Filling Tanks - How Hot is Too Hot?

[realdiver7]

Just put a hose on the top to overflow outside. Something like the corrigated hose used for boat bilge pumps. You can also use a plastic boat thru hull fitting on the top to connect the hose to. If both inlet and drain both are at the bottom you just may cool just bottom with no flow up the side of the tank.

Captain....OK, that makes sense.

To everyone else....thanks for the replies!

 

[oxyhacker]

There's a screed that has been floating around the dive community by Fred Calhoun, in which he denounces water baths as causing rust in tanks while offering no benefits. Fred is a scuba instructor and writer, and a very smart and knowledeably guy, but also enjoys being a gadfly, and has a number of fairly controversial theories, of which this is one.

Most of the problems he attributes to water baths can be avoided by not immersing the tank up the the valve, and by exercising a little extra caution to see that water does not get in the valve, however since this is something dive shops are often too rushed to do, Fred's arguement has quite a bit of validity. He also disputes the value of the water bath for cooling, suggesting that it just doesn't make enough of a difference in final fill pressure to be worth the complication.

You can find the article at Fill Express's website, http://www.fillexpress.com/library/fillfaq.shtml, and there's a gloss of it by Bill High at the PSI site.

Filling in a water bath is out of fashion? I noticed that my "new" LDS has no water bath, but then I found out they plan to add one, and the former LDS had several water baths...

I'm confused here... What's the reason to not use a bath?

 

[Couv]

….The negative water bath sentiment came about when some dive shop owners felt that water baths were too much trouble ... aimed at gullible readers who lacked understanding of physics.

There's a screed that has been floating around the dive community by Fred Calhoun, in which he denounces water baths as causing rust in tanks while offering no benefits. ...
Most of the problems he attributes to water baths can be avoided by not immersing the tank up the the valve, and by exercising a little extra caution to see that water does not get in the valve, however since this is something dive shops are often too rushed to do, Fred's arguement has quite a bit of validity. He also disputes the value of the water bath for cooling, suggesting that it just doesn't make enough of a difference in final fill pressure to be worth the complication.
You can find the article at Fill Express's website, http://www.fillexpress.com/library/fillfaq.shtml, and there's a gloss of it by Bill High at the PSI site.

Pesky and Vance,

Thank you for clearing that up. I was one of the aforementioned gullible readers who was taken in by this article in spite of common sense telling me better.

The above not withstanding, Fill Express is one of my favorite shops. Good products at a good price, great web site, friendly and helpful staff, gas fills at a reasonable price as they charge per cubic foot pumped.

couv

 

[crpntr133]

I told myself that I wasn't going to post but...I am.

I just filled three tanks last night with my Utilus. I haven't checked the fill rate but it is rated for 3.9. So we are in the same ballpark.
Last night I had shorts on and after filling, the tanks felt good on the bare legs. The outside air temp was around 60. They were far from what I would consider hot!! Warm maybe, but not hot. I think that most was from the compressor warming the air (which I never thought about before this thread).


As far as the merits for a water bath. My LDS filled one of my steels a bit to fast and he told me about it. It was warmer than I like but still not "hot". Since they have a pool I set the tank in the pool for a couple minutes, valve not in the water. Took it back in the shop and he topped it off. It had dropped 300psi which surprised me. He topped it off and I used it the next day. When I checked the SPG to see what I ended up with I had 100 psi MORE than what I had read in the shop. I figured that it was the SPG so I checked it using the same gauge that I used in the shop. Nope it was truly 100 more.
This wasn't one of those times were there was a huge swing in air temps nor was the tank exposed to any major temp swings. I have seen big swings from the water temps when diving in the winter and the tanks have been stored inside.

With that being said I'm not sure that a water bath would be worth it IF THE TANK IS FILLED AT THE PROPER RATE. Just my $.02.

 

[Luis H]

I told myself that I wasn't going to post but...I am.

I told myself the same thing...



There is no question that water has a higher heat transfer coefficient (and thermal capacitance, two different qualities) than air. But, based on some heat transfer calculations that I performed, if you do a quick fill (lets say, less than 5 minutes) the end result is not significant at all (from a dry or wet fill).

The heat transfer from the air inside the cylinder to the cylinder wall is a major heat flow impedance that is unaffected by the water bath.

A water bath does make much more of a difference if the tanks soaks for a while…like 10 minutes or longer.

One of the significant assumptions in my calculation is that both the tank and the water bath start the process at room temperature. If you bring a hot tank from the trunk of your car a water bath would help to cool down the tank itself.

An interesting observation is that a water bath tends to help a steel tank a bit more than an aluminum tank. The major reason for this is the high heat capacitance of an aluminum cylinder (the mass of the aluminum tank holds more heat). Some may have notice that the outside wall of a recently filled aluminum cylinder gets hotter a few minutes after the cylinder was actually filled.

This involves a transient heat transfer analysis not steady state.

My personal preference is a looong fill… preferably in a water bath, but only if I personally attach the fill hose. I have had personally bad experiences with others filling my tanks in a water bath…particularly bad experience when the bath was salt water in a fill station on a pier in a marina in Puerto Rico.

 

[pescador775]

If impedance were a factor your air conditioner would not work. Look, a hot tank can be dipped in 75F sea water for one minute and the tank pressure will drop 300 psi. If that doesn't convince you nothing will.

These theoreticians seem to forget that 35 pounds of steel or aluminum hold a lot more heat than 6 pounds of air. When that metal touches water, heat is going to move on and out, FAST. It will move faster with aluminum because of its conductivity. However, the steel tank usually starts out hotter so it is a close call since heat transfer depends on temperature differences. Also, it should be noted that total heat capacity is a function of mass, and that steel and aluminum tanks have about the same mass contrary to what you have been told. However, I seem to recall that capactance per unit mass is higher for aluminum but I would have to check that. I don't really care. Divers know that aluminum will cool faster than steel.

When that happens heat is pulled from the air inside at a rapid rate. This process is helped by convection currents inside the tank. It happens rapidly, especially with aluminum but don't hasten to pull the tank out of the water immediately. The tank will warm up if it is pulled from the water too fast. This is due to the fact that the metal is still capturing heat from the air inside the tank.

 

[ReefHound]

If impedance were a factor your air conditioner would not work. Look, a hot tank can be dipped in 75F sea water for one minute and the tank pressure will drop 300 psi. If that doesn't convince you nothing will.

One factor I've not seen mentioned is agitating the water to increase thermal cooling.

Anyone that doubts the effectiveness in cooling a block of aluminum or steel with water, I suggest draining the coolant from your car and seeing how far you can drive before your engine overheats. Bet you won't get far...

 

[crpntr133]

One factor I've not seen mentioned is agitating the water to increase thermal cooling.

Captain indirectly did this.

Probably a trivial amount of help doing this but you might feel obligated to do so.

 

[Luis H]

If impedance were a factor your air conditioner would not work. Look, a hot tank can be dipped in 75F sea water for one minute and the tank pressure will drop 300 psi. If that doesn't convince you nothing will.

These theoreticians seem to forget that 35 pounds of steel or aluminum hold a lot more heat than 6 pounds of air. When that metal touches water, heat is going to move on and out, FAST. It will move faster with aluminum because of its conductivity. However, the steel tank usually starts out hotter so it is a close call since heat transfer depends on temperature differences. Also, it should be noted that total heat capacity is a function of mass, and that steel and aluminum tanks have about the same mass contrary to what you have been told. However, I seem to recall that capactance per unit mass is higher for aluminum but I would have to check that. I don't really care. Divers know that aluminum will cool faster than steel.

When that happens heat is pulled from the air inside at a rapid rate. This process is helped by convection currents inside the tank. It happens rapidly, especially with aluminum but don't hasten to pull the tank out of the water immediately. The tank will warm up if it is pulled from the water too fast. This is due to the fact that the metal is still capturing heat from the air inside the tank.

It is true that I have done a substantial amount of theoretical heat transfer calculations during the last 30 years, but many (if not most) of my calculations have been confirmed with some kind of real experiment…some times very precise experiments, other times with just physical observations.

As I mentioned above one of the assumptions was that the water and tanks start at room temperature. A very quick hot fill was then performed. I did not analyze the heating cycle, just the cooling cycle since that is where the cooling water comes into play. This and all the other assumptions where selected as being reasonable condition.

Also the calculation uses the lumped mass transient heat transfer assumption… both the actual mass and thermal capacitance of the air and the metal are taken into account.

To make the calculations reasonably simple (after all, I did this just for the fun of it) I took some simplifying assumptions that IMHO are very reasonable.

I have not done any precise experiments to back these calculations, but it does seem to agree with my real life observations filling tanks.

There is nothing magic about heat transfer calculations, if you would like I will be glad to email you a copy of my Excel spread sheets.

BTW, thermal impedance is just the heat transfer term for the inverse of conductance. It is sort of parallel to resistance and conductivity in an electrical circuit.

The thermal impedance (or conductance) of any solid substance is calculated based on its thermal conduction coefficient, the heat flow area and distance heat has to travel in the geometry being analyzed. For a fluid it involves the convection coefficient and the interface area from the fluid to adjacent solid.

I highlighted part of your statement that confirms my calculations. I just tried to put some reasonable numbers behind those observations…like I said; you are welcome to review my calculations.

 

[Luis H]

One factor I've not seen mentioned is agitating the water to increase thermal cooling.

Anyone that doubts the effectiveness in cooling a block of aluminum or steel with water, I suggest draining the coolant from your car and seeing how far you can drive before your engine overheats. Bet you won't get far...

The convection coefficient into the water is already many times larger than inside a confined air space.


You are referring to a steady state heat transfer condition (during normal operation of a car engine that was explicitly design to be liquid cooled), not a transient condition. As you mentioned, it won’t be far before it overheats, but it is not instantaneous either.

Many internal combustion engines have been design to be air cooled…it is just a mater of the design and the heat flux density, etc.

BTW, the liquid cooled engine ultimately dumps all the heat into the air via the radiator.