How accurate is the CYA test?

[TomAtlanta]

I've been doing this for over a year and I still have trouble deciding on my CYA level. The black spot on the bottom becomes invisible, then I think I have added too much, then I take out a little and can't decide if I really see a shadow or not. Do other people have this problem? Maybe I am a slow learner, or just old and dumb.

If 10 experienced pool owners on the forum each tested the same water for CYA, do you think they would all get the same reading? If not, do you think the readings would all be with in 5 of each other? How far apart would you guess the highest and lowest reading would be if we could do this experiment?

 

[jblizzle]

It is +/- 15ppm. Are you pouring the solution back and fork to see if you keep getting the same reading? Or take an average.

 

[Richard320]

I would guess we'd all be within 5 of each other.

If you want to perfect your testing technique, order the CYA standard solution the next time you order refills. Mix it up as if it were your pool water and see if you get 50. Experiment with the lighting, how far you hold it, anything. Whatever it takes so that you hit 50 ⁺/_-every time. Then when you test your water, do it exactly the same.

 

[TomAtlanta]

Thanks for the info. I'll try the CYA standard solution.

 

[chem geek]

The +/-15 ppm is the range Taylor gives and is like a 95th percentile for any of the measured CYA levels. It most likely applies to a high CYA level where 15 ppm isn't a great distance in the tube so its much more likely to have that size of error. For lower CYA levels in the 50 ppm or so range, I'd expect the error to be less than 10 ppm and for experienced people they'll likely be within +/- 5 of their average among them. Once you get used to it, it's a pretty consistent test, assuming you've got reasonably consistent lighting conditions such as standing outside with the sun to your back with your body shading the tube.