Temperature-determining device and method for calibrating same and for determining a medium temperature

A temperature-determining device for determining a temperature (TMED) of a medium via a temperature of a surface surrounding the medium, the device comprising:

• an ambient-temperature sensor, which is arranged in surroundings of the surface, configured to measure an ambient temperature (TU);
• a surface-temperature sensor, which lies on the surface, configured to measure a mixed temperature (TM) lying between the temperature (TMED) of the medium and the ambient temperature (TU);
• and
• an arithmetic-logic unit having an approximation formula electronically stored thereon for calculating an approximation (TMEDN) of a temperature of the medium, the approximation formula being stored as a sum of the mixed temperature (TM) and a product of two factors, the first factor comprising a difference between the mixed temperature (TM) and the ambient temperature (TU) and the second factor comprising a ratio of a dividend to a divisor, the dividend comprising a difference between a calibration temperature (TMEDKAL) of the medium and a calibration mixed temperature (TMKAL), and the divisor comprising a difference between the calibration mixed temperature (TMKAL) and a calibration ambient temperature (TUKAL).