CYA Test Accuracy


LifeTime Supporter
May 30, 2008
NE North Carolina
Disclaimer - There is a proper and accepted way to perform CYA testing that will maximize the accuracy for the home user. I am NOT questioning the need to properly test for CYA. But.....

There always seems to be a lot of fretting regarding performing the CYA test. Understanding the ideal way to perform the test is important for consistency, but at the end of the day, does the typical deviation for a home user really matter? As long as you perform the test outside, in natural light, does it actually matter if the dot is completely, 100% obscured, ghosted or not ghosted, etc - a somewhat subjective end point for the test? I am talking being close to the end-point for the test, not off by 40 ppm. What is the accuracy of the CYA turbidity test? I guess what I am asking is - if the test is performed under natural light, can that subjectivity really result in more than 10 ppm or so difference in accuracy. At the levels of CYA we generally recommend for a BBB pool, that really only means a difference of 2 ppm in the target FC for 30 vs 50 ppm CYA (10 high or 10 low from 40 ppm CYA), which in my mind is insignificant as I never let my FC approach the lower limit for sanitization.

I tried searching the internet for the accuracy of the test and came up empty handed.


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
With significant amounts of practice, and proper outdoor lighting, most people can get repeatable results that give the same answer +-5. But the test it's self isn't really that precise. The CYA test is about +-15 when you are doing it correctly. Novices often stop early, when the black dot is kind of mostly obscured, which can give results that are 20 or 30 ppm higher than what they would get if they were doing it correctly. Reading the test indoors seems to result in numbers that are 10 or 20 ppm higher than otherwise.

I think that the main reason people fret about this test in particular is that there is some ambiguity about when the vial is sufficiently full. No one is sure exactly what "black dot disappears" really means at first. And even after you learn that, there is still ambiguity when it vanishes for a moment, and then comes back as you stare at it. All of this becomes obvious over time, but when you are just starting out it doesn't really make sense.

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