Conductivity Sensors

A conductivity sensor is a tool to measure the electrical conductivity of an electrolyte solution and is based on the material’s ability to conduct an electric current. It is used to measure conductivity in process, laboratory, or field applications.

Electrolytes in the sample dissolve to give ions that conduct electricity. The higher the ion concentration, the higher the conductivity. The measuring cell of the conductivity sensor consists of at least two electrically-conductive poles with the opposite charge to measure the conductance of a sample.

If the exact cell constant is unknown, then calibration must be performed. When the exact cell constant is known, then verification is sufficient. This is the case with sensors with a certified cell constant or sensors which have been previously calibrated.

Conductivity is strongly temperature dependent. As the temperature of a sample increases, the viscosity of the sample decreases, which leads to increased mobility of the ions. Therefore, the observed conductivity of the sample also increases even though the ion concentrations may remain constant. 

In good practices, every conductivity sensor result must be specified with a temperature or be temperature compensated, usually to the industry standard of 25 °C.

There are several ways to compensate for temperature.

Conductivity in an aqueous solution is highly affected by temperature (~2 %/°C). That is why it is conventional to link every measurement to a reference temperature. 20 °C or 25 °C are the commonly used reference temperatures in the case of conductivity measurement.

Different temperature correction methods have been developed to suit different users:

• Linear: for medium and highly conductive solutions
• Non-linear: natural waters such as groundwater, surface water, drinking water, and wastewater
• Pure water: ultrapure water, deionized water, distilled water
• None: some standards such as USP <645> prohibit any temperature compensation

The impact of temperature on different ions, and even varying concentrations of the same ion can be challenging. Hence, a compensation factor, called temperature coefficient (α), must be determined for each type of sample. (This is also the case for the calibration standards. All METTLER TOLEDO meters can automatically account for this compensation using preset temperature tables.)

Yes, it is possible. For example, organic substances also have dissociative properties, which allows the conductivity of organic compound solutions to be measured. Organic compounds like benzene, alcohols, and petroleum products generally have very low conductivity.