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Technical Bulletin: Differences in Test Kit Readings
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Joe Sweazy, Environmental Test Systems, Inc.
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Have you ever tested swimming pool water with two different test kits and gotten two completely different results? Then, which one do you trust? You would expect that two accurate test kits would give two accurate results, and that those results would be the same since the water did not change in the two minutes between tests. Usually that will be true, but not always. There are certain situations that you should be aware of where two different test kits may give you two different test results.
Free Chlorine
Liquid and tablet test kits use a chemical called DPD as the indicator to measure free chlorine. If you are testing for free chlorine using a DPD test kit, be aware that high levels of combined chlorine, or 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. But, occasionally levels of combined chlorine will 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. Usually a strong, foul chloramine odor will confirm that there is a high level of combined chlorine present. Utilizing another testing method, some test strips use an indicator called syringaldazine to detect free chlorine. Chloramines do not react with syringaldazine. Therefore, the liquid or tablet DPD kit may show there is some significant level of free chlorine present when the test strips show that there is no free chlorine in the water.
Bromine
Like chlorine bromine is present in swimming pool water in two forms, free and combined bromine. However, combined bromine is an effective sanitizer and does not give off such a foul odor, therefore we often use the term “bromine” referring to all forms of the sanitizer. Many test kits on the market will measure total bromine, while others will measure only free bromine. Be sure you know what your test kits are measuring. Test kits that measure only free chlorine and also measure bromine are only reading free bromine. Most test kits will report on the label or somewhere in the kit that it is measuring total bromine. If not, chances are you are only reading free bromine. Obviously, a test kit that measures total bromine will have a tendency to read higher than a kit that measures free bromine since it is measuring all forms of the sanitizer. So two different bromine tests may give significantly different bromine readings.
Monopersulfate
Monopersulfate is a non-chlorine chemical that is commonly used to shock treat swimming pool water. It is a powerful oxidizer which causes a DPD test for free chlorine or bromine to react to show a positive reading. A small amount of monopersulfate residual will show an indication that there is a free chlorine or bromine residual present when there is none, or show a higher residual than there actually is. Some test kit manufacturers have now successfully neutralized monopersulfate in this reaction. Once again, this should be stated somewhere in the kit, probably in the manual that monopersulfate is not an interference. Additionally, test strips for free chlorine that utilize the syringaldazine indicator do not have this interference. So monopersulfate will potentially cause some kits to read higher free chlorine or bromine than the actual level. Also, monopersulfate does dissipate off relatively quickly so waiting for a couple of hours after shocking with monopersulfate will generally guarantee that this interference will not be a problem.
Precision/Interpretation
Another factor to keep in mind if comparing test results is the precision factor. Most liquid/tablet kits allow for very precise readings for certain parameters. Total alkalinity and hardness, for example, are measured in increments of 10 ppm. This allows the user to determine within 10 ppm what the actual result is. Other test kits and test strips may not offer such precision. Although likely to give accurate results, these test kits may not allow you to determine within 10 ppm of the actual result. Because these tests are colorimetric (you use colors to determine the concentration) and open for interpretation, users may interpret the results slightly higher or lower than the actual concentration indicated causing a further discrepancy between the two test results. So keep in mind that interpretation may play an important role in getting similar test results with two different kits.
Technique
Technique is also important to every test kit. Of course this is more essential for some kits than it is for others. Generally, liquid/tablet kits require more precise user techniques to obtain very precise results. Test strips are usually much less technique dependant, but offer less precision. Additionally, directions are very important. Many inaccurate test results occur when individuals do not follow directions-or follow the wrong directions! Many test kits look very similar and may in fact use the same set of directions. However, do not assume that because they look similar, the directions are the same. This can be a significant source of variation. Make sure you know and follow the directions for each test kit, as directions change from time to time and from manufacturer to manufacturer.
© 2001
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