How humid is compressed tank air?

[pescador775]

More about "Grade E" criteria. The allowable "volatiles" is about the same as water, 25 ppm. Let's pick a typical volatile, toluene, and see how the CGA standard plays for purposes of filling an 100 cf SCUBA tank. Now, instead of two drops equivalent of water vapor we are considering the effect of about two drops (50 mg) of toluene placed inside a tank. Two drops of toluene is about the same amount which is dissolved in a teaspoon of wetsuit glue. Two drops placed in a wine glass should give you a pretty good buzz. I don't know what it would do at depth but the combined effect of high CO and toluene vapors would probably send the diver into a tailspin, and all perfectly legal under CGA Grade E standard. I'm not privy to the meetings and consults which lead to this standard or the rationale of the diving community in accepting it. At best, it is harmless to our community as most compressors probably do better, but not always. I've read about some incidents where compressors produced bad air and the problem was usually due to over heating or poor location. However, I don't think that the low end or worst case performance of compressors should be used as a guide to writing standards. I'm not sure what Grade E was originally intended for. I'll take a guess and submit that it is similar in concept to some OSHA standards, that is for low pressure compressors supplying air to men who work in sess pits, chemical storage tanks and the like. Somewhere along the line, people who work with high pressure air (firemen?) got into the act and influenced the CGA to make a change to the water criterion, and that is now down where it should be. However, we need a "divers' standard" for compressed air and not piggy back off of this CGA thing, whatever it is.

 

[rjack321]

I agree that CGA-E is inadequate, particularly for volatiles.

For instance, benzene is a possible volatile contaminant of breathing gas. Its a known carcinogen and its 8 hour workplace exposure limit is 0.1 ppm, which equals 0.325 mg/m3 - well within CGA-E limits. You'd be looped out of your gourd breathing this at 4 or 5 ATA. If you survived that, you'd get cancer - swell.

 

[donacheson]

Don,Where does 18/29 come from? I think I get the 18, the gram molecular wt of water. Is the 29 a fudge factor? TIA

Don, water vapor pressure is not "independent" of the total pressure. When the total pressure of a gas is increased so are the partial pressures, one of which, the partial pressure of water vapor, is included.

Sorry for the nit picking I'm still doing damage control. If I hadn't made a mistake in terms (I'll never live down "specific humidity"), most of this flap would not have occurred.

The 29 comes from the (approximate) molecular weight of air - 79% N2 (mw=28) and 21% O2 (mw=32): 0.21x32 + 0.79x28 ~ 29

When water vapor is a constituent of a gas, its partial pressure will increase until the vapor is saturated, then it will condense as a liquid. Removing this liquid is the function of interstage separators, since piston compressors don't like to try to compress liquids.

 

[pescador775]

Got it, thanks. Somewhere along the line, I got it into my head that 22 was the weight. Actually, that's only the nitrogen component. It doesn't have any effect on my calcs as far as I can tell. In a couple of the original calcs, when converting V/V to W/W I rounded down to be conservative so the result should still be close. What's a half drop of water, anyway? I need to be more careful when I read published lesson plans and course notes. That's where that came from. (G)