The accuracy of elec. systems for measuring differential and average values of 2 temps. was described. Two-output measurement systems with Pt sensors were taken into account. The limited errors and std. uncertainties of lab. and industrial measurements of the temp. difference were estd. and compared for the cases when (i) the measuring system was initially calibrated, (ii) was no calibration or (iii) was necessary to replace the Pt sensor during continuous operation.