Water Testing Errors - Further Reading

Common Testing Errors and Interference Issues

The most common mistakes when testing water are from human error.[1] Aqua Magazine has a good article on the subject.[2]

The thing to understand about testing is that, outside of a laboratory environment, there are a huge number of assumptions and environmental factors that go into testing that will lead to variance.[3] Statistically speaking, if you did the tests enough times (say a few dozen times), your answer would converge fairly quickly on an mean value that is accurate to the true value with a tolerance pretty close to what is specified on the test (+/- 0.2 ppm).

However, if you do two or three tests, you could see a very large spread between data points. The reasons are many but it mainly centers around operator error - droplet volume variation and where to call the endpoint. Even Taylor understands this and has designed the test, based on industry recommended levels, so that the number of drops needed is below 20. Why? Because a person gets pretty lazy after 10 drops and is definitely not getting good droplets after 20 drops. You squeeze too hard or too soft, your hands shake, you lose count, etc, etc. Each droplet is assumed to be 40uL but, in reality, the volume of each droplet is highly dependent on the individual.

Also, the test is not perfect in and of itself. The chemical indicator powder and titrating reagents are designed to make the test as accurate as possible but the chemistry of the test is such that the pink color slowly comes back over time even when you add nothing. This is why Taylor explicitly says that the ENTIRE test (FC and CC) needs to be completed in under 1 minute from the time you add the powder to the water to the time you finish counting the last drop for the CC test. Even taking time to write down numbers between tests could cause errors.

Given all of this, the mantra of TFP is this - trust your own testing. Try to not get hung up on "proving" how accurate things are or how your testing stacks up against a pool store. There is simply too much variation that will cause you to doubt everything.

As long as you are drawing water from the same depth and area of the pool and you are doing a reasonably good job of performing the test, then your one result is good enough for a day's work. Trying to squeeze out an umpteenth decimal place of accuracy is a fool's errand when it comes to pool water testing.

The other mantra to keep in mind is this - you're not cooking up rocket fuel or vaccines for COVID-19, it's pool water! People get into this mindset of thinking that, "OMG, if my pool water chemistry isn't exactly perfect and spot-on accurate to the 20th decimal place, my pool is going to turn into a green swamp and/or turn into a supermassive black hole and suck my house, dog and kids into oblivion...." I tell the truth, your pool water is fine and getting an extra decimal place isn't going to make it more-fine. Just follow the recommended levels as best you can and trust that you're doing the right thing.

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: [4]
    • 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.
  • Taylor would tell you none of them are good past their expiration but that’s being cautious because there’s no way you control how people store and handle them.[5]
    • General speaking the acid/base reagents (R-0005/R-0006, R-0015/16, R-0009, R-0010, etc) will last a long time if you are careful with them and store them properly.
    • R-0007 sodium thiosulfate is good for a long time.
    • pH indicators like R-0004, R-0008, etc, are fairly stable.
    • Titrants like R-0871 and R-0012 can easily get contaminated by air (oxidation) and so won’t last long.
    • R-0001 and R-0870 (DPD indicator) are easily oxidized by air and should be kept tightly sealed.
    • R-0011L (calcium indicator) will go bad over time and separate out of solution.
    • Finally R-0013 (CYA) ... indestructible unless you inadvertently contaminate it.
    • The silver nitrate titrant in the salt test kit is light sensitive. As long as it’s kept cool and dark, it will last a long time.

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.

Test Water Should be Between 50F and 90F

For consistency, reliability, and accuracy, Taylor Technologies recommends sample temperatures be between 50-90℉. Sample temperatures <50℉ or >90℉ may produce unexpected color development.[6]

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

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.[7]
  • 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.[8]
  • Specific types of interferences listed below can be overcome. Taylor Technologies has videos on General Test Interferences[9]
    • 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.[10] 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