3 replies to this topic

[Narwhal]

Some years ago I was working as an instructor for a LDS in Dallas when, on a Friday evening sometime in the early Spring, a lady came in and asked us to fill her cylinder as she wanted to go diving that weekend for the first time that year. I checked the hydrostatic test date (good) and the annual (still had a couple of months left) and proceeded to fill it. She paid and left. The next day she was back in and told us, "You have bad air."
We had just received our analysis from the lab the previous week so I was sure she was wrong and asked her what made her think we had bad air.
"Just smell it." She said. I did and it was putrid.
"Well", I asked, "Could it have been air left in the tank from last year?"
"No", she said "I stored the tank empty in my closet all winter with the valve removed and sitting on the shelf in my closet. I put the valve back in just before I brought the tank up here yesterday to be filled. There was no air left in the tank from last year."
Suspecting the worst, I took the tank outside, opened the valve and let it drain. I then pulled the valve. Mercifully, the little mouse that had built its nest in the tank over the winter died quickly (although somewhat messily) under 3000 psi of pressure. Obviously, that's the wrong way to store a dive cylinder.

A fairly experienced diver recently asked me how to store a tank over the winter when she would not be using it. Ok, OK no comments about "woose" divers in the South or Texas . She had been told in OW class that a tank should be stored with 500 psi in the tank to prevent water or moist air from entering while the tank was in long-term storage. I have taught this to my students because that is what I was taught also. But it got me thinking (a sometimes painful experience) about "truths" we teach our students and the logic behind them. I asked a lot of "experts" from tank manufacturers, to PSA Instructors, to hydrostatic testers and got a lot of different answers. What do you think? And, more importantly, why?

[Walter]

Reminds me of a story, but I digress.

As long as the pressure in the tank is greater than ambient, it will keep moist air out. OTOH, even the oldest valves are designed to hold 1800 PSI. As long as the valve is closed, nothing is getting in the tank. If the valve is open, it'll empty out soon anyway.

Back to the story..........

Once upon a time, I took my doubles in for a visual inspection. I took them to the same shop I'd taken them the year before. In the year that was just over, I'd made about 120 dives with those particular tanks. Tony set right to work while I was there. I sat down to chat as he pulled out a valve. Before he did anything else, he asked if anyone wanted to look inside. I hopped up and said, "I do." Tony dropped the light down in the tank and I looked inside the tank for the first time since they'd been vized the year before. On the bottom was a dead roach. Lovely.

[ScubaDadMiami]

Storing a tank at max fill pressure over time will stress the tank metal. It is best to leave the tank pressurized to some degree (such as 200 psi) in order to prevent the outside environment from entering the tank (humidity, pollution, etc.). Other than some pressurization, you don't need more than that.

If your tank is steel, it is very important to make sure that there is no water in the tank. Dry gas will prevent rust development.

[maninthesea]

Leaving the tank fully pressurized over time stresses the tank less than actually using it. It is the pressure cycles that stress it most. Leaving 500psi is fine also as it keeps everything out. storing it at around 1000 to 2000psi is not recomended especially for aluminium tanks becuase there is a possibility that in a fire the pressure in the tank would not burst the burst disc prior to the heat weakening the walls enough for it to explode.

So Full or 500psi both work whichever is more convinenet. I would go with full as you never know when you might find a non diveing use for compressed air.

Cheers Jim