I'm now attempting to bring my CYA up to 40 ppm after it dropped to zero (see http://www.troublefreepool.com/cya-f...ppm-t8686.html).
I know the CYA test is difficult and somewhat subjective. After carefully reading the test instructions, I'm still finding the test is very inconsistent. Some days I measure under 40 ppm CYA other days it's over 60.
I've finally determined that the issue is lighting. When it's sunny I can see the black dot through much more solution than when it's less bright (either overcast or before the sun is fully up or in the shade). So, if I measure the CYA at midday on a bright sunny day I read 35-40 ppm. If I measure in the evening or when it's cloudy I measure 60-65 ppm.
The pool stores do the test indoors which would be less ambient light then when it's overcast outside. This would mean their tests would read even higher.
Under what lighting conditions is the tube calibrated?