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Dew point of scuba air

Discussion in 'Compressors, Boosters and Blending Systems' started by BRT, Jul 2, 2015.

  1. cool_hardware52

    cool_hardware52 Dive Equipment Manufacturer

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    If you want dry air cool it.

    [​IMG]

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


    Tobin
     
  2. ti325v

    ti325v Dive Shop

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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.JPG DSCN3279.JPG
     
    Last edited: Jul 3, 2015
    Suzuki416, kr2y5 and cool_hardware52 like this.
  3. BRT

    BRT Giant Squid

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    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 ----------

    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.
     
  4. cool_hardware52

    cool_hardware52 Dive Equipment Manufacturer

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    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
     
  5. ti325v

    ti325v Dive Shop

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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...
    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 !
     
    Last edited: Jul 3, 2015
  6. BRT

    BRT Giant Squid

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    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.
     
  7. ti325v

    ti325v Dive Shop

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    Jeez..I gotta sleep more...P
    Point is even if your suppliers are pumping dry air, if they are filing wet, they are very likely gonna get water in your tanks...so....if it is not you it is them.
     
  8. BRT

    BRT Giant Squid

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    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 ----------

    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???
     
  9. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    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.

     
  10. BRT

    BRT Giant Squid

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    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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