Use of a sensor for measuring conductivity or impedance in a test substance, with said sensor being partially immersed in such test substance, said sensor comprising: a first and a second magnetic core (C1, C2), each one of said first and second magnetic cores comprised of one or more pieces of a magnetic material, and each one of said first and second magnetic cores describing a closed path, wherein said first and second magnetic cores are two magnetic cores in a single piece of magnetic material in the shape of the numeral "8" so that the first and the second magnetic cores share a common section; a primary wire coil wrapped around said common section and connected to an electrical power source applying a voltage signal to the primary coil; a first and a second secondary wire coil (L1, L2), each of which being wrapped around a section of the first magnetic core and the second magnetic core, respectively, and around one of the magnetic cores only, wherein the wiring arrangement being such that the primary wire coil as well as the first and second secondary coils are wired separately from each other, and such that the first and second secondary coils present a first pair of ends and a second pair of ends, or a first and a second magnetic sensor, each of which being affixed to the first magnetic core and the second magnetic core, respectively; a measuring device; and an electrical connection between the first pair of ends of the first secondary coil and/or the first magnetic sensor, on the one hand, and the measuring device on the other hand, and an electrical connection between the second pair of ends of the second secondary coil and/or the second magnetic sensor, on the one hand, and the measuring device on the other hand, wherein the sensor is arranged to measure the conductivity or impedance in the test substance with the first core being immersed in said test substance, and with the second core not being immersed in said test substance; and wherein the measuring device is arranged to measure a differential signal between each one of the first and second secondary coils and/or magnetic sensors with the aid of a differential amplifier or by digital sampling and software calculation. a first and a second magnetic core (C1, C2), each one of said first and second magnetic cores comprised of one or more pieces of a magnetic material, and each one of said first and second magnetic cores describing a closed path, wherein said first and second magnetic cores are two magnetic cores in a single piece of magnetic material in the shape of the numeral "8" so that the first and the second magnetic cores share a common section; a primary wire coil wrapped around said common section and connected to an electrical power source applying a voltage signal to the primary coil; a first and a second secondary wire coil (L1, L2), each of which being wrapped around a section of the first magnetic core and the second magnetic core, respectively, and around one of the magnetic cores only, wherein the wiring arrangement being such that the primary wire coil as well as the first and second secondary coils are wired separately from each other, and such that the first and second secondary coils present a first pair of ends and a second pair of ends, or a first and a second magnetic sensor, each of which being affixed to the first magnetic core and the second magnetic core, respectively; a measuring device; and an electrical connection between the first pair of ends of the first secondary coil and/or the first magnetic sensor, on the one hand, and the measuring device on the other hand, and an electrical connection between the second pair of ends of the second secondary coil and/or the second magnetic sensor, on the one hand, and the measuring device on the other hand, wherein the sensor is arranged to measure the conductivity or impedance in the test substance with the first core being immersed in said test substance, and with the second core not being immersed in said test substance; and wherein the measuring device is arranged to measure a differential signal between each one of the first and second secondary coils and/or magnetic sensors with the aid of a differential amplifier or by digital sampling and software calculation.