does CYA normal drift down throughout a swim season?

[Poolhouse]

Hi, first of all I would like to say thank you again to the moderators on this forum. I have been able to maintain clear pool water with no signs of algea by testing the water daily (FC, CC and pH) and following Ben's Best Guess Guidelines. Our pool was installed late last fall so this is my first time running the pool for a full swim season. We opened the pool in March with a CYA of 60. The first of June I noted the FC to drop faster over the day than it had in the past so I checked the CYA. It had dropped to approx 45. I added 1 lb of stabilizer and restested in 5 days to find CYA at around 50. Before I add more stabilizer I wanted to make sure a loss of CYA is "normal" and that it is not due to some sort of problem I should deal with first. I use the recommended test kit and have never had more than 0.5 CC.

I've also noted that I hardly have to use muriatic acid if I aerate the pool with the bubblers for about 2 hours per day!

Thanks in advance!

[Watermom]

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!

[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.