Every service technician has a preferred method for testing pool water — whether it’s test strips, a titration kit or some combination of each.

What many techs don’t know, however, is that test kits themselves often must be tested and cross-checked for accuracy. The reagents in test strips and titration kits can change under the influence of heat, cold or humidity — and small inaccuracies can lead to larger chemical imbalances in a pool’s water.

The good news is that all these problems are preventable, and fairly easy to detect. Here, experts weigh in on common errors that can develop in various types of test kits, and how to fix potential test kit inaccuracies.

Kits with purpose

To understand the different types of erroneous readings that test kits may produce, it’s first important to classify kits according to the range of conditions they’re designed to measure, and the methods they use to measure each one. “The best way to prevent potential misreads is to understand what causes inaccuracies in the first place,” says Joe Sweazy, technical sales and services manager at AquaChek/Hach Co. in Elkhart, Ind.

The test type that produces the most general readings — and the quickest results — is a test strip. Chemicals on these strips react with conditions in the water, such as chlorine, total alkalinity and pH, to produce colors that can be visually matched with a chart. Alternatively, these strips can be used with a photometer, which analyzes the color electronically and produces a precise number. In both of these methods, though, it’s important to be aware of the strips’ condition before using them.

“A lot of strips have a sell date, like the one on fresh produce,” says John Shonfield, owner of Shonfield Pool Service in Los Angeles. “Sometimes I’ve seen inaccurate readings on ones that were kept in storage too long.”

A titration kit, on the other hand, can generate readings that are more precise than the ones provided by test strips. However, the process is more involved and depends on the skill of the user.

“Titration gives you an almost infinite number of data points, rather than a general range,” says Richard LaMotte, vice president of sales and marketing at LaMotte Company in Chestertown, Md. “If each drop is equal to 5 or 10 ppm [of chlorine], you can keep adding drops, and accumulate very precise data.”

Because titration kits, like test strips, depend on chemical reactions to generate their results, it’s important to be aware of their age, and the conditions in which they’ve been stored, before trusting their measurements. The process of ensuring test kit accuracy begins long before you arrive on the job site, though.

Calibration and confirmation

Even if a test kit has just arrived fresh from the factory, that doesn’t necessarily guarantee its accuracy. If the kit has gone through freezing or sweltering weather during transport, the chemistry of the reagents may already have been altered. That’s why it’s crucial to test all kits against chemical standard solutions on a regular basis.

“We encourage people to ‘test your test kit,’” says Pat Fitzgerald, vice president of marketing at Taylor Technologies Inc. in Sparks, Md. This means using standard solutions in place of an ordinary water sample. These standards come in two basic forms: bottles of water with certain values — such as pH, alkalinity and hardness — guaranteed by the manufacturer, or reagents guaranteed to set the chemical values of a water sample at certain levels.

In either case, testing these samples with a test kit’s reagents will allow you to determine the accuracy of your kit. “By testing your kit against chemical standards,” says Dan Jonaitis, owner of Arizona Pool Specialists in Scottsdale, Ariz., “you can calibrate it, and find out if your reagents are producing accurate results.”

On the job site, noticing inconsistencies between the test kit and observed conditions is sometimes a matter of instinct. If a test’s reading is far out of the normal range, and a second test’s result differs significantly from the first one, something has gone wrong with the testing process, and it’s important to investigate what’s happened.

It’s also crucial to consider current test results in view of past records of the pool’s chemistry. “For example, if you haven’t been on site for seven days but you’ve got a higher-than-normal free chlorine reading, then something’s out of whack [with your results],” says Jeb Henn, owner of Henn Pools LLC in Darien, Conn.

Many service professionals agree that it’s important to confirm the results of one test kit through multiple testing methods, especially if the first test reveals an unexpected chemical condition. “If one kit’s results are off by, say, 10 ppm, and you keep trying to compensate for that, you’ll create a snowball effect over time,” Shonfield says. Attempts to compensate for inaccurate test results by further adjusting the chemistry can cause serious damage to a pool. By using multiple testing methods, though, it’s possible to determine whether a problem like this can be attributed to, say, the test kit’s reagents, or if the pool’s chemistry and circulation system need to be investigated further.

Awareness of a few common titration interferences can also provide an indicator of testing errors. For instance, “potassium monopersulfate-based oxidizing shock will interfere with any chlorine test that employs DPD #3, leading the analyst to believe there is a higher combined chlorine level than there actually is,” Fitzgerald says. Some manufacturers sell a “deox” reagent, which can be used to eliminate this interference.

Another example is the influence of metal ions on a calcium hardness titration test. “Most likely it’s copper from pipes, algaecide, or the fill water,” Fitzgerald says. Instead of the expected color change — from red to blue — the solution may turn purple. In this case, it’s prudent to retest the sample, adding several drops of titrant before adding the buffer and indicator, to overcome any interference from the metal. Remember, though, to count these extra drops when you calculate the hardness concentration.

In the field, it’s also important to check all test results against a consistent light source and background whenever possible. “I’ve found that lighting is a critical factor, especially for pH tests,” says Steve Toff, chief pool inspector at Toff’s Pool Inspection in South Brunswick N.J. “I’ve seen a pH reading variance of 0.3 between one light source and another.”

One common method is to take a small sheet of paper, or a white card, and put it behind the test kit, using reflected light off that white background to neutralize any other background colors. Another option is to use a photometric reader — properly calibrated, of course.

Storage and care

Many test strips and titration kits are guaranteed by the manufacturer to remain reliable for a year or more, under ideal storage conditions. As long as strips or drip bottles are being used up every few weeks, techs agree that they’re unlikely to develop problems. Still, it’s important to keep all test kits out of extreme temperatures and direct sunlight, especially when storing them for long periods of time.

“All reagents have a shelf life,” Fitzgerald says, “and shelf life is more than just how old something is.” Environmental conditions can create a major impact on the reagents in a kit. If they sit in the sun, or next to other chemicals, or if their bottles’ caps are left off for a prolonged period of time, reagents can generate inaccurate readings. Thus, it’s important to store test kits in a dry place, at a temperature between 36 and 85 degrees Fahrenheit.

Some techs have observed that freezing temperatures can cause ice crystals to form in the liquid reagents of a titration kit. “I live out in an area where we get frost and freezing temperatures,” Shonfield says, “and I don’t leave that kit in the truck overnight if it’s getting down into the 30s — I bring it into the house.”

Test strips can also be vulnerable to light and heat. “You have to remember that the pads on the test strips have phenol red, the same chemical used to measure pH in a titration kit,” Jonaitis says. “So if it’s sitting in the sun and the heat all day, it can bleach out.” If a test kit is exposed to a harmful level of heat or cold, it may still be usable, but it’s smart to check with the kit’s manufacturer for advice on your specific situation.

Another potential concern is calcium deposits within the titration chamber itself. “Sometimes the plastic on the bottom of the test kit begins to calcify, so you can’t see the little black dot clearly anymore,” Jonaitis says. The easiest way to prevent these deposits is to clean the kit’s titration chamber out with distilled water every few days — but again, if kits are being used up on a regular basis, these accumulations aren’t likely to create a serious problem.

Experts agree that the safest place to store test kits is in a closed, controlled environment, such as a cooler or an air-conditioned closet. If  the kit’s reagent bottles aren’t being used up every few weeks, it’s crucial to test them against chemical standards on a regular basis — the consensus is around every two weeks — and replace them if necessary.

In short, each testing method is adapted for a particular range, and a certain type (or types) of reading. If one test result doesn’t seem to line up with what’s observed in the pool, it’ll prove helpful to check those results against another type of test, or a set of objective standards, before acting on that data. When it comes to test kit accuracy, an ounce of prevention often equals a pound of cure.