Apparatus for determining and/or monitoring the temperature of a medium comprising at least - a first temperature sensor for determining the temperature 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 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.