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Calibration of conductivity sensors for applications in pure and ultra-pure water

Range of services
The primary measuring set-up developed by PTB is specifically devised for conductivity measuring cells to be integrated in closed pure and ultra-pure water circuits. The measurement range extends from ultra-pure water (0.055 µS∙cm-1) to approximately 150 µS∙cm-1. Calibration is currently only performed at 25 °C. The relative standard measurement uncertainty is 0.25 %. We usually calibrate the conductivity measuring cells together with the corresponding measurement devices. On request, however, we can characterize solely the measuring cell and its dependency on the measuring frequency. In that case, we measure and evaluate an impedance spectrum of the measuring cells in order to determine the optimum measuring frequency and its dependency on the conductivity.

Measuring principle
The measuring set-up consists of a closed water circuit fed by a commercial device for ultra-pure water production. By adding an electrolyte solution, the conductivity in the circuit can be adjusted. The conductivity in the circuit is measured by means of a primary cell that was developed by PTB. The entire circuit is located in a temperature-controlled box that is flooded with argon to prevent the set conductivity value from being changed by dissolved carbon dioxide.



The primary conductivity measuring cell consists of two interior cylinder electrodes of two different lengths and one exterior cylindrical electrode that concentrically encloses the two interior electrodes. Between each interior electrode and the exterior electrode, an impedance spectrum is recorded from which the resistance of the solution in the volume between the interior and the exterior electrode can be derived. The measurement on two electrodes that differ in length is made to eliminate the stray impedances at the electrode ends computationally. The cell constant is determined by precise measurement of the cell geometry. All measurement values used for determining the conductivity are traceable to the International System of Units (SI) so that, consequently, the conductivity calculated from the values is also traceable to the SI.

A measuring cell of the customer will be connected in series to the primary measuring cell. Thus, both cells simultaneously measure the conductivity of the same water which therefore serves as a transfer standard for the calibration and ensures the traceability of the conductivity measured by the customer's device to the SI.