Scuba Tanks vs. Heat???

[PotatoC]

Wow.. Thanks so much for the replies!! Have to say you guys are very nice on here. I am on a couple of Jeep and fourwheeling boards where I get laughed at when I ask rookie/newbie questions. Thank you all for your help!!

 

[iztok]

PotatoC, where in NC are you moving to?

 

[FritzCat66]

Regarding tank storage, just make sure you read this thread and follow the guidelines therein. It's an eye-opener.

 

[Stu S.]

Dive instructors teach Charles law, and tell us that when a tank is heated the pressure goes up. Some students figure that if a tank at 70 degrees F is taken to 140 degrees F the pressure will double and something might blow up.

The pressure increase will be small. The temperature has not "doubled" but gone up only a small amount relative to absolute zero (−459.67°F). That's why my tanks are in a Florida garage right now.

 

[GrumpyOldGuy]

Another helpful suggestion... I have informed my wife that scuba tanks need to have the air cycled at least once per month for safety purposes. This insures I get adequate diving in despite living in West Texas.

Seriously, the only real adjustment I make for the heat of summer is I try to get my tanks filled in the morning with cool air and a cool tank rather than in the heat of the day. It does tend to make a small difference, about 5lbs/degree or 150lbs total. Not a huge difference, but all else being the same I like full tanks.

 

[hbhobby]

definately do not leave in car. I have personal experiance with leaving a full tank in the car (in phoenix az temps in shade run 110's in summer) and burst disk ruptured. no harm done but had to have valve rebuilt (was time anyway) but a hassle none the less. and I do keep them in the garage all summer long with no problems.

 

[captain]

The simple answer that other than a fire or throwing it in a volcano normal everyday temperature in a car trunk, or home garage will not affect a tank.

 

[devilkitty]

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...

 

[acwest]

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...

The toughest part of the classroom part of my certification was keeping the gas laws straight. I learned the Ideal Gas Law a long time ago (PV=nRT) and trying to keep track of the special cases by name always seemed somewhat pointless...
-Craig

 

[Tigerman]

The 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.

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 day