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Mixing of two methods for the drop checker CO2 measurement

Discussion in 'CO2 Enrichment' started by Tom Barr, Dec 9, 2006.

  1. Tom Barr

    Tom Barr Founder
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    In effort to increase response time for the pH probe method using a KH reference solution, I realized that I needed to forgo the air gap common in drop checkers. Ininitally I adapted the air gap similar to that of a drop checker CO2 test.

    The velocity at which the flux of gas transfers across the water to air and then from air to water is compounded and rather slow. Perhaps 2 hours to equilibrate or so based on several folks measurements(use 2 of them, one in the tank as reference for the tank's CO2 ppm, then add another and time the color change till both the units are equally resolved). Test Ref tank (t) to equilibrate equally to tank ref = response time. You place the ref test inside the tank 1 day prior and assume the tank's CO2 ppm is stable(caveat: it's not stable at longer response times, but this is close enough. But.....this is suitable and avoids such assumptions if you use the pH probe method and where the response times are reduced with the membrane visual checker).
    So with the membrane method, there's a better way to measure the response time's accuracy as well.

    Note, this same test method works well for the below version I recently tried out and also for testing pH probe set ups(but you need two pH probes/meters).

    I have been suggesting a membrane approach to transfer, this involved no phase intereface changes which slow things down a good deal and prevents large gas bubbles from causing some potential issues.

    Seconds versus hours.
    Now apply this same idea conceptually to the drop checker method suggested by Vaughn.

    A thin sealed box with the membrane snapped in with a notch and a lip with an O ring. You need enough KH ref solution inside to get a decent color resolve and placing a white backing on the container would certainly help.

    Now you have only to the membrane itself to cross, and no phase shift changes.

    This idea is a lot like the SeaChem (Greg Morin, are you listening??) Ammonia Alert visual test "hang in the tank" idea and could certainly be commercialized and adapted.

    I think such a unit would only cost perhaps 5-10$ from a vendor.
    Response time estimations should be about 5 mintues or so.

    A bit better than the 2 hours.
    If you want to do DIY,a simple small thin container you can wrap the DO membrane around and keep the KH ref solution seperated from the tank water and place in a semi high current region. Change and clean as needed.


    Regards,
    Tom Barr

    www.BarrReport.com
     
  2. VaughnH

    VaughnH Lifetime Charter Member
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    I have been thinking about this for some time now, and still haven't come up with a DIY design that I like. Is Dupont's Tyvek acceptable as a membrane? I haven't yet tested the little piece I used grand larceny to obtain from Home Depot. What other easily available membranes are there? I'm still thinking about this, and have done several sketches only to dump them as impractical. Having only hand tools is sometimes a handicap for tiny little things like this.
     
  3. Tom Barr

    Tom Barr Founder
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    Several companies have contacted me for the materials.
    I may buy a large sheet and make some DIY.
    I can give you some to try out and detail it for folks.



    Regards,
    Tom Barr
     
  4. Professor Myers

    Professor Myers Guru Class Expert

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    Count me in !

    I'm definitely interested, and it looks like one of those materials that goes alot further than most of us would ever need. For the record I've got one hell of a proto-type shop. All I generally need is designs, and materials, but time definitely enters into things on the holidays. Prof M
     
  5. VaughnH

    VaughnH Lifetime Charter Member
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    Professor,
    I'm glad you mentioned your shop! If you have a Tap Plastics store near you or a similar store where you can get all sizes of acrylic tubes, cements and sheets, you could make the indicator shown below:
    [​IMG]

    If you do it, don't forget to add the pH reagent to the 5dKH water in the cavity! I left that out of my mini-instructions.
     
  6. Professor Myers

    Professor Myers Guru Class Expert

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    Thanks Hoppy !

    That's exactly what I need, but I am having a bit of trouble wrapping myself around the illustration ? The written description makes a bit more sense. O.K. now I get it...so did we basically just construct a clear two piece 1" snuff box with a gas permeable bottom ? One could only hope the membrane just happens to be white. Man that's just as simple as it gets... almost poetic ? The one thing I might do would be to radius the outside edge of the inner ring on the membrane frame to prevent tearing the membrane, and perhaps thread the two sections, but for the simplicity of the design it could be built as a sealed throw away unit. If the membrane is gas permeable does the indicator solution ever evaporate or taint ? Grtz, Prof M
     
  7. VaughnH

    VaughnH Lifetime Charter Member
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    Yes, you figured it out - a snuff box with a membrane cap on it. If the cap threaded onto the box it would be a better device, but with it glued together it is just a throwaway. The indicator solution should never evaporate away, but biofilm would eventually plug up the membrane pores unless it is cleaned regularly with peroxide or bleach. If the unit is left in the light forever I assume the dye in the indicator would fade, otherwise it should last a long time.
     
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