My Goldline PC-8 controller with T-Cell 15 works fine, i.e., no error messages, but free chlorine generation appears to cease if I drop the chlorinator percentage from 50 to 45. While at 50, my FC level holds at about 5 solid pink but if I change the controller to 45% the FC disappears in a day. I would like to get to the SWCG sweet spot FC level of 2 or 3. Any ideas?

CYA 70
pH 7.5
CH > 600

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Post a full set of test results and how you got them.
How does your water look?

It sounds like a possible nascent algae bloom that the 50% setting is keeping up with but as soon as you lower it the algae starts out growing it and consuming all the FC.

That's interesting because I have the same setup with similar experience. I was running fine at 60% during the summer, 4-8ppm chlorine at 80+ degrees and lots of usage. Recently when it got colder and pool wasn't used I dropped it to 40% and saw a big (disproportional) chlorine drop. I brought it back up to 50% and all was good again. I sort of assumed what Bama Rambler suggested about a tipping point for algae growth using up the chlorine, but there might be more to it.

You would assume the algorithm the controller uses produces a linear amount of chlorine per the setting, but who knows. The difference between 50% and 45% should be minimal but maybe there is a large jump there. Curious.

My water is almost crystal clear all year long. I say almost because at night with lights there are plainly visible specks in the water which I attibute to calcium. I sweep the pool sides once a week, vacuum the bottom twice a week, use the leaf bagger three times a week, and clean the skimmer baskets daily. I have never seen an algae bloom. The most suspicious I have become is a yellowish dusting on the sides and steps that floats off easily when I sweep so I tend to think it is pollen or dust (we have several, big pine trees in the back yard).

Here are my numbers, which haven't fluctuated much at all for the summer months, and "how I got them"; not sure that matters much as I get repeatable results.

80F < H2O temp < 90F
Sand filter
FC ~ 6 (DPD)
TC ~ 6 (OTO)
CC ~ 0 (simple math)
pH ~ 7.5 (red drops, Taylor or HTH) (managed every second day with muriatic acid)
CH > 600 (green drops, HTH, no magnetic stirrer)
TA ~ 60 (blue drops, HTH)
CYA ~ 70 (disappearing black dot test, Taylor)

MajorDIY:
We are on opposite corners of the continent but it is interesting how we have similar observations. Would you mind trying to drop your percentage to 45 and see what happens? I have dropped mine below 50 about three times this year and I get the same results (not good) each time. I have not done focused testing on percentages above 50 to see if FC gets really high as I crank up the cell time. I guess I can be happy at 50 and just add cheap bleach before parties rather than stressing the T-15.

The best thing to do is turn the swcg off and run an OCLT. That'll let you know if you have something consumming the FC besides the sun.