by Rob Cox, March 22, 2017
Chemistry was (naturally) a favorite subject of mine during my education, and so (organically), the act of testing pool water has always been of high importance, an almost sacred act, in fact.
For those who enjoy testing and tinkering with water balance, or for those that just don't want to make rookie mistakes, our discussion today covers the finer points of accurate water testing, and water testing blunders to avoid.
You can't do true backyard water analysis with test strips, sorry. They are too hard to discern, the ranges are too broad, and at best, are probably 20% off the mark, one way or the other. The first pool water test blunder to avoid - are test strips.
The gold standard in pool water test kits is the Taylor 2000 Series. For my own use in my low-chlorine pool, I use the Low Range K-2105 test kit, currently $46 - which runs 8 different water balance tests; Chlorine, pH, Alkalinity, Calcium, Cyanuric - everything except metals.
The K-1004, currently $18 - is a great starter test kit, a scaled down version of the 2105, which runs 5 tests, for Chlorine, pH and Alkalinity.
1. Very High Chlorine levels (20 ppm+) can turn your pH sample test a variety of colors in the red/purple spectrum. You can however, add a few (1 - 3) drops of Taylor reagent #7, sodium thiosufate to the water, to remove the chlorine from the sample before adding the pH indicator or phenol red solution. You may still however obtain an inaccurate pH test, until the chlorine level is below 10 ppm.
2. Very Low Alkalinity levels (-50 ppm) can cause falsely higher pH test results, due to the relative pH level of the pH indicator solution, which is about 7.5. In a pool with low alkalinity levels, the pH of the test sample may change rapidly or "bounce" upwards, because of the addition of just 5 drops of the pH reagent, with a pH level of 7.5. This can raise your pH test result.
3. Out of Range Testing. Testing at the upper or lower limits of the test range can produce false test results. Between 7.0 and 8.0, you can discern the small changes and accurately test within that range. Below 7.0 and above 8.0 however, the test colors do not change reliably, and what may look like 9.0 is actually 8.4, or what appears as 6.6 is actually 5.8, for example. Make small additions or adjustments with pool pH chemicals, when pH is outside of the 7.0 - 8.0 range, to avoid overshooting the mark.
1. Very High Chlorine levels, like after shocking the pool or spa, can produce a yellow or clear endpoint, instead of turning from green to red, like normal. As above, you can add 1-3 drops of Taylor reagent #7 (included in the kits I mentioned above), to remove the chlorine from the sample before testing for alkalinity levels.
2. Cyanuric Acid Contribution - when performing a test for Total Alkalinity, the result includes carbonates, bicarbonates and also cyanuric acid. If you have any amount of cyanuric acid (chlorine stabilizer) in the water, it will add to your total alkalinity readings and produce false high test results. To compensate for the contribution, you should deduct about 1/3 of your cyanuric acid level, from your total alkalinity test. For example, if your cyanuric test shows a level of 60 ppm of cya, deduct 20 ppm from your total alkalinity test reading.
3. Very High Algaecide levels - quaternary ammonium algaecides (which is most pool algaecides), in very high concentrations in the pool can affect the alkalinity test, producing false low readings. Biguanides like Baquacil or Aqua Silk can also produce similar low alkalinity results, when levels are abnormally high.
1. High Chlorine levels, once again, high chlorine levels may be so high that the test sample "bleaches out" or turns a cloudy-clear color, after adding the reagents and swirling. In my low-range (0.25 - 2.5 ppm) K-2105 DPD test kit mentioned above, I can bleach out the test sample at less than 10 ppm, but the high-range K-2005 kit, which tests from 0.5-5.0 ppm, will go twice as high before the sample bleaches out. The problem is that high chlorine levels which don't bleach out completely, will produce false low chlorine results. To measure very high chlorine, you can dilute the sample by filling the test vial half full of pool water, and half full of bottled water, and then double the test result.
2. Combined Chlorine levels, will begin to raise the Free Chlorine test result within 30 seconds. DPD test kits use 3 bottles to test for Free, Total and the difference between the two is combine chlorine, aka chloramines. After adding DPD #1 and DPD #2, take your test reading within 30 seconds, before the inhibitors stop working on the chloramines. Add #3 reagent also within 30 seconds, and if the sample turns noticeably darker, you have a measureable amount of combined chlorine in the pool. Rinse your DPD test vial completely after testing for free and total chlorine, any traces of DPD #3 reagent left can falsely raise chlorine test results.
3. Non-Chlorine Shock, in high levels can affect a chlorine test, producing false high readings. If you have recently shocked the pool with MPS (monopersulfate), usually labeled "Oxy" - something, it can end up in the chlorine test too, and contribute to the chlorine test result, giving falsely higher total chlorine readings.