Posted by Karl S. on October 23, 2001 at 14:50:48:
In Reply to: Re: PADI Propaganda- and the big bad cave divers in fl posted by MHK on October 23, 2001 at 09:45:44:
Tanks have burst discs. They are set to blow at various pressures. As long as you stay below those levels, I think overfilling the stamped working pressure is fine, since Charles' Law is going to reduce the tank pressure once you hit the water anyway. I believe most burst discs are variously rated, and normally that results in at least 400 to 500 psi leeway on a tank fill pressure.
If the burst disc has been removed and plugged, then the tank should be safe up to 5/3rds of working pressure per the last hydro test.
3000 x 5/3 = 5000 psi. 2400 x 5/3 = 4000 psi.
I dont believe you are endangering anyone specifically until you exceed that level or at least get very close to it. How much danger is there in blowing a burst disc? A lot if underwater. None if on dry land or a boat deck.
Storing a tank full with a plugged burst disc seems irresponsible, but other than that, I think burst discs are overrated.
A boat or store will have policies and rules. They will also have customers. I have found that valuable customers get more respect and flexibility than infrequent customers do. Thats almost a law of economics.
Charles' Law will result in your tank pressure reducing by about 5 psi for each degree F differential between the water temp you are diving in and the tank fill temp. For room temp on a cold fill, that amounts to about 100 psi here in California, more or less depending on where you are north or south. For a hot fill, it could be several hundred psi.
One way to remember Charles' Law is to remember the "C" as in "cold fills are better than hot fills" vis a vis, temperature and pressure vary directly with volume held constant, just like on the inside of a scuba tank.
So filling your tanks to about 100 psi over the DOT working pressure should be just fine. I fail to see other than a tempest in a teapot regarding the original post, and I completely agree with Mike Kane here, that it is the business of the boat or store owner and the certified diving customer what pressure tanks are filled to, and to a certain extent an issue of compromise, within reason.
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