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Hey guys I stumbled on this on another forum so thought I would copy it over.

*****orignally posted by Glouglou @http://www.plantedtank.net/forums/water-parameters/28119-co2-measurements-dont-add-up-post241450.html?mode=linear******

"Hanns-J Krause CO2 Test
Author Handbook Aquaria Technology
Krause recommends a different method to measure CO2 content. That method is insensitive to the presence of unusual buffers.

Step 1: Take another water sample with some pH indicator and run the hose from an air pump into the sampe for a few minutes. This sets the CO2 level at 0.6ppm.
—>Measure pH of the water (=X).

Step 2: Take small water sample with some pH indicator. Stick a straw into the water and exhale through the straw into the water sample for two or three minutes. This sets the CO2 level at 60ppm.
—>Measure pH of the water (=Y)

Step 3:
The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of the difference between X and Y:
pH,opt = X +.67*(Y-X).

As an example, if you measure pH 8 with the first sample, and pH 6.5 with the second sample, a tank pH of 7 corresponds to 30ppm CO2, regardless of any buffers that might otherwise confuse a CO2 test or distort the charted values.

PH Opti (6.995) = x(6)*(y(8)-X(6)"
 

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There have been a couple of threads recently dealing with alternative methods of measuring CO2 levels. Here's one of them: http://www.aquaticplantcentral.com/forumapc/science-of-fertilizing/14281-another-co2-chart-to-try.html?highlight=co2+measurements. What seems to be becoming the method of choice states that you should drop the pH of your tank (only via CO2 addition!!!) by one pH unit to achieve a 30ppm level.

So basically you measure the pH of your tank water after it has sat out overnight to degas completely. The pH you want to shoot for in your tank is one unit lower than that which you measured. For example, if your pH is 7.8 from the sample you allowed to sit overnight, then you want to have your tank pH to be around 6.8 to achieve 30ppm of CO2.
 

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Sorry to drag up an old thread, but just curious about this since I'm new to pressurized CO2. My degassed ph via recently calibrated and very new ph monitor is 7.0. My KH is 10. I just can't "see" dropping my ph to 6.0. That sounds pretty low to me, especially with a fairly high KH. According to another chart, those equal out to about 30 ppm. My tank tends to run between 2 to 5 ppm phosphates, so I'm at a loss at what the best way for me to figure CO2 measurement would be.

Somebody help me understand, please.......
 

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It works out mathematically. You can solve the equation that relates ppm of CO2 to the PH and KH for KH at a known ppm of CO2, then plug that equation into the original equation in place of KH, and you get the equation that says you want to drop the PH by about one unit in your tank. It seems counter intuitive, but the math says it is correct, and you can verify that it is if you do some careful experimenting. The joker in the process is that we really don't know accurately what the ppm of CO2 is in a tank water sample set out to degas over night. Theory says it is 0.6 ppm. Most people say it is 3.0 ppm. I tested and found it was 4 ppm +/- about 0.3 ppm. One unit of PH drop means the CO2 ppm equals ten times the ppm in the outgassed sample so even this test is not real accurate, just more accurate than the PH/KH test alone. I haven't seen a better way to measure CO2.
 

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My degassed ph via recently calibrated and very new ph monitor is 7.0. My KH is 10.
Something is screwy here, your numbers don't 'add up', so to speak. Typically for a kh of 10, your pH should be around 8. IMO, you need to be sure your kits are giving you accurate readings. Run some standards. My kh is 9.5, my pH out of the tap is 8.0.
 

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My degassed PH is around 7.90. I run my tank at around 6.4 and as you can see from my signature my KH runs a bit higher than yours. It does take a fair amount of CO2 through a reactor to get the PH down to 6.4. Fish are all fine and new additions do ok as long as they are aclimmated slowly when added.

JR
 

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Bert H said:
Something is screwy here, your numbers don't 'add up', so to speak. Typically for a kh of 10, your pH should be around 8. IMO, you need to be sure your kits are giving you accurate readings. Run some standards. My kh is 9.5, my pH out of the tap is 8.0.
That's exactly what I was thinking!! We just received the KH test a couple of weeks ago from an online vendor, and the Milwaukee ph controller is from the same place. Reliable KH test brand, anyone?? I'm not really believing mine much. :)
 

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If you are measuring PH using a probe you have to be careful about electrical interferrence. I have heard that the heater, a powerhead or any other electrical thing in the tank will prevent the probe from accurately measuring PH. KH test kits are almost fool proof, since all you do is count drops and watch for the color of the test tube of water (shaking after every drop) for a change from blue to orange. So, I suggest getting a PH test kit - Aquarium Pharmaceuticals makes the one I use.
 

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hoppycalif said:
If you are measuring PH using a probe you have to be careful about electrical interferrence. I have heard that the heater, a powerhead or any other electrical thing in the tank will prevent the probe from accurately measuring PH. KH test kits are almost fool proof, since all you do is count drops and watch for the color of the test tube of water (shaking after every drop) for a change from blue to orange. So, I suggest getting a PH test kit - Aquarium Pharmaceuticals makes the one I use.
I just happen to have one of those laying around, same brand, too. I just about threw it out when the ph monitor came in, but now I'm glad I didn't.

Thanks! Off to test.....
 
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