But a vis isn't going to prevent this. That's the problem. You could get a bad fill on the first pump out of vis. You're relying on your pre-dive check to catch that.
The issue is that a vis is literally ONLY good between the time the inspector spins the valve back on and the first time you pump gas in the bottle. Once it's hooked up to something, all bets are off. A vis isn't going to catch something like that, the diver will when their gas tastes like oil or they get a CO hit at depth, etc., and then they take the tank in for service. The issues that a vis WILL catch preventatively do not occur fast enough to require a 1 year recertification. The rest of the issues you will have with a tank are pretty damn instantaneous and are not preventable with yearly vis, so what's the point?
I get a vis when a tank gets hydro'd, and gets a vis when something happens like I dump gas to 0. No sense doing it any more frequently unless something has happened that could compromise the tank.
Here are some things that reconcile it for me
I cant show you the standards for it but I had it at one time
First vis is good for a max of one year. They are done at yearly intervals if the tank if filled 2 or less times a week. if 4 times a week every 6 months and quarterly if filled more often.
Opinion under the worst conditions of say never using the same fill station, the odds of getting a bad fill is much greater.
Things like pits are not a problem when found but under the worse condition of continuous bad fills at over presure etc there could be a failed tank in the 12- 24 month period.
Vis will tell you the overall health of the total of fill stations you are using. This includes repeated at sea fills form fill stations on dive boats, A little salt water and high pressure would speed up the minor flaws turning into failures on the tank walls. Absolute worse situation a tank with a pit exceeding x amount should be ok for say 2 years so you catch it at the (max one year) time frame. The tank should hold up safely for a year before the effects of bad fills makes the tank a liability for near failure. This is very much like the overfilling issue. You tank can hold 12,000 psi before one tank out of a hundred hundred tanks ruptures,,,,, so you use a 4x safety factor and set working presure to 1/4 that 12k burst psi.
Yes I think it is important to do anual VIS. Certainly i would not vis every 6 months if all i did was fresh water diving 4 times a week. There reaches a point where any further definition of vis criteria becomes useless as it can not be inforced. Imagine going for a fill and the FSO asking when if any time the tank has been in salt water after the last vis, and how many times it has been filled to since the last vis to dertermine if the vis date is still statistically saying it is safe to fill.
I look at a vis like i do a car inspection,,,,, If you have 3/32" of tread the tires are good to go. You leave it to the integrity of the driver to replace it when the wear calls for it. That tread depth could be good for 2-3 years depending on miles driven or it could be an indicator that you need to replace tires in next month after your finish an IE. 5k mile trip. Like tanks it much depends on the tire.
Given those premis's PSI says look for a reason to pass and not to fail. If; you are unsure if it is in a failing condition then you pass it and reevaluate it in future vis' till it is in a clearly failing condition. Tank destruction is at stake.
