I would also like to know more about the tank: manufacturer, coating process, last hydro, last ViP, last fill, where filled, history of the fill station (compressor, filters), etc. Basically, I want to know enough to convince myself that it can't happen to my tanks.
It CAN happen to your tanks. But, in my personal opinion, you can prevent it from happening by keeping your hydro and annual inspections up to date. Also, if you get a free-flow and drain your tank to zero PSI underwater, have your tank cleaned and inspected right away. I think it's really that simple.
Sorry, I don't have the detailed information that you seek. I obtained many of the references through inter-library loan and so i do not have them anymore.
We don't know that all of the rust was created during that last fill.
That's true. However, if you look at the University of Rhode Island studies, severe corrosion in steel tanks occurred when there was a lot of water in the cylinder (500 ml to be exact). That water is necessary to fuel the corrosion process, and the corrosion process is what consumes the oxygen. Without water, the corrosion process stops. In the U of RI study, all 500 ml of water was consumed in at least one of the cylinders.
So... I would make an educated guess that in the case of the hypoxia fatality mentioned above, most of that rust
was created since the last fill. The victim had used the cylinder 3 months prior to the accident without incident, so we make the assumption that the amount of oxygen in the cylinder was sufficient to support life. Then he put the cylinder aside for 3 months, with only 300 PSI remaining. There had to have been a lot of water in the cylinder at that time for corrosion to lower the oxygen content to only 2% to 3%. That's my thought, anyway.
It seems to me that if 3 months were the universal truth, we would have a bunch of people dying from hypoxia. Yet, in all of recorded history, we have just one documented case (that we know of) or, at least, there are no CPSA warning labels required on every steel tank. In fact, other than this thread and the previous link, I have never even heard of a problem with steel tanks as long as they are not abused.
Maybe we do have a lot of people dying from hypoxia. Annual inspections are much more universal now than they were in the 1970s and annual inspections catch a lot of small problems before they become BIG problems. In fact, both the Battelle and University of Rhode Islands studies recommended annual inspections to eliminate many of the problems that they were finding in their studies!
Furthermore, take a look at the link above: Compressed breathing air - the potential for evil from within. The authors sugggest that death from "bad air" is more common than we think. The cause of death is usually "drowning" but the true cause of death ("bad air") may be going unrecognized.
You would think that if rust was forming at such a rate, tanks would fail hydro in high percentage.
Steel cylinder do fail inspections at much higher rates than aluminum. Do a search on this board and you'll see all the problems with steel cylinders. But again, annual inspections catch problems early so the steel cylidners can be tumbled instead of condemned....
I can look into the PSI tank inspection program and begin monitoring my own.
I am a cylinder inspector, too, and that is one of the reasons I started looking back at all of the original data.
Good luck! Hopes this makes you think more about your cylinders!
Safe diving!
