simbrooks:
Thing you have to remember about fatigue/crack prop testing is that the rate of filling and emptying also comes into play....all sorts of variables beyond the testing they may have done that makes that claim a little misleading.
Agreed.
What most people don't realize is that the Accepted Practices in Engineering call for using a value of 10e7 to estimate a product's stress Fatigue Limit, and if you look at a typical fatigue cycling curve, they tend to look like this:
http://www.statisticalengineering.com/rfl3.htm
As you can see, 10,000 fills = 10e4, and the stress value there is roughly 100 on this scale, but by the time you move out to the fatigue limit, its less than 60% of this value.
Hmmm... 4000psi * .60 = 2400psi. Now where have we seen that number before?
And this is only half the story. Even if it wasn't illegal to overfill tanks...ask yourself what the long term implications of it are.
You see, as divers, we buy tanks and use them for a number of years. If they don't go bad, we sell them used. The next guy does the same thing. So does the guy after that and so on. The problem is that none of us keep any documentation except for a VIP/Hydro to indicate what the history of the tank is...everyone in the chain simply assumes that the guy who owned the tank before him used it within the DOT limits and that it was properly designed to last forever in the first place.
What upsets this practice are overfills. Now the tanks won't last forever anymore, but have a very much shortened life. And while 10,000 dives might sound a lot to some of us, but across two or three diver's careers, its no longer implausible.
The net result of us buying and selling tanks with no documentation means that when that tank's number finally comes up, its probably not going to be you or me who pays the price, but one of our grandkids who catches the metal in their face.
So what can be done about this?
Well, we can try to claim that we're not responsible, even though the original root cause were our overfills causing excessive fatigue damage. This argument will probably try to take the approach that this is why we have Hydro tests.
But since Hydro's are based on certain assumptions on how the tank was being used (ie, properly!), who knows if the Hydro really has any reliable chance of detecting the impending failiure in time. In other words, this is an lame excuse.
The other alternative is to make sure that it can't possibly ever become a problem. We do this by sucking it up and doing the ethical thing, which is to make sure that the tank never gets used by anyone else down the road. And this is actually very simple to accomplish: when we're done using the tank, we destroy the tank rather than allow it to leave our control by selling it.
While we're at it, it would be smart if we permanently marked the tank with an indicator of "NonStandard Use - Destroy" and keep a dedicated equipment logbook that documents every time the tank has a nonstandard fill, plus every standard fill after the first nonstandard one. Just in case we're not around to instruct people what to do with the tank.
Now this does require extra work and extra expense, but it is a way to
Ethically Do It Right to prevent some future innocent diver from possibly getting hurt or killed as a result of our actions of overfilling a tank today.
-hh
