Grades of air

[JEScholz]

Not to put too fine a point on it, the data given by "Star" and others is outdated. Fill Express is up to date. The E standard for moisture has recently been lowered to 24 ppm. This reflects the need to require compressed air which doesn't cause dangerous rust and corrosion in air bottles. The other "E" numbers are still too lenient for use in divers' SCUBA. For example, the effective fraction of CO at 200 feet (fsw, yeah) would be 10 ppm X 7 ata = 70 ppm. That is enough to initiate an Excedrin headache.

Pesky

esky, I won't say your logic is flawed but it does not make sense to me. I am not sure I am correct on this either. I see how the 70 ppm is derived, but in my mind, the concentration does not change, it remains 7 ppm, regardless of pressure. Another way to put it is that if you move the decimile 4 places, you can express it as .007 %. (.7 pp100,000, .07pp10,000, .007pp 1000, .007pp100 or .007%) Now think of it as any other gas in your mix. The % of gas does not change with pressure. 21% 02 on the surface is still 21% O2 at 10 ATM.

 

[H2Andy]

the partial pressure of a gas, not the percentage of that gas at the surface, is what determines toxicity, etc.

for example, the partial pressure of 100% O2 at the surface is 1.00 and it's a wonderful thing

take the same 100% O2 to 50 feet, and now the partial pressure is 2.51 and WAY above the 1.6 to 1.8 maximum allowed by various agencies

you are now in serious danger of getting hit by oxygen toxicity

or in other words, the more pressure a gas is under, the "more effect" it will have on you ,due to the increased partial pressure

 

[JEScholz]

Thanks Andy, so the issue is not so much the mix, but the compounding effect of the mix at depth.

 

[jhoey]

Also note that standards differ from country to country; in the US, Grade E is sufficient. In Canada, the more stringent Modified Grade E is required.