CYA - testing in "sunlight" vs. "indoors"

cody21 · Mar 9, 2014

OK, so this issue came up and I'd appreciate any clarification/thoughts on this from you experts. As most know, Leslie's Pools allows a monthly "free" water test, which includes the CYA value. So they don't do the test the same as I would do per the instructions on this site: Outside, back to the Sun. I've tried this myself - testing outside, then indoors. The results are wildly different. For whatever reason, when indoors, the black dot take longer to disappear - which suggests a much lower CYA. (and PS - the guy(s) at Leslie's are very quick with this test and seem to never get the same results that I get ... I know, it's subjective anyway, but just sayin')

So I guess my question is WHAT is the point of testing CYA in the Sunlight (or bright light source) vs. in some other light?

Smykowski · Mar 9, 2014

Brighter light means more solution before the dot is obscured means lower CYA value. In a "darker" environment, the test will read artificially high.

cody21 · Mar 9, 2014

Yea, I had that backwards ... ... Anyway, so I get that there *IS* a difference ... but WHY is one the "right" method vs. the other ...?? Reading the various test kits there seems to be no mention at all about "test outside in the sunlight or bright light source if indoors" .... That info was only here on this site in the Extended Test Directions section ... and given that Leslie's is providing 'free' tests for the inexperienced pool owners, aren't they possibly misleading them with possibly "higher" readings than actual ? Thanks for the explanations.

JasonLion · Mar 9, 2014

The solution gets cloudy, but it isn't really opaque. More light makes it easier to see through the cloudiness to the black dot, so more solution is needed to obscure the dot. The test is calibrated by Taylor to read correctly in indirect sunlight. Taylor picked that kind of lighting because that is a much more consistent light source (from person to person) than any regularly available kind of artificial light.

Don't get me started on Leslie's, as a moderator I would have to remove my post after I wrote it

Isaac-1 · Mar 9, 2014

Our point often is pool store testing is nearly pointless, they have NO INCENTIVE to get it right, most often it is used as a tool to sell chemicals, so as long as the test shows that you need to buy something it accomplishes their goal.

aussieta · Mar 9, 2014

there is a calibrated cya test sample that you can buy
one of the experts here will know where to buy from
with that i believe it is set to 50 ppm CYA
just fill the tube to 50 then find a light source that just blocks the dot

Patrick_B · Mar 10, 2014

JasonLion said:
Don't get me started on Leslie's, as a moderator I would have to remove my post after I wrote it

aussieta said:
there is a calibrated cya test sample that you can buy
one of the experts here will know where to buy from
with that i believe it is set to 50 ppm CYA

Indeed, you can get it from TFTest kits.

BoDarville · Mar 10, 2014

Smykowski said:
Brighter light means more solution before the dot is obscured means lower CYA value. In a "darker" environment, the test will read artificially high.

This is exactly what happened to me right after I joined TFP. Did the CYA test in indirect sunlight (using a TF-100) and then immediately took the same water sample to the closest pool store (about a mile away). My test read 40 ppm CYA and their test read 80 ppm. I know my result was correct since I dosed chlorine according to my reading of 40 ppm CYA and have never had algae or combined chloramines (CC's).