Which Conductivity Sensor
Suits Your Application? Conductivity tests a solution’s ability to conduct electrical current and serves as a quick and simple indicator of water quality. Assessing the level of water purity via conductivity can help to prevent contamination. Testing salinity levels may aid in sustaining aquatic life or signal when recycled rinse water is spent. Water measured for conductivity comes from aquariums, waste streams, cooling towers, steam boilers, or laboratory research, among other applications.  When measuring conductivity to determine the purity or salinity of a solution, consider the conductivity cell along with expected measurement range, sample type, accuracy, and general usage. While most cells included with portable or benchtop instruments will suffice, cells with cell constants (K) for high or low ranges may be a better match. Other common considerations are cell materials (usually glass or plastic) and the number of cells (four-electrode designs are more accurate than two). Special applications such as high-purity uses in the pharmaceutical industry or boiler water in power plants require additional considerations, such as temperature coefficients and instrument resolution. Finally, keep in mind the target species. Most organic compounds such as solvents, alcohols, oils, and sugar contain little or no ionic material and therefore are invisible to conductivity measurement, which measures dissociation of ions in water. Low-Conductivity and High-Purity Applications Temperature greatly influences conductivity; generally the colder the water, the lower the conductivity. “Measuring the low end of conductivity is tricky‚” said Frank Paparone, Global Product Manager for Water Analysis Instruments, Thermo Fisher Scientific. “Since temperature can vary the result by 5% or more per °C, it’s important to use the correct compensation factor or none at all,” he said. “Electrodes with low cell constant values incorporate improved designs allowing the sensors to be more sensitive in low ranges. When carbon dioxide in air causes variations, a universal flow-through adapter will help. When conductivity is used as a relative measurement—comparing two samples or identifying trends up or down—it is quite a bit easier than using conductivity as an absolute measurement, particularly in pure water.” High-purity water (0.055 to 1 µS/cm) is used to produce pharmaceuticals, cosmetics, food, semiconductors, and more. When measuring high-purity water, the inverse of conductivity, “resistivity,” is the norm. Greater Compatibility with Solvents Glass electrodes can handle high temperatures and are compatible with solvents. They are also easy to clean yet they are breakable. Plastic electrodes do not break, but can’t be used in the presence of solvents because of chemical attack that can occur on the plastic and plastic electrodes also have temperature limitations.  “For a robust electrode with repeatable results that lasts longer and can be used with many acids, consider a graphite sensor‚” said Paparone. “It is more expensive but graphite is both easy to clean and compatible with most solvents.” High-Salt Solutions “If you double the amount of salt in solution, you will roughly double the conductivity, but only to a point. If sample dilution isn't desirable, consider ditching traditional conductivity measurements for a hydrometer, which is based on density,” said Paparone. “Conductivity meters are best for solutions containing less than 5% salt.” For solutions that meet this requirement, but would still be considered on the high end of the salt range, a torodial conductivity transmitter may work best. It is an industrial transmitter with an electrodeless sensor. High-salt applications in the food industry—for example, using brine solutions or a plating bath—utilize this technology for their measurements. Electrodeless sensors eliminate contamination and reduce long-term maintenance costs. Small Volumes and Large Volumes A microprobe (or microsensor) or a flow-through electrode can be used in small sample sizes measured in vials or well plates. For large-volume samples, some pharmaceutical facilities use industrial cell types with fittings that connect to tanks or pipes. What About the Meter? While the sensor is key to achieving conductivity measurements, the meter offers several variables that can enhance usability. When selecting the meter, consider: • Battery operated or electric power • Waterproof or nonwaterproof • Temperature displayed or need to push a button to get temperature • Data printing capabilities Also, when multiple measurements are required, it may be more convenient and cost-effective to purchase a multiparameter meter that can measure any combination of pH, conductivity, dissolved oxygen, temperature, salinity, turbidity, and more. Cup-style meters allow analysts to pour the solution directly into the meter rather than dipping an electrode in the sample. Cup-style meters generally require less sample and their cable-free sensors offer convenience. For continuous measurement of conductivity in process applications, such as in a water plant, chemical processing, or plating tanks, transmitters or controller/transmitter combinations are an effective solution. Depending on the model, the transmitter can be wired to meet specific application needs. For simple water testing for hydroponics, pocket testers are an economical alternative to benchtop or handheld meters. Keeping the Meter Calibrated Most conductivity instruments today are microprocessor based and don't drift. However, small physical changes to the cell can change readings over time. For best results, verify performance with certified calibration standards at regular intervals. Typically, multifunction calibrators, standards-grade reference multimeters, and precision resistors (per the manufacturer’s procedures) are used to calibrate conductivity and resistivity meters by a calibration service provider. View our selection of conductivity meters, testers and electrodes and sensors.
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