Swimming Pool Blog
by Rob Cox, May 21, 2010
Accurate Pool Water Testing
I got an email the other day from a customer. "I had a local guy with a computer who did my water tests… It was great… I got a print-out report every week with suggestions by chemical. Worked perfectly and my water was like glass. They are no longer in business and everywhere else uses a 19 year old kid with test strips… the accuracy depending on a) how much he had to drink last night and b) whether he really G.A.S. that day. Can I buy my own computer (?) for under $1K?"
My reply showed my slight distrust of the printout. "Some of the software that operates these store testing 'printouts' are designed to sell more chemicals." I explained. Then we got into the discussion of accuracy. More than a printout (which I later had to admit was rather convenient), what was most important for our customer was having accurate measurements of his water balance, and an easy way to know what chemicals, if any, he needs to add to the pool.
The least accurate method of testing your pool water. Test strips are also the cheapest, simplest and quickest way to test your pool water. Just dip the strip in the pool water, swirling for about 30 seconds and then compare the colors on the test strip to the colors on the bottle. "Comparison" tests such as this rely on the user to match the color and are subject to mis-interpretation or educated guessing.
A more accurate Test Strip came to market a few years back. Digital strip readers take the guesswork out of reading a test strip. You still dip and swirl, but then insert the strip into the handheld TruTest Digital Test Strip reader and it will give you a digital read-out of your test results.
Reagent Test Kits
The next most accurate way to test your water. For Chlorine testing, there are OTO and DPD methods of testing. OTO is the yellow colored test result. It computes Free Available Chlorine but does not test for Total Chlorine, which means that you cannot compute the level of Combined Chlorine in your pool. DPD is more accurate.
Test kits use reagents that you add, drop by drop, to your water sample. Chlorine and pH tests are a Comparison test, which means that we are relying on the eye of the tester to make an accurate color comparison. Total Alkalinity and Calcium Hardness and Cyanuric Acid (Stabilizer) tests are "Titration Tests", which means that we count the number of drops added until we observe a color change. Then we multiply the number of drops by a certain factor, usually 10, to determine the reading, in ppm (parts per million).
Reagent test kits range from $5 "Duo" Test kits, which only test for pH and Chlorine, to the pool management standard K-2005 test kit (shown), to the Professional Field Lab test kit. The fancier test kits will have Comparator Slides, which are color impregnated glass slides that won't fade or glare in the sunlight.
Digital Test Kits
For the ultimate in accuracy, and the testing method chosen by our customer mentioned at the start of this post, take a look at the ColorQ test kits by Lamotte. ColorQ test kits eliminate the need to visually determine slight color variations or use look-up tables, thus taking the guesswork out of poolside water analysis.
Popular with health departments, municipal pools and YMCA's - they give highly accurate results and are very simple to use. An internal photometer measures the sample and provides the user with the next steps, or what to add to the pool. Still no printout, but you could keep a log if you wanted to.
Another popular digital Test kit is the Scuba+ electronic tester. Drop in a tablet, swirl under water (the Scuba+ is an immersible tester) and the photometric reader does its work. Displays to the user the measurement on the LCD. This item is popular with poolguys, fits in a pocket of your cargo shorts.
With test kits, the axiom holds true, you get what you pay for. If you want accuracy, you'll need to pay a few more dollars. For the average pool owner however, I would recommend the $50 K-2005 kit. This will test all of the parameters important to water balance.
One more thing...there are some water testing techniques or tips that will also help improve accuracy.
- Always use Fresh Reagents, most have a shelf life of 1 year.
- Store reagents in a cool, dry location, out of direct sunlight.
- Take your water sample from under the surface. Testing surface water may yield inaccurate results.
- Keep your testing vials clean and rinse well between tests.
- If you get an odd test result, test again to be sure before adjustment.