The DC is really a primitive way to measure CO2 and only gives you a ball park answer. You can use my method to calibrate your checker; however, if you really want to measure CO2 this is a dynamite way.
You need a good pH meter and a 1 liter seltzer siphon.
I have a pH controller; so, I can monitor the pH of my tank continuously. Here is how it works. I have a densely planted aquarium. I stop adding CO2. With lights on, the pH increases rapidly. After two photo periods the pH levels off around 8.3. This is the “0” CO2 level (at least for the plants).
While your tank is chewing up CO2 charge the 1 liter seltzer bottle with a 4 gram cartridge and put it in the refrigerator. This is your 4,000 ppm stock solution.
Now for each 10 gal of water in your tank carefully add 100 ml of your stock solution and measure the pH. This corresponds to a CO2 concentration of 10.6 ppm.
For each 10 gal of tank carefully add another 100 ml (total of 200 ml) of your stock solution and measure the pH. This corresponds to a CO2 concentration of 21.6 ppm.
For each 10 gal of tank carefully add another 100 ml (total of 300 ml) of your stock solution and measure the pH. This corresponds to a CO2 concentration of 31.8 ppm.
For each 10 gal of tank carefully add another 100 ml (total of 400 ml) of your stock solution and measure the pH. This corresponds to a CO2 concentration of 42.4 ppm.
You now have pH / CO2 calibration curve for your tank.
The only other thing you need to do is measure the alkalinity of you tank by checking the kH. This may change over time and affect your calibration curve; so, check it periodically and adjust to a constant kH ( I recommend kH = 2)
BTW I have a drop checker in my tank just to confirm the pH meter.
