Filling scuba tanks

[Akimbo]

My testing has shown that a water bath really helps on pressure drop later. That said, I don't bother and just fill to 3900 so they settle at about 3600.

It is all a question of how fast you fill, which dictates how much thermal energy develops, and how fast that energy dissipates. It mattered a lot more when the largest Scuba tank on the market was a called a 72, but held less than 72 Ft³ at 2250 PSI. I have worked with high volume gas banks that could fill a single 80 in less than 40 seconds if you weren't careful.

 

[BRT]

It is all a question of how fast you fill, which dictates how much thermal energy develops, and how fast that energy dissipates. It mattered a lot more when the largest Scuba tank on the market was a called a 72, but held less than 72 Ft³ at 2250 PSI. I have worked with high volume gas banks that could fill a single 80 in less than 40 seconds if you weren't careful.

There are few people that fill slower than I do. My compressor puts out 1.8cfm and I fill multiple tanks at the same time. I can water bath 2 at a time so I filled 2 in the water and one in air, roughly 3 hours fill time and then checked pressure after they sat overnight. The ones in the water were over 100 psi higher than the one in the air. It would be more but I don't have a cooler loop hooked up to the water in the bath so the water gets warm.

 

[papaouiee]

Wait, so you guys are refilling used tanks, instead of buying factory-fresh new ones that come prefilled with guaranteed clean factory air for each dive?

Isn't that, you know, like washing dirty dishes and re-using them, instead of having a fresh new set for each meal?

Ugh.

If you use a tank once and then throw it away, send it my way and I'll use it for the next 25 years or until I die.

 

[sh1513]

We discussed this when I did my cylinder testers course and the consensus of opinion was apart from being potentially more dangerous, that it is 'old hat' and a complete waste of time putting a cylinder in water to be filled.

Unless it is being cooled (which I have never seen), the water is going to be at more or less ambient temperature so being in water or air isn't going to make the slightest bit of difference.

 

[Schwob]

If you use a tank once and then throw it away, send it my way and I'll use it for the next 25 years or until I die.

Yeah, will you also take his used dishes?

 

[RayfromTX]

We discussed this when I did my cylinder testers course and the consensus of opinion was apart from being potentially more dangerous, that it is 'old hat' and a complete waste of time putting a cylinder in water to be filled.

Unless it is being cooled (which I have never seen), the water is going to be at more or less ambient temperature so being in water or air isn't going to make the slightest bit of difference.

You can hang out in shorts and a t-shirt in 50f all day. You can even work up a sweat and need to cool down. In 50f water you will be hypothermic in a short period of time.

When you are rejecting heat through convection, the mass of the fluid(air and water are both fluids) is quite important. Movement of the air or the water will greatly increase the movement of heat. What you are doing is employing the second law of thermodynamics. The molecules in the air or the water become more excited and the molecules in the tank become less excited. The water has much greater density of molecules available to accept the heat energy from the tank than the air does. Movement brings more molecules into contact with the surface of the tank and keeps the delta T of the molecules in contact with the tank at a higher level.

Greater deltaT (temperature differential) equals greater heat exchange
Greater density of the cooling fluid equals greater heat exchange
Greater movement of the fluid equals greater heat exchange
Longer dwell time equals greater heat exchange

If you fill the tank in a water bath but do it in 5 minutes and then pull it out then the benefit of the water will be limited by the low dwell time. If you do it in water that is room temperature then the effect will be greater than if you do it in water that is 90f but 90f water will still cool a 130 degree tank faster than 70f air. If you have a small volume of water and fast fill lots of tanks the water will get very warm. It has accepted a huge amount of heat but it's capacity to continue doing so is now diminished.

I disagree strongly with anyone that claims it isn't faster to cool a tank down in water than in air but the amount of benefit is dependent on the above factors.

I am ignoring radiant cooling in my description. It is a small factor with it's own set of variables but it does have a greater impact in the air cooled side of this equation. Actually because air cooling is SO ineffective, radiation does play a significant role in that version.

I apologize for the lecture. I'm not good at this.

 

[Akimbo]

I apologize for the lecture. I'm not good at this.

I beg to differ, nicely stated sir.

 

[Jonny Wishbone]

OK, lets take a trip back in time for a second. Way back in OW cert didn't we learn that water was a better conductor of heat than air? I think we also learned that water can conduct heat something like 800 times faster than air. So if this is true, why would it not be accepted that the heat from a filling scuba tank would be drawn away from the tank better in water than in air. I would think that keeping the tank temp as low as possible when filling would be the best practice not soley for the PSI but for the health of the tank.

 

[shurite7]

OK, lets take a trip back in time for a second. Way back in OW cert didn't we learn that water was a better conductor of heat than air? I think we also learned that water can conduct heat something like 800 times faster than air. So if this is true, why would it not be accepted that the heat from a filling scuba tank would be drawn away from the tank better in water than in air. I would think that keeping the tank temp as low as possible when filling would be the best practice not soley for the PSI but for the health of the tank.

Water is a just over 800 (depends where in the world you measuring) denser than air. According to PADI water absorbs heat 20 times faster than air, NAUI states 25 times faster, USN states 30 times, and Mythbusters determined the factor is 28.

I forget where I read this (either through PSI/PCI or the paper from the University of RI), it takes a tank, filled out of water, just under three hours to cool to ambient temperature after it has been filled. I haven't seen a report indicating how long it takes for the gas inside a scuba tank to cool to ambient temperate if it is filled in a cooled/cold water container.

 

[gcarter]

Water is a just over 800 (depends where in the world you measuring) denser than air. According to PADI water absorbs heat 20 times faster than air, NAUI states 25 times faster, USN states 30 times, and Mythbusters determined the factor is 28.

I forget where I read this (either through PSI/PCI or the paper from the University of RI), it takes a tank, filled out of water, just under three hours to cool to ambient temperature after it has been filled. I haven't seen a report indicating how long it takes for the gas inside a scuba tank to cool to ambient temperate if it is filled in a cooled/cold water container.

I took a tank out of the back of my truck the other day and the pressure was at 4000. The tank was hot. By the time I finished gearing up and got in the water it had not noticeably dropped - 20 or 30 minutes. Outdoor temp was ~ 36C.

Once I got in the water - 19C - my tank was reading 3500 within 5 minutes.

Water definitely cools faster.

I just don't buy that I am killing my tanks when a slow fill goes to 3900 - 4000.