CYA test: regular vs diluted

BowserB

Silver Supporter
Jul 29, 2018
648
"Old" Katy, TX
Pool Size
15000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
I've kept CYA at around 40 since our pool was built in 2018. Chlorinated using liquid chlorine, alternating with Trichlor when CYA got low, worked fine. Recently I had a Pentair IC40 installed, so CYA is recommended much higher--70 to 90--and so for the first time ever, I'm actually adding cyanuric acid to the pool. No problemo. I had plenty of old athletic socks. But now the testing. TF100 kit has the test. It's always been a little vague, even though I'm very careful and stand outside in the sun with the vial in my shade (Is the dot still there, or am I imagining it?)

Before, precision was not an issue, with large gaps between 30 and 40 and 50. The gaps in the higher ranges are smaller and smaller, so a small difference can be 60 vs 70 or even 90 vs 100. So I wondered, will the process suggested for >100 work for me even at lower CYA levels? Can I use the dilute method and double the result still valid? I tried and got unexpected results. Standard CYA test seems to be 70. When I ran the diluted test, it showed about halfway between 40 and 50, which doubled would be 90? Big difference. At 70, I might add some CYA. At 90, I'm done, and on the verge of overdone!

Has anyone else tried this for any reason other than over 100 CYA? Is this much ado about nothing?
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
29,140
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Diluting the sample doubles the error. You are over thinking the CYA test. Just get it around 70-80. Call it whatever you want. The actual number does not matter. Precision in the CYA test does not matter. Close is good enough.
 
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BowserB

Silver Supporter
Jul 29, 2018
648
"Old" Katy, TX
Pool Size
15000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Diluting the sample doubles the error. You are over thinking the CYA test. Just get it around 70-80. Call it whatever you want. The actual number does not matter. Precision in the CYA test does not matter. Close is good enough.
(y) Thanks. Good point.
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
5,151
NW Ohio
Diluting the sample doubles the error. You are over thinking the CYA test. Just get it around 70-80. Call it whatever you want. The actual number does not matter. Precision in the CYA test does not matter. Close is good enough.
^This right here.^

The CYA test is very accurate, but very imprecise. Unfortunately all the digital tests that supposedly add precision greatly reduce accuracy, and then there's test strips that are imprecisely inaccurate. Since accuracy is the key we go with that. There is a lot of wiggle room with TFPC, if you are worried then treat your CYA as 80. It will be really difficult to get your FC noticeably too high, so don't be afraid to play around with it.

As far as the diluted test... to be perfectly honest I'm not a fan of it under most circumstances. Decreasing accuracy in an imprecise test really starts to cloud the results, so to speak. Even when the CYA level is high I'd rather just see the person start draining water. Odds are they are going to do it in multiple steps anyway so it's perfectly reasonable to do 1 or 2 steps and then run a test to see where they are. Then people start talking about 3x or 4x dilution and the margin of error gets so insane... But anyway, just my opinion on CYA sample dilution as a whole.
 
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