machine for testing thermal resistance of plastic materials , comprising a tank configured , in use , to be filled for example with a heat - transfer fluid ; a heating coil for heating the heat - transfer fluid ; a temperature sensor generating a temperature signal of the heat - transfer fluid ; and a control unit calculating a degradation index of the heat - transfer fluid on the basis of the temperature signal . in particular , the degradation index is calculated by determining the temperature range associated with the temperature signal , updating the corresponding partial heating time , and calculating the weighted sum of the partial heating times previously saved in memory and pertaining to different temperature ranges . upon reaching one or more thresholds , signals are generated which indicate the need to replace the heat - transfer fluid .