How humid is compressed tank air?

[pescador775]

Thanks, mike. The term is volumetric humidity. Rick is correct, "specific humidity" refers to the relative weights or vapor pressures/pressures of water/air. However, the answers which I gave stand, eg humidity in a scuba tank varies with pressure with the highest moisture levels in high pressure tanks, all else being equal.

 

[Rick Murchison]

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.

Great resource. What it says is that at high pressure, relative humidity (the amount of water vapor that is present/the amount of water vapor that can be held without condensation) increases for any given specific humidity. I think this is the point pescador775 is trying to make. And that if the temperature gets low enough then you can get moisture - and damage - in the tank; that the temperature at which you can get moisture is considerably higher than the dewpoint of -50F specified as the standard for compressed breathing air, as *that* dewpoint is measured at 1 ATA.
As for the humidity of Scuba air at ambient pressures for breathing in the recreational Scuba depth range, it's real dry.
Rick

 

[pescador775]

relative humidity is a function of specific humidity and temperature, not pressure

Sorry, Rick. You are way off. Relative humidity is a function of pressure, or rather volume. Confining a large quantity of dry air in a scuba tank raises the humdity. The only way this can be done is by raising pressure in the tank. Hence, humidity in a tank is a direct function of pressure.

 

[Rick Murchison]

Sorry, Rick. You are way off. Relative humidity is a function of pressure, or rather volume. Confining a large quantity of dry air in a scuba tank raises the humdity. The only way this can be done is by raising pressure in the tank. Hence, humidity in a tank is a direct function of pressure.

You're right; get the pressure high enough and you'll "squeeze" the moisture out. My wording is as sorry as yours was when you cited "specific" humidity initially.
I think we have it straight now.
Rick

 

[redacted]

Specific Humidity

specific humidity is defined as

specific humidity = mass of water vapor/total mass of air

example:

in a given parcel, the mass of water vapor is 1 gm

the total mass of the parcel (N2, O2, AR, H2O, other trace gasses) is 1 kg


I don't see where pressure comes in to play??

 

[Rick Murchison]

Specific Humidity

specific humidity is defined as

specific humidity = mass of water vapor/total mass of air

example:

in a given parcel, the mass of water vapor is 1 gm

the total mass of the parcel (N2, O2, AR, H2O, other trace gasses) is 1 kg


I don't see where pressure comes in to play??

It doesn't. But when considering relative humidity, pressure must also be considered. At atmospheric pressures relative humidity is almost exclusively a function of temperature, and most references don't even mention pressure, but at the high pressures we deal with inside Scuba tanks, the pressure effect becomes significant, as pescador775 has said.
Rick

 

[donacheson]

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.

As a rough approximation, water vapor pressure is independent of the total pressure of the gas (air, nitrox) with which it's mixed, but it depends strongly on temperature. If the compressor has a decent condensate filter after it's last stage and the air has been cooled to say 30C (86F), the vapor pressure is about 32/760 = 0.042 atmosphere. If the cylinder is compressed to 3000 psi (about 200 atm), the water vapor occupies only 0.042/200 = 0.021 percent of the volume and about (18/29)*0.021 = 0.013 percent of the weight of the gas in the cylinder. That air would have a dewpoint of about -40F (or-40C) if expanded to normal atmospheric pressure. For many purposes, that's considered "wet". Dessicants, such as silica gel or molecular sieve are used to reduce it further. Years ago, I worked with compressors and dessicants which could reduce the dewpoint to about -75F.

 

[pescador775]

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.

 

[Notso_Ken]

-50F (not C).

In the area of -40 to -50, there isn't much difference between C and F

 

[Rick Murchison]

If I hadn't made a mistake in terms (I'll never live down "specific humidity"), most of this flap would not have occurred.

Ahhhh, sooo... do we instructors still crack you up??? Inquiring minds, don'tchaknow (sorry... couldn't resist)
Rick