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The way the DC works is by gaseous diffusion. CO2 will diffuse from a place of high concentration (your tank) to a place of low concentration (the checker). This will continue until the concentrations are equal. If the concentration of CO2 in your tank decreases the process will reverse. This is a fairly slow process so it takes a relatively long time for the DC and tank to equilibrate.
At night your tank will lose some CO2 but it doesn’t drop that fast so you don’t see much of a change in color. (The color change is not that accurate of a measure of the CO2 concentration). You can “check” your checker by taking it out of the tank. It will turn blue rather quickly.
I use one of those “cheap” Red Sea checkers and it works fine. The only thing you have to do that is different from the directions is to use 4 deg. kH water instead of tank water in the bulb.
 

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The way the DC works is by gaseous diffusion. CO2 will diffuse from a place of high concentration (your tank) to a place of low concentration (the checker). This will continue until the concentrations are equal. If the concentration of CO2 in your tank decreases the process will reverse. This is a fairly slow process so it takes a relatively long time for the DC and tank to equilibrate.
At night your tank will lose some CO2 but it doesn't drop that fast so you don't see much of a change in color. (The color change is not that accurate of a measure of the CO2 concentration). You can "check" your checker by taking it out of the tank. It will turn blue rather quickly.
I use one of those "cheap" Red Sea checkers and it works fine. The only thing you have to do that is different from the directions is to use 4 deg. kH water instead of tank water in the bulb.
Exactly, well put.

I should add that the Red Sea checker having in essence a white background for the checker liquid, is a lot easier to see the color than the pretty and more elegant looking glass Cal Aqua checker (double or single bulb). I sometimes use an old white plastic credit card I keep in my under-tank cabinet and put it behind the Cal Aqua glass bulbs to be able to read the colors definitively. The Cal Aqua bulbs being all glass and clear, and having plants, wood and fish in the background makes it harder to see the liquid color sometimes.

The Red Sea checker (this photo shows it empty, but you can see that with liquid it does have a white background to help see the liquid color):

 

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I just got the drop checker today. I need to get/make the liquid. I bookmarked the tom barr report post thingy

Anyway, reading what you folk say about the use of a credit card, gave me a little idea.

I just got mine from Aquaticmagic and it comes with a PH colour strip. My thought was to glue the colour to a piece of white cardboard and get it laminated...

Anyway. this thread has been good.
 

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Anyway, reading what you folk say about the use of a credit card, gave me a little idea.

I just got mine from Aquaticmagic and it comes with a PH colour strip. My thought was to glue the colour to a piece of white cardboard and get it laminated...
Is this so you can match the color to the strip? It's really not needed. Lime-green is lime-green and is pretty easy to see. It's either blue, or a definitie blue-green, which means too little CO2, or a strong yellow, which is too much. My use of the white credit card was just because the completely clear bulb is too hard to see what color is in there. The white card gives the temp background to see the color.
 

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ok, so your color does change...can someone that has a single bulb DC chime in on how their color changes, if at all?
from what i understand, co2 will leave the water pretty quick of you stop the flow. if that is the case then the DC should turn dark blue just like it was originally when first added to the bulb. if the DC doesnt change back to blue then it doesnt really work like i expected and im going to be disappointed.

can anyone explain how it works?

i assume the co2 enters the DC and reacts with the fluid to change its color. what happens to the co2 gas at that time? does it break down into something else? what is the by product? how is it possible for the co2 or the by product to escape the DC? i dont see how it can. if it doesnt escape then how can the liquid change back to blue?

also, the directions dont say where to place the DC in your tank but i did read that you should place it on the opposite side of the tank from the diffuser. that is fine, but how far below the surface of the water should the DC be placed?

thanks.
From what I understand CO2 moves more quickly from air to water than O2. So if you have less CO2 in the water than the air it will go back into the water and be dissipated, replaced by the O2 that is now more prevalent in the water since the CO2 has lessened. Hence the drop checker reverts back to the blue color. The little air bubble reverts as well. When the CO2 rises it traps in the air bubble and the water and changes the solutions color back to green to yellow.

Guess I was a little late in posting.... LOL They were much more succinct anyway! :D
 

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Red Sea CO2 indicator works exactly the same as expensive drop checker from ADA, for accurate reading: instead of aquarium water KH=4dkh standard solution should be used.This solution available on the web.Also Azoo Co2 indicator does the same job.
 
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