Inconsistent CYA Tests

[cvgnus]

Hello,

So let me set up the background first. I have a LaMotte Spin Lab with 301 CL/BR disks ( I know how you all feel about this). The updated 301 disk regaents have been reformulated to be more accurate (according to the LaMotte chemist who helped me update the software to accept the new disks and my numbers for CH and Borate are quite different from the 300 disk series). I also have a Taylor k-2006 from two winters ago which I had used twice (pre TFPC education). It was kept closed in a climate controlled basement and I assumed the reagents expired so I didn't really again use it until yesterday. I recently purchased the TF-100 test kit to get a clean start on testing this season. I also went to Leslies Pool Supply for a test (first time for me, they use taylor reagents, taylor speed stir and the sales person seemed knowledgeable).

I had some issues with the CH tests and figured out that that I had a fading end point. After reading this and trying it all my CH testing became more consistent.

So I have 3 different test kits, a pool store test and have results from 9 tests in all. All results from the test kits and Leslies are very close. Within .5ppm for FC, .1 for pH, 10ppm for TA and 25ppm to 30ppm for CH. But the CYA is inconsistent and concerns me.



Spin Lab
K-2006
Leslies
TF100
TF100/K-2006

Test 1 Spin Lab x2
CYA 44


Test 2 K-2006 x2
CYA 45 (Halfway between 40-50ppm)


Test 3 Leslies x1
CYA 50


Test 4 TF-100 x2
CYA 20


Test 5 TF-100 R-0013 and K-2006 test bottle and CYA tube x2
CYA 45 (Halfway between 40-50pp)


I have read and re read instructions, watched videos, used consistent lighting, used one eye and used my belt buckle for consistent distance.
Any thoughts on this would be helpful. I plan to do another series of tests today as a sanity check. My R-0013 supply is taking a beating though.

Thank you

 

[Texas Splash]

This topic (CYA testing) takes a beating because it is the more subjective of the tests. But I wouldn't over-think it too much. Actually, with the exception of test #4 (not sure what happened there), they all seem to be right around 45. But if it were me, I would stay consistent with one test kit (either the TF-100 or K2006) and stay with it. I suspect bouncing back & forth may only become frustrating.

When I do my CYA test with the TF-100, I do like many others. I mix the agents, shake, wait 30 secs, shake again, then read. After that first reading, I put it back in the mixing bottle and do it one more time. That way I double-check my own reading with the same kit.

My recommendation - keep it simple. One test kit ... consistent results. Don't beat yourself up over it. Get in the water and enjoy the day!

 

[cvgnus]

Thanks for the response. The purpose for this exercise was that I wanted to standardize on the TF100 since it is the recommended test kit. After doing several tests and getting 20 ppm on the CYA prompted me to do the comparison testing yesterday.

 

[pabeader]

please note that the scale for CYA is NOT linear! It is logarithmic. So halfway is more like 1 or 2 NOT 5. (if I have my log scale memory correct.)

 

[Griswald]

Considtsncy is better than individual numbers. If you test to "around 40" 4 out of 5 times, use that as your result. Close enough.

We arent dealing with blood chemistry here...

 

[cvgnus]

Sorry if I'm a little OCD. And maybe I'm not being clear. My point is is that the TF-100 appears to be flawed testing CYA (I suspect the CYA testing tube) when compared to the other tests, particularly against the K-2006 and its CYA testing tube. I want to use the TF-100 exclusively of course but I now have reservations as far as the CYA test is concerned. I know we are not splitting atoms (or doing a blood test) but 20 - 30 ppm deviation is pretty wide for CYA. Of course I could use the K-2006 CYA test tube with the TF-100 but I thought I should get this out for opinion.

 

[chem geek]

I've compared the CYA tests between the Taylor K-2006, the Taylor K-1720, and the TFTestkits TF-100, and I have not the kind of variability you are seeing. I don't think it is the tube in the TF-100 since it is the identical tube used in the Taylor K-1720 and should be at least as accurate if not more accurate than that in the Taylor K-2006 because it's volume is larger and the lines easier to read. I've also tested absolute accurate using the CYA standard solution at 50 ppm.

If there is any significant source of error between the tests it would be in the 50/50 markings of the mixing bottle in the TF-100 compared to the markings in the mixing tubes of the K-2006 and K-1720, but that isn't a large error. The other possibility would be if the CYA reagent you had from the TF-100 were bad for some reason, but from your tests it seems it's the tube (or the mixing bottle?) since the same reagent was used. I suggest you get some CYA Standard Solution that is 50 ppm so you can test for absolute accuracy as well. I suspect that the more open tube used in the TF-100 and K-1720 is causing differences with the lighting conditions you are using. For all tests you are doing are you standing outside with your back to the sun and have the tube in front of you waist-high looking straight down into the tube with your fingers near the top of the tube? For the K-2006 tube which is really part of the entire block so has more light obscured for it, are you holding it so that it is in full strong indirect light?

Try isolating the difference to whether it is the TF-100 mixing bottle vs. the Taylor 9191 bottle or the TF-100 CYA viewing tube vs. the Taylor K-2006 comparator block. Then get the standard solution to see which one is "right" under the proper viewing conditions. A difference between 20 and 45, when done consistently and with the same bulk water samples, is pretty substantial.

I should note that this thread is also reporting a "too low" CYA problem so I wonder if there's something wrong with the melamine solution though I believe you were using the same solution in both of your tests, is that right?

 

[cvgnus]

Chem Geek, thanks for your response. I will obtain the CYA standard solution to establish a baseline for the TF-100 CYA viewing tube and the K-2006 comparator block and ensure my testing technique is sound. As far as your questions regarding full indirect light for the comparator and sun behind for the CYA tube I believe that is the method I used but will do a sanity check along with performing the isolation testing you describe.

Thank you

 

[cvgnus]

Sorry missed your last question. Yes I used the same solution from the tf100 in both the cya tube and comparator. I have yet to use the solution from the k2006 in the cya tube.