Measuring Cyanuric Acid - can this help?

[CarlD]

I agree with Evan. Something is wrong.
Do you always rinse your test vial, squeeze bottle and squeeze bottle top both before and after you run the test--every time?
Do you shake the pool water/reagent mix for at least 30 seconds before running the test?
Do you keep your reagent in a cool, non-sunny place?

Correct CYA technique involves holding the test vial at waist level, facing the sun, and you do the test in direct sunlight, if you can.

Fill the vial slowly WHILE HOLDING IT AT YOUR WAIST. I like to move the bottom back and forth to see the dot better--movement is easier to see through the cloudy water than non-movement. When I cannot make out the dot I'm done.

I repeat the test by pouring the fluid from the vial back into the squeeze bottle and repeating.

I always do it this way. Therefore my measurements vary from the amount of CYA, rather than my technique.

So even if my technique isn't perfect, the continuity of HOW I do it reveals changes in CYA fairly consistently.

[smallpooldad]

Thank you both and a happy 4th.

Well the mystery continues. Did two test as you perscribed, dry, very clean, and in the sunlight.

The first with a Taylor kit R-0013 the result was 30 ppm. Then with my neighbours Pentair Water kit the result 25 ppm.

Which should I believe.

[waterbear]

Pay very close attention. THE TEST IS NOT THAT ACCURATE. You have essentially the same results. Call it 30 ppm and call it a day! A differenece of 5 or even 10 ppm on a CYA turbidity test is nothing to lose sleep over. I would not even worry too much about a 15 ppm difference. A $1000 LaMotte Waterlink Express (as used in many stores) has an accuracy of +10/-25 ppm for this particular test!

[smallpooldad]

Thank you.

This makes it very clear I will split the difference and call it 27.5. Now I can finally get a good nights sleep.

Aloha.

[waterbear]

People get the precision (what is the test result number?) of a test mixed up with the accuracy (Is the result repeatable?) of a test all the time. Your number of 27.5 might seem precise but it is not necessisarily accurate. Ben spoke of this a bit on his Saturation Index calcualtor page.
http://www.poolsolutions.com/frm/calculator_SI.php
(Ignore the errors at the top of the page and scroll down to the section on Precision vs. Accuracy in Testing. If you enter values into the calculator and hit the submit button the errors will go away, btw.)
No matter how you cut it the CYA turbidity test is NOT a precise test. It is the most subjective of all the tests we commonly do on pools. Best advice is read it to the nearest line. The results you posted above ("Oh and I got it from doing repeated tests with a new Taylor kit eg. below 30, about 32, possibly 35, etc.") are all really 30 ppm. There is no way you could tell if the number is 32 or 35 since the divisions on the view tube are not divided into equal sized sections. Just because the dot disappears halfway between 30 and 40 does not mean the test is 35 ppm. It might be much closer to either 30 or 40 because the cloudiness does not necessarily form in a linear fashion between each graduation on the scale. If you test several times and it's closer to 30 than 40 then call it 30 ppm. It will be an accurate reading. To try and make it any more 'precise' than that is really just deluding yourself and complicating things.

[CarlD]

I agree with Evan (again).

The only reason to say "32" is because you do it the same way every time with the same kit and "32" means the same thing to YOU every time. You actually have NO idea if it's 30 or 40--just that it's between them. You can be no more precise than that, and, since it's a log scale, halfway in the middle does NOT mean "35".

I repeat the test 2 or 3 times just so I know which side of the line I'm on. I guess "32" because it's a lot closer to the 30 line than the 40, or "38" for the same reason it's closer to the 40. But even that is no more than a convention.