Filling scuba tanks

[papaouiee]

I have been diving now for over 45 years and have never encountered this until I hit my home town shop. All my life I have been told that tanks are to be filled emerged in water and filled slowly.
A tank filled out of water and quickly is a "hot fill" which:

1.) leaves you at the dive site short up to 500 lbs of air
2.) destroys the life of tanks
3.) is out right dangerous.

Yet the shop owners here insist that that they were taught in training that "hot filling" a tank was perfectly safe and the way to do it. I was even told that their insurance company dictates that is how they are to fill them. I believe this to not be correct information. I have even talked to a few manufacturers who have told me that "hot filling" destroys tanks lives and is dangerous; they should always be filled in water and at a slow pace; at least seven to ten to fill one.

I'm looking for other shop owners, experienced divers and those in the know to comment.

 

[tomfcrist]

There’s nothing wrong with dry fills...fill slowly...overfill a bit(experience helps) and it will settle to working pressure.

Hot fills are 30 second fills...

Wet fills have an opportunity to introduce water into the cylinder, which is obviously not good for a steel tank.

 

[boulderjohn]

Almost everyone I know (but not everyone) believes that a water bath does little to nothing of value when filling a tank. Of all the places where I get fills, only two use water baths.

The question of a hot fill is "How hot?" When I fill my tanks (no water), I try to do things reasonably slowly, but not absurdly so. It does not take me long to fill a tank from scratch (no, I haven't timed it), and if it is feeling hot, I will go 200-300 PSI above what I want, and I am usually close to spot on.

 

[gcarter]

My HP tanks are slow filled to 3900 psi, allowed to cool for 5 minutes, topped again to 3900. I arrive at a dive site with my advertised 3400 - 3500 psi. Filling in water is not necessary. The speed of the fill and how hot it is allowed to get are the important bits.

 

[tomfcrist]

I think the only merit I see in wet filling is when the operator of the fillstation doesn’t want to overfill/fill slowly...and also wants a full quick fill....in a hot climate where the water temp is significantly lower than the air temp.

Even then it would need to be chilled water and circulated.

I’ll stick with dry filling.

 

[Storker]

I never fill wet (we don't have the equipment for that at my club). I always overfill by some 10%. My (now sold) 200bar/2900psi tank used to settle pretty close to 200 at room temp; my 300bar/4350psi tanks settle at ~280bar at room temp. I usually top them up to ~310 bar, which settles pretty close to 310 at room temp and fairly close to 300 before submerging.

Since the test pressure here is 150% of service pressure and I'm using steels, I have no problem with overfilling hot at 110% of service pressure. On hot summer days, I might back off a little bit, but I usually pay for it with lower pressures after getting in the water and before I submerge.

 

[tursiops]

The point here, OP, is that a dry fill is not necessarily a hot fill; the latter are bad, the former are not. You can hot fill in a tub of water, too, and it might not feel as hot but the gas still mostly will be, and will cool off, and the pressure will drop.

Hot (i.e., fast) is bad. Slow is good. Water helps mostly psychologically, but not physically, unless it was a hot fill, and then the help is minor. Water in the tank is worse than the occasional hot fill. See PSI-PCI - Filling Cylinders In Water - Time to Review.

 

[JamesBon92007]

I was told the same thing (way back when) and whatever dive shop I used they always had them in a tank filled with water. Now I have my own compressor and at first I was filling them in water but after reading many threads here I determined that it was not necessary. My compressor is rather small and it takes a while to fill a tank so they don't get very warm. I overfill them about 200 psi and when they cool they are just about right. If not, I check my tanks a day or two before a dive and top them off if necessary. It's been quite a while since I took a tank in to get it filled so it looks like things have changed. As far as the danger goes, as long as they have been visually inspected and hydroed every five years and have the correct burst disc then I don't think there is any real danger. I suppose it would be possible for an inspected and hydroed tank to explode while being filled but it's probably one in a billion or something like that.

 

[Jonny Wishbone]

My LDS back in the day (mid 80's) had a 6 inch thick cement tank with copper coils going round and round inside which chilled the water. Not sure of the set up though.They were never in a hurry and you always left with a cold tank filled right on the money. I think i remember back then a fill was 2.75 if you bought a card and they punched it each time you got a fill just like the old subway sandwich cards. If you bought 1 fill at a time it was 3.00 i think. Now they just fill tanks in a horse trough with water. Times have sure changed....

 

[Seaweed Doc]

Interesting thread. I might have to fill two tanks at my LDS in and out of the water bath and compare results. Now y'all done got me curious if the myth I learned is reality, or the truth is false, or something.