Tank re-fill questions

[Hatul]

Yes. The shop is right. A slow fill allows the tank to stay at lower temp and more air in the end. It's called adiabatic heating and cooling and is also responsible for some of the regulator freezups people get in low temp diving.

Adam

 

[Scared Silly]

From PSI Cylinders on Wet Fills

 

[spectrum]

I like the sound of the second shop so that sounds like the way to go. But a few points......

The filling of any cylinder will result in expansion of the gas. This happens as the gas expands into the less full cylinder and is subsequently re-compressed as it fills. Simplistically.... as the air enters the cylinder it is doing the happy dance and the molecules are are less constrained. As the cylinder fills they get crowded together and friction, lack of freedom results in too much energy so it is released as heat. heat results in expansion and the air is in effect fluffed up. If you do a fast fill you can drive the volume of a given amount of air up to where it pressurizes as 10% more. A fast fill to 300 may easily be only 2700 when cool. This can be managed to some extent by:
* Slow fill
* Filling slow from cooled banks of air
* Bringing cool cylinder in to fill never hurts
* Filling with the cylinder in a water bath, whether you like it or not.

This is commonly attributed to Charles law but some consider Gay Lussac Law a better fit for the cylinder fill scenario.

Now the water bath does have a very limited effect I have come to believe and as such probably is not worth the risk of introducing water through sloppy technique. Remember that air is an insulator so once you have that hot ball of gas in there it's not in a big hurry to transfer energy to the cylinder walls. Air in direct contact will cool and there will be convection and very limited conduction across the gas mass. All told a cylinder needs about 4 hours to get from excited to steady state. I wait until the next morning to do a final check Y tag of my fills for this reason.

The shop that allows an hour for a top-off can come reasonably close if the initial fill was gentle. Better still if that add an extra 100- - 200 PSI above the working pressure. The only pressure that really matters is steady sate at 70F. A hotter cylinder should be at a higher pressure. it's no different than of you took a perfect fill at 70F and left it in the sun in a vehicle on a hot sunny day so don't worry about the cylinder.

Slow fills are your first line of defense, followed by a top-off. Better yet is dropping them off and coming back a day latter so the top-off can be done on a cool stable cylinder. If the shop has the cylinders waiting in the sun all bets are off!

I strongly suggest that you make sure to check each cylinder after it is off the whip. Do not just view the panel gauge. I have seen repeated human errors of party opened valves resulting in grossly short fills while the panel said things were at equilibrium and up to pressure. The time to discover this is as you accept the cylinders, not at home or at the next dive site. It also sends a message that you have expectations.

Pete

 

[Rogersea]

Filling in a water bath while unnecessary is a common and acceptable practice. Filling from a Cascaded Air Bank is good way to get a quick fill. It is relatively uncommon to get exactly 207Bar or 3000PSI, over the years I have grown to expect fills between 2850 and 3100 PSI. Anything outside of that range and I would mention it to the shop owner or manager(they appreciate it).

I fill my own tanks and do not use a water bath.

Cheers
Roger

 

[Scuba-dan]

Yeah i did a wreck dive yesterday. When I check my SPG i had 3200 PSI. When the tank hit the water it went to 3000 PSI... Because i bought the tank new at the 1st LDS i got 34 free re-fills. But once I've gone through all those I'll try the other LDS with the slow re-fill to see the difference.

Thanks again for all the great info.

 

[KarolinaKid]

I would never take my tanks to a shop that uses a water bath. It's more than merely pointless, it can be one more way to introduce moisture into the tank.

Yea, that's why I leave my tanks on the boat and dive with a really, really long hose...if I dunk them under water I fear they will get a rude introduction to moisture...

 

[ClayJar]

Yea, that's why I leave my tanks on the boat and dive with a really, really long hose...if I dunk them under water I fear they will get a rude introduction to moisture...

If you're actually *adding* air to your tanks as you breathe underwater, your SAC is even better than my buddy (nicknamed "The Compressor" because their SAC is so low). On the other hand, if you're like the rest of us and consuming gas from your cylinders during the dive, there's no possibility of anything getting *into* your cylinders during the dive, so your quip doesn't *quite* work.

If the filling procedures are proper and followed without error, water would not be introduced into your cylinders even in a water-bath fill station. On the other hand, if someone has a momentary brain fart at a wet fill station, water *could* be introduced, whereas at a dry fill station, there's obviously much less chance of water being around to be forced into the cylinder (although a dry fill station filling a cylinder with water on the face of the valve could still happen). The question, then, merely is whether the thermodynamic benefits of a water bath (which we have not experimentally examined as of yet) are significant enough to merit the additional risk water may be introduced into the cylinder (a risk which should be quite low, indeed).

Right now, we don't really have any good data on which to base that decision, so we're stuck with anecdotes and opinions. That being the case, it's probably not reasonable to consider either side worthy of sarcasm (which I prefer to reserve for cases in which "rightness" is patently obvious from well-documented scientific analysis).

 

[Rogersea]

never tried it, maybe next .......

Yea, that's why I leave my tanks on the boat and dive with a really, really long hose...if I dunk them under water I fear they will get a rude introduction to moisture...

 

[captain]

Any wet fill station should be constructed so the tank valve and the fill whip can never ever possibly be submerged. That is how mine is and I have 40 and 50 year old steel tanks that have always been filled in the water bath and none have ever failed a hydro or VIP.

 

[trailblazer1229]

I can tell who filled my tanks at the shop when I turn them on at the dive site. Same thing in the fire department with the bottles. Slow fill then top off. Nothing like getting cheated out of a few minutes of excitement in the fire or under the water.