WO2015062766A1

WO2015062766A1 - Device and system for determining refractive index

Info

Publication number: WO2015062766A1
Application number: PCT/EP2014/068208
Authority: WO
Inventors: Thomas Blödt; Sergej Lopatin; Ronja Binczyk
Original assignee: Endress+Hauser Gmbh+Co. Kg
Priority date: 2013-10-31
Filing date: 2014-08-28
Publication date: 2015-05-07
Also published as: DE102013112026A1

Abstract

The invention relates to a device for determining the refractive index which is created on the boundary surface to a medium (4), comprising a self-excited system which consists of - an active high-frequency assembly (5) for generating electromagnetic waves of an operating frequency within a predetermined frequency band, - a sensor (2) which is supplied with said electromagnetic waves and which is at least temporarily arranged relative to the medium (4) such that the phase position of the electromagnetic waves reflected on the boundary surface changes under the influence of the medium (4), and - a feedback loop (6, 7, 8, 9) which feeds back the electromagnetic waves to the sensor (2), thereby determining the frequency of the self-excited system. The device further comprises a frequency detector which detects the frequency of the self-excited system, and an evaluation unit (21) which determines the refractive index on the boundary surface to the medium (4) on the basis of the detected frequency.

Description

Device and system for determining the refractive index

The invention relates to a device and a system for determining the

Refractive index, which occurs at the interface to a medium or between two media with different refractive indices.

The refractive index is also called the refractive index and corresponds to the ratio of the speed of light of electromagnetic waves in the vacuum to the propagation speed in the medium. The complex refractive index is linked to permittivity and permeability. Here, the permittivity is a measure of the permeability of a material for electric fields and the permeability is a measure of the permeability of a material for magnetic fields. Permittivity and dielectric conductivity are synonymous terms. The relative permittivity, also called

Permittivitätszahl or referred to as the dielectric constant, describes the permittivity of a medium to the permittivity of the vacuum. Magnetic substances are classified by permeability as diamagnetic, paramagnetic or ferromagnetic substances.

In automation technology, in particular in process automation but also in factory automation, field devices are used which are used to determine and monitor process variables. The term 'field devices' in the context of the invention, both actuators and sensors understood.

Sensors record, for example, the various process variables such as level, flow, pH, turbidity, substance concentration, pressure, temperature, humidity, conductivity, density and viscosity. A large number of such field devices are offered and distributed by the Endress + Hauser Group.

Important process variables are also the humidity and the

Dielectric constant. More generally, the present invention relates to the determination or monitoring of the refractive index that occurs at the interface to a medium. Depending on the material properties of the medium to be monitored, it is therefore possible with the invention to obtain both statements about the relative permittivity and about the permeability. An important field of application of the present invention is described below: Field devices have both electrical / electronic and mechanical components. For electrical / electronic and mechanical components water condensate or water vapor is often problematic because condensed water has a high dielectric constant and a high conductivity. Therefore, deposits in the form of condensate usually interfere with the normal operation of the field devices. In addition, condensate can also lead to corrosion of the metallic parts of the field devices. Therefore, the protection against water condensate and

Water vapor plays an important role in automation technology.

To protect the field devices from water condensate and water vapor, various types of water adsorbers are used. Suitable water adsorbers are silica gels and zeolites. Both adsorbers are able to effectively bind water molecules, thus keeping the electrical and mechanical components dry.

Silica gel is one of the simplest water adsorbers. Silica gel is an amorphous silica with irregular pores. Have such pores

typically a broad size distribution ranging from 1 nm to 5 nm or from 5 nm to 30 nm, depending on the manufacturing process. Silica gel therefore adsorbs all molecules that fit into the pores because of their size. It also has an increased affinity for the polar water molecules. Silica gels are one of the moderate water adsorbers: they can easily adsorb and desorb the water depending on humidity in the ambient atmosphere. The fine-pored silica gels absorb up to 40-50% by mass of water. The adsorbed water can be released almost completely under light vacuum or at elevated temperatures again. This is the

Scope for silica gel restricted.

Zeolites are inorganic aluminosilicate-based crystals with nanochannels that can bind molecules of a given size in the channel lattice. Such substances are also known by a term 'molecular sieves'. As zeolites, a group of substances with over 200 known individual substances, which are usually produced artificially, called. These substances have a complex crystal structure and can adsorb different molecules from water to amino acids and peptides. For selective water adsorption two zeolites with a channel opening of 0.4 nm (type 4A) and 0.3 nm (type 3A) are used. These substances are widely used in industry as drying agents and can effectively adsorb up to 25% by mass of water. A major advantage of zeolites type 3A and 4A compared to the desiccant silica gel is the very narrow pore size distribution. This guarantees excellent selectivity and efficiency for the adsorption of water molecules. In addition, zeolite binds the water strongly exothermic (67 kJ / mol at room temperature) and binds the water to 100-120 ° C. Complete dehydration under normal atmospheric pressure takes place at 300-450 ° C. For effective use of the drying agents in electrical / electronic and mechanical equipment, the water absorption capacity of the silica gel or zeolite adsorbers is usually constantly monitored. For silica gel in particular colored indicators are known. These are substances that change their color as soon as the moisture content in the silica gel reaches a certain limit. Such

However, indicators do not allow for constant monitoring of the adsorption process.

In a preferred field of application of the present invention, the effect is exploited that the water absorption in adsorber materials such as zeolite or silica gel changes the electrical properties of the respective materials. Therefore, the electrical properties can serve as a measure of the water adsorption: by determining the relative permittivity, the degree of saturation of the adsorber can be determined very accurately and continuously. It is also possible, the present invention for

Surface analysis or to analyze a medium. In particular, statements can be made about the purity of media.

For an observation of the water absorption capacity in zeolite-containing materials, various electrical measuring methods have already been proposed. In particular, it is known that there is a clear dependence between the conductivity and the water content in zeolite membranes. In US 3,186,225 a sensor has been proposed for the purpose of moisture control. In the known sensor, the conductivity is determined in a zeolite body by means of built-in measuring electrodes. US 2009/0261987 A1 describes an analytical sensor which includes the adsorbing zeolite particles in a matrix between two measuring electrodes.

In WO 201 1/124419 A1, a method for determining the proportion of an adsorbed substance which is contained in a shaped body, granules or powder of a zeolite, a zeolite compound or silica gel as an adsorbent material is presented. In the event that the adsorbent material is in the form of a shaped body, at least two electrodes are spaced from each other on a surface of the shaped body applied and / or firmly inserted into the molded body. In the case where the adsorbent material is in the form of a powder or granules, a molded article of the same material is prepared and two electrodes are spaced apart on a surface of the molded article or fixedly inserted into the molded article; Subsequently, the shaped body is introduced into the powder or granules. The electrodes are subjected to an alternating current, whereby an electrical characteristic of the adsorbent material is determined. On the basis of the determined electrical parameter, the proportion of the adsorbed substance in the

Adsorber material determined. The known methods for measuring the proportion of water in zeolite have two

Disadvantage:

1 . The zeolite probe with electrodes is electrically connected to the actual measuring unit. Electrical connections may be due to e.g. the oxidation of the

Electrodes be problematic. Furthermore, the drying agents usually exist in granules (diameter to 3-4 mm) or powder form. For corresponding free-flowing substances, a measurement of the conductivity or capacity with immersed electrodes is uncertain.

2. The electrical measurement is carried out with at least two electrodes, which are burned or sputtered onto the measuring body. If the electrode is released from the material due to liability errors, the measured values are evaluated incorrectly. In the worst case, a water-saturated state is then not detected by an electrical measurement. In addition, increased by the

Water absorption the volume of zeolites. As a result, critical mechanical shear stresses can occur in the electrode area, which can also lead to detachment of the electrodes.

The invention is based on the object, a device and a system

to propose, over which the refractive index, which occurs at the interface between two media, can be determined with high accuracy.

The object is achieved by a device for determining the refractive index, which occurs at the interface to a medium

with a self-excited system consisting of

- An active high-frequency module for generating electromagnetic waves lying within a predetermined frequency band

Operating frequency,

a sensor, which is acted upon by the electromagnetic waves and which is arranged at least temporarily with respect to the medium in such a way that the phase position of the electromagnetic waves reflected at the boundary surface changes under the influence of the medium, and

a feedback circuit which returns the electromagnetic waves to the sensor, thereby determining the frequency of the self-excited system, with a frequency detector detecting the frequency of the self-excited system, and an evaluation unit which, based on the detected frequency, assigns the refractive index at the interface determined the medium. The device according to the invention is also referred to below as a microwave resonator.

Depending on the conditions prevailing at the interface with the medium, the device according to the invention is able to make statements regarding the electrical or magnetic properties or the changing electrical or magnetic properties of the medium. The possible uses of

erfindungsemäßen microwave resonator are many. In addition to the applications already mentioned, it is e.g. also possible to use the microwave resonator for the detection and differentiation of operating materials, e.g. to use of oils in production.

The self-excited system oscillates in the steady state on a defined oscillation frequency, which is located in the microwave range. The self-excited oscillating frequency of the self-excited system, that is the microwave resonator, depends on the refractive index at the interface with the medium, wherein the microwave resonator must be arranged with respect to the interface so that the

Oscillation frequency is influenced by the properties of the medium. Thus, the microwave resonator must be located in close proximity to the medium or in the medium. In the case of determining the moisture content of an adsorbent material, e.g. the determination of the water content in a water-adsorbing

Shaped bodies, e.g. From zeolite, the oscillation frequency changes depending on the moisture content of the molding, since with increasing moisture content the

Dielectric constant increases. Measured in connection with the invention, the oscillation frequency or the frequency change due to the influence of the medium.

For this purpose, a frequency counter is preferably used as the frequency detector.

According to an advantageous embodiment, the sensor consists of an electrode which is arranged on an insulating material. The electrode and the insulating material and optionally a shaped body in this case form a measuring cell.

An advantageous embodiment of the device according to the invention provides that the frequency band in which the self-excited system is oscillatable outside the

Natural frequency of the sensor is located. The natural frequency of the sensor is reflected by the geometric dimensions of the sensor or the transit time of the sensor

determined electromagnetic waves within the sensor. As a result, the self-excited system has a low quality; this is very advantageous for the self-excited system. In addition, the frequency deviating from the natural frequency prevents the sensor from acting as an antenna and absorbing or emitting large amounts of energy. Thus, interference is reduced. The medium is preferably a liquid or gas-adsorbed solid, liquid or gaseous substance.

In the system according to the invention is a previously in different

Embodiments described device on the outside of the wall of a

Container or disposed in the wall of a container, wherein in the container is the medium to be monitored. The wall consists of a transparent material for the electromagnetic waves. The thickness of the wall of the container between the sensor and the medium is dependent on the

Dielectric constants or the magnetic permeability of the medium to be monitored. The higher the dielectric constant of the medium, the thicker the wall can be made.

Preferably, the container is a tank or a bag. In the pharmaceutical or biotechnological application often disposable or disposable bags are used, which are disposed of after a single use. This poses no problem, since the measuring cell of the device according to the invention or of the system according to the invention can be produced cost-effectively and if necessary can be recycled. In addition to the above-described integral solution of sensor or inventive device and container is provided that the sensor is mounted on the outside of the wall of the container or is temporarily brought into contact with the outside of the wall of the container. An advantageous embodiment of the system according to the invention proposes that the measuring cell is shaped so that it is adapted in the region of attachment to the geometry of the container or in the case of a solid medium to the geometry of the medium. Preferably, the device according to the invention or the system according to the invention is used to determine and / or monitor the adsorption of the water fraction in a water-adsorbing material, wherein the water

adsorbing material is preferably a zeolite or a silica gel. As already mentioned above, the invention is furthermore suitable, for example, for analysis purposes. By way of example, statements about the composition of the medium influencing the oscillation frequency of the microwave resonator can be generated via the change in the refractive index. In particular, it is possible to make a statement about the purity of a medium or a surface coating to meet. Decisive for use in possible

Areas of application is that the material to be monitored or determined has an electrical or magnetic characteristic which also changes as the composition of the material changes, and which subsequently affects the

Oscillation frequency of the device according to the invention or the system according to the invention influenced.

Below are some examples of applications for the inventive solution called:

1. Measurement of the moisture content and determination of the moisture absorption capacity of the drying agent in the form of a solid, granules or powder.

2. Constant observation of a water adsorber for the protection of

electronic and mechanical devices is used.

3. Continuous Moisture Dosimeter.

4. Observation of adsorption in zeolites or other adsorbers intended for the adsorption of defined molecules.

The invention will be explained in more detail with reference to the following figures. It shows:

1 shows an embodiment of the device according to the invention which is suitable for use in a solid adsorbent material,

2 shows an embodiment of the device according to the invention which is suitable for use in an adsorbent material made from a granulate or powder,

3 shows a diagram which reproduces the frequency characteristic of a microwave resonator according to the invention under the influence of a solid adsorber material; FIG. 4 shows a diagram which reproduces the frequency profile of a microwave resonator according to the invention under the influence of a granular adsorber material;

5 is a diagram showing the functional dependence of the frequency of a

microwave resonator according to the invention shows the proportion of water in a solid Adsorbermaterial,

6 shows a first embodiment of a circuit arrangement for detecting the frequency of a microwave resonator according to the invention and 7 shows a second embodiment of a circuit arrangement for detecting the frequency of a microwave resonator according to the invention.

Fig. 1 shows an embodiment of the device according to the invention, which can be used for example for determining or monitoring the moisture content of an adsorbent material. The measuring cell 1 is suitable for use in connection with a shaped body 4, which consists of an adsorbent material. In the case shown, the measuring cell 1 serves to determine or monitor the moisture content in a shaped body 4 made of a zeolite-containing composite material.

The measuring cell 1 has an electrode 2 which is arranged on a carrier material 13. The electrode 2, the insulating material 3 and the carrier body 13 form the

Measuring cell 1. The measuring cell 1 and the electronic components 5, 6, 7, 8, 9 form the microwave resonator according to the invention or the self-excited system. The carrier material 13 of the measuring cell 1 is preferably adapted to the geometry of the shaped body 4: In the case shown, the carrier material 13 is designed as a half pipe and is dimensioned such that it can receive the correspondingly configured cylindrical shaped body 4 of an adsorbent composite material. Preferably, the measuring cell 1 is used with the molding 4 for drying the air in an electronic device, in particular in a field device of automation technology. It goes without saying that the invention can be used in any application. The measuring cell 1 shown is suitable, for example, for controlling the degree of saturation in a zeolite or silica gel-based moisture adsorber. The determination of the refractive index or in this case the dielectric constant works as follows: With respect to the microwave with a defined oscillation frequency

acted upon electrode 2 of the measuring cell 1 is the shaped body 4 of a

zeolite-containing composite material or silica gel arranged so that the oscillation frequency of the microwave resonator is influenced by the composite material of the molded body 4. The microwaves are generated by a high-frequency functional unit 5 or an active high-frequency module 5. Together with a decoupling network 6, a sinusoidal signal S is generated whose period length or

Frequency component reflects the dielectric constant or the current saturation level of the moist-adsorbent molding 4.

The following modules are involved in the measurement: the electrode 2 or the

High-frequency element 2, the high-frequency functional unit 5 and the active

High-frequency module 5, the passive high-frequency module 7 and the

Decoupling Network 6. The majority-passive-frequency-determining element is the Electrode 2. This is connected to the active high-frequency module 5. The passive high-frequency module 7 is set as a minority frequency-determining passive element fixed to an operating frequency. The passive high-frequency module 7 includes the necessary for the generation of vibrations Beschaltungswerk 8 and the power supply 9. In one embodiment, for the purpose of

Frequency fine adjustment (minority adjustment) nor the supply of a

Control voltage provided. This is optional. About the decoupling network 6, the dependent of the degree of saturation of the shaped body 4 signal S is tapped. To the

To increase measurement accuracy, the input of the control voltage is a

Temperature-dependent voltage supplied to compensate for the influence of temperature.

The electrode 2 is made of an electrically conductive material, for example, it is a copper-etched structure in an elongated, round or square shape. In the case shown, the shape is square. Over the electrode 2, an insulating layer 3 is provided for the purpose of process separation, which consists for example of PP or PTFE. An electrical connection between the molded body 4 of e.g. zeolite

Composite material and the electrode 2 may, but need not exist. Will the

Insulating layer 3 is used, the electrode 2 and the molded body 4 are not electrically connected to each other. Preferably, the connection between the electrode 2 and the active high-frequency module 5 may consist of a periodically meandering structure. A

corresponding structure makes with respect to the heat conduction a relatively large

Resistance; however, the high-frequency waves are only a short connection line. In the apparatus according to the invention or in the

The system according to the invention preferably has the following operating or

Vibration frequencies used: 2.4 GHz, 433 MHz, 866 MHz or 5.8 GHz. In general, at least any frequency in the frequency range between 300 MHz and 30 GHz can be used. Fig. 2 shows a measuring structure which is suitable for use in a water adsorber, if it consists of a granulate or a powder. The measuring cell 1 a for the adsorbent granules or powder has a measuring electrode 2 a. In addition to the actual measuring cell 1a, a reference measuring cell 1b is provided with a reference electrode 2b. If the measuring cell 1a is to measure the moisture content in a zeolite or silica gel granulate or powder, the reference measuring cell 1b preferably contains a water-saturated or a dry zeolite or silica gel. The measuring cell 1 a high-frequency waves are supplied from the generator 5a; In the reference measuring cell, high-frequency waves are supplied from the reference generator 5b. While the frequency the reference measuring cell 1 b remains constant, the frequency of the high-frequency waves, which are supplied to the measuring cell 1 a, depending on the degree of saturation of the zeolite or silica gel granules or powder changes. The frequency difference between the high-frequency waves of measuring cell and reference measuring cell is in the

Frequency difference images 10 determined. The difference frequency is from the

Frequency detector 1 1 detected. Based on the temperature-compensated difference frequency, the evaluation unit 21 determines the degree of saturation of the granules or powder to be monitored. As already mentioned above in connection with the embodiment described in FIG. 2, the solution according to the invention can be used not only for shaped articles 4 made of an adsorbent material but also for granulated or powdered moisture adsorbers, such as silica gel and zeolite. In this case, granules are measured in a measuring cell 1 a, so to speak, as bulk material. The frequency change is referenced with a preferably identical measuring cell 1 b with a dry or water-saturated adsorber. The temperature-compensated frequency difference is a measure of the moisture content in the granulate or powder.

The measuring methods described above enable a cost-effective, continuous monitoring of the degree of saturation in a water adsorber. Preferably, the

Monitoring through a container wall or a packaging film therethrough. As already mentioned above, the device according to the invention can be used in a closed housing of a field device:

Adsorber material binds the water molecules in the closed housing and the moisture content is determined continuously or temporarily. The measurement data can be used to make an early statement as to when

Adsorber material goes into the saturation state and will no longer be able to bind additional moisture. The solution according to the invention is also suitable for detecting a leak in the seal and a saturated state of the adsorber material in good time before the device fails. On the basis of this information, appropriate countermeasures can be taken early. The

inventive solution is therefore for Predictive Maintenance and Advanced

Diagnostics measures ideally suited. Fig. 3 shows a diagram showing the frequency response of a fiction, according to the invention

Microwave resonator for a Adsorbermaterial reproduces. In the case shown, the adsorber material is a solid composite of 65% zeolite 4A and 35% PFA. The water absorption from the ambient air takes place in the case shown on a readable on the X-axis period at room temperature. At the starting point is the Moisture content of the adsorber material zero, at the end point of the saturation state of the adsorbent material is reached. It can be clearly seen from the diagram that the

Oscillation frequency of the microwave resonator decreases almost continuously with increasing proportion of water. The higher the water content in the adsorber material, the higher the dielectric constant of the adsorber material becomes and the more intense the interaction of the microwaves with the adsorber material. The drop between the starting point and the end point is in the case shown at 18 MHz, so that the microwave resonator according to the invention is characterized by a high measuring sensitivity.

The diagram shown in Fig. 4 shows the frequency response of a microwave resonator according to the invention for a granular adsorber material. In particular, FIG. 4 shows the typical behavior of the oscillation frequency in a measuring cell with a dry silica gel (1-0, 1 mm) during the absorption of water from ambient air at room temperature. Again, the frequency change of the oscillation frequency is a measure of the respective degree of saturation of the adsorbent material.

FIG. 5 shows a diagram which shows the functional dependence of the frequency of a microwave resonator according to the invention on the degree of saturation or the proportion of water (in%) in a solid adsorbent material, e.g. in zeolite shows. With increasing proportion of water in the zeolite adsorber, the oscillation frequency of the microwave resonator decreases continuously. The measuring sensitivity is particularly high when the proportion of water in the zeolite is 5-6%. Therefore, the solution according to the invention is particularly effective for e.g. for detecting a leakage at a field device in the initial phase (e.g., immediately following manufacture) when the adsorptive power of the adsorbent material has the highest efficiency.

6 shows a first advantageous circuit arrangement for detecting the frequency of a microwave resonator according to the invention. On the line 12 is a

High-frequency signal given. This high frequency signal is at least approximately sinusoidal. The high-frequency signal originates for example from the electronic part 14 of a field device, in particular a sensor of the automation technology, which works on the basis of microwaves. For example, the sensor is a radar level gauge.

The circuit is designed so that it realizes the frequency of the high-frequency signal with the least possible effort and with the lowest possible cost of assemblies and power consumption. The high-frequency signal is supplied to the high-frequency mixer 15. The high-frequency mixer 15 is further a high-frequency signal supplied in the high frequency oscillator 16. The high-frequency oscillator 16 is a VCO whose output frequency is

according to a control voltage. In the technical embodiment, for example, a cost-effective VCO from the WI_AN segment can be used. At the output of the high-frequency mixer 15 is a signal, which consists of a superposition of the sum and the difference between the two frequencies

High-frequency signals exist that of the Feldgeräteelektronik14 and the

High frequency generator 16 come. In the simplest case, there is the

High frequency mixer 15 from a single diode.

At the output of the high-frequency mixer 15, a low-pass filter 17 is attached. in the

In the simplest case, the low pass 17 is a passive low pass consisting of a resistor and two capacitors. The low-pass filter 17 is dimensioned so that only one frequency is transmitted to 0 Hz (DC voltage) with high amplitude. The low-pass filter 17 is followed by a rectifier 18, which acts as a detector. The rectifier 18 consists for example of at least one diode, which is advantageously supplemented by a capacitor / of a capacitor. The interconnection of low pass 17 and the rectifier 18 causes a

DC signal transmitted and the usually sinusoidal

High frequency signal is increasingly attenuated with increasing frequency. The output signal of the two modules 17, 18 is fed to a control circuit or a control circuit 19. This adjusts the high-frequency oscillator 16 so that the amplitude of the output signal of the rectifier 18 is maximized. Consequently, the frequency of the output signal 20 (or the output voltage) of the VCO 16 corresponds to the frequency of the high-frequency signal applied to the line 12

Field device electronics 14.

In a modification to the circuit arrangement described above, instead of the low-pass filter 17, a narrow-band bandpass filter can be used. As a result, the maximum amplitude at the rectifier 18 is not at 0 Hz (DC voltage), but at the transmission frequency of the bandpass filter. Consequently, the high-frequency oscillator 16, which is preferably designed as a VCO, is set to the difference frequency between the high-frequency signal delivered by the field device electronics 14 and the high-frequency signal present at the output of the bandpass filter. The measured oscillation frequency can thus be calculated as follows, the evaluation in the

Evaluation unit 21 takes place:

Frequency of the field device electronics 14 =

Frequency of the VCO 16 + transmission frequency of the low-pass filter 17 In order to further reduce the cost of the circuit arrangement, it is provided that the VCO 16 is replaced by a much cheaper high-frequency oscillator with a fixed operating frequency. In this case, the bandpass filter is replaced by a tunable bandpass filter. This can be realized, for example, by replacing the capacitances associated with the bandpass filter with varactor diodes. The varactor diodes receive a control voltage. The control unit 19 and the output signal 20 are then arranged between the rectifier 18 and the bandpass. By the way, the high-frequency mixer 15 is preferred in all cases

described embodiments operated advantageously in saturation.

FIG. 7 shows a further advantageous circuit arrangement for detecting the frequency of the microwave resonator according to the invention. The circuit arrangement shown is much simpler and thus cheaper, but the

Measurement accuracy also lower. The high frequency signal 12 provided by the field device electronics 14 is fed to a bandpass filter 22. Of the

Bandpass filter 22 has a frequency-dependent characteristic. Preferably, the

Bandpass filter 22 is designed as a passive filter to reduce temperature dependencies. The high-frequency signal 12 must have a constant amplitude or be stabilized by a limiter amplifier (not shown separately in FIG. 7) connected between the field device electronics 14 and the bandpass filter 22. The frequency dependence of the bandpass filter 22 leads to a frequency-dependent amplitude of the output signal 23, which is present at the output of a rectifier 18. The DC voltage of the output signal 23 clearly corresponds to a frequency. In a further variant, which starts from the circuit arrangement according to FIG. 6, the control unit 19 is saved. An inexpensive high-frequency oscillator 16 can not be changed in frequency. A bandpass filter (or low-pass filter) is applied to the difference frequency between that supplied by the field device electronics 14

High frequency signal 12 and the supplied from the high-frequency oscillator 16

High frequency signal fixed and thus set without control. If a low-pass filter 17 is used instead of the bandpass filter, the two high-frequency signals have the same frequency. If the frequency of the high-frequency signal 12 is varied, the amplitude at the output of the low-pass filter 17 and the rectifier 18 decreases. The amplitude of the signal at the output of the rectifier 18 is now in a fixed state

Transfer function to the frequency of the high-frequency signal 12 and can be subsequently supplied, for example, an A / D converter (not shown).

If a 17 is used as the filter, then only the difference frequency of the two is used

High frequency signals detectable. When the frequency of the high-frequency signal of the high-frequency oscillator 16 becomes a band limit of the one to be detected However, a clear assignment of the

Frequency band possible.

In all the embodiments described above, the control unit / control unit 19 can be implemented both in analog technology and digitally in a microprocessor. If it is accepted that the frequency of the high-frequency signal supplied by the high-frequency oscillator 16 deviates from the frequency of the high-frequency signal applied to the low-pass filter 17, it is possible to use very inexpensive components which have been developed for markets which require mass production. By way of example, the areas GPS, WLAN or UMTS can be mentioned.

LIST OF REFERENCE NUMBERS

cell

reference cell

Electrode / high frequency element / sensor

measuring electrode

reference electrode

insulation material

moldings

High frequency generator / active high frequency module

High-frequency generator

reference generator

coupling-out

passive high-frequency module

snubber

power supply

Frequency difference images

frequency detector

management

support material

Sensor electronics / field device electronics

RF mixer

High-frequency generator

lowpass

rectifier

Control circuit / control circuit

output

evaluation

bandpass

output

Claims

claims

1. Device for determining the refractive index which occurs at the interface with a medium (4),

with a self-excited system (2, 5, 8) consisting of

an active radio-frequency module (5) for generating electromagnetic waves having an operating frequency lying within a predetermined frequency band,

- A sensor (2) which is acted upon by the electromagnetic waves and with respect to the medium (4) is at least temporarily arranged such that the phase position of the reflected at the interface

electromagnetic waves under the influence of the medium (4) changes, and

a feedback circuit (6, 7, 8, 9) which feeds back the electromagnetic waves to the sensor (2), whereby the frequency of the self-excited system (2, 5, 8) is determined,

with a frequency detector (1 1), which detects the frequency of the self-excited system, and

with an evaluation unit (21), which determines the refractive index at the interface with the medium (4) on the basis of the detected frequency.

2. Apparatus according to claim 1,

wherein the sensor consists of an electrode (2), which is arranged on an insulating material (3), and

wherein the electrode (2) and the insulating material (3) form a measuring cell (1).

3. Apparatus according to claim 1 or 2,

wherein the evaluation unit (21) determines or monitors the dielectric constant or the magnetic permeability of the medium (4) on the basis of the refractive index.

4. Apparatus according to claim 1, 2 or 3,

wherein the frequency band, in which the self-excited system (2, 5, 8) is oscillatable, outside the natural frequency of the sensor (2), wherein the natural frequency of the sensor (2) by the geometric dimensions of the sensor or the duration of the reflected electromagnetic Waves within the sensor (2) is determined.

5. Device according to at least one of claims 1-4,

wherein the medium (4) is a liquid or gas adsorbing solid, liquid or gaseous substance.

6. Apparatus according to claim 5,

wherein the medium (4) is a liquid or a gas adsorbing solid, liquid or gaseous substance.

7. System consisting of a device according to at least one of claims 1- 6 and a container having a wall of a transparent material for the electromagnetic waves, wherein the thickness of the wall of the container between the sensor (2) and medium (4) in dependence the dielectric constant or the magnetic permeability of the medium to be monitored (4) is dimensioned.

The system of claim 7, wherein the container is a tank or a bag.

9. System according to claim 7 or 8, wherein the sensor is mounted on the outside of the wall of the container or temporarily with the outside of the wall of the

Container is brought into contact.

10. System according to claim 7 or 8,

wherein the sensor is integrated in the wall of the container.

1 system according to at least one of claims 6-10,

wherein the measuring cell (1) is shaped so that it is adapted in the region of the attachment to the geometry of the container or in the case of a solid medium (4) to the geometry of the medium (4).

12. Use of the device as described in at least one of claims 1-6, or the system as described in at least one of claims 7-1 1, for determining and / or monitoring the adsorption of

Water content in a water adsorbing material, wherein the water adsorbing material is preferably a zeolite or a silica gel.