Firstly, to take the safety issue out of the way, I will not be using this SPG on dive trips, it is just a fun little experiment. On the technicla side, you're theoretically right. But little chemistry that I remember, the few molecules of acid in a drop of 6% dilution of acetic acid will react with way fewer molecules of copper. So yes, there will be a variation of thickness, but i bet it will be in the .01% range. Look, I have 3 different spg-s. On a full tank one shows 200bar, one 210 and one slighlly under 200. When they go to zero, the needles hover around the zero mark or rest on the stop pin, so what are we talking about
) ?