Filling scuba tanks

[papaouiee]

I am afraid that this really isn't true. When a tank fails, it usually splits and lets all the air out at once. If it is being dry filled at the time, the air can escape in every direction equally and looses it's pressure everywhere. However, once you put a tank into a water bath, that failure will have the expanding air pushing against water which is not compressible. So, instead of air rapidly expanding into the surroundings, it is rapidly displacing the water. The water has nowhere to go but up and that is usually not enough to soften the pressure. So the water also expands to the sides causing a drastic failure of the walls of the container. Most dive shops that fill in water use standard horse troughs and those are usually galvanized or heavy plastic. That means some gnarly shrapnel is flying along with the water.

Just my $.02

Your explanation is defying the basic laws of physics. When the air is released from the splitting tank, the force is going to go in the direction of least resistance and since water is denser than air, the majority of the force will be going up (the spot of least resistance). I'm not saying it will all go up, but the greater amount of it will go up. This will leave the rest of the force that does go in other directions less than if it was not in the tub. Also, the water bath will actually absorb some of it and the majority of it will be going up. What's left, will of course go in all directions. The container will also absorb some of the force, again reducing the damage done by the explosion. Tubs will not throw shrapnel, but in all likelihood will split, leaving a rip or rips in the tank. Bottom line, if the operator is in the room when a tank does go being filled, the operator in most cases will not know what hit him or her; the percussion alone will knock them out, if not do them in (and a tank of water will again reduce this percussion, absorbing some of it). This is not opinion but nothing more than a basic understanding of the laws of the physics.

 

[papaouiee]

We have had a Bauer Junior II for the past 16 years and filled over 4,000 tanks. We have always filled using a water tank and can say that for steel tanks it makes a big difference in final fill pressures. For aluminium tanks it is less of a difference. If you have a small compressor and need to fill in temperatures over 25C (US figure out what that is!), then I reckon use a water tank if you can.

But that difference in aluminum tanks disappears when filled out of water and especially when filled hot. I have seen cases (filled in front of me, my dive buddy's aluminum tanks), filled at a dive shop and done in about three minutes or less (the usual way most dive shops fill tanks, regardless of what they tell you). They'll put 3300-3500 in them, claiming when they cool (they are very hot to the touch), they will be at 3000. When we get to the lake the next day, he's got 2500-2700 in the tanks. Following the general rule of thumb, the tank will at a full fill, go up or down 100 lbs. every 10 degrees F. Assuming the tank is at 80 degrees at the lake (the temp they'd about be here coming from air conditioning from the home and then the car the pressure in the tanks is changing about 700-800 lbs, or the temperature is changing 70-80 degrees. Meaning the tank after the hot fill must be around 150-160 degrees. Sudden changes like this, up and down in temperature and under stress from the pressure, in time changes the characteristics of the metal over time. This is the last thing I want done to a tank I strap on my back.

 

[papaouiee]

I have never heard a real valid reason for filing dry. The goals were never get water in your tank. dry filling supports that. fill slow, dry filling helps that. filling with hot gas from a compressor. dry is useless, enhancing cool down time after fill , dry filling does nothing. precooling the tank, dry filling does nothing.

I normally fill wet when I can. whether it is in a bath of with wet towels wrapped on the tank. They are not wet till the fill line is attached and the line is not attached till the valve is blown out by cracking the valve. A rix 6 will heat up a lp95 very fast and make it hot. Yes the internals are not cooled by wet filling bu the rate the heat is drawn out is sped up. next putting them in a bath lowers tth tank temp prior to filling. so yo are not filling a 110f tank to 140F. The best thing to do as I can see is to get a cooler for the filliing gas to cool it prior to entering the tank.now you can take a coil of say 100 ft of steel line and put it in a bucket with water and that will cool the gas a whole lot. do that prior to the moisture separator and that is even better. AS IT IS I EITHER WET BATH OR USE A TOWEL WRAPPED AROUND THE TANK AND KEEP ADDING WATER TO IT TILL THE TANK IS FILLED TO 2-300 OVER intended pressure. One day I will be able to afford a cascade.

What I dont do is say that i can fill faster with a wet fill. wet or dry makes no difference in a 30 second fill.

You are very right about fast filling making no difference if in air or water when filling. Technically, the water will still do more cooling, but in a short period of time where the tank is rapidly heated, it would take sensitive equipment to see the change, I would think. Reading between the lines, the best filling situation, is in a water tank and filled slowly. The worst way is to fill in air and do so quickly. Big question here is: Why do so many dive shop owners defend the air fill, when everyone else, defends the water bath; especially those who have their own compressors and fill their own tanks? Also, being a former dealer of Pressed Steel Tanks and having talked to their engineering people, they told me never let someone dry fill them, especially fast enough to heat them up. I don't know about y'all, but I have always been one to listen to the manufacturer of a product; they know the ins and outs of their gear better than anyone.

 

[Namreg 58]

Big question here is: Why do so many dive shop owners defend the air fill, when everyone else, defends the water bath; especially those who have their own compressors and fill their own tanks? Also, being a former dealer of Pressed Steel Tanks and having talked to their engineering people, they told me never let someone dry fill them, especially fast enough to heat them up.

The simple answer is that by dry filling, the tanks need minimum handling when compared to having to put them into and out of an above ground or in ground tub of water which can lead to back injuries etc.
From this, one could also surmise that dive shop tank fill personnel are spineless and lazy.

I once had my tanks filled for a week by a dive op. that used a 25 cfm compressor and only dry filled 2 tanks at a time from the compressor. About a week later one of the yoke o rings blew out and when I tried to change it, it had been vulcanized into the ring groove. If I ever go there again, I'll take my portable with me and fill my own.

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