a): the percentage losses as a function of the temperature difference between T exc and T ch . b): The measured power (in red) and the power applied to the electrical heater (in black) are shown as a function of the time. The yellow line corresponds to the fit function done by using the eq.2, performed both on the set and on the measured power. The green line corresponds to the error band of 0.2% on the measured power. In the inset the difference between the applied and the measured power during the time is shown.