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Calculator to make a KH reference for any target CO2 ppm (Drop Checkers)

Discussion in 'CO2 Enrichment' started by Wet, Dec 30, 2008.

  1. Wet

    Wet Lifetime Members
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    Hey fellas. Here's a calc that will let you input any target CO2 ppm and will spit out how to make its KH reference. It assumes you want to use NaHCO3/Baking Soda and just inserts its 30mg/L NaHCO3/dKH into the standard pH formula then dilutes for your container's volume.

    drop checker for target CO2 calculator

    But this is part of a bigger idea: Soon I will be getting my first drop checkers to work out an idea where one can get a better idea of their actual CO2 by using the maximum limit of one drop checker and the minimum limit of the second drop checker. This may not be practical but will at the least be interesting.

    Do you happen to know what the +/- is in terms of CO2 ppm or pH for the green color? Is there a formula?

    Thanks.
     
  2. scottturnbull

    scottturnbull Junior Poster

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    Providing you know the pH range where Bromothymol blue indicator is green coloured, you would use the standard CO2 formula:

    CO2 (ppm) = 3 * kH (degrees) * 10^(7-pH)

    That's 10 to the power of (7 - pH value). So using a known kH, and a known pH range for green, it should be trivial to work out the +/- of CO2 in ppm.

    However, most pH test kits with Bromothymol Blue provide rotten colour comparison charts, so you might end up measuring the error range for the chart.

    My guess is approximately +/- 5ppm. From the formula it should be apparent that tiny fluctuations in pH have a significant impact on the calculation, much more than with kH.
     
  3. Wet

    Wet Lifetime Members
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    Thank you.

    This is exactly what I am asking for :) Your points about the variability in the Bromothymol blue source makes me stop wanting to worry about it, though
     
  4. Tom Barr

    Tom Barr Founder
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    If you could shoot light into the color soultion and then reflect it back into a colorimeter for a reading, then you could measure it very accurately.
    Then all you'd have to deal with is the location of the DC and the time delay.

    YSI's dissoloved CO2 meter sort of does this but uses something different and is faster/less time delay. It's also 8500$.

    The more direct method is using a pH probe in a reference KH solution or a DC without any color pH solution, adding a gasket, then using that. You still have 2 hour time delay at least. But you can accurately measure the pH.

    Which is all the color bromo blue does.........

    If you used a good gas permable memberane that allows CO2 across the memebrane, but not leak out water and KH, then you have a decent method as well if you use a pH meter/probe.

    Regards,
    Tom Barr
     
  5. Wet

    Wet Lifetime Members
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    I'm cheap, Tom :) I hope you do not mind me crossposting the below across a few sites. You and I are currently active on an odd menagerie of forums and I suspect neither of us are going to change ;) :

    Again, I don't have a drop checker yet, so this is a thought experiment/bounce, and I hope you can find to time to bounce back.

    On another forum a superstar made the point that he thinks, at best, he can read the green color of his Drop Checker vs pH charts at +/-.2pH. This sounds about right so let's run with it for a second.

    Let's say I have a drop checker with 3dKH (23ppm CO2 target) and its green. It says I have ~15-35ppm CO2.

    I have a second drop checker with 6dKH (45ppm CO2 target) and it is green. It says I have ~30-70ppm CO2.

    I use both. Now I know I have 30-35ppm CO2.

    (With a 4dKH sample, using the .2pH/color margin of error ballpark, you have between 20 and 50ppm CO2. I am using the 6.6 pH/4 dKH/30ppm CO2 in this post. The calc derives from the formulas we all know and love.)

    I could build this margin of error into the calc, as I think everyone understands all we're trying to do is build a smaller ballpark for little coin, and the +/- .2pH/color thing is another (good?) guesstimate. What do you think?

    Anyone is welcome to the current source code or whatever calc we end up with, of course.
     
  6. VaughnH

    VaughnH Lifetime Charter Member
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    Your idea is a good one, in my opinion. I don't think, though, that most of us can truly say we can read the color that indicates 6.6 pH accurate to +/-.2 pH. It is often very difficult to even tell if the color is blue or green when the drop checker is in the tank. I have been estimating that a regular drop checker, that is "green" means you have 20 to 45 ppm of CO2, but I know that range is optimistic for many of us.

    So, just brain storming, suppose you have two bulbs, as you propose, so that when one is still clearly more blue than green, and the other is clearly more yellow than green, the midpoint between the two gives 30 ppm. What would the expected error be? I will play with that awhile and edit this if I can find a good estimate.

    EDIT: Here is a combined chart of bromothymol blue solution color vs pH:
    [​IMG]

    The problem with this chart, which is a combination of two charts I found on the internet, is that in making the solution, the last step is to adjust the color of the solution, by adding sodium hydroxide, so it is blue, not green. That tells me that various indicator solutions will differ slightly in the pH at which they will be green.

    I did some trial calculations, and the net result is that if you take two numbers each with a +/- X% error, and combine them in any way, by adding, subtracting, dividing, multiplying, the result will have an error of +/-2X, twice as big. So, using two drop checker bulbs makes the potential error twice as big, not smaller.

    The example I tried was with one solution being 2 dKH, and the other being 10 dKH. I was trying for having one not quite green, still yellowish, and the other not quite green, still bluish, both at 30 ppm of CO2. That gave me a much bigger error range than either of them separately.

    I think there is still a way to make this work, but my brain is refusing to concentrate enough to find it yet.
     
  7. VaughnH

    VaughnH Lifetime Charter Member
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    After a good night's sleep:
    It looks like if you make the two drop checkers so one is definitely not yellow and the other is definitely not blue, at 30 ppm of CO2, one will have 2.5 dKH water, and the other 10 dKH water. That means, at 30 ppm, one will be at 6.4 pH and the other at 7 pH. Now, if your eyes aren't that good, and you misjudge the colors, so the one is really at 6.2 pH, the other would be at 6.8 pH, and the real concentration would be about 45 ppm. If you misjudge so the high KH bulb is at 7.2 pH, the other would be at 6.6 pH and the real concentration would be about 20 ppm. So, the drop checker accuracy would be at least as good as 20-45 ppm when reading 30 ppm. And, judging that reading would be a lot easier. Unlike with a single drop checker it should be easy to stay within that accuracy.

    So, I think this idea will work well, but the total accuracy will still mean it will be very, very difficult to reduce the error below the 20-45 ppm range. If your color judgement is really good, and you judge the high KH one as being 7 PH when it is really 7.1 pH, the low KH one would be at 6.5 and the CO2 will be at about 25 ppm. If you judge the low one to be at 6.4 when it is really at 6.3, the high one would be at 6.9 and the CO2 at about 40 ppm. So, with really good eyes, you would still only get to 25-40 ppm when you think you have 30 ppm, but again, this would be a lot easier to achieve than with a single drop checker.

    Ideally this would be a drop checker with two bulbs, one with 2.5 KH water and the other with 10 KH water. Much more work to maintain, but with better accuracy too. The remaining problem is that there is no "standard" pH reagent using bromothymol blue. That final step of adjusting the reagent color to blue remains a potential problem to achieving even these accuracies.
     
  8. Wet

    Wet Lifetime Members
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    Thank you very much, Vaughn. I believe you state such in your morning after post: I do not think we'll be subject to 2x the error since effectively we only use one limit from each drop checker, so minus the variability in drop checker location, etc, our assumptions and error should be the same as one drop checker. (In the theoretical make believe!)

    I need the drop checkers to apply what you're showing me. Distinctly yellow and blue does seem smarter and more realistic and can always build a web calc for it if folks want to try but skip the math, if only to see what more folks think about practical use... Will post then or when new thoughts happen.

    This idea came about when stumbling across Cal Aqua's Double Check units: CAL AQUA LABS Double Drop Checker | Green Leaf Aquariums Unfortunately, I did not realize the reference sample does not equalize gas.
     
  9. Tom Barr

    Tom Barr Founder
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    I have no issue posting other sites or vendors, save just one vendor(PSW).
    As long as they engauge, post on topic stuff, not just post to promote their site, etc.
    I have no policy about web sig pages either.
    But if it's not on topic, that will get moderated harshly.

    Simple rule, I do not have vendor ads here.
    So there's none of that BS.

    Try making a small pH probe reference cell like a drop Cehcker or a membrane based version.

    Regards,
    Tom Barr
     
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