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interference starts above 10 ppm ….

interference starts above 10 ppm and is significant above 20 ppm. If bleaching is suspected, add extra DPD, reduce the sample size, or do a dilution.

Combined chlorine will interfere in the DPD test for free chlorine if the reading is not taken within 30 seconds. This will result in high readings. If the free chlorine reading cannot be taken immediately, Steadifac can be added to freeze the free chlorine reading. Steadifac is 0.25% thioacetamide. Steadifac is also useful in preventing high readings from oxidized manganese. Another thing that can cause combined chlorine to show up in the free chlorine is residue from DPD3 reagent. This reagent speeds the reaction of DPD with combined chlorine. Any residue of DPD3 on testing equipment can cause a problem in testing for free chlorine. So after testing for total chlorine, immediately wash test equipment.

Potassium monopersulfate (MPS or potassium peroxymonosulfate) is a non-chlorine oxidizer that will interfere in the total chlorine DPD test. Treating recreational water with MPS to remove bather waste can result in false high combined chlorine or total chlorine readings. To prevent this interference:

• Test free chlorine before adding MPS.

• After adding MPS, wait at least 12 hours before testing free chlorine.

• Use a test kit specified for MPS-treated water.

Cyanuric Acid

There are two important reasons to know a pool’s cyanuric acid levels. The first is that cyanuric acid moderates chlorine’s strength: more chlorine may be necessary to perform the same operations at high cyanuric acid levels. The second is that cyanuric acid is unfortunately measured in the total alkalinity test and must be subtracted out from the test result to get the necessary alkalinity information.

Cyanuric acid is most commonly tested with a turbidity test. This test uses a chemical reaction where the water sample becomes cloudy in proportion to the original concentration of cyanuric acid.

The latest Model Aquatic Health Code requires testing cyanuric acid on a monthly basis for commercial aquatic venues.

For residential pools, the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends that the service tech test cyanuric acid monthly.

When testing for cyanuric acid with a melamine-based turbidity test, the most significant interference is water temperature. High temperatures, above 90 °F, can result in readings as much as 15 ppm low. Low temperatures, below 60 °F, can result in readings that are 15 ppm high. The ideal temperature is about 75 °F.

A problem with cyanuric acid test strips is that they are sensitive to the pH of the water. For the best result, adjust the water pH to the ideal range, 7.4 to 7.6, before testing for cyanuric acid.

Calcium Hardness

Calcium hardness a measure of the calcium concentration of the water, expressed as ppm calcium carbonate. It affects both the clarity of the water, as well as whether the water will be scale forming or corrosive to surfaces.

The ideal range for calcium is recommended to be within 150 to 250 ppm, although the acceptable range extends to 1,000 ppm.

Experts recommend a lower level (100 to 800 ppm) for spas because hot water promotes scale.

The latest Model Aquatic Health Code and the Association of Pool and Spa Professionals 4th Edition Service Tech Manual recommends testing calcium hardness on a monthly basis for both commercial and residential aquatic venues.

Calcium hardness is usually tested by titration.

A buffer and indicator reagent are added to a sample of water, and the solution is swirled to mix. The solution will turn red in the presence of calcium compounds. Next, a calcium hardness reagent (EDTA) is added, and every drop is counted as the solution is swirled. When the solution turns blue, the endpoint has been reached. The number of drops is then multiplied by an equivalence factor and recorded in parts per million calcium carbonate.

A high level of metals, such as copper or iron, is the most common interference in calcium hardness titrations. Normally the endpoint of a hardness titration is a color change from red to blue. If the color change is red to purple, a high level of copper is usu- ally the cause. The addition of metal sequestering agent to the pool or spa can minimize this problem. Many manufacturers provide directions for avoiding such metal interference.


Metals in water exist in three major forms: insoluble (metal oxides and hydroxides), free, and complexed. Test methods used to determine the concentration of metals in pool water may not measure insoluble metals and, depending upon whether the method measures free or complexed dissolved metals, may exhibit interferences from chelators and sequestering agents.

Iron (Fe) Test Methods: Most total iron test methods require the reduction of iron to its ferrous form. Because a reducing agent is used in these tests, typical concentrations of chelators and sequestering agents may not interfere with these total iron tests. Elevated levels of these chemicals in the pool or spa water, however, may interfere with the iron test, resulting in an inaccurate lower value. Additionally, depending upon the reducing agent used, insoluble suspended iron such as rust may or may not be reduced to the ferrous form of iron and be measured. Consult the manufacturer’s test instructions to determine if insoluble iron is measured by the test method.

Copper (Cu) Test Methods: Most copper test methods measure free unsequestered copper. Typical concentrations of chelators or sequestering agents interfere with these tests, resulting in an inaccurate lower value for total copper. To determine the concentration of total dissolved copper (both free and sequestered), the use of a total copper test procedure is required. Consult the manufacturer’s test instructions to determine if the test method used measures free copper or total copper.


Colorimeters are becoming more common for running water tests. The main advantage of colorimeters is that they give an objective measurement of water tests. They also allow color-blind people to make accurate measurements. According to the National Eye Institute, as many as 8 percent of men and 0.5 percent of women with Northern European ancestry have the common form of red-green color blindness.

Colorimeters work by measuring the passage of light through a sample. Anything that interferes with the passage of light through the sample will interfere in the test. This includes scratches, dirt, stains, fingerprints, and water droplets on tubes or in the reaction chamber. Always clean the reaction tube or cell and rinse with distilled or deionized water immediately after use. Follow manufacturer’s instructions for maintenance.

Portable electrochemical sensors and probes can also be subject to interferences. There are few interferences in pool water for pH probe measurements, other than dirty or poorly maintained probes. The interferences for pH testing with reagents mentioned above can be avoided with a pH probe. Calibrate probes on a regular basis with calibration solutions as recommended by the manufacturer. Rinse probes with distilled or deionized water before and after use. Most pH probes need to be stored in a storage solution or moist environment.

Salt and TDS meters are really conductivity meters with special calibrations. They measure any conductive ions in the water, including sodium, chloride, calcium, magnesium, sulfate and others. The interferences for salt meters are other ions and dirty and poorly maintained probes. Rinse these probes with distilled or deionized water after use and store dry. Do not allow fingerprints or other residues to remain on the probe.

The main interference for ORP probes is a change in pH. Another interference may be elevated cyanuric acid levels. As the pH drops, the ORP reading can increase, and as pH increases, the ORP reading can drop. Dirty probes are also a problem. Rinse probes with distilled or deionized water and store in a moist environment. In all cases, follow manufacturer’s instructions for cleaning and maintenance.

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