A radiometric for measuring a physical, measured variable of a fill substance located in a container and for outputting a measurement signal, which corresponds to a measured value of the physical, measured variable. The measuring device has available a single line-pair, via which energy supply of the total measuring device occurs, and via which transmission of the measurement signal occurs. The measuring device includes: a radioactive radiator, which, during operation, sends radioactive radiation through the container; a detector having a scintillator and a photomultiplier and serving for detecting, and for converting into an electrical output signal, radiation intensity penetrating through the container dependent on the physical, measured variable to be measured; a measuring device electronics, which serves, on the basis of the electrical output signal of the detector, for producing the measurement signal and for making such available via the line-pair; an energy storer supplied via the line-pair; a controller, which, as a function of energy available via the line-pair and the energy storer, triggers measuring phases, during which the measuring device measures the physical, measured variable, and which places the photomultiplier in operation exclusively during the measuring phases, wherein high voltages required for operating the photomultiplier are produced during the measuring phases by means of a high voltage cascade.