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?

[Watermom]

The K-2006 does use the vanishing black dot test.

(Hi, BTDG ....... uhhh, I mean, son!)

[waterbear]

The difference is most likely because of the lighting that was used to read the test since it is read from the top of the tub AND because the second store did not clean the tub out after each test (just bad practice and all the more reason to get your own kit). Get yourself a K-2006 and test the CYA yourself and clean the tube after and you will be fine!

FWIW, pool store salt tests are about as accurate as pool store CYA tests, which means they are not. If they are using a meter they most likely are NOT calibrating it often enough ( because the standard calibration solutions are not cheap). If they are doing a titration they are very possibly overtitrating, which is very easy to do with the standard chroamte/silver nitrate titration for chloride (such as the Taylor salt test kit). If they are using strips they are most likely not letting them sit long enough (since strips need about 10 minutes according to info on the AuqaChek/ETS/Hach website that is not on the bottle of strips and most stores are not going to take that long to test your water, particularly if they are busy!)

Best bet, test the water yourself!

[BigTallGuy]

Hi Mom and thanks Evan. I am going to finally order my own kit. I just wanted to be sure exactly how the Taylor K2006 worked. Pool Stores, Bah.

[chem geek]

The expected lighting for the Taylor CYA test is strong indirect lighting. They recommend that one stands outside with one's back to the sun so that the tube is shaded by your body and you look down straight into it. You can also buy standard solutions of 50 ppm if you want to practice, but that's overkill and only for those who are really curious or picky. I've always gotten the expected result when using the method recommended by Taylor.

[BigTallGuy]

This confirms what everyone has told me all along, even the pool store guys. The test is totally subjective. However, the condition of the testing tube (yellowed and tarnished vs. clean and clear) has to be added to the list of variables.

My expeience showed the "Pool Store" reading be higher than actual, and thus taking away the "Need" to buy more chemical.

So this kinda works in reverse. It would behoove the pool stores to use new or clean testers in good lighting in order to sell more CYA.

[famousdavis]

I have a new Taylor K-2006 kit, and tried doing my own CYA tests this weekend. I do find it hard to know how much of the black dot must be obscured to say that it has "disappeared". For instance, it pretty much disappeared at about the same level reported by one of the pool companies I've taken water samples to, but I *could* still manage to see a very faint outline of a black dot (because I knew a black dot was at the bottom). Even after filling up the tube with the remaining CYA fluid I could still kinda-sorta see a black dot at the bottom of the tube, although it was quite hard to do so. So, unlike the FAS-DPD chlorine test where "colorless" means absolutely without any color at all, I'm interpreting the black dot test to be mean that you stop when you can't easily see a black dot anymore. At the least, I'm able to replicate the pool store's numbers with a home-test kit.

[aylad]

famousdavis,
The important thing is that you do the test in consistent conditions, and if you "stop to when you can't easily.....anymore", then you do it to that same point every time. The test is very subjective, and is really a good ballpark, so if you are just consistent about it from test to test, you should be fine.

Janet