WO2023280748A1 - Conductivity sensor - Google Patents

Abstract

The invention relates to a conductivity sensor (3) for determining and/or monitoring an in particular specific electrical conductivity of a free-flowing medium (4) in a container (5), in particular in a tank or through a pipeline, comprising: - an excitation coil (6) for generating a magnetic field in the medium; - an operating circuit, which is designed to feed an electrical input signal into the excitation coil, the input signal being designed in such a way that the magnetic field thereby generated by means of the excitation coil (6) excites movable charge carriers in the medium (4) to move; - a magnetic field-sensitive unit (7), which is designed to provide a measurement signal, in particular a fluorescence signal, which correlates with a change and/or a strength of a magnetic field generated by the movable charge carriers of the medium (4), said magnetic field-sensitive unit (7) comprising a sub-unit (15), and said magnetic field-sensitive unit (7) having an optical excitation unit (9) for optical excitation of the sub-unit (15) and an optical detection unit (10) for detection of the measurement signal; and - an evaluation unit (8), which is designed to determine the in particular specific electrical conductivity of the medium at least by means of the measurement signal provided by the magnetic field-sensitive unit (7).

Description

conductivity sensor

The invention relates to a conductivity sensor for determining and/or monitoring the specific electrical conductivity of a medium in a container.

In general, conductivity sensors that work according to an inductive or a conductive measuring principle are often used in process automation to measure the electrical conductivity of a medium. A conductive conductivity sensor is known from the prior art, for example, which comprises at least two electrodes that are immersed in a medium for the measurement. To determine the electrical conductivity of the medium, the resistance or conductance of the electrode measuring section in the medium is determined. If the cell constant is known, the conductivity of the medium can be determined. In order to measure the conductivity of a medium using a conductive conductivity sensor, it is absolutely necessary to bring at least two electrodes into contact with the measuring liquid.

With the inductive principle of determining the conductivity of process media, sensors are used which have an excitation coil and a receiving coil arranged at a distance from it. An electromagnetic alternating field is generated by means of the excitation coil, which acts on charged particles, e.g. ions, in the liquid medium and causes a corresponding current flow in the medium. This flow of current creates an electromagnetic field at the receiving coil, which induces a received signal (induction voltage) in the receiving coil according to Faraday's law of induction. This received signal can be evaluated and used to determine the electrical conductivity of the liquid medium.

Typically, inductive conductivity sensors are constructed as follows: The excitation and receiving coils are generally designed as ring coils and include a continuous opening that can be acted upon by the medium, so that medium flows around both coils. When the excitation coil is excited, a closed current path is formed within the medium, which passes through both the excitation coil and the receiving coil. The conductivity of the measuring liquid can then be determined by evaluating the current or voltage signal from the receiving coil in response to the signal from the excitation coil. The principle itself is established in industrial process measurement technology and is documented in a large number of documents in the patent literature, for example in DE 198 51 146 A1. In order to determine the conductivity, it is necessary to know the temperature of the medium to be measured; the conductivity usually increases as the temperature rises. A temperature sensor immersed in the medium, eg an Rt100, Rt1000, NTC etc., is used for temperature measurement, which is usually separated from the medium by a housing wall or a casing.

A more recent development in the field of sensor technology is represented by so-called quantum sensors, in which a wide variety of quantum effects are used to determine various physical and/or chemical measured variables. In the field of industrial process automation, such approaches are of particular interest with regard to the increasing efforts towards miniaturization while at the same time increasing the performance of the respective sensors.

Quantum sensors are based on the fact that certain quantum states of individual atoms can be very precisely controlled and read out. In this way, for example, precise and low-interference measurements of electric and/or magnetic fields and gravitational fields with resolutions in the nanometer range are possible. In this context, various spin-based sensor arrangements have become known, for which atomic transitions in crystal bodies are used to detect changes in movements, electric and/or magnetic fields or gravitational fields. In addition, various systems based on quantum-optical effects have also become known, such as quantum gravimeters, NMR gyroscopes or optically pumped magnetometers, the latter in particular being based on gas cells, among other things.

For example, in the field of spin-based quantum sensors, various devices have become known which use atomic transitions, for example in various crystal bodies, in order to detect even small changes in movements, electric and/or magnetic fields or gravitational fields. Typically, diamond with at least one nitrogen defect, silicon carbide with at least one silicon defect or hexagonal boron nitride with at least one defect color center is used as the crystal body. In principle, the crystal bodies can have one or more defects. DE 102017 205 099 A1 discloses a sensor device with a crystal body having at least one defect, a light source, a high-frequency device for applying a high-frequency signal to the crystal body, and a detection unit for detecting a magnetic-field-dependent fluorescence signal. The light source is arranged on a first substrate and the detection device on a second substrate, while the high-frequency device and the Crystal bodies can be arranged on both interconnected substrates. External magnetic fields, electrical currents, temperature, mechanical stress or pressure can be used as measured variables. A similar device is known from DE 102017 205265 A1.

DE 102014 219 550 A1 describes a combination sensor for detecting pressure, temperature and/or magnetic fields, the sensor element having a diamond structure with at least one nitrogen vacancy center. DE 102018 214 617 A1 discloses a sensor device which also has a crystal body with a number of color centers, in which various optical filter elements are used to increase effectiveness and for miniaturization. A sensor device has become known from the previously unpublished German patent application with the file number 10 2020 123 993.9, which evaluates a process variable of a medium on the basis of a fluorescence signal of a crystal body with at least one defect. In addition, the status of the respective process is monitored using a variable that is characteristic of the magnetic field, such as the magnetic permeability or magnetic susceptibility. A limit level sensor is also known from the previously unpublished German patent application with the file number 10 2021 100223.0, in which a statement about a limit level is determined on the basis of the fluorescence.

Another sub-area in the field of quantum sensors concerns gas cells, in which atomic transitions and spin states can be queried optically to determine magnetic and/or electrical properties, among other things. A gaseous alkali metal and a buffer gas are usually present in the gas cell. Magnetic properties of a surrounding medium can be determined by Rydberg states generated in the gas cell.

For example, gas cells are used in quantum-based standards that provide physical quantities with high precision. They have long been used in frequency standards or atomic clocks, as is known from EP 0 550240 B1.

US Pat. No. 1,0,184,796 B2 also discloses a chip-sized atomic gyroscope in which a gas cell is used to determine the magnetic field. An optically pumped magnetometer based on a gas cell is known from US Pat. No. 9,329,152 B2. By manipulating the atomic states in gas cells, further Open up fields of application for gas cells. For example, JP 4066804 A2 describes the use of gas cells to determine absolute path lengths. In addition, gas cells are also used as the starting point for microwave sources, as described in EP 1 224 709 B1.

Many measurement principles known from the prior art allow the respective medium to be characterized with regard to its magnetic and/or electrical properties. In this context, both invasive measuring devices, in which the respective sensor unit is brought into direct contact with the respective medium, and non-invasive measuring devices, in which the process variable is recorded outside the container, are used. Non-invasive measuring devices basically offer the advantage that no intervention in the process is necessary. However, such measuring devices have only been available to a limited extent so far, since many different factors, in particular relating to the measuring accuracy, have to be taken into account with regard to the achievable measuring accuracy and possible interference, for example due to the container wall or the environment.

Another aim is to continue to miniaturize while simultaneously increasing the performance of the respective sensors. Thus, such sensors are desirable that enable a comprehensive characterization of the respective medium with regard to many different physical and/or chemical properties. With regard to magnetic and/or electrical properties, precise devices for detecting changes in magnetic fields, magnetic fields and, depending on the sensor type, possibly also gravitational fields are required in this context.

The object of the present invention is therefore to specify a conductivity sensor which offers an alternative to conventional inductive and conductive conductivity sensors and is preferably suitable for detecting foreign bodies in the medium to be conveyed.

According to the invention, the object is achieved by a conductivity sensor for determining and/or monitoring an in particular specific electrical conductivity of a flowable medium in a container, in particular in a tank or through a pipeline, comprising:

- an excitation coil for generating a magnetic field in the medium;

- an operating circuit, which is set up to feed an electrical input signal into the excitation coil, the input signal being designed in such a way that the magnetic field generated thereby by means of the excitation coil excites mobile charge carriers in the medium to move; - a magnetic field-sensitive unit, which is set up to provide a measurement signal, in particular a fluorescence signal, which correlates with a change and/or a strength of a magnetic field generated by the mobile charge carriers of the medium, the magnetic field-sensitive unit comprising a sub-unit, which has an optically excitable material, the magnetic field-sensitive unit having an optical excitation unit for optically exciting the subunit and an optical detection unit for detecting the measurement signal, in particular the fluorescence signal; and

- an evaluation unit which is set up to determine the particular specific electrical conductivity of the medium at least on the basis of the measurement signal provided by the magnetic field-sensitive unit.

Analogously to conventional inductive conductivity sensors, the conductivity sensor according to the invention uses an excitation coil in order to set the charge carriers in the medium in motion. In contrast to inductive conductivity sensors, however, the conductivity sensor according to the invention is not based on measuring an induction voltage of a receiving coil, but instead directly measures a change and/or strength of the magnetic field that is generated by the movement of the charge carriers. Today's magnetic field-sensitive units, also called magnetic field sensors, are characterized by a very high sensitivity to magnetic fields, so that a precise measurement of the magnetic field is made possible both with an invasive and with a non-invasive arrangement of the magnetic field-sensitive unit. The electrical input signal of the excitation coil is, for example, an alternating current. The magnetic field-sensitive unit can consist exclusively of the sub-unit or of a large number of sub-units of the same type and/or other, in particular electronic, components such as electronic circuits, photo-sensitive units such as a photodiode, measurement signal conductors such as an optical waveguide, and/or structural components such as a carrier substrate.

The sub-unit of the magnetic field-sensitive unit preferably comprises a crystal body with at least one defect center or a gas cell. Under appropriate optical excitation, crystal bodies with at least one defect center or gas cells show a fluorescence signal which is dependent on a magnetic field present at the crystal body or the gas cell. Optionally, the crystal body is additionally excited with a microwave signal. Both the crystal body with the at least one defect center and the gas cell lead to an improvement in the measuring accuracy of the conductivity due to their high sensitivity to magnetic fields. In one possible configuration, the crystal body comprises a diamond with at least one nitrogen vacancy center, a silicon carbide with at least one silicon vacancy or a hexagonal boron nitride with at least one color vacancy center.

In a further embodiment, the gas cell comprises a cell enclosing at least one gaseous alkali metal.

The conductivity sensor comprises an excitation unit for optically exciting the subunit, i.e. the optically excitable material or the crystal body or the gas cell, and a detection unit for detecting a fluorescence signal from the crystal body or the gas cell, which correlates with the magnetic field acting on the subunit. Filters and mirrors as well as other optical elements can optionally be used in order to direct an excitation light to the crystal body or to the gas cell and/or the fluorescence signal to the detection unit. The crystal body can optionally have a frequency-dependent microwave signal applied to it, which is generated by a microwave unit that is part of the magnetic field-sensitive unit or that can be integrated in the magnetic field-sensitive unit or designed as a separate unit.

An advantageous embodiment provides that the conductivity sensor has a field guide body for guiding magnetic fields, with the magnetic field-sensitive unit, in particular the sub-unit, being arranged in a partial area of the field guide body.

By means of the field guide body, the magnetic fields generated by the medium to be guided and/or magnetic field changes caused can be better captured and guided to the magnetic field-sensitive unit. Thus, the specific electrical conductivity of the medium can be determined over a measuring tube cross-section through the appropriate number of magnetic field-sensitive units and a suitably designed field guide body.

Furthermore, small changes in the magnetic field caused by contamination in the medium can be detected by the field guide body even if the contamination does not directly pass through the magnetic field-sensitive unit.

An advantageous embodiment provides that the field guiding body is configured in sections in such a way that the magnetic field to be conducted is concentrated in the partial area. An advantageous embodiment provides that the partial area is designed as a narrowing of the field guide body.

An advantageous embodiment provides that the field guide body has a receptacle in the partial area, with the magnetic field-sensitive unit, in particular the sub-unit, being arranged in the receptacle.

An advantageous embodiment provides that the partial area has a gap, with the magnetic field-sensitive unit, in particular the sub-unit, being arranged in the gap.

The advantage of the previous configurations is that events which cause even the smallest changes in the magnetic field generated by the eddy currents in the conductive medium can be detected using the magnetic field-sensitive unit, since the magnetic field is guided by the field guide body to the magnetic field-sensitive unit.

An advantageous embodiment provides that the field guide body is designed as a sleeve which at least partially encloses the exciter coil, the exciter coil being arranged at least partially between an outer wall of the container and the field guide body.

The advantage of the configuration is that no openings are required in the container or in the pipeline for the conductivity sensor. In addition, a conductivity sensor that can be clamped on can also be implemented, which can be attached to the container or to the pipeline exclusively in a non-positive and/or positive manner.

An advantageous embodiment provides that the conductivity sensor comprises a medium-contacting housing in particular, wherein the field guide body, the excitation coil and the magnetic field-sensitive unit are arranged in the housing, wherein the

Field guide body is designed as a pot core, wherein the excitation coil is arranged in the pot core, in particular is wound around a portion of the pot core.

In the alternative embodiment, the excitation coil and the magnetic field-sensitive unit are arranged inside the housing. The housing can be on an outer

Lateral surface mounted or in an opening of the container can be arranged so that a front portion of the housing is in contact with the medium. In the second case, the conductivity sensor is designed as a plug-in sensor that can be plugged into the container, such as existing pipelines, and by means of the connection sockets provided is fastened and / or - is immersed by the operator in the medium to be examined - such as in laboratory and / or environmental applications.

An advantageous embodiment provides that the field guide body is designed such that the magnetic field to be conducted is concentrated in at least two sub-areas, with a magnetic field-sensitive unit, in particular a sub-unit, being arranged in each of the at least two sub-areas.

An advantageous embodiment provides that the evaluation unit is designed in such a way that it carries out a differential evaluation of the measurement signals provided by the at least two magnetic field-sensitive units, in particular the at least two sub-units, in particular the fluorescence signals.

The advantage of the two previous configurations is the improved sensitivity to events that do not result in a spatially homogeneous event

lead to magnetic field changes. If the conductivity of the medium changes uniformly over the measuring range of the conductivity sensor, this leads to a uniform change in the measuring signals. However, if the eddy currents in the medium change locally, for example due to the presence of non-conductive impurities, this can be detected with the differential evaluation.

An advantageous embodiment provides that the evaluation unit is set up to interpret a deviation of the measurement signal or a measurement variable dependent on the measurement signal from a monitoring criterion as caused by a foreign body.

According to the invention, the conductivity sensor is used to detect foreign bodies such as glass, metal splinters, etc. in the medium. For this one makes use of the fact that the essentially constant or only continuously changing eddy currents in the medium are disturbed by the contamination. This disturbance affects the generated magnetic field and can thus be detected with the magnetic field-sensitive unit and filtered out of the measurement signal determined.

An advantageous embodiment provides that the field guide body has exactly two, in particular exactly four sub-areas, in each of which exactly one magnetic field-sensitive unit, in particular exactly one sub-unit, is arranged.

In this way, the sensitivity of the conductivity sensor to contamination can be improved, so that even in large containers, especially in Monitoring of the medium can be guaranteed in pipelines with large nominal diameters.

The present invention will be explained in more detail below with reference to FIGS. 1-4. They show:

1: a simplified energy scheme for a negatively charged NV center in diamond;

2 shows a first embodiment of the conductivity sensor according to the invention;

3 shows a second embodiment of the conductivity sensor according to the invention;

4: a perspective view of a third embodiment of the conductivity sensor according to the invention;

5 shows a cross section through a further embodiment of the conductivity sensor according to the invention; and

6: a cross section through a further embodiment of the conductivity sensor according to the invention used in a pipeline.

A simplified energy scheme for a negatively charged NV center in a diamond is shown in FIG. 1 in order to exemplify the excitation and the fluorescence of a defect in a crystal body. The following considerations can be transferred to other crystal bodies with corresponding defects.

In diamond, each carbon atom is typically covalently bonded to four other carbon atoms. A nitrogen vacancy center (NV center) consists of a defect in the diamond lattice, i.e. an unoccupied lattice site, and a nitrogen atom as one of the four neighboring atoms. In particular, the negatively charged NV centers are important for the excitation and evaluation of fluorescence signals. In the energy scheme of a negatively charged NV center, in addition to a triplet ground state ³ A, there is an excited triplet state ³ E, each of which has three magnetic substates m _s =0,±1. Furthermore, there are two metastable singlet states ¹ A and ¹ E between the ground state ³ A and the excited state ³ E.

Excitation light 1 from the green range of the visible spectrum, e.g. excitation light 1 with a wavelength of 532 nm, excites an electron from the ground state ³ A into a vibrational state of the excited state ³ E, which emits a fluorescence photon 2 with a wavelength of 630 nm returns to the ground state ³ A. An applied magnetic field with a magnetic field strength B leads to a splitting (Zeeman splitting) of the magnetic sub-states, so that the ground state consists of three energetically separated sub-states, each of which can be excited. However, the intensity of the fluorescence signal depends on the respective magnetic substate from which the excitation took place, so that the distance between the fluorescence minima can be used, for example, to calculate the magnetic field strength B using the Zeeman formula. Further options for evaluating the fluorescence signal are provided within the scope of the present invention, such as evaluating the intensity of the fluorescent light, which is also proportional to the applied magnetic field. An electrical evaluation can in turn be carried out, for example, via a photocurrent detection of magnetic resonance (PDMR for short). In addition to these examples of evaluating the fluorescence signal, there are other options which also fall within the scope of the present invention.

The excitation of gas cells is not explicitly shown, but in the case of gas cells, too, excitation with light of a defined wavelength leads to an excitation of an electron, which is followed by the emission of fluorescent light. For example, the intensity and/or the wavelength of the fluorescent light emitted is used to determine the magnetic field.

2 shows a first embodiment of the conductivity sensor 3 according to the invention and a first embodiment of the measuring system 13 according to the invention, in which the conductivity sensor 3 is arranged outside the container 5, ie non-invasively. In the example of FIG. 2, the container 5 is a pipeline through which the medium 4 flows. Alternatively, the container 5 can be a tank.

The conductivity sensor 3 has an excitation coil 6 arranged around the pipeline, which feeds an electrical input signal into the medium 4, which is shown as the fed-in magnetic field B1. The fed-in magnetic field B1 induces an electric field E1, which leads to a partial movement of the mobile charge carriers of the medium 4 and thus to a current flow. This in turn results in a magnetic field B2. The magnetic field-sensitive unit 7 is arranged and designed in such a way that a change and/or a strength of the magnetic field B2 generated by the mobile charge carriers of the medium 4 can be determined. Furthermore, an evaluation unit 8 is provided in order to determine the specific electrical conductivity of the medium 4 at least on the basis of the magnetic field determined by the magnetic field-sensitive unit 7 . The arrows on the field lines of the magnetic fields B1, B2 and the electric field E1 indicate the direction of flow. As a rule, the excitation coil 6 outputs an alternating magnetic field, so that consequently the resulting electric fields E1 and magnetic fields B2 are alternating fields. For reasons of simplification, no alternating fields are shown in FIGS.

The induced electric field E1 leads to the flow of an electric current, i.e. to the flow of mobile charge carriers. An insulator 11 is optionally provided on the inner wall of the pipeline 5 . The insulator 11 is arranged on an area of an inner wall of the container in such a way that the mobile charge carriers of the medium 4 have to move across the insulator 11 . The container enclosing the medium is often made of a conductive material, so that the current generated by the movement of the charge carriers can propagate through the wall of the container. However, in order to correctly determine the magnetic field and the conductivity, the current must flow at least partially through the medium. For this purpose, an insulator is attached in sections to an inner wall of the container. The non-invasive variant in particular, in which only the insulator is arranged inside the container, offers the advantage of simple and hygienic attachment of the conductivity sensor. The magnetic field B2 caused by the movement of the charge carriers is determined by the magnetic field-sensitive unit 7 .

A crystal body with at least one defect center or a gas cell, for example, is used as the magnetic field-sensitive unit 7 . In the first case, it is, inter alia, a diamond with at least one nitrogen vacancy center, silicon carbide with at least one silicon vacancy or hexagonal boron nitride with at least one vacancy color center. A cell enclosing at least one gaseous alkali metal, for example, can be used as the gas cell. Both the crystal body with at least one defect center and the gas cell each require an excitation unit 9 for optical excitation of the crystal body or gas cell and a detection unit 10 for detecting a magnetic field-dependent fluorescence signal of the crystal body or gas cell. The excitation unit 9 and/or the detection unit 10 can optionally be arranged in the area of the magnetic field-sensitive unit 7, but they can also be arranged away from the magnetic field-sensitive unit 7, for example using optical light guides.

The measuring system 13 for determining and/or monitoring the specific electrical conductivity of a medium 4 in a container 5 consists of at least the conductivity sensor 3 and a temperature sensor 14 for determining the temperature of the medium 4, which is here, for example, outside the container 5 is arranged. In principle, it is also possible to attach the temperature sensor 14 inside the container 5 . For example, a platinum resistance thermometer or a crystal body with at least one defect can be used as the temperature sensor 14 . The evaluation unit 8 uses the magnetic field determined by the magnetic field-sensitive unit 7 and the temperature determined by the temperature sensor 14 to determine the specific electrical conductivity of the medium 4 at a defined temperature.

A second embodiment of the measuring system 13 and the conductivity sensor 3 is shown in FIG. In this case, an invasive application of the

Conductivity sensor 3 and the temperature sensor 14 shown. The excitation coil 6 is optionally provided with a magnetic shield 12 in order not to disturb the magnetic field B2 in the area of the magnetic field-sensitive unit 7 . Fig. 4 shows a third embodiment of the conductivity sensor 3 for determining and / or monitoring a particular electrical conductivity of a flowable medium in a container 5 - in the case in a pipeline - comprising an excitation coil 6 for generating a magnetic field in the medium, a Operating circuit 19, which is set up to feed an electrical input signal into the excitation coil 6. The input signal is designed in such a way that the magnetic field thus generated by means of the excitation coil 6 excites mobile charge carriers in the medium to move. Furthermore, the conductivity sensor includes a magnetic field-sensitive unit 7, which is set up to provide a measurement signal, in particular a fluorescence signal, which is generated by the mobile charge carriers of the medium with a change and/or a strength

correlated magnetic field. In the case shown, the conductivity sensor 3 includes exactly four magnetic field-sensitive units 7, with two magnetic field-sensitive units 7 being covered by the container. According to the invention, the magnetic field-sensitive unit 7 includes a sub-unit 15, which includes an optically excitable material and an optical excitation unit 9 for optical excitation of the sub-unit 15 and an optical detection unit for detecting the measurement signal. The measurement signal can be a fluorescence signal that is generated by the material and by the detection unit. Furthermore, the conductivity sensor includes an evaluation unit which is set up to determine the specific electrical conductivity of the medium at least on the basis of that provided by the magnetic field-sensitive unit 7

to determine the measurement signal. The sub-unit 15 of the magnetic field-sensitive unit 7 can comprise a crystal body with at least one defect center or a gas cell. According to the case that the subunit 15 has a crystal body, this comprises a diamond with at least one nitrogen vacancy center, a silicon carbide with at least one silicon vacancy or a hexagonal boron nitride at least one defect color center. In the event that the sub-assembly 15 comprises a gas cell, this comprises at least one cell enclosing a gaseous alkali metal. The conductivity sensor 3 shown has a field guiding body 16 for guiding magnetic fields. The magnetic field-sensitive unit 7, in particular the sub-unit 15, is arranged in a partial area 17 of the field-guiding body 16. The field guiding body 16 is configured in sections in such a way that the magnetic field to be conducted is concentrated in the partial area 17 . According to the illustrated embodiment, the partial area 17 is designed as a taper. In addition, a receptacle can be provided in the partial area 17, in which the magnetic field-sensitive unit 7, in particular the sub-unit 15, is arranged.

Alternatively, the partial area 17 can have a gap in which the magnetic field-sensitive unit 7, in particular the sub-unit 15, is arranged. The field guide body 16 is designed as a sleeve which encloses the excitation coil 6 at least in sections. The exciter coil 6 is arranged at least in sections between an outer wall of the container 5 and the field guide body 16 . The conductivity sensor can be used to detect foreign bodies. For this purpose, an evaluation unit 8 is set up to interpret this as caused by a foreign body if the measurement signal or a measurement variable dependent on the measurement signal deviates from a monitoring criterion.

FIG. 5 shows a cross section through an embodiment in which the field guide body 16 is designed in such a way that the magnetic field to be guided is concentrated into four independent partial areas. The field guide body 16 has a taper in each of the partial areas. A magnetic field-sensitive unit, in particular a sub-unit 15.1, 15.2, 15.3, 15.4, is arranged in each of the partial areas. Alternatively, a gap can be provided in the partial area in which the sub-unit 15 is inserted. According to the configuration shown, the evaluation unit 8 is designed in such a way that it carries out a differential evaluation of the measurement signals, in particular sub-unit measurement signals, provided by means of at least two magnetic field-sensitive units or by means of all magnetic field-sensitive units, in particular their four sub-units 15.1, 15.2, 15.3, 15.4 . In this case, the evaluation unit 8 is set up to interpret a deviation of the measurement signal or a measurement variable dependent on the measurement signal from a monitoring criterion as caused by a foreign body. Events caused by contamination can thus be detected. For this purpose, the evaluation circuit 8 can take into account a conductivity value provided by the operator. Furthermore, the excitation coil 6 is arranged between the field guide body 16 and the tube wall of the container 5 . In the case shown, the excitation coil 6 is wound around a section of the pipeline. FIG. 6 shows a further embodiment of the conductivity sensor 3, which is inserted into an opening of a container 5. In this case, the conductivity sensor 3 comprises a medium-contacting housing 18 in which the excitation coil 6 and the magnetic field-sensitive unit 7 are arranged. The field guide body 16 is designed as a pot core, with a receptacle in which the magnetic field-sensitive unit is arranged. The exciter coil 6 is arranged on the pot core, in particular the exciter coil 6 is wound around a portion of the pot core. The conductivity sensor 3 is inserted into an opening and connected to the container 5 by means of a connecting piece 20 . A temperature sensor 14 is arranged in a front section in contact with the medium. This can be in contact with the medium.

Alternatively, the housing 18 of the conductivity sensor 3 can also be attached to the outer surface of a pipeline.

reference list

1 excitation light

2 fluorescent light 3 conductivity sensor

4 media

5 containers

6 excitation coil

7 magnetic field-sensitive unit 8 evaluation unit

9 excitation unit

10 detection unit

11 insulator

12 magnetic shielding 13 measuring system

14 temperature sensor

15 subunit

15.1 Subunit

15.2 Subunit 15.3 Subunit

15.4 Subunit

16 field guide bodies

17 section

18 housing 19 operating circuit

20 connecting pieces

B1 magnetic field fed into the excitation coil E1 electric field

B2 Magnetic field resulting from the movement of charge carriers

Claims

patent claims 1 . Conductivity sensor (3) for determining and/or monitoring an in particular specific electrical conductivity of a flowable medium (4) in a container (5), in particular in a tank or through a pipeline, comprising: an excitation coil (6) for generating a magnetic field in the Medium, an operating circuit which is set up to feed an electrical input signal into the exciter coil (6), the input signal being designed in such a way that the magnetic field generated by means of the exciter coil (6) excites mobile charge carriers in the medium (4) to move, a magnetic field-sensitive unit (7), which is set up to provide a measurement signal, in particular a fluorescence signal, which correlates with a change and/or a strength of a magnetic field generated by the mobile charge carriers of the medium (4), the magnetic field-sensitive Unit (7) comprises a sub-unit (15) comprising an optically stimulable material sst; wherein the magnetic field-sensitive unit (7) has an optical excitation unit (9) for optical excitation of the subunit (15) and an optical detection unit (10) for detecting the measurement signal, in particular the fluorescence signal; and an evaluation unit (8) which is set up to determine the specific electrical conductivity of the medium at least on the basis of the measurement signal provided by the magnetic field-sensitive unit (7). 2. conductivity sensor (3) according to claim 1, wherein the sub-unit (15) of the magnetic field-sensitive unit (7) comprises a crystal body with at least one defect center or a gas cell. 3. Conductivity sensor (3) according to the preceding claim, wherein the crystal body comprises a diamond with at least one nitrogen vacancy center, a silicon carbide with at least one silicon vacancy or a hexagonal boron nitride with at least one vacancy color center. 4. conductivity sensor (3) according to claim 2, wherein the gas cell comprises at least one cell confining a gaseous alkali metal. 5. conductivity sensor (3) according to at least one of the preceding claims, wherein the conductivity sensor (3) has a field guide body (16) for Comprises guiding magnetic fields, wherein the magnetic field-sensitive unit (7), in particular the sub-unit (15) in a portion (17) of the field guide body (16) is arranged. 6. Conductivity sensor (3) according to claim 5, wherein the field guide body (16) is configured in sections such that the magnetic field to be guided in the partial area (17) to concentrate. 7. conductivity sensor (3) according to at least one of the two preceding claims, wherein the portion (17) is designed as a tapering of the field guide body (16). 8. Conductivity sensor (3) according to at least one of claims 5 to 7, wherein the field guide body (16) has a receptacle in the partial area (17), the magnetic field-sensitive unit (7), in particular the sub-unit (15) being arranged in the receptacle is. 9. conductivity sensor (3) according to at least one of claims 5 to 8, wherein the portion (17) has a gap, wherein the magnetic field-sensitive unit (7), in particular the sub-unit (15) is arranged in the gap. 10. Conductivity sensor (3) according to at least one of claims 5 to 9, wherein the field guide body (16) is designed as a sleeve which encloses the excitation coil (6) at least in sections, the excitation coil (6) being at least in sections between an outer wall of the container ( 5) and the field guide body (16) is arranged. 11. Conductivity sensor (3) according to at least one of claims 5 to 10, wherein the conductivity sensor (3) comprises a housing (18) which in particular comes into contact with the medium, the field guiding body (16), the excitation coil (6) and the magnetic field-sensitive unit (7) being arranged in the housing (18), the field guiding body (16) being designed as a pot core, the excitation coil (6) in the pot core is arranged, in particular is wound around a portion of the pot core. 12. Conductivity sensor (3) according to at least one of claims 5 to 11, wherein the field guide body (16) is designed in such a way that the magnetic field to be guided is concentrated in at least two partial areas (17.1, 17.2), wherein in the at least two partial areas (17.1 , 17.2) in each case a magnetic field-sensitive unit (7.1, 7.2), in particular a sub-unit (15) is arranged. 13. Conductivity sensor (3) according to the preceding claim, wherein the evaluation unit (8) is designed such that a differential evaluation of the measurement signals provided by the at least two magnetic field-sensitive units (7.1, 7.2), in particular their sub-units (15.1, 15.2). , in particular to carry out subunit measurement signals. 14. Conductivity sensor (3) according to at least one of claims 5 to 13, wherein the evaluation unit (8) is set up to interpret this as caused by a foreign body if the measurement signal or a measurement variable dependent on the measurement signal deviates from a monitoring criterion. 15. Conductivity sensor (3) according to at least one of claims 5 to 14, wherein the field guide body (16) has exactly two, in particular exactly four partial areas (17.1, 17.2, 17.3, 17.4), in each of which at least exactly one subunit (15), in particular exactly one magnetic field-sensitive unit (7.1, 7.2, 7.3, 7.4) is arranged.