Apparatus for determining and/or monitoring the temperature (T) of a medium comprising at least - a first temperature sensor for determining the temperature (T) of the medium , - a reference element for the in situ calibration and/or validation of the apparatus , wherein said reference element is made at least partially from a material for which, within the temperature range that is relevant for the operation of the apparatus , at least a phase shift occurs at least at a predefined phase shift temperature (T Ph ) and the material remains in the solid phase at said phase shift, wherein at least a first and a second electrically conductive electrode are arranged on the reference element , wherein said first and second electrode are electrically isolated from one another, - a first connecting cable to contact, particularly electrically, the first electrode of the reference element , and - a second connecting cable to contact, particularly electrically, the second electrode of the reference element , characterized in that - a third connecting cable is provided, which is made from a material which is different at least from the material of the first or the second connecting cable, and - wherein the third connecting cable is arranged in such a way that, with the first or the second connecting cable, it forms the first temperature sensor in the form of a first thermocouple. - a first temperature sensor for determining the temperature (T) of the medium , - a reference element for the in situ calibration and/or validation of the apparatus , wherein said reference element is made at least partially from a material for which, within the temperature range that is relevant for the operation of the apparatus , at least a phase shift occurs at least at a predefined phase shift temperature (T Ph ) and the material remains in the solid phase at said phase shift, wherein at least a first and a second electrically conductive electrode are arranged on the reference element , wherein said first and second electrode are electrically isolated from one another, - a first connecting cable to contact, particularly electrically, the first electrode of the reference element , and - a second connecting cable to contact, particularly electrically, the second electrode of the reference element , - a first connecting cable to contact, particularly electrically, the first electrode of the reference element , and - a second connecting cable to contact, particularly electrically, the second electrode of the reference element , characterized in that - a third connecting cable is provided, which is made from a material which is different at least from the material of the first or the second connecting cable, and - wherein the third connecting cable is arranged in such a way that, with the first or the second connecting cable, it forms the first temperature sensor in the form of a first thermocouple. - a third connecting cable is provided, which is made from a material which is different at least from the material of the first or the second connecting cable, and - wherein the third connecting cable is arranged in such a way that, with the first or the second connecting cable, it forms the first temperature sensor in the form of a first thermocouple. <EOS>