A method for the in-situ calibration and/or validation of a thermometer with at least a temperature sensor and at least a reference element, said reference element consisting at least partly of a material, for which material at least one phase transition occurs within the temperature range relevant to the operation of the thermometer at at least one prespecified phase transition temperature (T ph), during which phase transition the material remains in the solid phase, comprising the following process steps - Detecting and/or recording at least one measured value (T m) received by the temperature sensor, in particular as a function of time (T m (t)), - Detecting and/or recording at least one characteristic physical or chemical reference variable of the reference element, in particular as a function of time (Tm(t)), - Detecting the occurrence of the phase transition based on a change in the reference variable, in particular a dramatic change, - Determining a phase transition time point (t ph) at which the phase transition took place, - Determining a sensor temperature (T m) from a measured value received by the temperature sensor at a measurement time point (t m) which is the closest to the phase transition time point (t ph), and Comparing the sensor temperature (T m) with the phase transition temperature (T ph) and/or determining any deviation (ΔT) between the sensor temperature (T m) and the phase transition temperature (T ph), wherein a model of dynamic heat convection is taken into consideration to determine any deviation (ΔT) between the sensor temperature (T m) and the phase transition temperature (T ph), said model considering an inhomogeneous and/or dynamic heat propagation within the thermometer, i.e. at the location of the temperature sensor and/or at the location of the reference element, and effects of heat dissipation which may be caused by changes in the ambient temperature. - Detecting and/or recording at least one measured value (T m) received by the temperature sensor, in particular as a function of time (T m (t)), - Detecting and/or recording at least one characteristic physical or chemical reference variable of the reference element, in particular as a function of time (Tm(t)), - Detecting the occurrence of the phase transition based on a change in the reference variable, in particular a dramatic change, - Determining a phase transition time point (t ph) at which the phase transition took place, - Determining a sensor temperature (T m) from a measured value received by the temperature sensor at a measurement time point (t m) which is the closest to the phase transition time point (t ph), and Comparing the sensor temperature (T m) with the phase transition temperature (T ph) and/or determining any deviation (ΔT) between the sensor temperature (T m) and the phase transition temperature (T ph), wherein a model of dynamic heat convection is taken into consideration to determine any deviation (ΔT) between the sensor temperature (T m) and the phase transition temperature (T ph), said model considering an inhomogeneous and/or dynamic heat propagation within the thermometer, i.e. at the location of the temperature sensor and/or at the location of the reference element, and effects of heat dissipation which may be caused by changes in the ambient temperature.