and pressures such that 224C/435F should be thought of as an extreme upper end temperature - the true pressure at 224C/435F will be higher than the ideal gas case of 5000psi. This corresponds to the bursting pressure being lower than the 5000psi ideal gas case. Just a guess, but I would allow up to a 20% departure from ideal gas behaviour (this is a bit extreme) at 5000psi leading to
TCf = (5-5*.2)/3 * (TCi+273) - 273
= 4/3 * (25+273) -273
= 124 C or 256 F
Now the tank bursts at 124C/256F rather than 224C/435F. 20% here and there goes a long way...
The true departure from ideal gas behaviour of dry air is quantified, but I don't have the number in front of me right now.