The Right Kind of Test

May 29, 2018

About the author: Joe Sweazy is technical sales and services manager for Hach Co. Sweazy can be reached at 574.262.2060 or by e-mail at [email protected].

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Do you know if you are using the most accurate water testing kit available? You may be; however, regardless of how accurate your test kit is, there are circumstances that will cause it to read inaccurately or at least differently than you would expect. It is up to you to recognize these situations so you can save yourself confusion, time and most importantly, money.

Liquid kits, test strips and even electronic devices will give you results at times that will make you scratch your head. The following is a look at the top causes for such situations and an explanation of how to recognize when this is happening so you do not spend time and money trying to fix a chemistry issue that you have not fully diagnosed.

Liquid Kits

Most liquid test kits use a chemical indicator called DPD to measure free chlorine. If you are testing for free chlorine using a DPD test kit, be aware that high levels of combined chlorine (chloramines) can cause false positives when using a DPD test for free chlorine. Low to moderate levels of combined chlorine is neutralized in most DPD kits.

Levels of combined chlorine will occasionally build up to such a level that the free chlorine test result will show that there is some level of free chlorine when in fact there is none present. A strong, foul chloramine odor may confirm that there is a high level of combined chlorine present.

Pay close attention to the chlorine reaction when adding DPD indicator (usually DPD No. 2). Free Chlorine reacts immediately with DPD to form a pink color, while combined chlorine generally reacts more slowly; therefore, if you do not get an immediate reaction and color develops after a few seconds or minutes, you are likely seeing combined chlorine react with the DPD indicator.

Elevated sanitizer levels can also cause problems for liquid test kits. First, high chlorine (greater than 15 ppm) or high bromine (greater than 20 ppm) can cause the pH indicator to form a purple color that does not match the pH color scale. This color is similar to the high pH colors, 8 to 8.4 ppm. Be careful not to mistake this for a high reading and add acid when it is not necessary.

If chlorine or bromine levels appear on the high side of the scale, and pH is reading a purple color, the best thing to do is wait for the sanitizer level to drop to an acceptable level and test again. If pressed for time, however, you can add a drop of sodium thiosulfate to neutralize the chlorine. This is less than ideal because thiosulfate solutions have a high pH, meaning that you are increasing the pH of the sample and again getting potentially high pH results.

Another problem caused by high sanitizer levels is that the DPD indicator can be “bleached out.” A common indicator that this is happening is a flash of color when the indicator is added that quickly becomes colorless. If you see this happening, dilute the sample and retest. You may, for example, use half sample water to be tested and half fresh water, ideally distilled water, as it contains no chlorine and little or no chlorine demand. Then run the test again using the same procedure. This will provide a result that is half the actual value, so you will need to multiply the result by two to obtain the correct reading. Higher sanitizer levels may require greater dilution in order to get an accurate reading.

Tablet Test Kits

Tablet test kits are another method for testing pool and spa water. These kits are similar to liquid kits and many of the chemical reactions are the same. You can, therefore, get inaccuracies similar to what is reported in the liquid section. Additionally, you can determine if a tablet is unusable by inspecting it before use. If it appears to be more powder than tablet, or if it is discolored, this usually indicates that the reagent within the tablet has been contaminated and is no longer usable.

Test Strips

Similar to DPD test kits, elevated sanitizer levels can also cause problems for test strips. Because test strips and liquids use the same pH indicator, phenol red, high chlorine or high bromine levels can cause the pH indicator to form a purple color that does not match the pH color scale. This tends to be more obvious with test strips, forming a purple color that does not closely resemble the 8.4 or other high-end color blocks.

In addition to the purple color pH, the total alkalinity may also be affected. As with the pH, elevated sanitizer levels can cause a color that does not match anything on the color scale, in this case a royal blue color. The royal blue alkalinity generally accompanies the purple pH, making it easier to determine when this problem is occurring. The best way to treat this problem is to wait for the sanitizer level to drop to an acceptable level and test again.

Another common issue for test strips is faded or washed out results. You can identify this situation when test strips react to form colors that appear to be in the same color family as the colors on the color chart, but appear significantly lighter or less intense than those colors on the comparison chart. This happens most commonly when test strips have been contaminated, generally by heat or humidity.

Test strips should be stored at room temperature. Prevent moisture from getting into the bottle by using dry fingers when handling strips and by sealing the bottle immediately after use.


Electronics, unfortunately, will not provide the same indicators as liquid kits and test strips to alert you of inaccurate readings—electronics users have to be aware when false readings occur.

The general rule is if you suspect that your meter is not reading accurately, you are probably right. There are two major things to consider with most electronic devices. First, is it calibrated? Most units require a single-point calibration, meaning you have a single solution of a known concentration to set your meter in order to ensure it is reading accurately. Some devices require a multiple point calibration with two or three solutions. If you are getting questionable readings with your meter, calibrate it to make sure. Some units are factory calibrated and do not require periodic calibration.

The next step is to determine if the device and any accessories are clean to prevent interference. For example, test tubes or vials used with a hand-held colorimeter might become discolored or faded over time. Recalibrating can help, but may not prevent this from causing falsely high or low readings. Faded or discolored vials or tubes should be replaced, especially if the coloration is not uniform across the entire surface.

Regardless of the electronic device, test kit or test strip you are using, you can still rely on a proven method to verify the accuracy of a questionable test. Double-check the readings you are getting against another type of test to confirm that they are in the same general ballpark. If one method tells you that the chlorine is reading low while the other shows the chlorine is acceptable, one of them is not reading accurately.

The bottom line is this: understand the testing equipment that you are using and the circumstances that would cause it to report false readings. You can then be on the lookout for questionable test results and know what to do in order to get more accurate results.


Ten Things to Know about Pool Chemistry & Water Testing

1 If an algae bloom occurs in a pool, check the nitrate level. Nitrates can get into a pool from a variety of sources including fertilizer overspray, bird droppings, rain and well water. Unfortunately, algae love nitrates; if the nitrate level is too high, you may encounter algae blooms even though the water appears to be perfectly balanced. Keep the nitrate level below 10 ppm—if your test strip reads above that level, you should partially drain and refill the pool.

2 Even if you are using an ozonator or mineral purification system, you still need to maintain a chlorine residual level. Most of the ozone in a pool will dissipate within 30 minutes. After the ozone becomes depleted, you need some residual chlorine to protect the water from additional contaminants that may be introduced. Similarly, chlorine supplements a mineral system by oxidizing contaminants and preventing algae growth. You should maintain a minimum of 0.5 ppm free chlorine in pool water if using one of these alternate sanitizing systems.

3 Keep the cyanuric acid level in the right range to avoid chlorine loss. In outdoor pools, the sun’s ultraviolet radiation quickly breaks down chlorine. Cyanuric acid stabilizes the chlorine against the effects of sunlight, acting like sunscreen for the pool; however, too much cyanuric acid can reduce chlorine efficiency. It will also elevate the total dissolved solids (TDS) level, which may cause scale formation, stains or cloudy water. Use test strips frequently to keep the cyanuric acid level in the ideal range of 30 to 50 ppm.

4 If the water is balanced in a hot tub but you are still having trouble, test the TDS level. The TDS increases every time you add chemicals to the water. In hot tubs, where jets and high temperatures speed up evaporation, the TDS level builds up more rapidly. Elevated TDS can promote bacteria growth or corrosion, even when the water seems to be properly balanced. To avoid these and other problems, use a conductivity meter or TDS test strip regularly.

5 In a brominated hot tub or pool, be sure you test for total bromine, not free bromine. Total bromine includes free and combined bromine (also called bromamines). Unlike chlorine, both the free and combined forms of bromine are effective sanitizers, so you need to measure them both.

6 Learn the pH of various sanitizers—they can help you balance the pH in the pool. For example, if you normally treat a pool with trichlor tablets (very acidic, with a pH of 2.8), then use sodium hypochlorite (pH of 13) when you shock the water. They will offset one another, helping to keep your pH in the ideal range of 7.2 to 7.6.

7 Know the uses and limitations of your testing method. If using an OTO test kit, keep in mind that it only tests for total chlorine, whereas free chlorine is the effective sanitizer in a pool. DPD kits measure free chlorine; however, they will yield false-positive readings for free chlorine in the presence of high combined chlorine (chloramines) levels. This is a frequent problem when opening pools in the springtime. Finally, all liquid reagents and test strips will expire. For accurate results, use products before the expiration date. Some liquid reagents do not have clearly marked expiration dates; these should be replaced at the start of each season.

8 You do not have to dip your test strip 18 in. below the surface to get accurate test results. With any circulation at all, the chemical levels at the surface will be nearly identical to the levels at the bottom of the pool; therefore, testing at fingertip depth is all that is required.

9 Chlorine demand is often higher in the spring when opening pools. When a pool is not properly closed or winterized, microorganisms that are not destroyed by chlorine, or some other sanitizer, will grow and multiply in the water. This is common when a pool is reopened in the spring and there is no free chlorine present. Under these conditions, you may need to add as much as 200 ppm of chlorine to reestablish a free chlorine residual.

10 Whatever type of testing product you use, be sure to follow the manufacturer’s directions. Most inaccurate test results occur when individuals do not properly follow directions. Not all manufacturers’ test strips are the same; updated versions of products may also use different test procedures.

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About the Author

Joe Sweazy

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