How humid is compressed tank air?

[dlwalke]

I'm assuming it must be pretty dry as you wouldn't want any moisture to get in tank. But it must be more than 0%, right? Also, does anyone know if the humidity of what you exhale (as in clearing your mask) is appreciably affected by the air that you breathe or are your wet lungs going to pretty much determine the humidity irrespective of whatever you are breathing.

 

[padiscubapro]

I'm assuming it must be pretty dry as you wouldn't want any moisture to get in tank. But it must be more than 0%, right? Also, does anyone know if the humidity of what you exhale (as in clearing your mask) is appreciably affected by the air that you breathe or are your wet lungs going to pretty much determine the humidity irrespective of whatever you are breathing.

How dry it is depends on the gas standard your shop is trying to meet... Many gas standards don't specify a moisture content..

The moisture content all ANDI shops have to meet is a dewpoint of -50c.. Shops that deal with icediving also strive for numbers in thsi range otherwise you get reg freeflows..

By the time the gas hits your lungs its at 100% humidity and then heated to body temperature..

 

[oxyhacker]

The CGA grade E (scuba air) and grade D (scba air) standards both specify <67ppm water vapor, which is about .007%, or a dew point of at least -50F (not C).

A decently maintained shop air system will produce air this dry, and if it doesn't, then a probablity exists that the filter stack has become saturated, and it is also letting oil and other contaminants by, bad news indeed.

 

[Rick Murchison]

The KISS answer is that Scuba air is essentially dry, exhaled air is essentially saturated, and the difference is sucked out of your body.
Hence... Hydrate.
Rick
P.S. while we're hashing nits, -50F is -46C... approximately

 

[Scuba St3ve]

Or you could buy this bio-filter and put it on your reg. They say it makes your air more humid. Dont know if it works but its a cool idea.
http://www.apollosportsusa.com/documents/prol-biofilter.pdf

 

[pescador775]

dwalke, as usual, I have not a clue as to the meaning of the question you ask much less the answer. However, I'm pretty sure I'm not alone. See above. A volumetric humidity of 67 ppm is relevant to part of your presumed question and answer ; it would be quite drying to the mucous membranes to inhale and any diver would know this. Of course, the fog contained in your exhalation is easy to observe and answers the physiological part. With regard to a Scuba tank with a pressure of 3500 psi, the volumetric humidity in that tank, which was small at atmospheric press, would now be 16 parts per thousand, quite humid by any measure. The relative humidity would depend on temperature, eg about 60% at 22C. I hope this satisfies your curiosity. If not, please narrow the question a bit.

Edit: Originally, I used "specific humidity" in the wrong context. This sparked a controversy. So, I edited the terminology. In another matter, after recalculating, I changed the temp from 21C to 22C.

 

[Rick Murchison]

A specific humidity of 67 ppm is relevant to part of your presumed question and answer ... with a pressure of 3500 psi, the specific humidity in that tank would be 16 parts per thousand, quite humid by any measure. The relative humidity would depend on on temperature, eg about 60% at 21C.

Negative... specific humidity is mass/mass and independent of pressure. 67ppm at 1 ATM is still 67ppm at 235 ATM. Likewise, relative humidity is a function of specific humidity and temperature, not pressure.
Rick

 

[pescador775]

Rick, are you a dive instructor? You guys crack me up. I assure you the humidity in that tank is as I've stated. However the definition of humidity, I grant you mass/mass is one thing and I am referring to mass of water per unit volume which is different. .

 

[miketsp]

If anyone got lost there is a very nice document here

http://www.hse.gov.uk/research/rrpdf/rr427.pdf

which explains where the limits come from. Dew points, frost points etc.

 

[Rick Murchison]

Rick, are you a dive instructor? You guys crack me up. I assure you the humidity in that tank is as I've stated. However the definition of humidity, I grant you mass/mass is one thing and I am referring to mass of water per unit volume which is different. .

Then you are *not* talking about specific humidity, which has a specific meaning.
I am gratified you find that amusing.
Rick