Inaccurate test results are not always the fault of the water-testing equipment. Whether you use a test kit, test strip, portable meter or countertop lab, sloppy technique or poor housekeeping can interfere with results.
Fortunately, the situation can be rectified easily — and usually at no expense.
The first rule of thumb is to read the instructions every time when making a purchase. Even if it’s the same test equipment from the same manufacturer, something important in the procedure may have changed since you last bought the product. If a “simple” test strip has a required technique for successful use, you can bet the farm other testing supplies do, too.
Keep in mind that test results are not an end in themselves — they are used to make treatment decisions. If the sample employed for the test is not representative of conditions in the whole pool, the results will not be actionable.
First, use a clean sample container. And avoid sampling near return lines, chemical feeders and dead zones. Instead, try for the midpoint of the pool, as you want to take the sample at the specified depth.
In addition, when a treatment chemical has just been added, allow at least two filtration cycles to pass before retesting in order for the product to circulate.
Timing also is important. After taking a water sample, don’t let it sit for too long. The sanitizer residual is particularly apt to change, resulting in a false low because it tends to dissipate as the sample sits. If the instructions say to observe a wait time after adding reagent(s), do not omit this step. Copper, iron, manganese and nitrate tests often need extra time for proper color development. The cyanuric acid turbidity test is time-sensitive as well.
Using the wrong sample volume is another common mistake. In “teaspoon chemistry,” a little difference can have a major impact. Always have the bottom of the meniscus sitting on the fill line of the sample tube. If using a pipettor, remember to check for improper fills due to air bubbles. A volume-related mistake with liquid reagents commonly involves using the wrong drop size. In addition, the dropper bottles are meant to be held vertically when dispensing. If a technician holds the bottle at a slant, the amount of reagent released will be more than the test design calls for. Moreover, if static is allowed to build up at the tip, the drops dispensed will get progressively smaller, resulting in less reagent being added.
Another testing pitfall lies in incomplete mixing. Reagents in tablet or powder form must be completely incorporated into the sample water for the proper reaction to take place. In particular, if a test vial is square, make sure nothing is stuck in a corner.
The fixes here are easy: Hold the dropper bottle upright; discharge static by wiping the dropper tip with a clean, damp cloth; and swirl until any clumps of reagent dissolve.
It’s paying close attention to what’s happening during each and every test that’s the real challenge.
Ambient light is another important consideration in testing. Most manufacturers recommend conducting color-matching tests in natural light (but not looking into the sun). This is because artificial lighting skews color perception — as do sunglasses. If testing indoors, where finding natural light is inconvenient, buy an inexpensive illuminator, much like photographers use to simulate daylight, and perform color comparisons in front of it. Colorimeter users must be aware that stray light will interfere with a test — be sure to close the sample chamber lid/cap the chamber as directed.
Contamination also will foul up test results. Check to ensure there is no residue in your sample container or test cell left over from a previous test. Do not switch caps between reagent bottles, or let them become dirty when you remove them. For that matter, replace caps immediately after use so that air and humidity don’t get inside to spoil reagents or test strips. Never allow finger oils to come in contact with reagents — this means not using a finger as a cap for a test cell, and not touching the pads on a test strip.
Finally, all tests have range limits, and testing outside the limits will result in error. For example, phenol red indicator is widely used to determine pH. It works in the range of 6.8 to 8.4, which is where pH normally is for pools and spas. A pH below or above that range can result in a color reaction that does not match anything on the color chart, or a deceptive “match” to the first or last color standard.
(When in doubt, for an extreme low reading conduct a base demand test, or for an extreme high reading do an acid demand test, using the treated sample.) The remedy is to use another indicator, appropriate for that pH level; although most of us in this scenario would treat to raise/lower the existing pH into the desired range using the demand test results as a guide, and then retest with phenol red for confirmation.
If you see room for improvement in your testing technique, or in the way you care for your testing supplies, turn over a new leaf today. It’s just as easy to develop a good habit as a bad one. Your test results will reward you for it.