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A potential free temperature measurement in gas insulated switchgear and lines (GIS/GIL) was investigated within an HVDC research project, because the temperature is of high importance during development and operation of HVDC equipment. Infrared spectrum measurement of the heated inner conductor by pyrometer sensors has emerged as the best technical solution for HVDC GIS/GIL. To optimize the infrared emission of the inner conductor, it was painted with black varnish. The sensor had to be adjusted to the infrared transmission of the pressurized insulating gas atmosphere. Experiences as well as methods to perform such a calibration are shown in the paper. A calibration by adjusting transmission and emission coefficients inside the sensor software did not succeeded, because the coefficients differed for variable sensor ambient temperatures. A successful calibration was possible by considering two major variables: the pyrometer output as well as the sensor ambient temperature. However, certain limits of the pyrometer measurement in GIS/GIL were observed as well. The EMC performance of the pyrometer was improved by an additional electronic unit, which protects the commercial electronics. Overall, the potential free temperature measurement could be performed during the long-term operation of the regarded HVDC GIL system. Typical temperature curves based on the calibrated pyrometer sensor are shown.