Water Testing Errors - Further Reading

Common Testing Errors and Interference Issues

The most common mistakes when testing water are from human error.[1]

False water testing results can be avoided by following these guidelines


  • Wash your hands, then keep them as dry as possible, and keep fingers out of vials and from optical chambers.
  • Using a new testing kit?
    • Even if it is the same brand and labeling, review the directions. Sometimes a procedure or part of the product may have changed.
    • If purchasing a new brand, read the directions and be sure to discard all the old components since most kits will have differences in the vials, droppers, and other components.
    • When reordering components, use the product numbers printed on the originals.
    • Static electricity can build up at the bottle tips, decreasing drop size. Simply wipe the dropper tip with a damp, clean cloth to eliminate the static.
    • Calibrate the instruments, like the pH meter, before you start testing.
    • Check the vials to ensure they are still clean from the last time you used them.

Expired Test Reagents

All water test reagents have a shelf life. Powders and crystals are very stable if kept dry, and acids last a long time.

  • For example, Taylor reagents can be recognized as going bad: [2]
    • R-0002 DPD #2 - should be colorless; as it goes bad it will change to pink then brown.
    • R-0003 DPD #3 - should be colorless; as it degrades it will become yellow.
    • R-0004 pH Indicator - as it degrades it turned from red to yellow or purple.
    • R-0008 TA Indicator - should be dark green; begins to stain the milky bottle as it goes bad.
    • R-0011L CH Indicator - should be deep blue; any other color indicates it is bad.
    • R-0718 Silver Nitrate Indicator - should be colorless; no easy way to tell if bad and recommended to replace annually.
    • R-0871/R-0872 FAS DPD - Should be colorless; if bad will become increasingly yellow.

Notes about Eyesight and Vision

  • Are you red-green colorblind? It may make it hard for you to distinguish between shades of pink with a DPD test. This style of test only requires seeing a change in color with the test sample.
  • Make sure to complete color matching in natural light without sunglasses.
  • If readings must be done indoors, purchase a small daylight simulator.

Cold Test Water Should be Room Temperature for CYA Test

If your water is below 70 degrees read the procedure in the CYA Testing wiki page.

Only the CYA test is sensitive to cold water.

Testing Process

  • Before testing, allow the pump to run at least 30 minutes. This allows the chemicals to be well distributed throughout the pool water.
  • Where you take the water sample matters. The sample should be taken away from return lines, chemical feeders, and dead zones.
  • The depth of the sample is important. Take as according to directions, or at a minimum of 14 inches (about elbow length) into the water, midpoint between the ends of the pool.
  • When taking the sample, hold the container upside-down as you immerse it into the water, then turn it over to fill it up.
  • Be certain the proper sample amount is used for testing. Sometimes too much or too little can really affect the results. The lowest point of the curve seen when you hold the sample at eye level should be on the fill line.
  • Test the sample right away. The longer it is exposed to air before you test, the more likely the chlorine and/or pH levels can change.
  • Ensure each part of the testing chemicals during the process are properly used:
    • Dropper bottles must be held vertically. If held at an angle, the drops of reagent will be the wrong amount. In fact, depending on the angle of the dropper, it could be like using 1.5 drops rather than one.
    • Let the drops form and roll off the tip. Don’t squeeze the bottle to pop the drop off prematurely.
    • Tablets must be completely crushed for the proper chemical reaction to occur.[3]
  • Allow the full amount of time necessary for:
    • Cyanuric acid turbidity test to complete its course. Side note: CYA is commonly known as your pool stabilizer, pool conditioner, or chlorine stabilizer.
    • Melamine to mix properly with the CYA in the water.
  • Mix the reagents thoroughly between drops to help them bind by swirling or using a Speedstir Magnetic Stirrer.
  • Keep in mind that swishing a strip is different than dipping or swirling, and each have separate purposes.
  • Hold test strips horizontal to the ground, wait to flick off extra water only according to instructions.
  • Stray light that may interfere with test results can be hindered by the meter’s light shield.

Potential Water Test Interferences

  • Sometimes elements (including copper, chlorine, bromine, or iron) in the water can affect the testing and result in the wrong color or no color at all. In this case, be sure to understand and use the beta testing product or call the customer support if you are not sure.
  • If parameters being tested are outside the manufacturer’s means, the results will not be interpreted correctly. For example, a high level of chlorine may cause the pH test to give false readings.[4]
  • Specific types of interferences listed below can be overcome. Taylor Technologies has videos on General Test Interferences[5]
    • Calcium Hardness Test Interference - interference from copper and iron can cause a fading endpoint and some color changes before the final endpoint.
    • Chlorine Test Interference - high levels of chlorine > 10 ppm can cause a false low or 0 chlorine reading.
    • pH Test Interference - high levels of chlorine > 10 ppm can cause a false pH reading, usually bluish purple.
    • Total Alkalinity Test Interference - high levels of chlorine or bromine can cause the reaction to change from blue to yellow instead of green to red.
  • Potassium Monopersulfate Compounds aka MPS or "non-chlorine shock' will interfere with the Combined Chlorine test.[6] Taylor Technolgies has the R-0867 Deox Reagent that will neutralize MPS effects on the CC test. R-0867 Deox Reagent is available in the K‑2041 (.75 oz) and K‑2042 (2 oz). It’s also available as a stand-alone kit, the K‑1518 FAS-DPD chlorine/monopersulfate test.

Cleanup & Storage

  • Rinse out test residue when you are done with the testing, then wipe it dry with a clean cloth.
  • Recap the reagent immediately after use and keep them closed tightly to prevent contamination, or spoilage from air and humidity. This will also keep the original caps on their correct bottles.
  • Store reagents at a consistent temperature between 36F-85F (2C-29C).
  • Keep reagents out of prolonged direct sunlight
  • NOTE: Extreme fluctuations cause reagents to deteriorate