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Gas Permeable Membrane Drop Checker

Discussion in 'Articles' started by VaughnH, Jan 18, 2007.

  1. VaughnH

    VaughnH Lifetime Charter Member
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    Thanks to underwurlde, I have a small supply of Goretex "vents", which are half inch diameter gas permeable membranes, with an adhesive backing. These should be ideal for making a DIY drop checker. So, yesterday I made one!

    This was a 1/4" long piece of acrylic tube, 1/2" in diameter, glued to a long flat strip of acrylic, to act as a handle, as well as to convert the tube to a cup. Then, I loaded it with 5dKH distilled water having indicator reagent mixed in. Finally, using tweezers, I stuck one of the "vents" on the open end. Then, being a man, I read the directions! It seems that the adhesive on the vents needs to cure for 24 hours before subjecting it to use. So, I laid the assembly on top of my aquarium overnight.

    Guess what? It did exactly what it is designed to do. The water inside passed thru the vent as water vapor, since the concentration of water vapor was higher inside the cup than outside. And, since the concentration of O2, N, etc. in the air was higher outside than inside, it passed into the cup. So, this morning the cup had a drop of very blue water in it, but was mostly just air. Lesson #1 learned: Do not ever allow such a device to stay out of water more than a very few minutes. Lesson #2 will never be learned: always read the directions first.

    So, today I am on to my next attempt - further reports to follow.
     
  2. VaughnH

    VaughnH Lifetime Charter Member
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    Here is where I am now:

    [​IMG]
    [​IMG]

    That is a piece of 1/2" dia x 1/16" wall acrylic tube, with a small hole on the side as a fill port. The hole will be plugged with the small diameter acrylic rod, which will also serve as a handle. The second photo is the tube with two Goretex vents stuck on either side. Now, I have to wait a day for the adhesive to cure.
     
  3. VaughnH

    VaughnH Lifetime Charter Member
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    While waiting for the adhesive on the Goretex vent to cure, I set up my first one with a Cole Parmer O2 membrane, using the O-ring included with the membrane kit to hold the membrane on the cup:
    [​IMG]

    This time I am keeping it in a glass of water to avoid losing all of the water in the cup. As soon as my tank gets up to full CO2 concentration I will try this one out to see what time it takes to reach equilibrium.
     
  4. VaughnH

    VaughnH Lifetime Charter Member
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    Here is the Cole Parmer version in the tank, after 5 minutes:
    [​IMG]

    It obviously requires more than 5 minutes to reach equilibrium.
     
  5. Frolicsome_Flora

    Frolicsome_Flora Guru Class Expert

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    wow I wish my drop checker was as pretty as that glass one!

    cool project Vaughn :)
     
  6. dapellegrini

    dapellegrini Lifetime Charter Member
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    Well, no hope of that membrane working with a pH meter, if it takes more than five minutes to balance... Still a very cool idea for a different kind of drop checker, if you can get it work reliably.

    Love the photos
     
  7. VaughnH

    VaughnH Lifetime Charter Member
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    5 minutes would have been nice. But, it took about 3 hours to reach equilibrium!! Tomorrow I will do some more testing with different diameters and fluid volumes to try to establish the parameters that would give us a short response time. Plus, the Goretex checker will be ready to test, in case it is the Cole Parmer membrane that is slowing down the reaction. I'm having fun with this, in case no one noticed!
     
  8. Tom Barr

    Tom Barr Founder
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    I'd suggest the DO membranes vs the Goretex.
    But it does not hurt either to try.

    Vaughn, if you are smart about this:

    A couple of things:
    Make a test reference tank, maybe 1 liter.

    Make a DI+baking soda ref solution, basically pure water + baking soda and add CO2 to that. Add the CO2 and wait for things to stabilize.

    Next, measure the CO2 via the pH meter and KH ref.
    Record that reading.
    Add the various pH/Drop checker designs you have and reference them against the known pH/KH CO2 solution and record the time it takes to achieve the desired correct ranges.

    This is a simple thing to set up, (easier than a tank) and simple method to verify your results further.

    What you want to do is produce good results that show it works well and within a precise time frame.

    This way you can predict what it does much better.
    You should use a pH meter to check and have a back up method to measure the CO2.

    This addresses that and is something you can do relatively easily.
    Set the 1 liter test solution up and drop the pH probe and drop checkers in there and wait and record.

    If you are even smarter, you'll use several 1 liter bottles and several pH drop checkers per 1 liter. Baking soda, 1 liter containers, DI water: all very cheap/simple.

    Regards,
    Tom Barr
     
  9. VaughnH

    VaughnH Lifetime Charter Member
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    Tom,
    I agree with the method you outlined, but my situation doesn't allow me to do it just that way. I have a single aquarium, with a single CO2 system set up for it. It is in the dining area of my condo, with little room to set up any tables, etc. that I could leave set up for the several days this stuff will take. So, I am kind of forced to do this piecemeal.

    Also, I am an engineer, not a scientist. I look for solutions and designs, and rarely data. The accuracy I use when I do things is rarely ever greater than I need. And, I change directions as soon as my testing says I am on a wrong track. Right now I am trying to figure out the following:
    How well would a membrane work in this application?
    Would it give a significantly better response time than the simple drop checker?
    What factors determine the response time?
    What simple design would allow any of us to DIY this, if it is practical?

    So, I don't really care to know the response times for any configuration with any accuracy beyond whether it is hours or minutes. And, for simplicity I am using the same 5 dKH water I already have, since that's what is in my existing drop checker - the "standard". I already know it's response time is of the order of 2 hours, so I am trying to get something that responds in 10 minutes or so.

    Last night I spent some time with a pencil - great invention! - and figured out the obvious - a cylindrical container of KH standard water responds only as a function of the length of the cylinder, not the diameter. That shot down my plan of attack for today, because I can't reduce that length much below the 1/4" it is now, certainly not enough to reduce the response time by an order of magnitude.

    So, my next idea is a "lens" of KH solution captured between two membranes, and only 1/32 of an inch or so thick. The obvious problem with that will be getting the solution color intense enough to even see when it is that thin.

    I'm also going to try the Goretex membrane just to see how it compares to the Cole Parmer one. It already has two strikes against it because it is neither transparent nor white.
     
  10. Tom Barr

    Tom Barr Founder
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    You do not need an extra aquarium, just a small 1 liter container per test.
    Fill with DI water/baking soda, just like the ref KH solution except 1 liter size.
    Add CO2(DIY can work) and add the drop checker to that.
    Don't tell me you cannot afford or have space for a 1 liter bottle:)

    See comments below.
    Very well with the right membrane
    Yes
    Surface area, diffusion coefficient(See Fick's 1st law), concentration gradient, distance etc.
    Yup, see below.

    See what SeaChem did with their Ammonia/pH alert, they used the entire housing as a membrane. I've asked Greg Morin to consider and look into it as well as selling a KH ref solution to folks which we know will be highly accurate.

    You maybe use a flat box model or tube etc, and look through the other short Depth side(say 5mm) while the L x W dimensions are very large (say 4 cmx4cm).
    It's a narrow sliver, but you have enough room to view.
    Placing the drop checker in high current will also help.

    I think this has a lot to do with why I have little issue with the pH probe method, I'm not bound by viewing color. The probe can be right on top of the membrane at very small distances and measures pH faster.

    You will have a trade off with resolution and color with the sizing of the drop checker. I think the best bet for a faster response time lies with Seachem (or someone else etc) etc making a pH sensitive skin that also diffuses gas.

    That would change the color and allow for a super thin "bag" for CO2/pH measure.
    I have my doubts as to speeding it up that much, but removing one or 2 gas liquid phase barriers should help.

    At least Fick's 1st law would certainly predict it.


    Regards,
    Tom Barr
     
  11. VaughnH

    VaughnH Lifetime Charter Member
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    Here is a test "drop checker" using Goretex "vents", which are gas permeable. It is a 1/4" long piece of 1/2" acrylic tube, with vents glued to both faces, and a hole for filling it, which is plugged with a acrylic rod, acting as a handle.
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  12. VaughnH

    VaughnH Lifetime Charter Member
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    The two latest membrane type drop checkers are in the tank now.
    [​IMG]

    The Cole Parmer one is a double layer of membrane with 5 tiny drops of 5dKH water and indicator solution in between. This was a bear to assemble. I ended up putting the solution on a piece of membrane, then freezing the drop in the freezer, placing it on top of the other membrane already installed, and adding the O-ring to hold it. Then I put the assembly in a glass of water to thaw it, and keep the air out. Tricky!

    EDIT: Results:
    Good! The Cole Parmer membranes with a tiny amount of 5dKH water and indicator reached near equilibrium in less than 30 minutes. Reading the color is a major problem - as the color changes from blue it also gets much fainter and harder to read.

    The Goretex "vents" device reached near equilibrium in 45 minutes! This is astounding considering that the quantity of fluid in it was as much as the single membrane Cole Parmer device I tested yesterday that took about 3 hours to reach equilibrium. This one has twice the membrane area, so it should have taken an hour and a half if the membranes were equivalent.

    My conclusion, for now, is that for a DIY device the Goretex vents are a very promising possibility. They are relatively easy to work with, pretty rugged, react rapidly and have adhesive already on them. Their disadvantage is that they are opaque, rather than transparent, making it more difficult to design a device that is easy to view the color on. If they were white it would help, because a single membrane could be used as a backdrop for viewing the color accurately thru a clear window on the opposite side. But, working with what I have, the next obvious step is to reduce the thickness of the slug of colored water in it, to speed up the reaction still more. (The response time should be proportional to the membrane area and inversely proportional to the volume of solution in it, making response time inversely proportional to thickness of the cylindrical slug of fluid.) I will next try a section of half inch diameter acrylic tubing about 1/16 inch thick, which might give a response time of about 10 minutes, a huge improvement over the regular glass drop checkers with their one hour or more response time. And, this would be a cheap device too, assuming a reasonable cost for the Goretex vents.
     
  13. VaughnH

    VaughnH Lifetime Charter Member
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    Another day of testing. Today I made a single membrane Tyvek device, a two membrane 1/16" thick Goretex device, and tried to make a two membrane Cole Parmer device.

    The Cole Parmer membrane is extremely hard to work with. It is extremely thin, but not delicate, just thin. Just seeing it is difficult. Unless the light reflects just right off of it, it just becomes invisible. Then, I have yet to find a way to cement it. Acrylic cement doesn't work. Super glue (methyl methacrylate) doesn't work. So, its DIY potential is pretty slim, even though you can secure it with an O-ring acting as a rubber band around the circumference.

    Gortex has me puzzled. The 1/16" thick device was about as quick to react to the high CO2 concentration in the tank as I expected. It took about 10 minutes to go from blue to green. But, two anomalies: first, removing the device from the aquarium and placing it in a glass of degassed tap water didn't work nearly as fast going back to blue. It took 1 hour to do so. Then it became apparent that water could pass thru the membrane - just a slight touch with a paper towel drained the solution from between the membranes. Also, the glue holdling the membrane to the acrylic cylinder was barely adequate. It took very little to pull it off. These problems combined with its dark gray color make me wonder if it is worth pursuing it any further for a DIY probe.

    The Tyvek was a surprise. I had previously sneaked a 2 inch square of the stuff out of Home Depot, so I cut a 1 inch diameter circle out of it and used an O-ring to hold it on my one membrane tester. The ruggedness of the Tyvek made that easy. And, the white color of Tyvek makes it easy to see and a good backdrop to see the color of the fluid inside. This single membrane, 1/4" thick cylinder of fluid took 1 hour and 20 minutes to become green. Remember, using this same device, with a Cole Parmer membrane took 3 hours to reach green. I wasn't much surprised at this, because you can blow thru the Tyvek easily, meaning it has either big pores or a lot of them. The Tyvek device took about 1 hour and 45 minutes to go back to blue when placed in a glass of degassed tap water. So, its reaction time is in the neighborhood of 1.5 hours per 1/4" thickness of fluid and single membrane. I would expect a 1/16" thickness of fluid, with a single membrane, to take about 20-30 minutes to reach eqilibrium with the water. And, with two membranes, one on each end of the cylinder, it sould be a 10-15 minute device. I consider that worthy of pursuing as a DIY device. Tyvek is very cheap (even if you don't just swipe it), easily available, and very easy to handle. Whether it can be glued is a question needing an answer, but using O-rings to hold it it place is workable. My next attempt will likely be trying to make an easy to assemble and re-assemble, thin cylinder device, followed by thinking how to make such a device easily usable in the aquarium. Ideas welcomed!
     
  14. Mr G

    Mr G Junior Poster

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    Been following this thread and finding it very interesting.
    Has anybody used one of these ( Aqua Medic Membrane Reactor ) and could they be modified into a CO2 tester ?

    I think the membrane should be ideally suited, but was wondering if the device as a whole could be used rather than just the membrane.

    Al
     
  15. Frolicsome_Flora

    Frolicsome_Flora Guru Class Expert

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    I have one of these and I dont think it would suit. If theres no gas pressure inside the unit, water leaks back through the membrane, its a pretty low quality product. Expensive for what it is, I regret buying mine.
     
  16. Mr G

    Mr G Junior Poster

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    OK thanks ...... never actually seen one up close - guess I'll keep it that way :rolleyes:
     
  17. Frolicsome_Flora

    Frolicsome_Flora Guru Class Expert

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    its all right I guess for what its designed for, but it really struggles if you have anything more than about 15 bubbles/min going through it, and its extremely hit and miss in getting the water flow across it right, if its not bang on, you loose alot of your CO2 due to it leaking out of the membrane in large floaty bubbles. I have a mister now and its so much better, and it was 1/2 the price of it.
     
  18. VaughnH

    VaughnH Lifetime Charter Member
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    I have a workable, DIY, easy to make design now for a membrane version of the drop checker. Tomorrow I will make a couple to do some more testing. Here is the sketch:
    [​IMG][​IMG][​IMG]

    This design will allow you to reduce the thickness of the KH water solution to as thin as you can use and still see color differences, thus making it react rapidly. It also will seal well, so leakage of the solution will not be a problem. And, you can use either Tyvek or Cole Parmer membrane, or just about any other membrane on it. Installing the membrane will be as easy as I can make it be. All of the materials are readily available from a Tap Plastic store and a hardware store (for O-rings), plus a 1/2" diameter probe holder/vacuum cup from the LFS. I feel good about this one!!

    EDIT: Add photo of finished device. I made three of these, so I can do side by side tests of 2 or 3 membranes if I wish. The first test will be a piece of a Priority Mail envelope, which is Tyvek, but thinner than building wrap. Of course we are supposed to only use those envelopes for mailing something by Priority Mail, so I have to strongly recommend that we search out used envelopes for this purpose. Being totally law abiding I would never, ever dream of picking up one of those envelopes at the Post Office for this purpose, since they are free.

    EDIT AGAIN: I ran a quick test with Priority Mail Tyvek. There was a major problem trying to load it with the solution. I ended up having to freeze a big drop of the solution in it, then add the membrane. Otherwise, the tiny amount of fluid would wick out before I could seal it. But, the good news is that this time it reached near equilibrium in 4 minutes! In 10 minutes it was as green as it would get. So, PMT (Priority Mail Tyvek) does work. And, reducing the amount of fluid in the device definitely speeds it up drastically. I wish I could think of a good DIY design that could be much more easily loaded with solution.
     
  19. VaughnH

    VaughnH Lifetime Charter Member
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    If a membrane type drop checker had acid free, neutral pH, blotting paper in the fluid chamber, would it work as well as pure fluid there? Who knows enough about chemistry or whatever specialty would be involved to make a good guess or knowledgeable answer to that? I can see using such blotting paper, white, soaked in 4 dKH fluid and indicator, with a clear membrane over it, as a sensor.
     
  20. Tom Barr

    Tom Barr Founder
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    It should work but the blot paper often has poor resolution.
    I think a white background would help with determining the liquid coloration though.

    10 minutes is getting down there and much better than the 2 hours you posted prior Vaughn! Now you have a simple working unit that is a good DIY project for all.

    Since many folks will never do the DIY and make the KH ref solution, I might suggest talking to Greg Morin at SeaChem and goading him into making these along with a ref solution (say 500mls of 40KH solution).

    Regards,
    Tom Barr
     
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