The present invention is intended to solve the problem of the reliable acquisition of accurate temperature data for a miniature calculator provided for the application of family planning methods. The calculator is provided with a receptacle removably carrying a thermometric probe for providing the temperature measurement in a digital form. The circuits of the probe comprise an oscillator including a temperature responsive resistance element to be placed in contact with the area of the temperature to be measured, Tx. The output frequency fx of this oscillator is a function of Tx, and is measured by a counter controlled by a signal formed from a reference frequency fr provided by a reference oscillator and further provided to a frequency divider. A comparator compares the contents of the counter to that of a memory, and in case the counter contents is higher than the stored contents, the comparator controls the entry thereof to the memory so that the probe operates as a maximum thermometer. The contents of the memory is read in serial fashion by the system of the calculator when the probe is replaced into its receptacle.