Strange CYA Test Results

[robhdrider]

I opened the pool in late May to a CYA of 30, added 3 lbs of stabilizer expecting to get to 45 and a couple of days later tested to 45, everything's good. Then added another 3 lbs expecting 60, a few days later it only tested to 50. Added another 1.5 lbs and waited a few days, still testing at 50. At this point I have added 7.5 lbs of stabilizer and should have been at approximately 65. I waited another week and the CYA was still testing at 50.

I added another 4 lbs of stabilizer from Walmart thinking there was something wrong with the stabilizer I purchased at Rural King. After a week CYA still measured at 50. Out of curiosity I thought I would do a diluted test to see if it gave a 25 ppm result, the diluted test was 40 for a total of 80. I left everything alone for another week and tested again, direct test was again 50, diluted test was again 40 for an 80 total.

This is my 3rd summer using the TF-100, the reagent was replaced last month. I have the 50 ppm standard and always get a 50 ppm result with the standard so I am confident in my testing. I always maintain at 70-80 ppm and haven't had this problem before this year. For the diluted test I am combining 40 ml each of pool and tap water to minimize dilution errors.

I have added only minimal water to the pool during this time and I am adding the stabilizer via sock in front of a return jet.

Based on the stabilizer added I should be around 85 ppm so I'm believing the diluted test but cannot understand why I'm getting such a discrepancy between the direct test and the diluted test.

Today's test results,
CL 8
CC 0
PH 7.5
TA 120
CH 225

Any ideas?

 

[needsajet]

Curiouser and curiouser!

All I can think of is temperature. You want the sample a bit warm (75F+), and maybe the dilution is changing the temperature? You could get the pool sample, and the tap water for dilution and et them sit a while before testing to either rule this possibility in or out.

I'm not a believer in the 7 ml dilution either. I like two cups pool water to two cups tap water measured to the same line on the same measuring cup.

I read an old post from Chem Geek where he mentioned that CYA does not diffuse quickly, so I'd also be sure to be using the same bulk sample for both tests.

If it were me, I'd stop adding CYA for a while, and run the pool based on the higher number, just as you are.

 

[MarianParoo]

I agree. Stick where you are. By any chance are the Rural King and Walmart brands the same?

 

[robhdrider]

The Walmart stabilizer was HTH. I don't have the containers that came from Rural King to get the actual name but it was a brand I had never seen before, it gave me the impression it was a rebranded product.

Next weekend when I test again I will pull bulk samples of pool water and tap water and let the temperature of each equalize before doing the direct and diluted tests to see if it has any effect on the test.

As my SWG is having no trouble meeting the demand I have no intention of adding additional stabilizer at this time, I would just like to understand the discrepancy that I am seeing.