If it is a steel tank with current hydro the shop should fill it, but an aluminium tank I would probably not, especially if it is luxfur or us divers. The most common tank you will see is marked 6498, in my experience those tanks have a very high failure rate. In fact about 10 years ago luxfer "recalled" them offering anyone q 50 or 75 dollar credit toward a new tank and they paid return shipping. If a company is willing to put out 75 or 100 toward a 20 year old product that sold for 100 in 1985 when it was sold that tells me they really what them "off the street".
I just want to keep it straight and stick with the facts before this goes downhill.
Facts:
1) 6498 is a special permit number given to Luxfer for production of Al 80 tanks made from 6351- T6 alloy.
2) SP 6498 was one of several special permits that were consolidated under the 3AL standard. Tanks with those permits were all supposed to have been stamped "3AL" at their next requalification - about 20 years ago. It is however still pretty common to find them un stamped.
3) Luxfer never recalled those tanks or any 6351-T6 scuba tanks per se, but they did offer a discount toward a new tank for a limited period of time. The supposed rationale for that articulated above is pretty good evidence of why it does not pay for companies to do that, and Luxfer, not surprisingly, stopped offering the discount. This also occurred in the era when the growth or prevalence of sustained load cracks was not clear, and when that was clarified, the perceived need for a remedy for replacing tanks was no longer present.
4) 6351 T-6 alloy scuba tanks are still DOT approved for service provided they pass requalification which now requires the tank inspector to conduct a visual eddy inspection in addition to the normal visual inspection and hydro test.
I agree with you that there is a failure rate for 6351-T6 tanks, in terms of sustained load cracks being found in the necks of 6351-T6 alloy tanks but it is still low and most importantly:
5) there has not been a single instance of a properly inspected 6351-T6 tank catastrophically failing since eddy current inspection protocols were implemented over a decade ago.
6) Based on field experience the DOT only requires the eddy current inspection even 5 years at requalification of the cylinder. And testing and field experience indicates a typical sustained load crack takes about 8 years to propagate from detection to cylinder failure. There is no substantiated evidence of rapid crack growth.
7) The industry practice was to conduct an eddy current/visual plus inspection every year. This was developed several years prior to the DOT final requirement for 5 year testing but it's still prudent.
The main informed objection (there are several non informed objections) to the use of 6351-T6 alloy tanks is that someone may screw up the eddy current/visual plus inspection and put a tank back in service with an un detected sustained load crack. However, with the DOT and scuba industry standards overlaid on the 8 year time line for crack propagation to the point of catastrophic failure, the crack would have to be missed on at least 1 DOT inspection (possibly 2 depending on timing) and at least 7 scuba industry / local dive shop performed eddy current/visual plus inspections.
So it's remotely possible that a tank could be poorly inspected for 8 consecutive inspections in a row and suddenly explode, but the odds on that are almost infinitely small. The fact that there are still millions of 6351-T6 tanks in service and that none have catastrophically failed due to an SLC in the 10 plus years since eddy current inspections were implemented supports that. So to operate under the assumption that properly inspected 6351-T6 tanks are unsafe to fill borders on the paranoid, but it's not generally paranoia it's just ignorance of the facts or a bias held over from the time when the facts were not clear and no one really knew the risks.
8) the reality is that many dive shops/fill station operators fall in one or other categories above - or just see it as great reason to get customers to buy new tanks. The latter is not likely if the tanks are not already on the show room floor as the profit in tanks is quite low, but if they happen to have half a pallet of not getting any younger tanks sitting along the wall for sale, you have to consider it.
Personally, I probably would not buy a 6351-T6 tank at this point, but I would not turn one I own into scrap metal and I am not hesitant to fill a properly inspected 6351-T6 tank.
