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Heat loss testing of parabolic trough receivers with heating cartridge is well-established. In that technique temperature is measured by pressing thermocouples against the inside surface of the absorber tube. As the temperature of the thermocouple is influenced by radiation temperature and air temperature in the annulus, the temperature of the thermocouple is higher than the absorber temperature. The paper presents a measurement of absorber temperature over- prediction as function of heating power between 89 °C and 577 °C for three different insulation configuration yielding over-prediction between 4.4 K and 15.3 K. This shows, that absorber temperature over-prediction is at a relevant order of magnitude for heat loss measurements of parabolic trough receivers. Interpolation using 2nd- and 3rd-order 2d polynomials with the additional condition of disappearing over-prediction at HL=0 for correction of heat loss measurements is demonstrated.