Dew point of scuba air

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Poor man without Tobins welding skills...
Air in 44*C, air out 10*C

Ice water in cooler..4 hours work....
DSCN3278.JPGDSCN3279.JPG
 
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If you want dry air cool it.






Feed this heat exchanger ~- 5F water and your air will be very very dry.


Tobin

Thanks Tobin. I owe you for making me think of cooling the air when you showed a picture of your heat exchanger earlier. I built one to put between the compressor and the moisture separator. Only problem is that if I go much below 30 degrees the condensate freezes, jamming up the works. How do you avoid that?

---------- Post added July 2nd, 2015 at 11:21 PM ----------

PMV pressure will increase the efficiency of the filter you have...If you are testing your air and coming up with high numbers, despite the small pump..you likely need more filtration or more dwell time..if you are rusting from your own fills, AND you are testing high for moisture assuming all other stuff is okay...you are pumping too fast for the filtration you have.
Most hyper filters designed for moderate flow...6-12 cfm, then the prices are just nuts.
I have not seen the Howell labs documents, despite a few errors and omissions..the 'Compressor filter lifetime calculator from scubaenginner.com will clearly show you you the effect of increased filter and separator pressure...
https://www.dropbox.com/s/xq6u07wwsz12sgt/compressor_filter_lifetime_calculator.xls?dl=0
Plug in the numbers. it has proven EXTREMELY accurate for me as I have played with all of the variables over the last 5 years and measurement bears out the theory. There is a place for PMV pressure in the calculations.

The only way to be sure it is not you, is test your air..a few little flash type rust spots are inconsequential. Most dive shops fail air tests on moisture and you are diving cold...likely it is their shop air that is getting you..if so, a small portable hyper filer between their whip and your tank will stop this problem. I DO NOT mean that stupid coltri "Personal filter".

Thanks, I'll sign up for Dropbox and look at this tomorrow. Probably I need to send some air away for testing. I have not done that. These are small rust spots. Certainly not much more than surface discoloration, but I hate to see them appearing.
 
Thanks Tobin. I owe you for making me think of cooling the air when you showed a picture of your heat exchanger earlier. I built one to put between the compressor and the moisture separator. Only problem is that if I go much below 30 degrees the condensate freezes, jamming up the works. How do you avoid that?

It's never been a problem for me.

Maybe flow and pressure? My pump is 18 cfm @ 5000 psi.

The real key is getting the air to temps below ambient ahead of the coalescing separator. I'd just run it a bit above freezing.

Tobin
 
just click drop box link...will open a browser window with the file..

If you are chilling your air, have enough filtration, enough dwell time, likely you have OCA as I do, with out a hyper filter. If all else is good, the moisture is coming form those foreign fills.

If any of these shops is wet filling..you are gonna get some water in your tanks...
Screw a wet whip onto a wet valve, pump a couple of drops of water in..drops can condense leaving rust spots in minutes....mystery solved...possibly !
 
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just click drop box link...will open a browser window with the file..

If you are chilling your air, have enough filtration, enough dwell time, likely you have OCA as I do, with out a hyper filter. If all else is good, the moisture is coming form those foreign fills.

If any of these shops is wet filling..you are gonna get some water in your tanks...and when the tank pressure is low..relative humidity is high.
Screw a wet whip onto a wet valve, pump a couple of drops of water in..low pressure...drops can condense leaving rust spots in minutes..pressure builds, r.h goes down, condense water goes into solution of air...mystery solved...possibly !

Don't you mean when tank pressure is high, relative humidity is high? As I understand it relative humidity is the percent of the air's capacity to hold water as vapor without it condensing into liquid. Low pressures tend to boil water into vapor and high pressures tend to condense vapor into liquid. High RH means the air is closer to dropping out liquid water.
 
No it is not wet fills doing it. I have tried that on a few tanks and it works great but I hooked up the tanks bone dry and lowered them into a water filled pipe with the valve out of the water. The "shop" fills I have bought almost all came from one shop and they fill dry.

---------- Post added July 3rd, 2015 at 12:56 PM ----------

PMV pressure will increase the efficiency of the filter you have...If you are testing your air and coming up with high numbers, despite the small pump..you likely need more filtration or more dwell time..if you are rusting from your own fills, AND you are testing high for moisture assuming all other stuff is okay...you are pumping too fast for the filtration you have.
Most hyper filters designed for moderate flow...6-12 cfm, then the prices are just nuts.
I have not seen the Howell labs documents, despite a few errors and omissions..the 'Compressor filter lifetime calculator from scubaenginner.com will clearly show you you the effect of increased filter and separator pressure...
https://www.dropbox.com/s/xq6u07wwsz12sgt/compressor_filter_lifetime_calculator.xls?dl=0
Plug in the numbers. it has proven EXTREMELY accurate for me as I have played with all of the variables over the last 5 years and measurement bears out the theory. There is a place for PMV pressure in the calculations.

The only way to be sure it is not you, is test your air..a few little flash type rust spots are inconsequential. Most dive shops fail air tests on moisture and you are diving cold...likely it is their shop air that is getting you..if so, a small portable hyper filer between their whip and your tank will stop this problem. I DO NOT mean that stupid coltri "Personal filter".

It appears this calculator is screwy. I dissected a filter to weigh the ingredients and entered the data. In the calculator the fill pressure doesn't affect hours of filter life. That is only changed by the Pressure Maintaining Valve setpoint and the ambient air temperature and the temperature rise on the filter. So I get 1/2 hour of filter life with no PMV and 128 hours if I set my PMV at 240 bar, my fill pressure. It is impossible to fill to 240 bar and maintain pressure over the filter at 1 bar. So if I tell it my PMV is set at 1900 psi/130 bar I get no credit for the time above that. I also think there is a chance the 13x would last longer than 1/2 hour into filling the first tank without a PMV.

In addition I don't see a way to adjust for ambient humidity. There almost has to be some gain from our dry desert air at 10-20% relative over 80-90% Mexico air unless the calculator is assuming the moisture trap gets either one to the same level???
 
I most likely don't have it any more. I had it when I was looking for drying agent for repackable filters. Its very likely you might have to use multiple towers to get there. The meat of the chemical is how much it will absorb. Pumping at 3k is not the same as pumping at 5k. 5K will yield lower dew points. It made no difference to me as I bought x13 because of the spec of how much it would absorb, which was greater than the other drying chemicals. If x13 will absorb to 30% rh at 5k then at 1atm one is looking at,,, slightly less than .1%rh at 1atm. Now put that in your tank at 220 atm and you have about 20% rh in your tank. 20% at 70f is a dew point around 25f.

Where did you get that info? I'd sure like to see some solid data!
 
I most likely don't have it any more. I had it when I was looking for drying agent for repackable filters. Its very likely you might have to use multiple towers to get there. The meat of the chemical is how much it will absorb. Pumping at 3k is not the same as pumping at 5k. 5K will yield lower dew points. It made no difference to me as I bought x13 because of the spec of how much it would absorb, which was greater than the other drying chemicals. If x13 will absorb to 30% rh at 5k then at 1atm one is looking at,,, slightly less than .1%rh at 1atm. Now put that in your tank at 220 atm and you have about 20% rh in your tank. 20% at 70f is a dew point around 25f.

Assuming your numbers are correct, and they probably are, I start to see the problem. The inside of the tanks will frost at 25 degrees F. The only question at that point is if frost will cause rust in a high O2 environment. My guess is yes, which gives no solution to the problem except to not worry about little rust spots. In reality the water will not freeze until somewhere below the normal freezing point of water while under pressure. I don't know how much lower.

Thanks
 

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