Thank you for this. Good to know about the CYA reading.Not worth bothering to interpolate between the graduation marks. The scale is logarithmic, not straight forward to read in-between the lines. If the dot disappears between 40 and 50, just round up to 50 and call it a day.
One more thing to consider, is to make sure to compare pH-readings at the same points within the chlorination cycle. When we say that bleach is pH-neutral, that means that the chlorination cycle is pH-neutral. When you add bleach, pH will rise. Once FC is back to where it was before adding bleach, that pH-rise will be compensated, as the "use" of chlorine (by killing germs, oxidizing stuff and by UV-decay) is an acidic process. Without the pH-drift by CO2-outgassing, pH would be back where it started.
That means that if you allow large FC-swings by letting FC come down to below min (with CYA 50, your min FC is 4, but you seem to let it come down to 1.5) and then adding a big splash of bleach, you will also have a considerable pH-swing withing your chlorination cycle (not a drift, but a cycle). If you start at pH 7.6 and add 81oz of 12.5% bleach (from your PoolMath log), pH will be nearly up at 8 (assuming TA 60). Starting at pH 7.9, these 81oz would get you up to 8.3. That is not due to the CO2-outgassing drift. But you might misinterpret this as part of the drift when comparing pH-readings taken at different points within the chlorination cycle.
Try not to let your FC go below min (which you should anyway as you are at risk of an algae bloom by allowing FC drop below min). Add smaller quantities of bleach more often, and always compare pH-readings from the same point in the chlorination cycle when estimating the pH-drift.
The effect on pH directly after adding chlorine gets sometimes forgotten, and that the statement of bleach being pH-neutral has to be seen over the complete chlorination cycle. Here a quick refresher from Chem Geek:
I think I follow you about the chlorination cycle. Most of my pH readings have been when the chlorine is also reading lower, prior to adding bleach. So I thought I was getting readings at the end of the cycle???
So you are saying that the lowest my chlorine should read is 4 and I should add more when it hits that? Should I shoot for a lot higher? I was thinking 4 was my goal and that I should try and maintain that on average but that as the chlorine is used it would drop below that and that’s when I should add more. I’ve been adding every couple of days. It seems to maintain at 4 or more for a couple of days and then overnight it seems to drop to 1.5-2. And at that point I add chlorine (and acid because the pH is still high).
again thanks for your reply. Still learning!