I wanted to make this point separately.
We have had many threads on ScubaBoard over the years about shops refusing to fill older aluminum tanks, and I always assumed it was the old alloy issue that was to blame. A thread last year changed my thinking. I did a Google search and found a whole bunch of old articles from past decades about the supposed dangers of filling older aluminum tanks. There really was a prevailing belief not all that long ago that aluminum tanks had a very short lifespan. Some articles suggested junking them after only 15 years. What was interesting was that you could see the change in thinking over the years in articles published citing the same authority. The authority that said 15 years a while before was later saying 20 and then later saying there was no specific lifespan. (I wondered if their estimate of the cost of replacing their entire stock of aluminum tanks might have been a factor.)
It is easy to see how someone who became part of that thinking years ago might have that thinking evolve from "maybe it's a good idea" to "it's the law." We saw something similar with reverse dive profiles. In 1972, the PADI OW student manual suggested that divers doing repetitive dives do the deeper one first, probably because doing so shortened the surface interval. Over the years, the language became stronger and stronger, and before long it was a hard and fast rule that the deeper dive must be done first, with the suggestion that you would die an agonizing death if you violated that rule. No one knew why the deeper one had to be done first, but, by golly, we knew it was a rule that had to be followed.
