300 bar cylinders

[Duncan Price]

In case it helps, I seem to remember a very useful discourse on the strange way gases behave at very high pressure in an issue of the CDG Newsletter.  It was written by Andy Goddard, an industrial chemist and experienced cave diver.

I remember the same article (also as a chemist and sometime cave diver).  From memory, the salient point was most breathing mixes are "ideal" up to around 230 bar.  The exception to this being pure O2 which shows a positive deviation from ideality which might be an issue for a rebreather diver.  A 300 bar fill of air has actually used up one third of the free volume of gas in the cylinder when the pressure is 180 bar not 200 bar.  This means that the "thirds rule" fails safe (though of course not all the remaining gas in your cylinder may be available).

As one fills to even higher pressures there are diminising returns in squeezing more gas in your cylinders and specialised three stage regulators are required at very high pressures (we are not going to see 400 bar diving cylinders).

Mostly its a problem in principle rather than practice although non-ideality has greater implications in mixing gases which is why it is always a good idea to analyse your mix.