Sensor for measuring the electrical conductivity of a fluid medium

#The present invention relates to an inductively operating sensor for measuring the electrical conductivity of a fluid medium, having an excitation coil to which an input signal is fed, and a receiver coil coupled to the excitation coil via the fluid medium, the receiver coil providing an output signal that is a measurement of the conductivity of the fluid medium.

#The excitation coil of such sensors can be designed as a toroid coil fed by an a.c. voltage. A ring-shaped alternating magnetic field is generated in the interior of the excitation coil. A receiver coil, which can also be designed as a toroid coil, is arranged in the same plane in which the excitation coil lies. The alternating magnetic field in the excitation coil causes mobile ions in the fluid medium to generate a ring-shaped current in the fluid medium to be measured, which, in turn, induces an output signal in the receiver coil whose strength is a function of the mobility and concentration of the ions, and therefore of the electrical conductivity of the fluid medium. The output signal is usually detected as an induced current.

#Sensors of this type are preferably employed in factories manufacturing food or drugs for monitoring the production processes thereof. The sensors must always provide an accurate and dependable output signal to rapidly detect excessive changes in the conductivity of the medium being measured and to trigger a correspondingly rapid reaction in order to be able to prevent deterioration of the food or drugs being produced. Appropriate reactions to a change in conductivity can be triggered either indirectly via the production crews or directly by the production installation itself.

#FIG. 1 shows a preferred embodiment of the sensor in accordance with the invention.

#An inductively operating sensor in accordance with the invention is identified in its entirety with 1 in FIG. 1. The sensor 1 is used for measuring the electric conductivity of a fluid medium 2. The sensor 1 has an excitation coil 3 designed as a toroid coil, which is fed by an alternating voltage UErr. A ring-shaped alternating magnetic field is generated in the interior of the excitation coil 3. A receiver coil 4 is also arranged in the same plane in which the excitation coil 3 is located and is also designed as a toroid coil. A ring-shaped current IMed is generated by ions moving in the fluid medium 2 in response to the alternating magnetic field in the excitation coil 3, which, in turn, generates an induction current Iind in the receiver coil 4. The strength of the induction current Iind depends on the mobility and concentration of the ions and therefore on the electric conductivity of the fluid medium 2.

#The sensor 1 has means for measuring the input current IErr, which are identified in their entirety by reference numeral 5. The means 5 for measuring the input current IErr have a resistor R and measure the voltage U dropping across that resistor R. Damage to the windings of the excitation coil 3, the receiver coil 4, or to the sensor connecting cables 15 can result in leakage currents or short circuits which can be detected early and dependably by monitoring the input current IErr of the excitation coil 3.

#The sensor 1 also comprises a measured value transformer 10 receiving the induction current Iind and connected with the means 5 for measuring the input current IErr. The means 5 for measuring the input current IErr generate a status signal, which is a function of the measured value of the input current IErr and supply that status signal to the measured value transformer. The measured value transformer can correct the induction current Iind as a function of the strength of the status signal and issue a corrected signal at an output 11 so that an error-free function of the sensor 1 is assured despite the fact that some damage to the sensor 1 has occurred. The measured value transformer can also trigger an alarm signal at output 11 should the status signal lie outside of a defined threshold range.