does CYA normal drift down throughout a swim season?

[Poolhouse]

First off, the CYA test is extremely subjective, so, I doubt your CYA has changed. It is just not a very precise test. Readings of 60, 50, and 45 are all within the limits of the accuracy of the test. I don't think you have a problem.

By the way, thank you for your kind words!

Thanks for the quick reply. I knew the test was subjective but that's a wide variance. So then, according to the Best Guess Guide, I can keep the FC level anywhere from 3 to 10? I guess I'll aim for a FC of 5-6-7 like I have been.

[aylad]

Besides the CYA being a subjective test, you're also going to lose CYA when you lose water (backwashing, splashout, draining rainwater, etc.)

[chem geek]

There is a slow breakdown of CYA from oxidation by chlorine, but for pools it's on the order of around 2-3 ppm per month (in hot spas it's more noticeable at around 5 ppm per month). Most of the loss is probably from the water dilution sources Janet (aylad) mentioned.