to be provided with current i 1 and i 2 with the aid of the voltage measuring instrument 6 the voltage u present across the temperature dependent resistance element 1 is determined at the same time the sequence of this method is controlled by a microcomputer 7 to the output of which there is also connected an optical acoustic display device 8 fig3 shows a sensor in a perspective representation this comprises two resistance elements 1 2 arranged in the immediate vicinity of each other in holding devices 11 12 the temperature dependent resistance element 1 expediently being arranged above the heating resistance element 2 for filling level or level measurement only the temperature dependent resistance element 1 is used according to fig4 a this resistance element may also be arranged vertically with respect to the fluid level or according to fig4 b it may be arranged parallel to the fluid level this resistance element 1 is operated as a heater and resistor simultaneously since the dissipation of heat from the resistance element 1 takes place to different degrees inside and outside the fluid a defined constant heating current i 1 is set in such a way that the different heat dissipation behavior of the resistance element 1 is reflected in a clearly measurable way by a change in its resistance value for quality measurement.