Big difference b/w Local Pool Store vs Taylor K2006 (DIY) Chemistry Readings

[HamSammich]

Out of curiosity, I took a pool water sample to my local pool chemical store for testing. I wanted to see how their numbers compared to my own, Taylor K2006. The sample I gave the pool store was no more than 15 minutes old. Here's what they came back with:

FC 3.24 ppm
PH 7.9
CH 300 ppm
TA 40 ppm
CYA 45 ppm

The FC number surprised me, as I've struggled to get FC levels that low and I've forgotten to turn the chlorinator down since the water has been getting colder. The pool is only 3-4 months old and I'm still on the learning curve. I decided to do my own test, about 2 hours later from the original sampling. Here's what I found:

FC 9.5 ppm
PH 7.4
CH 180 ppm
TA 60 ppm
I didn't check CYA this time but after adding CYA a few weeks ago my levels have routinely been in the 80-90 ppm range.

As you can see there are some wide variations, which begs several questions. Am I doing it wrong? I've watched many videos on water testing with the K2006 kit and I am very careful to follow the instructions. Which readings are real? Can the local pool store's readings be trusted?

Thanks for the help.

 

[zea3]

Your water sample can degrade after it has been sitting for awhile. A better apples to apples comparison would be to test the samples as close together as possible. When you have been doing your own testing and dosing the pool does the pool respond as you expect based on test results? If you can say yes, then you are testing correctly. If no, then run a set of tests, post the results and how you conducted each test and we can see if there is something not being performed correctly.

As far as the pool store goes, what where they using to test the water? Strips and computer analysis tests are often way off. Judging from the results posted they are using some sort of computer. Computer that run these tests have to be calibrated on a regular basis, and the person adjusting the machine has to know what they are doing for the tests to be accurate. That is just one reason why we tell folks to rely on their own testing.

 

[Divin Dave]

If you are confident in your testing skills, then trust the K2006.

As zea points out, apples to apples would be to perform tests at the same time using the same method.

A better analysis, (or perhaps even more confusing), would be take samples to 3 different stores. My money, says that all 3 pool store testing will be vastly different.

 

[pabeader]

I have a simpler answer. The fact that your numbers and their numbers are different, is a good thing. Only if they were the same, would I question my testing.

 

[HamSammich]

Thanks for the feedback.

I'm not certain what type of test they were performing. It was not automated. The reagent box looked a bit like the Taylor K2006, but much larger and orange. The water samples were placed into small vials and dropped into a small machine (a colorimeter?). In hindsight, I should have asked.

I do feel confident that I am performing the tests correctly. When adjusting with Taylor's recommendations I do see results that I expect and they are repeatable. I suppose the real question is where you put your confidence. It is troubling to see such vastly different numbers.

 

[JoyfulNoise]

I've seen pool stores use a variety of testing equipment from automated water testers like a commercial version of the LaMotte ColorQ to the Taylor titration reagents. The problem isn't so much the test equipment they use, it's the variability in their results caused by either inexperienced (or simply careless) staff or because they do not properly calibrate their test equipment (in the case of electronic machines). So there's both human-to-human variation as well as equipment issues. The net result of this is a error in analysis that is random and unpredictable.

By contrast, when you test on your own, you eliminate most of the variations that come from differences in people's testing skills and you minimize the equipment errors since you are more aware of old reagents. So the testing errors start to converge to what a manufacturer, like Taylor, has established for it's chemistries. Even if your personal error in testing deviates from the manufacturers established errors, the errors a single person produces are typically biased in one direction.

This is the number one reason WHY it is so important for pool owners to test their own water themselves - the only reliable results you will get are the ones you do yourself. So trust your own numbers; the more you test, the better and more confident you will become over time.