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Dew point of scuba air

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

  1. BRT

    BRT Giant Squid

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    The problem is I don't know what my dew point is. I need to send a sample off. An online calculator shows that a dew point of -85F at atmospheric pressure is a dew point of 11F at 3500psi. I need to wait towards the end of my assumed filter life and do the sample to see where I'm really at.

    ---------- Post added July 5th, 2015 at 01:30 PM ----------

    The moisture eye never changes and with my modifications over the last couple years the indicator strip on the filter never changes from blue. I just don't know what level of water that indicates.
     
  2. ti325v

    ti325v Dive Shop

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    I think it indicates damn dry gas, especially if you are using a 10/20/30...as i recall the filter strips are 40%..

    I gotta admit, my thermodynamics goes to bed long before I do.. some serious goof ups and outright errors on my part, anyway...

    Give Mike Casey a ring at Lawrence factor.
    I know he worked for the military, I think missile guidance systems, high pressure freezing cold gas, super pure....A walking encyclopedia and a great guy to talk with. I have talked and written to him for hours over this stuff.
    x13 is good to -104* but you need the dwell time..I will have a look around for the literature.

    L.F. says -100*f for x13 here..
    http://www.lawrence-factor.com/Library/Filters/XpendableFilters.pdf




    I have been suggesting a moisture test from long ago, you need that info and/or you may find you are pumping at -100*F.
    And who ever is filling your tanks is pumping at -50...now we do get into the realm of the near absurd but factual absorption of moisture by metals...Get a "wet" fill and your tanks may just be holding moisture till your next fill. It does take a while for most metals to give up water trapped in microscopic pores.. (When I open my filters my moisture eye goes white and I have to pump for 8 hours or so before it turns blue again) .....I understand S.S. is best..and they get worse from there..at least as far as metals used in diving..aluminum, brass, whip hoses could takes days or forever to dry down to -100.
    Moisture test..
    It is late and I am not gonna put my foot in my mouth further, but there are reasons....Good luck
    ..oh and why other then a few flash rust spots is this such a concern ? Regs freezing ?
     
    Last edited: Jul 8, 2015
  3. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    I going to really reach for this one. I think the moisture eye changes at 40% rh. Of course that is 40% at 3000 or 4000 what ever you use. When it changes it says your dessicant needs changed. From that point on, you may not have the -50 dew point to allow the rest of the filtering to work properly to prevent toxifying the air.

     
  4. IyaDiver

    IyaDiver Nassau Grouper

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    Hi BRT,

    Over a month old post, but let me assist you.


    No need air test, just buy a real dewpoint meter.
    -65F is easy to achieve even for not a new 100% dry filter catridge, if it is a twin tower of :

    First tower
    677 Lawrence Factor. All 13x
    August Industries - Filter Cartridge for MAKO PD-1503

    2nd Tower
    247 Lawrence Factor. 3 medias
    August Industries - Filter Cartridge for MAKO PD-1803

    -100F is when filter cartridges are very very new.


    P5 is Bauer USA name for big twin tower of the size as seen on photo.
    See -81 Farenheit is easy, even for not new filter cartridges

    System belongs to my friend.

    Note : air flow rate is between 200-400 liters a minute.
    To read this dry, it takes hours of compressor running.
    This is because the dew point sensor is so sensitive to changes when there is no air flow.
    So after overnite shutdown, to get its true reading, at least run compressor for 1 hour or more.
    Photo of reading is when compressor has done 3+ hours running filling tanks for customers.
    This dew point sensor can read extremee minute moisture on pipes, high pressure seat of line valves, hoses and most importantly, I am sure the 13X when not under pressure will also release its water vapor....a tiny tiny bit.


    See the chemical based humidty sensor. Its 20% is still super blue ( dry ).
    At approx -52 to -53 Farenheit, the portion of 20% will start to loose its blue.
    -52F at 163BAR is equal to 21.27% RH at 1 BAR/ATM.


    Sorry, the wide photo became resized by system.
    The text box wrote :
    ======================================

    Dewpoint sensor reads high pressure output of filter towers

    -18.8 Celcius at 163 Bar is equal to -63.1C at 1 ATM/BAR.

    -63.1C is -81.58 Farenheit

    =======================================


    Dive safe....



    ..
     

    Attached Files:

  5. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    BRT

    I dont know how low an air test will measure water content. It does not come in temp when you get the result is comes in a reading like parts per million. You will need a chart to say what parts per xxxx is what temp. I may be wrong, i dont concentrate on these reports very often.
     
  6. nadwidny

    nadwidny Instructor, Scuba

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    Our hygrometers are accurate to 1ppm (-76C or -104F). EDIT-I just checked. They are actually good to .1 ppm (-90C or -130F) but we never see anything in that range. 2 or 3ppm is pretty much as low as we see.

    Depends on the company. Our reports give both ppm and degrees C. Last I looked RPC and Maxaam gives degreesC. I think Trace gives degreesF.
     
  7. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    thanks for the correction.


     
  8. BRT

    BRT Giant Squid

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    Thanks for the info. According to the interweb 2.55 ppm is a -94 dewpoint which would do it for me. Next time I fill tanks I'll have a more sensitive moisture eye in place. The one I had till now NEVER changed from blue. And my indicator strip in my filters had stopped changing from blue since I made changes to drain the water trap on a more intelligent basis. I'm pretty sure that my air is dry enough but I always mistrust my own stuff first.
     
  9. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    BRT

    Ii just looked on ebay for moisture disks that change color. they are for identifying 10-20-30 % RH. desicant is supposed to take things down to 40% at pressure, would it not make sence to put a BP reg on the filter output to keep filter pressure as high as possible to make the driest air? So you get 40% at 3500 psi and not 40% at 3000. I never gave it any thought before today. If so it would have to be on the output of the filter and not the water separator for the lowest %RH. Granted a BP on the separator increases the water drained out of the separator. but if the desicant will only work down to 40% then one would have to have one on the filter output. Am I missing something here? My understanding is that the desicant is the limiting factor because it has a limit on low it will take the humidity. Higher psi in the filter should increase dwell time and make 40% RH a lower over all percentage by volumn.
     
  10. BRT

    BRT Giant Squid

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    Of course, my BP regulator is after the filter and moisture eye. I have considered raising the BP regulator pressure but it appears you run into somewhat diminishing returns and more load on the compressor as well as I'd have to buy an adjustable one. I'm going to see what my 10-20-30 moisture eye tells me first.
     

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