The procedure presented here is important for another reason also. The sensors for industrial standards can be pur- chased with a characteristic table in which an ‘allowance’ is given. The allowance is not always adequate for precise work. In such situations, additional calibrations need to be carried out to obtain higher accuracy. However, it is unclear which temperatures should be selected as the calibration points. Following the procedure here, one is given the best allocation of the calibration temperatures according to the requirements in terms of measurement accuracy. The present approach gives the optimum locations of the calibration temperatures. Therefore, it is applicable only to a practical thermometer which can be calibrated against a standard at arbitrary temperatures. In contrast, the cali- bration points of standard thermometers, such as those used in ITS90, are limited to a very reproducible fixed point, such as the triple point of pure materials.