Confusing CYA readings

[Simmons99]

I am leaning toward believing Ben's kit because I also find I have no chlorine after the pool sits all night. But I have about 5ppm Chlorine while the pump (and SWG) are operating. Sounds like low CYA doesn't it? I should be able to maintain chlorine overnight - right?

Actually if you are losing the chlorine at night - that would not be due to CYA - CYA protects against sunlight - and there is no sunlight at night. Sounds like you have organics in the pool using up chlorine at night.

As far as which to believe - I have the same problem as you - Ben's kit tests <20, and pool store tests at 40. I think it might be user error on my part. Are you testing in a brighter/lighter area than you were before?

[mshumack]

I generally get a water sample early in the morning (before sun up) and take it inside to test. My kitchen lights are pretty bright but I think the instructions say to view the "dot" in outdoor light. I don't know if this is important - and I have not tried to verify whether or not this really matters - although it would be easy enough to do.
Thanks for your reply.