CO2 probe meter

This is an instructional post to show how to make a DIY CO2 probe. This can be done in order to avoid tannins, water treatment phosphate buffers to prop up the alkalinity during seasonal tap water changes or any other non carbonate alkalinity interferences to the system. the trade off is that it is slower response times, but you can see how slow that is, generally a few minutes and unlike a drop checker, the accuracy is GREATLY improved. It is also relatively cheap to make, maybe 50$ for a few DO screw on membranes and then the cost of the extra pH probe/meter.
So for about 150-180$, for new parts, you can make one DIY.


If you can use a drop checker solution and calibrate a pH meter and probe, you have the hard part down.

Now for the easy part.

Parts list:

A propane torch
A pair of pliers or a small vise
A large female threaded brass pipe nut 1/2" ID threads(3/8")
A replaceable Dissolved Oxygen membrane(you should buy several) with 1/2" female threads
4 degree dKH reference solution (71.44 ppm)
Pippette
pH probe(A flat tip pH probe is best, I choose a commonly used pinpoint probe here)
pH meter






Step 1:
Heat the female thread, this will be used to make the threads on the pH probe. Do not over heat.





Step 2:
Thread the heated female brass thread onto the pH probe tip to make the screw threads. Best to repeat this a few times to slow get the threads worked into the tip of the plastic. reheat a little and then re-thread again. Do not over heat the threads and melt through the plastic housing. There is no rush.
Use a vise or a pair of pliers to twist the heated brass thread female end over the pH probe carefully.

Step 3:
Test to see if the threads work well with the DO membrane cap.





If so, then move on to step 4.

Step 4:

Add 4 degree(or some known reference KH) KH reference solution.
Fill the membrane up so that when you screw the cap on, there's over flow and no air bubbles.
Careful not to bust the membrane off from the pressure when threading the cap onto the probe.



Step 5:
Check for a good seat, leaks and air bubbles, try to remove the air bubbles. You can use a little silicon on the threads if the seal leaks.

Now you have a CO2 reference cell probe.

To convert the pH to CO2, you use the Henderson-Hasselbach equation or the pH/KH/CO2 table. So for a KH of 4 degrees, you now can safely assume the pH is not influenced by any other factors other than KH, since the probe and membrane isolate the CO2 probe from the aquarium's water.

Now for the rest of the story: in order to compare the response time of your DIY CO2 probe versus the pH probe without the DO membrane and the KH reference solution, you can compare them side by side and adjust the CO2, generally when the CO2 1st starts being injected to the aquarium in the morning when the lights come on.

Another way to measure this and to calibrate; use a 1 gallon container and a small powerhead to disperse the CO2 gas over a wide range with a 1 gallon volume of a 4 dKH reference solution. Use the pH probe and then the DIY CO2 probe. Compare the response time and the stability and then plot the response times over the increase/decrease of CO2. Say over 1-2 hours and then say go from 3 ppm to 30ppm and back down again with vigorous current in the container.

 

Uses a pH meter and has a gas membrane. Conversion to ppm is easy, you have a referenced table to say what the ppm's are for any given pH.
The unit is independent of KH and aquarium water.

As long as the unit is kept clean and stored correctly, it should last a 1-2 years.

 

No, you should be able to move it from tank to tank. Response times will be about 1-5 minutes thus far.

Yes.

 

There's a simple way to check the system with a Reference RODI water with Sodium Carbonate for the KH to say 71.44 ppm alkalinity and then check it against a pH probe.
They should match and the response time lag is able to be measured also.

I am NOT selling these, I am showing folks how to DIY

 

I'll do a video how to.

 

Mine is a lot tougher and easier to make and has far less liquid required.
Try the band down at the tip, not the top of the probe.