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There are a lot of potential factors. The pH of most dichlor products is around 6.7. Even if you are using buffered MPS -- if it isn't buffered to the exact pH you are running, it will affect pH/TA over time. As others have mentioned, sweat, oils, even the air injected into your tub can affect pH/TA. Since the TA serves as a buffer, any acid introduced to the tub will have to reduce TA before you start to notice pH changes. While you think your water has been "perfect" for 4 weeks, TA has likely been creeping down slowly, but hasn't reached a value that the sensitivity of your test method can detect, so you don't notice it. Once the TA gets low enough, it stops buffering pH and thats when things appear to change quickly. I'm guessing that a lot of Taylor kit users only look at sanitizer and pH levels on a regular basis, so TA changes go unmeasured until pH starts to move. I know that I only test TA when I'm balancing the water, or when I need to adjust pH and have to decide between carbonate and bicarb.
Btw, I do check both my ph and alk once a week. It's weird they always stay at 7.5 and 100 for about 4 weeks then all of a sudden they both slowly start to creep down together. Once alk gets to about 80 the ph really seems to fall quite rapidly.
You are welcome. I guess the point is really just to know that is part of the natural progression of a fill, so don't sweat it. As far as testing TA every week, I can't help but wonder why? Once you have it balanced, save the reagent until you see something happening to pH. FYI, in my tub a TA of 80 seems to be the equilibrium point. Above that, pH tends to be stable/drift up. Below 80, and pH drifts down.