Why CYA drop?

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cfclay

cfclay

Well-known member
Apr 24, 2010
279
Lexington,Ky
Take it easy on me pool Forum gods. I have several posts asking for help. I started with wondering why not just close pool in Arkansas in September- specifically what costs less, operations till water goes 60 degrees or the cleanup of a algae filled pool next spring. You convinced me to leave it open, and with our highs in the 90s we've enjoyed it.

I keep FC at 6 with my CYA at 40, PH 7.5 and TA 100.
This week we haven't had swimmers in everyday- maybe 30 minutes with 2 people, maybe 45 on the days we did. Battled a little algae, brushed the sides. I'm losing 3-4 ppm of chlorine each day in this full sun. No rain of any measurable amount in 2 weeks. Check CYA today during weekly check and it's probably 30- which explains loss?

How do you lose 10 points CYA in a week. No water added. Thanks for the tips.
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Yes, that's correct. Unless you has a significant water exchange, I doubt you actually loss 10 ppm of CYA in just one week. A very small amount of CYA may degrade each month, but not that much. We know the CYA test has about a +/- 15 ppm variance in testing, so it's very possible it's just that - a testing variance. Perhaps something very small effected the two results (outside lighting, positioning of viewing tube, etc). On your next bright, sunny day, I'd check again and perhaps compare your viewing tube reading with someone else to double-check.
 
cfclay said:
Come again? Do you mean the CYA can vary 15 ppm?

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The test can vary by as much as 15ppm. It relies on your eyesight to distinguish a black dot disappearing. It's highly dependent on how good the lighting is and how repeatable the lighting conditions are. I proved this to myself a few weeks back - my pool water has 80ppm CYA in it based on what I have added historically and where I like to keep my CYA level. I tested as I normally do each time - bright, noon-time sun, back to the sun with the comparator block at waist height in my shadow with my fingers not blocking any incoming light. Repeat the test three time and I get 80ppm on the spot. Take the water sample and simply move under the shadow of the awning over my side door. All the same conditions with the comparator exposed to as much light as is available under the awning. New reading -

60ppm (measured three times)

Just by adding a little extra shade, I can throw the test off by as much as 20ppm.
 
In addition to test variability CYA does degrade at a rate of 2-3 ppm per month and up to 5 ppm per month in the middle of summer when the water is warmer.
 
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