I can tell you this.
My degassed pH/KH readings indicate a CO2 concentration between 3.0 and 4.0.......like clock work.
With my tank disaster, my KH has been all over the place for over a month. I can test KH and accurately predict degassed pH every time.
And some time back this topic came up and I calculated a bunch of members degassed CO2 levels based on their posted tank spreadsheets........and yep somewhere around 3.0 to 4.0 every time.
Now whether our KH readings are totally accurate is another question.
And in no way is this proof, just an observation. Still would love to see high end test equipment used to delve deeper.
Actually, I’ve been one of us that have also supported the 3-4 ppm equilibrium, which I thought was proven by Henry’s law but, recently, I’ve started to question it based upon some of the stuff that @Edward
is showing, as well as what @cl3537
discussed. I’ve looked around and found that the variables and math do seem to support this, but I also still see compelling explanations to expect 3-4 ppm. Frankly, right now, I’m not sure what the real answer is or how someone might verify (calibrate) the math without reading the CO2 directly, which would be pretty expensive and involved. I’m not talking about the CO2 indirect inference-type tests that hobbyists use and are <$100.
However, although a dc won’t tell us precisely where CO2 is, it should be reliable in terms of proximity, according to what I’ve read and knowing how it works with only the one variable involved. This assumes that we truly know the KH of the indicator solution and this decays, as you know, in a few weeks.
I’m not so sure that I can recommend the 1-point drop approach anymore. It would be nice to be able to have a chart that would show how much a 1-point drop equates to x-amount of CO2 based upon the variables that are supposed to influence it, if the simple 1-point drop from any pH level is no longer (or never was) valid. For example; if you are confident that a 1-point drop from your degassed pH is 30 ppm, do you also believe that a further 1-point drop from your fully gassed level would be another 30 ppm?