Temperature compensated dielectric constant measurement device

Abstract

The invention relates to a high-frequency-based measuring device (1) for determining the temperature-compensated dielectric constant of a medium (2), the measuring probe (11) of the measuring device comprising an electrically conductive inner conductor (111) and an outer conductor. The inner conductor (111) is at least in sections along an axis (a) rod-shaped. The inner wall of the outer conductor (112) is symmetrical with respect to the axis (a) of the inner conductor (111) such that the inner wall extends along the axis (a) towards the medium. For temperature compensation, the measuring device (1) comprises a temperature sensor (113) which, according to the invention, is positioned in a first end region of the inner conductor (111) towards which the inner wall of the outer conductor (112) extends. One of a plurality of sensor terminals (1131) is at the electrical potential of the inner conductor (111). As a result of this integration of the temperature sensor (113) according to the invention, the temperature of the medium (2) is measured directly without impairing the high-frequency-based measurement of the dielectric constant. Thereby, a high-accuracy measurement of the dielectric constant and a high-accuracy temperature compensation are possible.

Description

TECHNICAL FIELD

[0001] The present application relates to a measuring device for determining the temperature-compensated dielectric constant of a medium. BACKGROUND

[0002] In automation technology, in particular for process automation, field devices are frequently used which serve to detect various measured variables. The measured variables can be, for example, the fill level of a medium, the flow rate, the pressure, the temperature, the pH value, the redox potential, the electrical conductivity or the dielectric constant in a process plant. In order to detect the corresponding measured values, the field devices respectively comprise suitable sensors or are based on suitable measuring methods. The Endress+Hauser Group produces and sells various types of field devices.

[0003] The determination of the dielectric constant (also referred to as "specific inductive capacity" or "relative permittivity") of various media is of great importance for solid loads as well as for liquid loads and gas loads, such as fuels, waste water, gases, gas phases or chemicals, since this constant can be a reliable indicator of the contamination, the moisture content, the material concentration or the material composition. Possible measuring principles for determining the dielectric constant are the measurement of the amplitude, the phase shift or the signal transit time of a high-frequency signal. A high-frequency signal with a defined frequency or in a defined frequency band is thus coupled into the medium, and with regard to the emitted high-frequency signal, the corresponding received signal is evaluated with respect to the amplitude, the phase position or the signal transit time of the corresponding received signal. In the context of the present patent application, the term "high-frequency signal" relates to a corresponding signal whose frequency is between 10 MHz and 100 GHz.

[0004] A phase-based dielectric constant measuring device is described, for example, in the published document DE 10 2017 130728 A1. The effect utilized in this case is that the signal velocity of a high-frequency signal and thus the phase position along the measuring probe depends on the dielectric constant of the medium prevailing along the measuring probe. In principle, a distinction is made between a relative phase measurement and an absolute phase measurement, wherein in the case of an absolute phase measurement a so-called quadrant correction is additionally performed.

[0005] For example, the TDR ("Time Domain Reflectometry") principle can be applied to determine the dielectric constant by means of a pulse transit time measurement. In this measurement principle, a sensor transmits a pulsed high-frequency signal having a frequency between 0.1 GHz and 150 GHz along an electrically conductive measurement probe and measures the transit time of the pulse until a reflected high-frequency signal is received. Here, the effect is utilized that the pulse transit time depends on the dielectric constant of the substance surrounding the measurement probe. The functional principle of a TDR sensor is described, for example, in the publication EP 0622628 A2. For example, TDR sensors are marketed by the company IMKO Micromodultechnik GmbH in several embodiments. Furthermore, TDR sensors have the advantage that, in addition to the dielectric constant, the dielectric loss factor of the substance can potentially also be determined.

[0006] In process automation, the dielectric constant is often to be determined in applications that are not operated at room temperature, such as moisture measurement in the food industry. However, since the dielectric constant is strongly dependent on temperature, in particular in the case of aqueous media, the dielectric constant measurement thus requires temperature compensation depending on the application. In particular, in order to be able to determine the moisture content of a medium via the dielectric constant, a very precise and repeatable temperature measurement is thus required. However, a suitable temperature measurement on the medium is often not possible, since it is difficult in practice to integrate a temperature sensor into the dielectric constant measurement device: if the temperature sensor is integrated directly into the measurement probe of the dielectric constant measurement device and thus at the location of the medium to be checked, this at least in the case of high-frequency-based measurement devices significantly distorts the dielectric constant measurement. However, the implementation of a temperature sensor remote from the measurement probe in turn can have the result that, for example, the temperature of the medium in which the medium is located is measured instead of the temperature of the medium to be checked. SUMMARY

[0007] The present application is therefore based on the object of providing a measurement device for determining the dielectric constant, by means of which the dielectric constant can be measured as precisely as possible and temperature compensation can be carried out.

[0008] The present application achieves this object via a measurement device for determining the temperature-compensated dielectric constant of a medium, the measurement device comprising the following components:

[0009] - a measurement probe, the measurement probe having:

[0010] o an electrically conductive inner conductor, which is rod-shaped at least in sections along the axis,

[0011] o an outer conductor, which is arranged around the inner conductor and comprises at least one electrically conductive inner wall, wherein the inner wall is symmetrical about the axis, such that the inner wall extends along the axis,

[0012] a temperature sensor,

[0013] ■ the temperature sensor is positioned in a first end region of the inner conductor, the inner wall of the outer conductor extending towards the end region, and

[0014] ■ the temperature sensor comprises at least two electrical terminals, wherein a first terminal is at the electrical potential of the inner conductor,

[0015] The measurement probe is designed such that, in the installed state of the measuring device, the first end region of the inner conductor, in which the temperature sensor is positioned, is oriented towards the medium.

[0016] For controlling the measurement probe, the measuring device comprises:

[0017] a control / evaluation unit, the control / evaluation unit being designed to

[0018] ■ determine the temperature of the medium via the second terminal of the temperature sensor,

[0019] ■ couple a high-frequency electrical signal into the inner conductor or the outer conductor and to receive a corresponding reception signal, and

[0020] ■ determine the dielectric constant of the medium in a temperature-compensated manner using the determined temperature and using the reception signal.

[0021] The measuring device according to the application thus exploits, on the one hand, the fact that the location of the temperature measurement coincides with the location of the dielectric constant measurement. On the other hand, the implementation according to the application is used, i.e. the dielectric constant measurement based on high frequencies is not influenced as long as the temperature sensor is at the electrical potential of the inner conductor on one side. For this purpose, the temperature sensor can be designed, for example, as a capacitive sensor or as a resistance-based sensor, in particular as a PT1000. Precise dielectric constant measurement and precise temperature compensation of the dielectric constant can thereby be achieved.

[0022] Within the scope of the application, the term "unit" refers in principle to any electronic circuit designed in a manner suitable for the intended purpose. It can thus be an analog circuit for generating or processing corresponding analog signals, depending on the requirements. However, it can also be a digital circuit, such as an FPGA, or a storage medium that interacts with a program. In this case, the program is designed to carry out the corresponding method steps or to apply the necessary calculation operations of the respective unit. In this context, the various electronic units of the dielectric constant measuring device according to the application can also potentially access a common physical memory or be operated by means of the same physical digital circuit.

[0023] Within the scope of the application, it is irrelevant which high-frequency-based measurement principle is implemented in the measuring device in order to determine the dielectric constant. For example, the measuring device or the control / evaluation unit can be designed to determine the dielectric constant of the medium using the phase position of the received signal, using the amplitude of the received signal and / or using the transit time, in particular the group transit time or the phase transit time, of the received signal. However, the control / evaluation unit can also be designed accordingly in order to determine the dielectric constant of the medium by means of a pulse transit time method, in particular by means of a TDR method or a FMCW method.

[0024] The frequency of the high-frequency signal is in principle selected depending on the dielectric constant measurement range. In the case of a measurement range of the dielectric constant of between 1 and 140 (including, in addition to aqueous media, gases of 1 to 1.5 and oily measuring media of 2 to 20), the control / evaluation unit will be designed accordingly in order to generate a high-frequency electrical signal with a frequency of between 0.1 GHz and 30 GHz. In the case of a water-containing medium whose dielectric constant is approximately 80 to approximately 100, high-frequency signals with a frequency of between 5 GHz and 8 GHz are preferably generated and evaluated accordingly. In this regard, the length of the inner conductor and the geometry of the outer conductor should be designed depending on the frequency of the high-frequency signal. For example, the inner wall of the outer conductor can be formed symmetrically about the axis such that it tapers conically, exponentially or elliptically along the axis in order to direct the near field of the high-frequency signal towards the medium. In order for the propagation of the field towards the medium to be optimally unaffected, the inner conductor and / or the second terminal of the temperature sensor can be contacted with the control / evaluation unit via a second end region of the inner conductor opposite the first end region of the inner conductor.

[0025] In order to be able to reduce the size of the measuring probe without reducing the measurement accuracy, it is also possible to introduce an electrically insulating filler between the inner conductor and the outer conductor. The higher the dielectric constant of the filler, the smaller the measuring probe can be designed. It is therefore advantageous if the filler has a dielectric constant of at least 2. Polyethylene, PEEK, PTFE or polypropylene can therefore be cast as a filler for this purpose, for example.

[0026] Corresponding to the dielectric constant measuring device according to the application, according to one of the previously described embodiment variants, the object on which the application is based is also achieved via a corresponding method for operating the measuring device. The method therefore comprises at least the following method steps:

[0027] - coupling a high-frequency signal into the measuring probe,

[0028] - decoupling a corresponding received signal from the measuring probe,

[0029] - determining the dielectric constant using at least the received signal,

[0030] - measuring the temperature by means of a temperature sensor, and

[0031] - temperature-compensating the dielectric constant using the determined temperature.

[0032] The temperature compensation can be performed, for example, on the basis of a compensation function or using a look-up table. In this context, the compensation function or the look-up table can be created by training or calibrating the measuring device with respect to a medium pair having a known dielectric constant and a known temperature. BRIEF DESCRIPTION OF DRAWINGS

[0033] The application is explained in more detail with reference to the following drawings. The following shows:

[0034] Figure 1 : dielectric constant measuring device according to the application on a container, and

[0035] Figure 2 : cross-sectional view of a measuring probe of the dielectric constant measuring device. DETAILED DESCRIPTION

[0036] For a comprehensive understanding of the dielectric constant measuring device 1 according to the application, Figure 1 A schematic arrangement of the measuring device 1 on a container 3 filled with a medium 2 is shown in Fig. 1. For determining the dielectric constant of the medium 2, the measuring device 1 is attached laterally to a terminal of the container 3, such as a flange terminal. Optionally, the measuring device 1 can be connected to a superordinate unit 4, such as a process control system. For example, "PROFIBUS", "HART" or "Wireless HART" can be implemented as an interface. Thus, the dielectric constant can be transmitted as an absolute value or as a complex value having a real part and an imaginary part. However, further information about the general operating state of the measuring device 1 can also be transmitted.

[0037] The medium 2 can be a liquid, such as a beverage, a paint, a cement or a fuel, such as a liquefied gas or a mineral oil. However, it is also conceivable to use the measuring device 1 for a medium 2 of the type of a bulk material, such as granules to be dried. Depending on the application, the medium 2 can undergo an exothermic reaction in the container 3 in addition to a drying process, such as a fermentation process. For this purpose, the container 3 can be optionally appropriately climate-conditioned, for example by means of a heating element 31 located on the container 3. Thus, the temperature of the medium 2 can deviate from the room temperature at least during the respective process. Since the dielectric constant strongly depends on the temperature depending on the medium 2, the measuring device 1 has to be designed in such a way that the dielectric constant of the medium 2 is determined in a temperature-compensated manner.

[0038] The measuring device 1 according to the application is based on the fact that the dielectric constant of the medium 2 is determined by means of a high-frequency signal s HF , r HFThe dielectric constant can be determined using measuring device 1 in a temperature-compensated manner. For example, measurement principles based on transit time, such as the TDR method or FMCW method, can be implemented as a measurement principle for determining the dielectric constant. However, measurement principles based on amplitude or phase can also be implemented. To make the high-frequency signal s... HF The corresponding near-field interacts with medium 2 to determine the dielectric constant, such as Figure 1 As schematically shown, the measuring device 1 therefore includes a measuring probe 11 oriented towards the medium. The measuring probe 11 is controlled by the control / evaluation unit 12 of the measuring device 1, wherein a high-frequency electrical signal s HF It is coupled to the measurement probe 11. Therefore, the control / evaluation unit 12 can convert the corresponding received signal r HF Decoupling is performed so that the current, uncompensated dielectric constant of medium 2 can be determined accordingly.

[0039] exist Figure 2 The structure of the measuring probe 11 according to the invention is illustrated in more detail in the form of a cross-sectional view, by means of which temperature compensation can be achieved. The core of the measuring probe 11 consists of a conductive inner conductor 111 and an outer conductor 112 arranged around the inner conductor 111. The control / evaluation unit 12 transmits high-frequency signals s HF Coupled to two conductors 111 and 112 so that the corresponding received signal r can be received. HF After interacting with medium 2, the corresponding received signal r HF Decoupling. In principle, it is irrelevant which of the two conductors 111, 112 serves as the signal ground. Furthermore, from a functional perspective, it is not important whether the inner conductor 111 and the outer conductor 112 are made entirely of conductive material. It is only necessary that the inner conductor 111 has a conductive outer wall relative to the outer conductor 112, and the outer conductor 112 has a conductive inner wall relative to the inner conductor 111. However, since the medium 2 to be inspected may also be a hygiene-sensitive substance, it is advantageous to make the inner conductor 111 at least of stainless steel. As long as the outer conductor is not entirely made of stainless steel either, the inner wall of the outer conductor 112 can be metal-coated, for example, by sputtering or by chemical vapor deposition.

[0040] As from Figure 2 As shown, the inner conductor 111 is rod-shaped, and its cross-section is preferably circular. With the measuring device 1 installed, the rod forms a straight axis a oriented towards the medium 2. Furthermore, the axis a of the inner conductor 111 defines the geometry of the outer conductor 112 because the inner wall of the outer conductor 112 is symmetrical about axis a, such that the outer wall of the outer conductor 112 extends along axis a towards the medium 2. With respect to axis a, the inner wall of the outer conductor 112 is further preferably designed to have a circular cross-section.

[0041] existFigure 2 In the embodiment of the measuring probe 11 shown, the extension of the inner wall of the outer conductor 112 is tapered. Typically, the shape or angle of the extension is adapted to the range of dielectric constants to be measured and the high-frequency signal s. HF The frequency of the high-frequency signal s HF The near field penetrates into the dielectric 2 with maximum intensity. This maximizes the sensitivity of the dielectric constant measurement. Therefore, the inner wall of the outer conductor 112 can also be extended, for example, in an elliptical or exponential manner.

[0042] From the height of axis a extending from the inner wall of the outer conductor, the two conductors 111 and 112 are separated by the insulating part 114. Figure 2 In the illustrated embodiment variant, electrical contact between the two conductors 111, 112 and the control / evaluation unit 12 is made via this end region. In this case, the insulation 114 can be implemented, for example, based on PTFE. This cable guide or conductor guide prevents any cable within the interior from being guided within the extension between the inner conductor 111 and the outer conductor 112, thereby maintaining the sensitivity of the dielectric constant measurement. Alternatively, an electrically insulating filler with a dielectric constant greater than 1, such as that made of PEEK, can also be introduced inside between the inner conductor 111 and the outer conductor 112. Therefore, the dimensions of the measuring probe 11 relative to the length and diameter of the outer conductor 112 can be reduced without compromising the sensitivity of the dielectric constant measurement.

[0043] For temperature compensation in dielectric constant measurement, a temperature sensor 113 is arranged in the end region of the rod-shaped inner conductor 111, the end region facing the dielectric 2. Figure 2 In the illustrated embodiment variant, the inner conductor 111 or its end region facing the medium 2 extends beyond the outer conductor 112 relative to axis a. Therefore, in the installed state of the measuring device 1, the temperature sensor 113 protrudes beyond the container wall into the medium 2, ensuring that the measurement of the current medium temperature is preferably not distorted by the container wall. Specifically, the temperature measurement and dielectric constant measurement are performed at the same location, thus enabling direct compensation for medium temperatures deviating from room temperature relative to the dielectric constant in the control / evaluation unit 12.

[0044] Temperature sensor 113 can be designed as a capacitive sensor or a resistance-based sensor, particularly as a PT1000. Within the scope of this invention, such as Figure 2It is important that the temperature sensor 113 shown has at least two electrical terminals 1131 and 1132 because, according to the invention, one of the two terminals 1131 is connected to the potential of the inner conductor 111. As a technical advantage, and most importantly, this has the effect that only one terminal 1132 of the temperature sensor 113 must be led out from the inner conductor 111. Therefore, the diameter of the inner conductor 111 can be kept small, thus simplifying impedance matching at the second end region 114, for example, with a 50-ohm coaxial cable.

[0045] via another terminal 1132, the control evaluation unit 12 can use the corresponding evaluation signal s T The temperature at the location of medium 2 is directly determined. Using the determined temperature and the previously determined dielectric constant of medium 2, the control / evaluation unit 12 can thus compensate for the dielectric constant very accurately relative to the temperature, for example, relative to room temperature. For example, the control / evaluation unit 112 is capable of performing temperature compensation, wherein the control / evaluation unit 112 compares the measured dielectric constant and the measured temperature with a lookup table or compensation function, for example, obtained via a medium-specific calibration measurement.

[0046] In such Figure 2 In the exemplary embodiment of the measuring probe 11 shown, the terminal 1132 of the temperature sensor 113—by which the control / evaluation unit determines the temperature of the medium 2—is guided out of the measuring probe 11 via the second end region 114 from the inner conductor 111. Compared to guiding the terminal 1132 internally between the inner conductor 111 and the outer conductor 112, this guidance of the terminal 1132 has the advantage that the internal components are not interfered with in high-frequency technology, thus preserving the dielectric constant measurement. Figure 2 In contrast to the illustration, the second terminal 1132 is guided through the interior and then through the outer conductor 112 to the measuring probe 11, thus providing the advantage of simplified manufacturability of the measuring probe 11.

[0047] List of reference numerals

[0048] 1. Measuring equipment

[0049] 2. Medium

[0050] 3 containers

[0051] 4. Upper-level unit

[0052] 11. Measuring probe

[0053] 12 Control / Evaluation Units

[0054] 31 Heating element

[0055] 111 Inner conductor

[0056] 112 outer conductor

[0057] 113 temperature sensor, first end region

[0058] 114 second end region, insulation

[0059] 1131 first terminal

[0060] 1132 second terminal

[0061] r HF received signal

[0062] s HF high-frequency signal

[0063] s T signal of the temperature sensor

Claims

1. A measuring device for determining a temperature-compensated dielectric constant of a medium (2), the measuring device comprising: - a measuring probe (11) having: o an electrically conductive inner conductor (111) which is rod-shaped at least in a section along an axis (a), o an outer conductor (112) arranged around the electrically conductive inner conductor (111) and comprising at least one electrically conductive inner wall, wherein the electrically conductive inner wall is symmetrical about the axis (a) such that the electrically conductive inner wall extends along the axis (a), o a temperature sensor (113), • the temperature sensor (113) being positioned in a first end region of the electrically conductive inner conductor (111), the electrically conductive inner wall of the outer conductor (112) extending towards the first end region, • the temperature sensor (113) comprising at least two electrical terminals (1131, 1132), wherein a first terminal (1131) is at the electrical potential of the electrically conductive inner conductor (111), wherein the measuring probe (11) is designed such that, in an installed state of the measuring device (1), the first end region of the electrically conductive inner conductor (111), in which the temperature sensor (113) is positioned, is oriented towards the medium (2), - a control / evaluation unit (12) designed to o determine a temperature of the medium (2) via a second terminal (1132) of the temperature sensor (113), o coupling a high-frequency electrical signal (s HF ) into the electrically conductive inner conductor (111) or the outer conductor (112) and receiving a corresponding reception signal (r HF ), and o determining the dielectric constant of the medium (2) in a temperature-compensated manner using the determined temperature and using the received signal (r HF ) 2. The measuring device of claim 1, wherein, The control / evaluation unit (12) is designed to determine the dielectric constant of the medium (2) using the phase position of the received signal (r HF ), using the amplitude of the received signal (r HF ) and / or using the transit time of the received signal (r HF ).

3. The measuring device of claim 1, wherein, The control / evaluation unit (12) is designed to determine the dielectric constant of the medium (2) using the group transit time or the phase transit time of the received signal (r HF ).

4. The measuring device of claim 1, wherein, the control / evaluation unit (12) being designed to determine a dielectric constant of the medium (2) by means of a pulse transit time method.

5. The measuring device of claim 1, wherein, the control / evaluation unit (12) being designed to determine a dielectric constant of the medium (2) by means of a TDR method or a FMCW method.

6. The measuring device according to any one of claims 1 to 5, wherein, the electrically conductive inner wall of the outer conductor (112) is symmetrical about the axis (a) such that the electrically conductive inner wall extends conically, exponentially or elliptically along the axis (a).

7. The measuring device according to any one of claims 1 to 5, wherein, the temperature sensor (113) is implemented as a capacitive sensor or as a resistance-based sensor.

8. The measuring device according to any one of claims 1 to 5, wherein, the temperature sensor (113) is implemented as a PT1000.

9. The measuring device according to any one of claims 1 to 5, wherein, The control / evaluation unit (12) is designed to generate and evaluate high-frequency electrical signals (s HF ) with frequencies between 0.1 GHz and 30 GHz.

10. The measuring device according to any one of claims 1 to 5, wherein, The control / evaluation unit (12) is designed to generate and evaluate high-frequency electrical signals (s HF ) with frequencies between 5 GHz and 8 GHz.

11. The measuring device according to any one of claims 1 to 5, wherein, the second terminal (1132) of the temperature sensor (113) is in contact with the control / evaluation unit (12) via a second end region (114) of the electrically conductive inner conductor (111) opposite the first end region of the electrically conductive inner conductor (111).

12. The measuring device according to any one of claims 1 to 5, wherein, an electrically insulating filler having a dielectric constant greater than 2 is introduced between the inner conductor (111) and the outer conductor (112).

13. A method for determining a temperature-compensated dielectric constant of a medium (2) by means of a measuring device (1) according to any one of claims 1 to 12, the method comprising the following method steps: - coupling a high frequency signal (s HF ) into the measurement probe (11), - decoupling a corresponding received signal (r HF ) from the measurement probe (11), - determining said dielectric constant using at least said received signal (r HF ) - measuring a temperature by means of a temperature sensor (113), and - temperature-compensating the dielectric constant using the determined temperature.

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