O/T but an interesting corollary subject.
Any idea if this is still relevent?
Is the probe available for sale? Have not seen any updates on this for a while.
barrreport.com
Improved PH probe to be gas specific..
I just had a discussion not too long with Tom about this. He sent some additional instructions and pics to me and suggested I build one. While it sounded interesting, after I looked it all over I decided it wasn't worth the effort, ast least for me.
So absent that, we are left with the next best thing, which is monitoring/controlling pH drop. And you are correct, with that method we are not measuring CO2. You will never know the abosolute value of CO2 without having a CO2 meter. But we can still do a pretty good job by control/monitoring relative values.
As to the dKH/pH chart, it would indicate I have about 100 ppm CO2. Is that true? I highly doubt it. That chart is very flawed and is of little use.
Same for drop checkers. In my experience, green means not enough CO2. With fully optimized CO2 it's more like pure yellow. While it is a secondary visual indicator, for me it's not precise enough as a primary measure.
Another advantage of pH controllers that is under discussed is that they adapt to your situation. If plant mass gets thick, there is more uptake of CO2, and the controller supplies it. If there a large reduction in mass, there is less uptake of CO2, and the controller adjusts. Same for changes in surface agitation. A controller keeps things stable, which is a good thing.
A controller also allows for fine tuning of pH drop. I often change my setting by 0.05 increments. It's very hard to do that with a needle valve. They also act as a fail safe for fish, which gives me peace of mind as I have a tank full of expensive hard to replace Rainbowfish.
So if you get the impression I am pro controller, that is correct. I personally would not be without one.
.... but I may want it still 
