Originally Posted by militantpotato
There are lots of reagentless online free chlorine monitors, though unless I'm missing a consumer grade one they start around $3000.
Would teeing off the line prior to the membrane for regular testing be feasible?
Well, yes, plus the waste line is already exactly that -- that's why I thought this was easy, just put a test tube under the waste and check chlorine - if it's detectable, time to change the carbon blocks.
But the pool tests, at least to my read, are aimed too high to be useful, so I think this is not very viable. By the time you start seeing 0.5ppm +/- I suspect the membrane is toast.