Addressing the challenges of traceable electrolytic conductivity measurements in water

Abstract This paper addresses the challenges of making accurate and traceable electrolytic conductivity cell constant calibrations and sensor system calibration measurements over a wide range of conductivity, from 5.5 µ S m −1 (ultrapure water) up to 140 000 µ S m −1 , where stable and reliable commercial certified reference materials (CRMs) are available. The challenges of making electrolytic conductivity measurements in the time domain versus the frequency domain are discussed and the relationship between the two domains is analysed in terms of a Gaver–Stehfest algorithm for inverse Laplace transform. Data from a commercial current-step-pulse-based sensor reader operating in the time domain are compared with accurate and traceable data from a closed-flow-loop electrolytic conductivity measurement system operating in the frequency domain and traceable to the SI length unit. In the high conductivity range, the readouts of the sensor reader are observed to deviate from traceable conductivity values by up to 10 times the estimated standard measurement uncertainty, which is in the range 0.07%–0.17%. If accurate conductivity calibrations are required (<1%), the usual method of calibrating a conductivity sensor system at a high conductivity level, where CRMs are available, and relying on the same calibration for low conductivity measurements is, therefore, risky.