5 Sources of Error in Water Testing

And What To Do About Them

All questions and answers on this page have been used with permission from Taylor Technologies, Inc.

Reader Note: The article below by Patricia Fitzgerald first appeared in the journal of the Northeast Spa & Pool Association, The Edge.

Water testing is not rocket science. Even 16-year-old lifeguards do it successfully all the time. But on the other hand, you can't sleepwalk through a procedure and still expect to get accurate results. Here are the five sources of error in water analysis. Common mistakes are listed under each, along with the solution. Resolve to eliminate any of these impediments to your testing prowess.
  1. The sampling location

    Take a sample of water that is representative of the whole pool or spa:  not from the surface, which is subject to special conditions, and not in the immediate vicinity of a return line for the same reason. For pools, sample midway between the shallow and deep ends. Use a clean plastic bottle. Insert it bottomside up so the trapped air keeps out the water momentarily, and then turn it rightside up when you get to elbow depth and let it fill.
  2. The operator

    It may seem obvious, but experience shows people still need to be reminded to read the testing instructions completely before beginning to test. You should do this:  A) whenever you buy a replacement test kit or bottle of strips, just in case the manufacturer has changed something in the procedure, and B) whenever you buy a brand for the first time because manufacturers' products all are a little different, including our test strips. You should continue to read over the instructions before starting the tests until you are completely familiar with each step.

    Don't let your sample sit around exposed to the air before you test it, even hot water from a spa. The sanitizer level is particularly apt to change if you wait.

    Always use the volume of sample water specified for the test. Even a little difference will mean a lot, resultswise. In the test vial, the low point of the meniscus (curvature of the water seen when you hold the sample at eye level) should rest on the fill line.

    Holding a dropper bottle at an angle will result in the wrong amount of reagent being dispensed. Hold it vertically to maintain uniform drop size. Similarly, incompletely crushing a tablet will result in too little reagent being available for the desired chemical reaction.

    A related mistake is not mixing reagents into the sample thoroughly. This doesn't just happen with liquid or tablet tests either. Not exposing a test strip to the water in the manner and for the time specified has the same effect. Note that dipping, swirling, and swishing a strip are different motions for different purposes.

    If you use an instrument for testing, you probably know one sure source of error is failing to calibrate it as directed by the manufacturer. But did you also know that, for many meters, there is a right way and a wrong way to position the test vial ("cuvette") in the optical chamber? Always align markers on the test vial and the instrument, otherwise variations in the vial's wall thickness and/or pattern may cause an inaccurate reading as well as inconsistent results over multiple trials.

    Tests can be ruined by a dirty test vial or optical chamber, or by contaminating the solution with an oily finger. Reagents can be spoiled by air and humidity, or by switching dropper-bottle caps, or by wet fingers causing the reagents pads on a test strip to react prematurely. Allowing the reagent from one wet pad to run into another will also void a test strip result. To prevent contamination and preserve freshness, keep reagent containers tightly closed. Don't use your finger for a stopper and don't interchange reagent caps. Hold your test strip horizontal to the ground and don't flick off excess water unless so instructed. Rinse out any test residue then wipe all equipment with a clean, dry cloth after use.

    Red-green colorblindness can make it difficult for some testers—mainly men—to distinguish between the shades of pink that develop with a DPD sanitizer test. Choose a kit that employs the FAS-DPD method instead. With it, all you need to see is the change in color in the treated sample; the actual colors are not important.
  3. The lighting

    Doing color-matching tests in artificial light, or with your sunglasses on, is a no-no. You can buy an inexpensive, portable daylight simulator, like photographers use, if you must read colorimetric tests indoors. Some countertop testing stations have these daylight lamps already built in.

    Use your meter's light shield to prevent stray light from interfering with test results.
  4. The water chemistry

    If you've just treated the water, wait to retest until the proper amount of time has elapsed (as specified by the treatment chemical manufacturer, typically at least one but usually two complete filtration cycles).

    There also could be interferences in the water that cause the wrong color to develop, or no color to develop—for example, a high level of sanitizer, the presence of a monopersulfate-based oxidizer, or metal such as copper from an algaecide or iron from your fill water. The best testing products tell you what to do in these cases. Call for technical support when in doubt.
  5. The testing tool

    Buy equipment from a reputable manufacturer. With that caveat, these other sources of error are easily remedied.

    Most test methods are limited by an operational range. Testing a parameter outside of the range specified by the manufacturer means the results cannot be interpreted correctly. For instance, chlorine over ≈10 parts per million (ppm) and bromine over ≈20 ppm may partially or completely bleach out the pink color of a conventional DPD color-matching sanitizer test. In this instance, one can dilute the sample and retest to obtain an approximate sanitizer value. But dilution is not always the answer. For example, a high level of either chlorine or bromine can also cause the pH test and the total alkalinity test in both liquid kits and test strips to give false readings. Here, one must wait until the sanitizer level returns to normal to retest. Another example is phenol red. Whether used as a liquid reagent or on a test strip, this indicator only works between pH 6.4 and 8.4. If the pH of a sample is beyond this range, use another test method (e.g., an electronic meter or another indicator that will test as high, or as low, as needed).

    With the notable exception of DPD liquids and tablets, test solutions from various manufacturers cannot be interchanged, nor can test vials. Sometimes you'll even find different strengths of a reagent from the same manufacturer; phenol red indicator for a pH test is a good example. These components are uniquely designed to work with a family of products. When reordering, use the product numbers printed on the originals as a guide.

    A buildup of static at a dropper bottle's tip will cause a visible, progressive decrease in drop size. In the case of a buffer or indicator, this results in less reagent being added to the test sample than intended. In the case of a titrant, this results in more drops being required to reach endpoint than the drop equivalency was set for. Either scenario can lead to incorrect test results. To keep static at bay, periodically wipe around the dropper tip with a clean damp cloth or paper towel.

    A major source of error is using reagents—liquid, tablet, or impregnated on a test strip—that are beyond their useful life because they are old or because they have been stored improperly. A good rule of thumb is to buy new chemicals at the beginning of the swim season. Date them and follow the FIFO rule of first in, first out. A use-by date on a product can give a false sense of security since storage conditions play a greater role in longevity than age. After an afternoon sitting in a blazing hot truck (or a couple days in a warehouse stored next to volatile treatment products like chlorine), even new reagents or test strips can go bad.

    Using stained, faded, or cracked equipment would be an obvious mistake. But did you realize even moderately scratched test vials can lead to unreliable results from your meter? Good housekeeping will eliminate this source of error.