is overfilling why DIR sanctions low pressure steel tanks?

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Posted by Karl S. on November 05, 2001 at 14:30:22:

I didnt realize that it was possible to over-fill low pressure steel tanks moreso than high pressure steel or aluminum ones. You would have to replace the burst-disc with a higher rated pressure burst disc first.

Overfilling by 100 or 200 psi will get your tank to the working pressure when you hit the water and the temp in the tanks goes down to ambient water temp of about 50 degrees F. Personally I believe this is not "overfilling" per se, because by the time the tank is "working" it will be 20 or 30 degrees F lower, and the pressure will go down commensurately, approximately 5 PSI per degree F.

Most of the really good dive shops will put about 100 to 200 extra PSI in your tanks for you anyway, knowing how Charles' Law works. [Charles Law: to wit, cold fills are better than hot fills because temp and pressure of a gas vary directly when volume is held constant.]

Overfilling by 500 to 5000 is more like "overfilling."

Is that the idea behind DIR's sanction of low pressure steel tanks?

It has always sounded lame to me that "inertia" or "streamlining" or "lower boosts on dive boats to 3000 psi on fills" were actual reasons to forego the luxury of smaller high pressure steel tanks. But if youre going onto a dive boat and refilling your LP steel tank with 3000 psi, well, that explains it completely.

A low pressure steel tank seems more likely to handle the strain of overfilling than tanks that are already at high pressure would. 3000 / 2400 x 104 = 130 cu ft of gas, and with doubles that would be 260 cu ft. This would be even more air than double HP 120s carry!

The burst disc better have been taken into account. Not all burst discs are rated the same. They come loose too, just like anything else that is made of metal and which undergoes temperature changes. They need to be tightened occasionally, just like all metal fittings need to be, and much more than just once a year during annual inspection. Before every dive is a good idea.

Now, let me see, I dont see any hazards in asking a sincere question of the DIR camp. I dont think anyone is going to die and need to be hauled out of the water over this question. There have been no recommendations in this post, other than to tighten all metal fittings before each dive.

I think this post is "safe" to post, even with a plethora of flaming strokes attacking others in a feeding frenzy.

MikeK or some other DIR might actually even answer the question, for a change?!

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