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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.
is the color will be the same of PH test color??

and the water inside the DROP CHECKER with KH of 4.. i do add 7 drops of PH test.. and do i've to add KH test drops?
 

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The color will be the same as for the pH test kit, since that is what you are doing - measuring the pH of the bit of water in the bulb. It works because that water is "perfect" water for using the pH/KH/CO2 chart, unlike the tank water, which is far from "perfect". Don't add any KH test reagent to the bulb.
OK.. im understand

SO i put 5 ml of water with KH of 4 and 7 drops of PH test.. as written in my PH test kit,

then.. i can't get this glass of drop checker.. it's not available in my country..

and till now.. i didn't find anything mach it.

could you help me?
 

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I did some glass blowing in college, many years ago, but I still recall it being a lot of fun. You could actually make a usable version of this that looks almost like an ADA product, just by starting with that same test tube and heating the closed end until it shrinks a bit, then blow a bubble - not too big - heat the neck and bend it, then anneal the whole thing by heating all of it and letting it slowly cool. You could then heat the open end and use a carbon rod to open it to a larger diameter. Of course you could burn yourself, the garage, the house in the process, but it would still be fun for awhile!
Hmmm.. that's way to dangerous!!!
 

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I dug down deep and bought another 2.5 oz bottle of aloe to try to make a drop checker. These pictures show my progress:

First, the bottle still filled with goo, with the razor cut across the top:


After making another cut to get rid of the excess height:


After using sissors to trim the pieces:


The pieces ready to assemble:


After "welding" the bottle parts together, and attaching the suction cup, plus adding a bead of silicone to make it leak tight: (as it sits here, it is upside down - the opening will be on the bottom in the water.)


Welding isn't easy. I used a knife blade heated with a torch, using the scrap part of the bottle to judge when it was hot enough. Then I gently rubbed the joint with the hot blade to melt it together. Unfortunately, that left a couple of slight gaps which could not be sealed by any further welding. So, I used some aquarium silicone sealant to seal the entire joint.

The suction cup is installed in a hole that is melted into the side using a drill bit heated by the torch. I started small and kept going larger until the suction cup could be forced into the hole. That is a bit tricky too.

Now, you, along with me, are going to have to wait for the silicone to dry to see if this will work. (My faithful assistant, Igor, is circulating now accepting bets!)
Looking forward to see i t working!
 

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OK, it works! It isn't beautiful, but it works and it is dirt cheap. I had to use a bit of silicone sealant to seal the hole where the suction cup mounts, but once that cured for a couple of hours it has been water and air tight. It took about 2 1/2 hours to reach the same green color as my ADA style drop checker. Here they both are, in the tank - still green with GDA for another day.


Your tank glass look's like green!

DO you've an algea bloom?
 

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Those of us who were believing the pH/KH/CO2 chart and thought we had plenty, if not too much, CO2 in the water, can now tell if we really do have the 30-40 ppm we should have. And, most of us will have to crank up the bubble rate to get there - to get the blue to change to green. That is the advantage of this method - it truly is an accurate way to measure the ppm of CO2 in the tank water. The only errors will be our judgement of the color green, and our mistakes in setting the distilled water KH at 4 or 5 dKH. We can reduce the effect of those mistakes by being very careful in how we judge the color and the KH of the distilled water.

When the bulb is yellow, with 4dKH distilled water, that means a pH of about 6.0 to 6.2, or 75 to 120 ppm of CO2! That is too much! I doubt anyone believing that even 75 ppm is tolerable in the tank. Of course if the bulb is only slightly yellowish green, the ppm may still be ok.
I found out the reason.

i've an external filter that built in with the tank hood, and it's type wet dry that came with spray bar..

i stopped the filter for about 4 hours and notice the color of my drop checker is turned to slight green..

with KH of 5 and look's like PH of 6.5 means i've enough CO2,,

even the plant's it's pearling like it never was!!...

then.. now i know the "Drop-Checker" give's the accurate amount of Co2 in my tank..

when testing the water with usually test kit.. test KH and PH and estimate how much CO2 you have.. that's not accurate... DON'T BELIEVE IT!!

Now by MR.Drop-Checker you'll be able to know how much of Co2 you have..

"LIKE YOU NEVER KNOW"
 

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Is that a shot glass? How did you make it? Once you understand how simple this device is, there must be a dozen ways to make one. The absolute simplest I have heard yet is to take a plastic tube, bend it into a "U" shape and cap one end. Of course a suction cup is still needed, but there are lots of options for that too.
what do you mean with a shot glass??

it's just a small cup..
 
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