Diluting CYA Test for MORE accurate result

[goody222]

Looking at the stabilizer scale of the Taylor 2006 test kit, the ppm markers for the 60-100ppm range are very close together. I recently made a stabilizer add to the pool. It seems to only take several drops to go from 90 to 80 and depending on who reads it, I'm getting answers of 90+ down to 75 (yes, I'm very familiar with exactly how to prep and read this test).

I was looking at the ppm markers for the 30-50ppm range which are very far apart making it easier to target your exact value (probably why my neighbors who use the standard advice of 30-50ppm never complain that the test is difficult). So in that 30-50ppm range several drops difference isn't going to change your result much at all. My question is, if I PROPERLY dilute my pool sample with tap water in an EXACTLY 50:50 ratio, can't I perform the CYA test as usual then multiply by 2? Wouldn't that give an OVERALL more accurate result being that I would now be reading in that expanded band? BTW - I'm talking about using this dilution method when stabilizer levels are 60+ppm of course. Thanks!!

 

[mknauss]

Read Step 8 in the CYA test instructions in Pool School. It is what we recommend when CYA tests at 90 or above.

I would not call it more accurate, as the dilution method doubles the potential error that inherently exists in the testing procedure. But in the realm of pool water chemistry, your thesis above is acceptable.

 

[ajw22]

The scale for the CYA markings is logarithmic, not linear. You cannot estimate the values between the markings. You simply round up to the next 10. That is good enough to set your FC minimum and target levels according to [FC/CYA][/FC/CYA]

 

[Donldson]

I suppose if you have a way to dilute a perfect 1-to-1 mixture, then your theory might be reasonable. However if you are off by even a tiny bit then you will be lowering your overall accuracy. You could probably counter this by using much larger samples than necessary, say 1 gallon of pool water and 1 gallon of tap water.

Really though, given the nature of the CYA test you aren't gaining much. Just do the test and round up, too little FC is an issue but a couple ppm too much is not. The active chlorine in a TFP pool is so low that you have a LOT of wiggle room upwards, doubly so with SWG targets.

 

[goody222]

Thanks Marty, it says "The precision of the test, when done correctly, is around plus or minus 15 for levels up to 90 and plus or minus 30 for levels between 100 and 200. Novices often have problems reading the test correctly, and tend to get higher than actual readings."

That is a huge error range! I had no idea it was that big. I wonder if it takes into account the user 'reading' the test (unlike, say, measuring for TA where there is no question when a color change takes place.)
I'd be willing to send a one time small sample to a lab for a more accurate reading just to see how far off I am, if such an option existed.

and Donldson, I understand. I do round up. I'm just a type-A engineer and like to know exactly what I'm dealing with. Thanks all!