Abstract: A dew-point measuring instrument has a capacitive dew-point sensor 1 which is cooled by a cooling device 17 to the dew-point temperature measured by a temperature sensor 8. A phase measuring circuit 14 measures the phase angle of the impedance of the capacitive dew-point sensor 1. The measured phase angle is used as a gauge for the contamination of the dew-point sensor 1.

Abstract: The resistance thermometer comprises a temperature dependent measuring resistor (10) and a measuring circuit (11) for measuring the voltage drop at the measuring resistor which is produced by a current (I) flowing across the measuring resistor (10). The measuring resistor (10) lies in the feedback circuit of an operational amplifier (A.sub.1) whose inverting input is applied via a resistor (21) to a constant reference voltage (E). To linearize the characteristic of the measuring circuit a voltage (k.U.sub.T) is applied to the non-inverting input of the operational amplifier (A.sub.1), which voltage is in a predetermined ratio (k) to the output voltage (U.sub.T) of the measuring circuit (11).