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Old 09-11-2015, 01:57 AM   #1
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CO2 Measuring question

If I decide to experiment with CO2 enrichment, what is the safest way?
Most accurate way to measure it? I hear CO2 is very tricky to measure accurately. I want my fish to be safe.
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Old 09-11-2015, 08:05 AM   #2
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The short answer is you can use the drop checker to give you a ball park but personally i push co2 into the yellow with no ill effects. It depends on you goal honestly. Are you looking to see if more co2 will help or the highest your fish can tolerate.


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Old 09-11-2015, 12:19 PM   #3
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My preferred method is just a calibrated pH pen.

Some people use the tank's KH and pH to calculate co2 concentration, but it's not necessary nor an accurate way to measure. I just compare the tank pH to the baseline pH, which I measure by taking out a cup of tank water, and de-gas it before checking pH by running an airstone in the cup for 30 minutes or so. A difference of 1.0 degree means that co2 concentration is close to 30 ppm, and if that's not quite enough, then a 1.2 degree drop will put the co2 concentration at around 48 ppm.

I re-test the baseline pH about once a month or so just to be sure, but it's always the same (7.8)
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Old 09-11-2015, 04:58 PM   #4
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I have a nice pHep 5 pH pen from Hanna Instruments. I keep it calibrated. Is there someplace I can see how many points of pH = how much CO2?

My goal is honestly just to learn. I have never did CO2 injection before. t is one of the few things I haven't done Tom Barr's thoughts on the relationship between CO2 and Algae are very interesting.
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Old 09-11-2015, 05:17 PM   #5
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Here's a diagram I have saved. I've heard that a pH drop of 1.0 equals roughly 30ppm. This would be comparing a completely degassed sample to a sample with CO2 at its peak. Most people just buy a drop checker.

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Old 09-11-2015, 05:35 PM   #6
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Originally Posted by jarrod0987 View Post
I have a nice pHep 5 pH pen from Hanna Instruments. I keep it calibrated. Is there someplace I can see how many points of pH = how much CO2?
As you can see somewhat from the chart posted above, the rough estimates are as follows:

0 degree drop (baseline) = around 3 ppm
Then for each 0.1 degree drop it goes (roughly) 4, 5, 6, 8, 10, 12, 15, 19, 24, 30, 38, 48, 60, 76 ppm, etc.
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Old 09-11-2015, 05:44 PM   #7
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I guess I don't understand. I already use this chart, I thought DKH was down the left and PH was at the top and there is your CO2? Your talking about off gassing the co2 from the sample then comparing the difference of PH only? Maybe I don't understand the chart or your method after all?

OK I think I figured out what your doing. Does KH change the amount that 0.1 drop means or no? looks like it does. So I have a DKH of 8 in my tank. My pH is 8.01 usually. So I need to get it to drop to around 6.9 to be around 30 ppm?

Is that right?
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Old 09-11-2015, 06:20 PM   #8
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I guess I don't understand. I already use this chart, I thought DKH was down the left and PH was at the top and there is your CO2? Your talking about off gassing the co2 from the sample then comparing the difference of PH only? Maybe I don't understand the chart or your method after all?
Yes, you're correct about the chart, however the chart is only accurate if the KH is entirely carbonate, which might not be the case. It should be mostly carbonates, however at higher KH readings especially, there could be a few degrees of non-carbonate KH that will throw off the calculation, likely by as much as 10 ppm or greater. (This is why a reference solution is used for drop checkers.)

By off-gassing a sample of tank water, the co2 concentration will be at equilibrium with the surrounding air, and we can assume that this concentration is roughly 3 ppm of co2. If you look for a 3.0 ppm value anywhere on the chart, and follow the same row to the left to reach a pH drop of 1 full degree, you'll see a value of 30 ppm. I hope this helps.
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Old 09-12-2015, 01:02 AM   #9
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Yes, you're correct about the chart, however the chart is only accurate if the KH is entirely carbonate, which might not be the case. It should be mostly carbonates, however at higher KH readings especially, there could be a few degrees of non-carbonate KH that will throw off the calculation, likely by as much as 10 ppm or greater. (This is why a reference solution is used for drop checkers.)

By off-gassing a sample of tank water, the co2 concentration will be at equilibrium with the surrounding air, and we can assume that this concentration is roughly 3 ppm of co2. If you look for a 3.0 ppm value anywhere on the chart, and follow the same row to the left to reach a pH drop of 1 full degree, you'll see a value of 30 ppm. I hope this helps.

I'm struggling with your method because .1 drop at 8 is no where near .1 drop at 7 etc. So if I drop .2 at a higher KH that means more CO2 then if I drop .2 at a lower KH doesn't it?


OK so ignoring KH lets say I start with a ph of 7.0 and when I off gas I have 7.6. Does that mean I had 9 ppm of CO2?
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Old 09-12-2015, 10:13 AM   #10
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What would be interesting is to find someone who has the co2 test equipment (thats really expensive) and measure the different methods we try to use to guesstimate co2 and see which one is closest.


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Old 09-12-2015, 03:02 PM   #11
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Try sifting through this thread. Tom Barr puts his two cents in trying to explain.
http://www.barrreport.com/forum/barr...o2-ph-kh-table

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Old 09-14-2015, 11:58 AM   #12
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I'm struggling with your method because .1 drop at 8 is no where near .1 drop at 7 etc. So if I drop .2 at a higher KH that means more CO2 then if I drop .2 at a lower KH doesn't it?
No, it's the same.


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OK so ignoring KH lets say I start with a ph of 7.0 and when I off gas I have 7.6. Does that mean I had 9 ppm of CO2?
That would be a drop of 0.6, which means the co2 concentration would be roughly 12 ppm (not 9 ppm).

Here's a more detailed chart:
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Old 09-14-2015, 07:05 PM   #13
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No, it's the same.



That would be a drop of 0.6, which means the co2 concentration would be roughly 12 ppm (not 9 ppm).

Here's a more detailed chart:
I get the drop of .6 part. I don't see where the 12 comes from. The change in CO2 between 7 and 7.6 is not 12 ppm. I am confused
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Old 09-14-2015, 07:09 PM   #14
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I get the drop of .6 part. I don't see where the 12 comes from. The change in CO2 between 7 and 7.6 is not 12 ppm. I am confused
At equilibrium, there is already around 3 ppm. 12 ppm is not the change in co2, it's the total concentration.
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Old 09-15-2015, 01:08 AM   #15
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At equilibrium, there is already around 3 ppm. 12 ppm is not the change in co2, it's the total concentration.
OK, I understand what your doing now, Thank You.
That's a very different number then what the chart gives when using KH.

I always read that CO2 % by volume in air was 0.033%. 3 ppm is by weight right?
Would indoor air be slightly higher then that?
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Old 09-15-2015, 02:24 PM   #16
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OK, I understand what your doing now, Thank You.
That's a very different number then what the chart gives when using KH.

I always read that CO2 % by volume in air was 0.033%. 3 ppm is by weight right?
Would indoor air be slightly higher then that?
I'm not sure. Parts per million is unitless, like percentage (i.e. parts per hundred).

Anyway, 0.033% would equal 330 ppm. Co2 concentration at equilibrium does not mean the concentrations in the air and water are the same (obviously, since water does not have anywhere near 330 ppm of co2 at equilibrium), it is the concentration that is reached due to the exchange of gasses that occurs at the surface. Equilibrium is reached when co2 is outgassing from the water and dissolving into the water at the exact same rate.
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