Surprised by CYA test results

[Ripple]

I purchased a home with a pool 5 years ago. I continued the same pool service the previous owners used. They came weekly and used 3" trichlor tabs in a chlorinator and/or skimmer baskets. I made a decision to take over management of the pool this summer. I purchased a TF-100 test kit, read everything I could find on this website (multiple times) and immediately switched to the TFPC method.

I was expecting very high CYA test results given the fact chlorine tablets have been used every day for the entire 5 year period I have owned the home.

Here were my numbers:

FC: 6.5
CC: 0.5
pH: 7.2
TA: 70
CH: 200
CYA: 40

I am confident in my testing procedure and results. I performed the test roughly half a dozen times and even ordered the standard sampler to have as a reference point. I was able to achieve repeatable results. The results were actually between 30-40 but closer to 40 so I rounded up.

With everything I've read, I don't quite understand why it is so low? I do have significant evaporation and a lot of rain this summer. Would that be responsible for diluting and helping keep the CYA low despite chronic tablet use (prior to me taking over)?

 

[mas985]

Bacteria and chlorine can degrade CYA:

Degradation of Cyanuric Acid (CYA)

This thread explores some of the ways that Cyanuric Acid (CYA) can degrade since many users report seeing CYA drop under various conditions. CYA Degradation by Bacteria Some pool users find that their CYA levels drop over the winter when their pool is let go (i.e. no chlorine added). Many of...

www.troublefreepool.com

 

[Texas Splash]

Ripple, consider yourself lucky. You're right, we would not have been surprised to see you post a very large number. While evaporation-refills don't change CYA, the link Mark posted does explain some of it. If you had some water overflow from heavy rains that could account for some of it as well. The other culprit is sometimes a pool leak, but let's not think of that. If you had a leak, your CH would probably drop as well, so that's probably not even an issue. Good to know your own numbers.