CYA Testing. Oh Brother!

[BigTallGuy]

I recently took a sample of water to a Pool store to be tested for CYA levels (and Salt) where I was told the CYA reading was 25. I bought a 1.5 pound bottle of Stabilizer and did the math. For my 11,000 gallon pool, according to the bottle should have brought me to around 45 ish or so. I added the product and a week later took another sample to a different Pool store because the first Pool store couldn't test my salt.

That's when the whole thing went nuts.

The second pool store told me that my CYA was now at 80. My jaw about hit the floor. It was chemically, physically impossible to hit a reading of 80 from 25 with 1.5 pounds of CYA in an 11,000 gallon pool. I thanked the man and took the remaining 1/2 of my sample directly to the first pool store for a "second opinion". The first pool store tested the exact same sample 5 minutes later and got a reading of 45, which is exactly where it should have been.

And that is when it hit me.

The First pool store that originally told me that I was at 25 and then 45 after adding the CYA, had just opened their doors to the public and had a brand new vanishing black dot tester, while the pool store that told me that I was reading 80 also had a vanishing black dot tester, but it looked like it had been through the war. It was old and yellowed and was badly "tarnished" inside and out.

The difference had to be the amount of light that penetrates through the plastic wall of the testing tube. I also have to wonder about ambient lighting in each of the pool stores.

The way the vanishing black dot tester works is by gauging when the dot becomes visible, which is totally subjective and directly affected by the light, not to mention the person doing the test. I made each store do 3 different test and there was no change in their respective readings.

Sooo, My question is, Does the Taylor K2006 Test kit use the vanishing black dot test, or do I need to buy a different tester?