The pH fluctuation in the chart above shouldn’t be because of plants uptake because the pH goes down when lights are on. Lower pH higher CO2. I know my CO2 pressure is not perfectly stable when room temperature changes, something the CO2 tank pressure regulator does. But still, having +/- 0.05 pH stability 24 hours a day without electronic pH controller is pretty good. Don’t you think? Plant CO2 uptake doesn’t affect it, evaporation does.
Yes, that does seem to be pretty stable. I was under the impression that the drop in pH was temp related, but you explained that and, if room temp of the canister is varying, certainly the pressure is as well. In my case, my canister is in the basement and the ambient temp does not vary. So, is it your opinion that your CO2 level, whatever it is, does not change throughout a 24-hour period?
The probes we use have very high resolution and terrible accuracy, and our KH test kits have poor resolution and poor accuracy. When we combine these two and understand that many properties are affecting their results like unknown acids and carbonates, then we either use questionable formula or hoping for debatable pH drop theory to give us some kind of ppm of CO2? Too many variables can skew the result.
I’m using a Hanna alkalinity colorimeter which, supposedly, has a tolerance of +/- 5%. When I calibrate to a 4 dKH solution (dilluted 40 dKH reference solution, since I’m getting back into dc’s), the colorimeter hit exactly 4 dKH and also calibrates exactly to the Hanna calibration solution they provide. So, I have very high confidence in the dKH level. They say that the only significant possibility for interference is chlorine (not chlorides) and I have no chloramines in my water.
PH could be another matter, as you said. However, I start with RO/DI and only add what we all know to add. How many weak acids the plants and other activity may be producing is an unknown, but with regular w/c’s may not be significant enough to skew pH readings. My meter calibrates quite well, but …who knows. As you and @cl3537
have mentioned, we may simply not be able to know how much of an influence all of the unknown variables may have on some of our test equipment, but can we get within a reasonable tolerance of a CO2 reading? Almost a rhetorical question.
Drop checkers are very slow, have poor resolution but have very good accuracy. They are displaying exclusively what we are interested in, CO2. Sure they cannot be wired to pH controllers but with continuous CO2 injection they work well.
I don’t use 4 dKH reference solution for simple reason my tap is always at 5 dKH. So it is simplicity for me, wash fill set. What is my CO2 target? Well, nobody knows how to measure dissolved CO2 gas or carbonic acid H2CO3 concentrations so what CO2 target … it comes down to plants not growing or fish in a bad mood. No not that. It takes little CO2 to have plants growing well, and it takes a huge amount of CO2 to have plants growing little faster.
What color is your 5 dKH dc showing?