Using a TF100 I have been reading my CYA at around 45 ppm.
Every is going well- I adjust to about 6ppm FC each evening and the next evening FC is down to about 3ppm - then I adjust back to 6ppm.
Anyway I thought I would double check the CYA reading by buying the 50 ppm CYA sample.
Well big surprise - with the sample I would have read it at 70 - 100 ppm - evidently it really means the black dot TOTALLY disappears in direct bright sun. Even using the method of standing with the (bight California) sun behind me the block dot seems to disappear well above 50 ppm with the sample.
Testing my pool water in direct bright sunlight the block dot only totally disappears at close to 20 ppm - (with the sun behind me it is closer to 40 ppm).
If the pool is really 20 ppm then I probably should add some CYA to get to 30 ppm and can set lower chlorine targets. However I would have thought I would lose more FC each day - the pool gets direct sun for most of the day. Other readings are OK PH 7.5 TA 80-90 CH 220.
With the pool water clear and at 85F - air temp at ~100F and family and friends coming over during the holiday weekend I am reluctant to change anything for now. However does it seem possible that with a CYA of 20 ppm I would only lose half the FC each day in direct bright CA sun?
If I do need to get the CYA up I have trichlor tablets I can use.