Interpreting Test Results

[Cyberwip]

If you are taking a sample of your pool water to the local pool shop for testing and suggestions, you are probably spending far too much on pool chemicals.

I have both a LaMotte digital and a Taylor test kit, and I take a water sample to the local "Pool Care Professionals" every week for testing (and their suggestions on WHAT TO ADD).

Under laboratory conditions, taking a 1 gallon sample from the center-most portion of the pool and from that measuring accurately into a sterilized vial and performing the tests with ultimate care... not only can I not get the same result twice, but the two different test kits never agree... and LaMotte even shows you 2 decimal digits to try and convey accuracy.

So, I need a referee! and I go down to the local pool store to get the water tested.
Nice lab setup, looks very professional... but it has it's own opinion and doesn't agree with either!

So who's right?

[chem geek]

Trust your Taylor test kit, assuming you are doing the tests correctly. The results are consistent for most users. There is a difference between resolution (2 digits) and accuracy. Just because a digital readout gives you 2 decimal digits does not mean that they are accurate. The Taylor TA and CH tests have an accuracy of 10 ppm or 10% of the reading, whichever is higher. The FC and CC tests have an accuracy of 0.5 ppm for the 10 ml sample or 0.2 ppm for the 25 ml sample or 10% of the reading whichever is higher. The pH can usually be read to within 0.2, sometimes 0.1 with practice. The CYA test is hardest to read, but is around +/- 15 ppm though with practice you can often get to +/- 10 ppm.

There are standards you can purchase to validate your test kits. See this page for TA, CH and CYA standards.