chrpai:Your missing the point. A non certified diver doesn't know what FO2 or MOD is. The contents label is completly meaningless to them. All they know is that big green and yellow sticker means DO NOT TOUCH.
They know what FO2 is. Its taught in the basic OW class. Composition of AIR - 21% Oxygen (ok, ok, 20.9%), 79% Nitrogen, 0.1% "all the rest." Covered in the basic gas laws portion of ALL open water classes.
If they see a label that says "FO2 21%" they know that it is air and can be dove, provided there is no other gas than Nitrogen on the label (some contents labels list nitrogen, some imply it - I don't list FN2 on the label, for example)
Teaching someone this takes 20 seconds. It is the ONLY way they will ever KNOW what is in the tank they grab - if there's a contents label on it (preferrably one they analyzed and wrote on themselves!)
No, they will know that it is a tank that cannot be dove AT ALL, because it lacks a required piece of information - what's in the tank, when it was filled, and who certified that it has in it what is supposed to be there (initials.)That sticker doesn't come off easily. What happens if your contents label accidentally gets pulled off? They will think its a regular old air tank.
The "Nitrox label" tells the potential user of the tank NOTHING, and it leaves entirely exposed the possibility of an air diver grabbing an "air tank" that was MISHANDLED and NOT analyzed.
This is foolhardy beyond words.
A "Nitrox wrapped" tank could have ANY FO2 in it from a HYPOXIC mix to 100% O2! That wrap tells you NOTHING. There is NOTHING wrong with a basic OW diver using a "Nitrox-wrapped" tank IF the FO2 inside the tank is 21%.
NO tank comes off my compressor without having a contents label on it. Never. Once the tank has been dove the contents label is REMOVED. That tank is then UNUSABLE until another contents label is affixed. If I have a set of doubles with 2500 psi in them (LP 72s) and come up with 1800psi left, I will typically affix a NEW contents label showing that, as that's a usable amount of gas for a second dive. If, on the other hand, the amount of remaining gas is not reasonably usable on its own, then I remove the label and do not replace it.
The label contains the FO2 of the gas, the MOD, the date and initials of the person who analyzed the gas. It also contains the COLD fill pressure.
If you grab the tank and find that the pressure is significantly (beyond what can be accounted for by ambient temperature change) different, you KNOW the label is bogus (someone screwed up) and the tank is unusable.
If the label is missing, the tank is unusable, despite what else may (or may not) be on it.
This is the RIGHT way to do it and the RIGHT way to teach it.
You hand me a tank of air, you had damn well be certifying that it IS air. If you're wrong, I am the one who dies, so I insist that you prove to me (or that I prove to myself!) that you're not wrong.
The only way you can prove that to my satisfaction is to analyze the contents and stick a label on the tank with your results, initials and date, or for me to do so personally just prior to going in the water.