accuracy of CYA testing

[adridimo]

Hello,

I'm curious about the accuracy of measuring CYA (Cyanuric Acid) using the disappearing dot method, especially when adding a standard +10 ppm adjustment. How reliable is this approach? Is there a more accurate way to test CYA levels, perhaps with a drop test similar to the method used for measuring Free Chlorine (FC)?

 

[wireform]

I've been using the CYA test you describe from the day I met TFP. Once you get comfortable with it it works very well and I'll run with whatever it tells me is my CYA level.

 

[ajw22]

CYA testing does not need to be very accurate for CYA to be effective and do its job.

Getting CYA +/- 20 ppm is usually good enough. Error on the high side to make up for the testing inaccuracy. A bit too much CYA is better then too little.

 

[SoDel]

To my mind, there’s very little downside to erring toward the high side when doing the CYA test and thus possibly upping the FC ranges a little bit. A little bit “extra” FC seems a lot better than a little bit too little. Also if you track your daily FC loss over time, you’ll get a feel for what’s typical and what shows more CYA might be needed. Exclude high pool usage days and get a feel for just the effect of the sun baking your FC. You can ask here what is “typical” range of FC loss is for your area. For me, with CYA around 50 to 60, pool with solar cover on loses about 0.5 ppm FC per day; cover off on sunny day loses about 2.5 ppm. If I get a lot of rain and have to remove some water, between the CYA test and daily FC loss, it tells me if more CYA is needed.

The upshot is, it’s ok to guesstimate the CYA test result a little high as this will ensure your FC ranges are high enough. With that in mind, the test is accurate enough.

 

[adridimo]

Thank you for the reply. You are all right, but I would like to know the correcte value in order to put the right ammount of LC and do not waste it.

I have the following measurment:
TFTestKit >> 30ppm (when the black dot disapper completely)
Scuba 3s photometer >> 59ppm (it's brand new and calibrated)

I'm loosing almost 5ppm during the day, the pool take direct sun from 9am to 5pm. Is this cloring loss normal?

 

[ajw22]

Trust your TFT test of 30ppm.

5ppm of chlorine loss with 30ppm is not unusal.

I would add at least 10ppm of stabilizer which should reduce your FC loss a bit.