Currently, conductivity sensors for pure and ultra-pure water applications are calibrated at 25 °C. In some applications (for example in power plants), the sensors are also used at temperatures up to 100 °C. To be able to relate the results to a defined temperature, the measurement results are usually converted to 25 °C. For this purpose, semi-empirical correction functions are used which are partly determined by measurements and partly by model calculations. For measuring the conductivity in pure and ultra-pure water in the temperature range of up to 100 °C, however, no reference values traceable to the SI have been available to date. Since temperature has a significant influence on the properties of the measurement device – in particular on the measurement of the water's resistance in the measuring cell, the type of the device can have a significant influence on the conductivity values measured up to now at higher (and lower) temperatures. This can result in measurement errors that may easily reach several 10 %. The measurement error can only be verified by comparing the measurement results of a device with reference measurements that are traceable to the SI, independent of the measurement device. Without measurement devices that are calibrated in this way, the measurement results of conductivity measurements in the range of pure and ultra-pure water must be put into question the more the measuring temperature deviates from the reference value of 25 °C. It is consequently necessary to also put in question the validity of the correctional functions that are implemented in most industrial conductivity measurement devices.
We are currently extending the measurement set-up for these reference values in order to be able to offer reference values which are traceable to the SI also for temperatures other than 25 °C. After this extension, it will, on the one hand, be possible to calibrate commercial flow sensors for conductivity measurements in pure and ultra-pure water in an extended temperature range between 5 °C and 80 °C. On the other hand, it will be possible to determine in how far the conductivity of ultra-pure water that is blended with additions of different electrolytes depends on temperature. These measurements can be used to validate correction functions that are implemented in the devices.