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How Measuring Conductivity Over a Wide Range Works

Conductivity measurements provide fast, continuous, inexpensive indications of changing electrical and material properties.

by Ilium Technology
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Problem: In non-aqueous systems, conductivities may be very low and can easily swing over many decades with only relatively small changes in concentration of additives or impurities. The ability to cover a wide range with a single meter permits the user to follow parameters such as solvent purity, additive concentration, and total dissolved solids, as well as track processes, titrations or other experiments where the conductivity varies widely. From a practical point of view, a single meter that covers a wide range is less expensive than multiple meters each covering a limited range.

The electrodes of a system designed to measure conductivity in aqueous samples are often configured as two rings separated by an insulator mounted coaxially on a long thin cylindrical probe. The surface area of the electrodes and the distance between them are fixed. During measurement, a potential difference is applied to the electrodes and the current emerges from one electrode and follows a curved path to return to the other electrode. The length and shape of the current path are dependent not only on the configuration of the probe, but also on the electrical characteristics of the liquid. Clearly, sample dependent changes in path length will affect conductivity measurement accuracy. The effects of changes in path length can be reduced by configuring the electrodes as concentric cylinders, however fringing at the edges of the electrodes can still introduce unacceptable variability over a wide measurement range.

Related Article: Resistivity / Conductivity Measurement of Purified Water

For measurements over a limited range, the variation in path length can be accommodated by calibrating the probe with an appropriate standard of known conductivity. The calibration procedure determines the cell constant (the effective ratio of path length to electrode area) for a particular probe. However, calibration becomes problematic at low conductivities since it depends on the availability of accurate standards. Good conductivity standards below 10 micro-Siemens/cm are notoriously difficult to obtain. At 10 micro-Siemens/cm, available standards have an accuracy of ± 2-3%. At 1 micro-Siemen/cm that accuracy degrades to ± 25%. For DI water in the nano-Siemen/cm range, no standard is available.

Solution: One example of a product that solves such problems is ILIUM’s conductivity meter which uses adaptive wave forms, dynamic signal compensation, and adaptive noise suppression techniques to provide highly accurate measurements over a range of 12 decades from milli-Siemens/cm down to 1 femto-Siemen/cm. The smart probes use concentric cylinder electrodes and incorporate guard electrodes to completely eliminate the effects of sample dependent variations in current path length on measurement accuracy. The fully guarded probe design permits accurate measurements over the full design range of the probe without recalibration. Probes are calibrated using an NIST-traceable procedure at the factory. All calibration data is stored in the probe from where it can be immediately retrieved by the meter. Probes can be swapped as needed without recalibration. In addition, smart probes are extremely easy to clean. They can be disassembled and reassembled in seconds with no change in the calibration or cell constant, providing complete reproducibility.

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