Avoiding Faulty Readings: Good Habits Of Highly Effective Water Analysts
Inaccurate test results are not always the fault of the water-testing equipment or something about the water chemistry interfering with the test method. Most often, sloppy technique or poor housekeeping is to blame. And that’s good news, because you can rectify the situation easily and usually at no expense.
The first rule of thumb is to read the instructions every time you make a purchase. For instance, you may think instructions for how to use a simple test strip are superfluous. As a test strip manufacturer, I can tell you there are important differences between brands. Some say dip (AquaChek® Yellow 4-in-1: "dip for one second and remove immediately"), some say swirl (Insta-Test®5: "swirl two times"). Taylor's sureTRACK® test strips specifically caution: "dip strip (don’t swirl)"; that’s because they are different motions designed to expose the reagent pads to the water in a specific way for a specific time. Some instructions go on to say after wetting the pads to shake off the excess water. Others warn against this. If you are a regular InstaTest user who uses an AquaChek strip in the accustomed manner, you are going to get an unreliable result. This is not the fault of the strip!
Even if it is the same test equipment from the same manufacturer, something important in the procedure may have changed since you last bought it. The takeaway here is that if a "simple" test strip has a required technique for successful use, you can bet the farm all your other testing supplies do too. Stay informed by reviewing the instructions.
Can we agree 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, you will not get actionable results. So, use a clean sample container. Avoid sampling near return lines, chemical feeders, and dead zones. Try for the midpoint of the pool. Take the sample at the specified depth. When you've just added a treatment chemical, allow at least two one turnover before retesting in order for the product to circulate
Water at the surface is interacting with the atmosphere and also may carry debris and oils. For a more representative sample, hold the sample container bottom-side up until you reach elbow depth then turn it right-side up to fill.
Timing is important. After taking your water sample, don’t let it sit for any length of time. The sanitizer residual is particularly apt to change if you do; you can get a false-low because it tends to dissipate as the sample sits. If you are instructed 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. But did you realize that multiparameter test strips are too? When you are instructed to read the test results in a specific order that is because those at the end of the sequence require the longest reaction time.
Manufacturers often give explicit instructions about the time needed for test results to develop properly.
Using the wrong sample volume is a 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 your sample tube. If you use a pipettor, remember to check for improper fills due to air bubbles. A volume-related mistake with liquid reagents is using the wrong drop size. The best manufacturers carefully control the size of the hole in their dropper tips so that a precise amount of chemical is dispensed with every drop. The dropper bottles are meant to be held vertically when dispensing. If you hold the bottle at a slant, the amount of reagent released will be more than the test design calls for. If you allow static to build up at the tip, the drops dispensed will get progressively smaller, resulting in less reagent being added than you think.
Static buildup on a dropper bottle tip will cause drops to become progressively smaller. When you notice this happening, eliminate the static by wiping the tip with a clean, damp paper towel.
Another testing pitfall is 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 your 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; 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. Zone out, lose out!
Ambient light is an important consideration in testing. Most manufacturers recommend doing color-matching tests in natural light (but not looking into the sun). This is because artificial lighting will skew your color perception. So, for that matter, will wearing sunglasses. If you are testing indoors where finding natural light is inconvenient, buy an inexpensive illuminator like photographers use to simulate daylight and perform color comparisons in front of it. Colorimeter users need to be aware that stray light will interfere with a test—be sure to close the sample chamber lid/cap on the chamber, as directed.
As I alluded to earlier, contamination will foul up test results. Check 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 get dirty when you take them off. For that matter, replace caps immediately after use so that air and humidity don't get inside to spoil your reagents or test strips. Never allow finger oils to come in contact with reagents—this means not using your finger as a cap for a test cell and not touching the pads on a test strip
All of the rules I’ve mentioned apply to electronic test instruments. Other cautions are peculiar to them, such as:
- Calibrate your meter on the manufacturer’s recommended schedule.
- Follow instructions for soaking the electrode on your tester when it's in storage. Depending on your particular model, allowing it to dry out or oversoaking it will degrade its performance.
- Rinse the electrode between measurements with distilled or deionized water to avoid cross-contamination, as well as long-term damage from aggressive solutions.
- Don't use a scratched or dirty (including fingerprints) cuvette in your colorimeter as this can cause excessive light scattering resulting in erroneous readings.
- Since the thickness and diameter of these cuvettes will vary slightly, be sure to set them into the sample chamber with the same orientation every time. Some colorimeters have markings on the chamber and cuvette you must line up to help ensure this.
- Make sure you lay the multiparameter test strip down on your digital reader exactly as directed. Sliding it across the glass window may result in incorrect alignment and/or cause cross-contamination between the tests on the strip.
Finally, all tests have range limits. Testing outside the limits will result in error. For instance, 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 do 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 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!