Please solve this mystery...

[beezwax]

A year ago a dive buddy of mine bought an ex-air force Davey compressor. It works great. We've probably done a couple hundred fills on it so far. So, I've got a 19 cu. ft. aluminum pony bottle. For several months the compressor topped it up to 3000psi, like every other tank. But for some reason I just cannot figure out, the last five times I have tried to fill it it will only take on 2200-2300psi. The compressor has three fill whips and the other two tanks load to 3000psi, but the pony bottle comes up short. This only happens on my pony bottle. I have checked it with multiple pressure gauges, and also double-checked the gauges on other tanks. It is not the gauge. Also, it's not a matter of heating/cooling, since I used to get a full fill in the pony during the summer. The o-ring is fine. Anyone have any idea why this is happening? Could I have some kind of partial obstruction in the pony valve?

?????

 

[WVDiver]

Having eliminated all the other variables, I would get the valve looked at. That's what it sounds like. When was it's last visual inspection?

 

[beezwax]

The tank was new in Feb 2006, so it's due for its first annual next month. I'll certainly ask them about it when I drop it off...

 

[rjack321]

That has got to be one of the craziest things I've ever heard. So everything else will get up to 3000, just not the pony bottle regardless of which whip you put it on??

My only suggestion is to replace the HP seat in the valve.

 

[beezwax]

Yeah it doesn't matter which whip I use, so it's got to be something in the tank valve. The first couple times it happened I just assumed it was "operator error," but five times in a row? I'd like to give myself more credit than that. I've got a couple spare tank valves so I'll breathe down the pony on my next dive and swap out the valves, and if it doesn't fill to 3000psi after that I'll have to blame it on ghosts or something....

 

[pescador775]

I agree with the others. The soft seat has popped out of the valve body. Overhaul the valve. Pressure gets underneath the plastic seat. It would probably fill normally at slow fill rate but....check it out.

 

[beezwax]

That makes a lot of sense-- so if I was using a banked cascade system I could probably get a full fill, but with charging it straight off the compressor the OP valve pops off before it has enough time to fully load.

 

[300bar]

Funny, I've tanks 20 jears old and NEVER had an overhaul on the valves so on a tank less then one year old :huh:.
The only time we see this kind of problem is when the valve is overthighted to many times.Remember closed=closed if you keep on thightening it you will damage the threads in the valve.When they got damaged then you will get this problem.
Try to empty your pony completly,if you find you can't you defenetly have broken the thread,in that case you have to loosen the valve very carefully to let the remaning air/gas out.

 

[beezwax]

It is strange that it happened to such a new valve, I don't know what the problem is. But at least I know it's not from overtightening too many times-- since it's less than a year old I've never even had the valve off. But I seem to have weird luck with tank valves-- I had another one where the valve stem kept falling off somehow (several times) and rolling around inside the tank.

 

[mudhole]

Hopefully not your problem. . . a local fire department had a tank that was always "low" when it was being used. I was at the test center when it was being hydro tested. A crack in the neck, would open up and drop the pressure by 1,000 lbs, then close up. Check the valve overpressure plug first, but if you don't find an obvious problem there have the tank tested.

A 70's aluminum tank of mine, wouldn't hold the overpressure test steady, it lost pressure but never fell below the 3,000 psi. there was a very slight split in the neck that wasn't visible to a normal VIP. That tank neck is now used to teach dive shop personnel how close they have to look.