I was at a LDS yesterday where the owner is vintage friendly. I had brought in an "oddball" tank I have to see if he had one kicking around he might be willing to sell. He had never seen the particular tank I brought in, but he did advise me not to look too hard because they changed the HYDRO process which has resulted in over 50% of the old steel tanks he sends out failing. Can anyone confirm this? I hate to have to get rid of my 72's.
I've had a few steel tanks fail hydro in recent years, but most have passed.
There is a problem with newer high-pressure galvanized steel cylinders. The layer of zinc bonded to the outside of the steel would keep the cylinder from returning to its original dimension. This led to failures due to permanent expansion.
The hydro facility I use is familiar with the procedure to properly test these cylinders. I'm not completely sure of the process, but it consists of cycling the pressure during the test.
The way he described it to me was they used to bring the pressure up some, then down then up to test pressure. That may be the cycling Charlie is referring to. The procedure they're using now, they just go directly to test pressure, which results in an over stretch, and a failure.
CJ- The way he phrased it was they used to bring the pressure up some lower it, then bring it up to tests pressure. Under the new rules they go direct to test pressure, which causes an "overstretch" and failure.
We are a small shop in a rural area, so don't do enough hydro's to make it profitable to have our own hydrostatic test equipment. I take our's to an Air Liquid operation. I've talked with the operators there about these procedures. All I can tell you is that they know more about it than I do.
We've seen the pictures and heard about the deaths and injuries that have occurred during scuba cylinder filling. When tanks fail, they get made into bells or used for target practice.