An apparatus for determining and/or monitoring temperature of a medium, comprising at least one temperature sensor and at least two reference elements for in situ calibration and/or validation of the temperature sensor, wherein the first reference element is composed at least partially of a first material, in the case of which at least one phase transition of at least second order occurs at least a first predetermined phase transition temperature in the temperature range relevant for calibration of the temperature sensor, wherein the second reference element is composed at least partially of a second material, in the case of which at least one phase transition of at least second order occurs at least a second predetermined phase transition temperature in the range relevant for calibration of the temperature sensor, and wherein the at least two reference elements are contacted via exactly two connection wires.