cause a secondary air flow across the evaporator 3 the vapour compression system 1 may further comprise a defrost element not shown arranged for heating the evaporator 3 in the case that defrost is required a number of sensor devices are arranged at various positions in the vapour compression system 1 temperature sensor 7 is arranged at the outlet opening of the evaporator 3 and it thereby measures the temperature of refrigerant leaving the evaporator 3 t ref out the temperature measurements performed by temperature sensor 7 can advantageously be used during the normal operation of the vapour compression system 1 the superheat is defined as the temperature difference between the temperature of the refrigerant leaving the evaporator 3 and the dewpoint of the refrigerant it is normally desired to control the vapour compression system 1 in such a manner that the superheat is as small as possible but positive thereby the refrigeration capacity of the evaporator 3 is utilised to the greatest possible extent while it is prevented that liquid refrigerant leaves the evaporator 3 in order to obtain this the temperature measurements performed by temperature sensor 7 constitute an important control parameter temperature sensor 8 is arranged in the secondary air flow across the evaporator 3 being caused by the fans 6 the temperature sensor 8 is arranged at a position before the air flow reaches the evaporator 3 and it thereby measures the temperature of air being supplied to the evaporator 3 t.