What is an exemption on a tank?

[Jamesmb]

I was just reading about the XS Scuba tanks and mentioned a DOT Exemption and wonder what that meant. I had bought a used older Spar Air that did not have to be hydroed think was 1800 psi, I guess anything under 2000 psi is exempt from hydro? But the above question is for a HP tank.

[ScaredSilly]

An exemption is a permit issued by DOT for the production of a specific cylinder that does not fit in the normal specs in the 3AA steel with a 10% overfill or 3AL.

The bitch with exemption cylinders is that it must be renewed. If the exemption is not renewed techically the cylinder can not be hydro. Look for threads on PST cylinders. PST went out of business so folks with 3500psi HP cylinders are a bit under the gun because the exemption expired. A new company has bought PST but it is still not clear if the new exemption is in place. (Some can correct if I am wrong on this as I have not followed this closely).

BTW the early Al cylinders were made under an exemption later DOT gave them their own catagory 3AL so some (the exemption number) were over stamped with 3Al.

Another type is special permit such as the SP6498 Al cylinders made from 6351 alloy which gives many people heart burn. SP cylinders unlike exemptions are good for their life in many cases.

[MEL-DC Diver]

Well, with regard to the PST cylinders... The outcome for people who had the cylinders already was never really in question - people with those tanks would ultimately have been able to have them hydro'd and filled even in the worst case scenario if they got a 'to use' exemption in the individuals name. I followed the issue pretty closely up to that point b/c I was about to get some PSTs at that time. Alas, that is also when I discovered Worthington and was able to avoid the issue altogether. I never did determine what the conclusion was with the "New PST", but the guys I dive with have 'em since before that time and they haven't experienced even a hiccup with regard to fills, etc. Perhaps the new PST was able to go forward and renew the original exemption after all??