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#51828
Why do scuba tanks gets hot when filled, but cold when let out quickly?
Greg - 5/22/2013 6:32 PM
Category: Equipment
Replies: 15

I think someone tried to explain this too me once, but I was either not paying attention, or I didn’t understand it. When filling a scuba tank, it tends to get hot. But if you open the valve on a scuba tank and bleed it out quickly, it gets cold. Any physics or air tank gurus out there that care to explain this crazy phenomenon?
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hoofpick - 5/22/2013 7:53 PM
As I understand this issue, when filling a cylinder the air is being compressed which creates tremendous friction ergo heat but when the air is released the air is expanding at a high rate without the friction. It is the same thing with a propane tank as the propane is release the valve will frost over. I think this may be over simplifying the process but its the easiest way I know to explain it.
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UWnewbee - 5/23/2013 4:44 AM
The heating process is common in filling compressed gases/air, theres major factors involved, the compressor, friction of the moving parts and friction of the filling which is known as Adiabatic process, which there are 2 sides Adiabatic heating, and Adiabatic cooling, upon releasing the air/gas, basically when you see the freezing its actually creating dry ice.
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LatitudeAdjustment - 5/23/2013 6:44 AM
Air/Nitrox is a precious thing for divers, it gets excited (hot) getting ready for a dive and each breath should be savored, not wasted so empting a tank by just opening the valve should be a cold day in hell!
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Eric_R - 5/23/2013 9:09 AM
This should help.
Gay-Lussac’s Law
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Greg - 5/24/2013 12:49 PM
Good answers, that makes perfect sense now. As pressure increases (as it does when you fill a scuba tank), the temperature increases. As pressure decreases (as it does when you empty a scuba tank), temperature decreases.

I suppose that’s also why scuba tanks may show 3000 psi directly after a fill, but drop to around 2800 psi when the tank and air cools.
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Greg - 5/24/2013 12:51 PM
Is that one reason some dive shops put scuba tanks in a water container while they fill it? To provide a better air fill?
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Eve - 5/24/2013 12:53 PM
Sounds right, by the way, my answer to your original question was provided by your dad !
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lerpy - 5/24/2013 1:09 PM
Yup some shops put the tank in the water to cool the tank for a better fill, however from what I have been told the tank ( if putting it in water) should go in before the fill starts, once the tank gets hot from the fill dunking it in cool water can be a shock to the tank and have affects on the metal. Same I have been told about a hot fill then not giving it time to cool before diving. I dunno how true this is, but seems to make some sense.

As well for the cooling, opening a tank wide open to get that icing then placing a beer bottle in front of it (full of course) is a great way to get a nice cold beer quick on a hot day. Making cold beer is not a waste of air.
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Greg - 5/24/2013 6:54 PM
From lerpy: As well for the cooling, opening a tank wide open to get that icing then placing a beer bottle in front of it (full of course) is a great way to get a nice cold beer quick on a hot day. Making cold beer is not a waste of air.
I’m not so sure an $8 air fill is worth a cold beer :) I suppose it depends on how thirsty I am and how hot it is outside.
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UWnewbee - 5/24/2013 10:14 PM
Yep,,im with greg on this one,,ill spend the $2.00 on ice since thats what its made for
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DiveBuddyChgo - 5/28/2013 5:26 AM
Thats why I use a low pressure 2450psi/85cu.ft steel tank. The air does not heat up as much. With a much better fill then does an AL80. I will have 80cu.ft. even after the tank cools down after a quick fill to 2450psi. The same tank filled to 3000psi will give me 100cuft after cooling down. Air does not like to be disturbed. Air passes threw the 1st stage squeeze point way below 0 degrees during a dive also. Even cold 40degree lake water is actually warming up that air again before it gets to your 2nd stage lips.
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D_Vince - 7/08/2013 10:19 PM
there are a lot of replies here, but none of them are really answer the question Greg asked...

replies like the one that says that a gas under pressure, or is being pressurised has less space between molecules. hence more collisions / Friction between them causes heat, Sounds good and are quite close to being correct.
However the question was referring to a "Scuba Tank" which would include the Gas passing through a "restriction, Orifice, Valve"

so the effect you are all referring to is "Joule-Thomson effect"

http://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect

http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Joule%E...3Thomson_effect.html

It explains the phenomenon of the heating of a gas during compression, as well as the Cooling effect of a gas when it is expanding after passing through an Orifice.
Its one of my favourite questions for Divemaster students, to ask them if they can find the answer to why even the High pressure hose heats up when a regulator is pressurised, the same as a Tank when it’s filled etc.

By the way, putting a scuba tank in a water tank when they are being filled is not good for the tank at all, and since the water is not moving cools the tank very little, while it does cause a temperature difference within the tank itself.
I have also heard that it is done fore safety, in case of an explosion it would channel the explosion if the tank were to burst.

First off what the water would do is cause a difference in temperature in the tank wall when it’s being filled.
This can actually cause some of the moisture that is left in the compressed air to come out of the vapour state and condense on the cooler part of the tank wall, this results in mosture in a liquid state in droplet form.
These would roll down to the bottom of the tank, and in the High partial pressures of O2 within the tank easily cause corrosion / Pitting at the bottom of the tank (this is one of the things you have to inspect and measure when doing your Tank VIP’s, the amount and depth of pitting in the bottom of the tank)

secondly, According to the State of Florida fire department, a tank that was filled to approximately 2/3’s full exploded when it was being checked for pressure (not being filled)
the Tank was in a Water bath when it exploded however there was significant structural damage to the building.
The concrete pad the water bucket was on was about 3 feet lower than the rest of the floor, there were "Walls" missing, luckily the young fellow (Chris) that was working with the tank only lost part of his hand, (2 finger and the Thumb) as well as other blast related injuries (I don’t remember what all happened to him without looking the indecent up) but I do remember that they said it was enough energy released to lift one of their articulating fire engines up in the air about 20 meters in the air.

I spent a long time working with explosives myself, and you can take my word for it, if you were to put a charge in a large Bucket of water and set it off, if you were close enough to it, you would be dead, maybe a little Damp, but dead all the same.
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D_Vince - 7/08/2013 10:21 PM
More on the Tank explosion here:
scuba tank explosion web page dedicated to Chris
Hawkins

http://www.scubabomb.freeservers.com/

http://www.scubabomb.freeservers.com/Cscuba.htm