I prefer LP tanks for a number of reasons, but one that is often over looked is PP filling nitrox from a cascade. The lower WP pressure means the O2 pressures required are less, and therefore you can recover more of the O2 in the supply bottle. HP tanks OTOH are handy for transfilling into LP tanks.
Regarding over filling; I do not advocate it, but it's well to remember some basic facts.
As long as you remain below the elastic limit steel will return to it's original shape when the stress is removed. If you impose stresses greater than the elastic limit you enter the "plastic" range and get permanent deformation that does not recover when the stress is removed.
Is a hydro test just pumping up the bottle to a given pressure and looking to see if it split? NO What is measured is how much the tank expands under the test pressure, and how much of that expansion is returned when the pressure is removed. This tells the tester if the tank is still behaving in the elastic region, or if it has, due to damaged or corroded walls etc. thinned to the point where it "yields" and suffers permanent deformation.
If Hydro pressures aren't causing the tank to yield, filling to a pressure above the WP, but below the test pressure is unlikely to either.
Think about a cable on a hoist. Lets say it's rated to 2500 lbs, Proofed at 10,000 and will fail above 20,000 lbs.
What happens if you hoist a 4000 lbs load? Is it less safe than staying under 2500 lbs? Yes of course, but only because you have reduced the margin of safety.
Now if all the above is true why are "Working Pressures" so much less than Hydro Retest Pressures? Safety margin. Scuba tanks can live a very hard life. Corrosion being a key risk, but many other factors can contribute to erode this safety margin.
The margin between the WP and the Retest pressure allows for multiple "mistakes" with a very low chance of failure.
Tobin
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