DIY Drop Checker

The reason for setting the KH of the indicator solution to 4 is so that with 30 ppm of CO2 the indicator color will be green, and an unequivocal green. That gives the maximum accuracy. If you had a lower KH, the 30ppm color would be near yellow, almost impossible to judge, and if you had a higher KH, the color would be blue green, again very hard to judge. Actually, any KH that is within .5 of 4.0 will work, but 4 seems to be just about the perfect KH. Since it is easy to get the KH to whatever you want it to be, as accurate as you want it to be, why not shoot for 4.00 KH? (You can even use 10X the usual water sample size, so that each drop of KH solution will equal .1 degrees of KH. I used 4X and could judge it even closer by noting how nearly each drop came to tipping the color over to yellow. (I use AP test kits)

This works because a sealed container of air in contact with water will reach an equillibrium amount of CO2 that is at least proportional to the ppm of CO2 in the water. So, two containers with their water in contact with the same sealed container of air will reach the same ppm of CO2. The "drop checker" has distilled or DI water in it, with a known KH obtained with bicarbonate only - no other source of alkalinity, and no source of acidity. That makes the equation that is behind the pH/KH/CO2 table work correctly (see Chuck Gadd's website). So, if we can measure the pH of that little bit of water in the "drop checker", and we can, by using a pH indicator solution, we can accurately calculate how much CO2 is in that container of water, which will be the same as is in the tank water, since they are connected with a sealed air column. At KH=4, and 30 ppm of CO2, the pH will be 6.6 which turns the bromothymol blue indicator, used in almost all pH kits, green. So, green means you have the right amount of CO2 in the water, and yellow green means you have too much, while blue green means you have too little.

There is no way for the indicator solution to migrate to the tank water, since there is an air gap separating it from the tank water. If it did get into the tank water, it would be about 4-8 drops of a 4% sodium hydroxide in water solution with a little bit of bromothymol blue dye in it. It wouldn't be desirable to have it mixed into the tank, but I doubt it harming anything if it were mixed in.

So, to use this:
First, get some distilled water from the grocery store.
Using clean glass container, pour about a cup of that water into the container.
Next, add a very small amount of bicarbonate of soda - baking soda - Arm and Hammer soda, etc. to the water and stir it up with a very clean stirrer.
Test the KH of that water with your test kit.
Most likely it will be a higher than 4 KH. So, add some more distilled water and repeat the test. (If the first KH comes out to 8 degrees KH, double the amount of water. If it is 6 degrees of KH, add 50% more water, etc.)
When you get close to 4 degrees KH, repeat the test using twice as big a water sample in the vial as the kit calls for, then count each drop of KH reagent as being half the degrees of KH that the kit normally says - for AP test kit, that makes each drop equal to .5 degrees KH. I found I can get very near "exactly" 4 degrees KH by doing this carefully.
Use a syringe to squirt some of this 4 degrees KH water into the bulb of the "drop checker". Fill the bulb about 2/3 full of the water.
Add a few drops of your pH test reagent - use enough to get a strong blue color, but not so much that the water becomes opaque with the blue dye.
Use the suction cup on the "drop checker" to suspend the device two or three inches below the tank water surface, with the "horn" of the "drop checker" pointing down so it traps air in the horn.

After a couple of hours or so the color of the "drop checker" fluid will be at about the equillibrium color, and that should be green if you have 30 ppm of CO2 in the water. If the color is yellow, you have at least 70 ppm of CO2 (and your fish are in serious trouble!). If it is blue, you have about 10 ppm of CO2, which is not nearly enough.

Functionally, they have to all be the same. But, esthetically there is a difference. I think, from looking at the photos, the "type 1" is a more direct copy of the ADA unit, while the "type 2" is produced by a different glass blower, and less effort was put into matching the appearance of the ADA unit. The teardrop shaped one is still another glass blower's interpretation of the idea. It looks very good, but I doubt it having as fast a response to changes in tank water CO2, since it seems to have a much smaller tank water interface (no "horn" to increase the surface area of the tank water being interfaced.) That last one would be much easier to load with water and indicator solution, and might satisfy some people much more for that reason. It is pretty hard to get the water and indicator into the "type 1" verson, and nearly impossible without a syringe with a bent "needle".

What is encouraging about this is that apparently this fellow in Hong Kong is selling these well enough to branch out into other versions. Maybe our discussions involving ppm of CO2 in the water will become more meaningful as more people use these.