There has been testing to prove that hydroing steel cyl. is not cost effective with the movement and drying of the flasks. So with that being said you should do undestructive testing (udt) and shear wave testing on these flasks every six years. These test for metel thickness (type of flask you have should have a min. wall thickness), stress cracks in the flask and welds. Also you can have these tests done in place. You should also do anual visual inspections of the exterior ( take pics) so you can compare each year. Im not sure what kind of these bottles are but usally have a max alowable pressure like 5000 psi (mawp 5000 psi @ __ F stamped on neck), so you should have a relief valve set @ 4500 psi.(10% under). The relief valve also needs to be able to flow enough air for your set up and have no stop valves between flasks and relief valve. Relief valves need to be tested at least every 3 years. High pressure rubber hose needs to be inspected every year for cracking, cuts, and any signs that its pulling out of the fitting. replace these hose's no matter what every 12 years.

Please dont take my word for this info. Do a research for navy flask inspection, and you will find a ton of info you can implement into your maint. schedual. One good one I found was (tm-cheng 05-010-sca rev b) lots of good info. Good luck to all you dive shops out there to make good air safely....