I've looked at the list before (I was seriously looking at opening a hydro facility) many of the certs are private business that only test their own equipment (there is a couple real close to me that make aerospace bottles that have to be tested but wont touch outside stuff), many of those listed are low pressure for fire extiguishers. There is a shop out in Lancaster that bank their tanks and hydro every other week or so, theres a shop out towards Ventura, theres a shop down the 5 south of the 60... cant think of the town, I KNOW for a fact he does not do +(when I asked if he did, the guy went off on a rant asking if I thought he was a feekin math genius or something... steel tanks come back with some surface rust, "if you want me to hook up nitrogen for drying them without rust, itll take me time to hook the effer up and your going to pay for it! A company that is mobile picks up the tanks from most all the diveshops in the Valley, I think everyone uses the same guy. Are you really interested in doing all this research, spending time driving far distances twice just to save $20?