How humid is compressed tank air?

[pescador775]

Yeah, go instructors, heh.

 

[all4scuba05]

don't stop now. I'm writing all this down for my thesis...lol

 

[CAPNVINNY]

and now class to address the difference in drag coeffecient of a string bikini in difference to a 3 mil shorty..aw heck I lost my place...lol

 

[pescador775]

I took a look at the British standard which was kindly referenced by another poster. This is EN 12021. I got tired of doing calcs which required conversion of PPM (v) to PPM (w) and vice versa. Using the metrics is nice because the terms are uniform and allows one to do only a few calcs and grab data from various tables, a lot easier. The outcome is not much different from the previous but since a few things which came to light were not discussed, I decided to offer those up. The absolute humidity permitted by this std is 0.035 grams/meter ^3 at one atmosphere. They call an atmosphere a "bar" which it isn't, really, but close enough. Anyway, multiplying 0.035 grams by 240 bar gives an absolute humidity of 8.4 g/meter ^3 at 3500 psi. This translates to a relative humidity of 50% at 22C and a dew point of 10C (49F). So, to answer the thread starters question about "dry air" in a tank--it ain't necessarily that dry. For, in addition to the humidity from the air compressor, one must account for any atmospheric air left over from a visual inspection. Fortunately, filling and draining a tank turns over the air and subsequent fills should be progressively drier, but not below 8.4 g/meter ^3. As I said, that is a relative concept. Although the air inside a tank can start to condense at only 50F, the actual amount of liquid moisture inside the tank is not large as all the moisture will not turn into liquid, only a small proportion but this condensate increases as temps drop below 50F. In numbers, the total amount of moisture inside this hypothetical tank (let's say a 12 liter steel tank) is about 0.1 gram, mostly vapor except at very low temps (a "drop" weighs 0.025g). Because this moisture is ethereal and disappears as the air pressure drops (now you see it, now you don't), a visual examination might not show visible liquid but possibly some faint powder rust. However, repeated fills over several years could result in more substantial rust, maybe enough to require tumbling of the tank. Hopefully, putting numbers to conventional wisdom repeated oft on Scubaboard will add perspective for those who wondered. The answer to powder rust is to dry the air even more. On "old ironsides", my Bauer Capitano, I added an extra filter which can be packed with the good stuff. I believe this filter came off an old Bristol compressor which would mean it was made by MDI in England. The filters that were made in that era were probably good for about 50,000 cycles. I dont' know how many times that bad boy has been cycled but I digress. So far, no rust in sight and that's what you want. To me, this does look like the typical output of a compressor which lacks a final chemical filter, the way "old ironsides" used to be. That would be the very definition of a "minimum requirement" reflecting the decision of a learned committee.

 

[SparticleBrane]

...maybe we could all just agree that the air is pretty dry...?

 

[Scuba St3ve]

Yea what pescador775 said...:huh:

 

[Charlie99]

...maybe we could all just agree that the air is pretty dry...?

.... until it gets compressed. the point of Pescador's post just before yours is that the very dry air at 1bar ends up having a 10C/50F dewpoint once it is compressed to 240 bar in the tank.

What I take away from this discussion is that it would be very easy for a fill station to be pumping air that would condense out moisture in a tank and cause rust problems.

 

[pescador775]

.... until it gets compressed. the point of Pescador's post just before yours is that the very dry air at 1bar ends up having a 10C/50F dewpoint once it is compressed to 240 bar in the tank.

What I take away from this discussion is that it would be very easy for a fill station to be pumping air that would condense out moisture in a tank and cause rust problems.

Thanks, I like it when someone reads my stuff. I could paste a title like "instructor" or "GUEIANTDANDI" after my name and charge $200/head for a lecture at a luxury hotel, and pack the place. Info offered free on Scubaboard just doesn't seem to cut it.

PS: How does one paste a goofy smiley at the bottom of the post? Mine end up at the header.

 

[rjack321]

50% RH for compressed breathing gas is way too high. To make sure the activated charcoal in the filter is working well you need the compressed gas below 30% RH.

CGA grade E specs are for 24 ppm (v/v), -65F dew point at 1 ATA which is about 20% RH at a typical backpressure reg setting of 1900 psi.

Here's a nice conversion chart someone sent me:
http://www.afcintl.com/pdf/Dew.pdf

 

[Luis H]

Thanks, I like it when someone reads my stuff. I could paste a title like "instructor" or "GUEIANTDANDI" after my name and charge $200/head for a lecture at a luxury hotel, and pack the place. Info offered free on Scubaboard just doesn't seem to cut it.

PS: How does one paste a goofy smiley at the bottom of the post? Mine end up at the header.

This has been a very good thread.

I would also like to thank miketsp and rjack321 for the good references.

I just which I could get a copy of the relevant CGA specs. I know they are not cheap. They are not public property and therefore not published on the web like the Codes of Federal Regulations.

About paying $200 for a lecture...I don’t think so.

The smilies are on the right hand side and at the bottom of the list there is a link for more. I think you clicked at the bottom under Post Icons.