Large Koesters interferometer for long gauge block measurements is known as a unique instrument, which for many decades is being used for the realization of the SI length unit with extremely low uncertainty values. High-precision certification of the temperature measuring system of the original Koesters interferometer has been realized, when using two alternative systems, operating simultaneously. The uncertainty level of each of the reference system is well below 1 mK. Our systems are based on platinum resistance thermometers (PRTs), calibrated directly on a gauge block with the
application of the correction on velocity error. Advantages and some drawbacks of the original system are outlined. We show that in the original Koesters interferometer of INMETRO, as a result of systematic thermocouple offsets, which are specific for each particular thermocouple and the way how it is located inside the instrument, the accuracy of the system is limited to about 2 mK. The basic result of this study is that the chamber of the interferometer permits to improve gauge block temperature measurements, when the fine effects, associated with a small overheating of the gauge block surface by the measurement current of resistance thermometer, can be detected. Those effects are energy dissipation of a heat wave during its propagation in the artifact and the velocity effect in self-heating measurements. Local overheating of long gauge blocks by PRT current, which lies in range of ~0.1 mK, is found to be one of the limiting factors in precise temperature measurements of the blocks.