NL1041088B1

NL1041088B1 - Method and device for measuring dielectric properties of a fluidum in a modified coaxial stub resonator.

Info

Publication number: NL1041088B1
Application number: NL1041088A
Authority: NL
Inventors: Hoog Natalia
Original assignee: Stichting Wetsus Centre Of Excellence For Sustainable Water Tech
Priority date: 2013-12-16
Filing date: 2014-12-10
Publication date: 2016-05-19
Also published as: NL1041088A

Abstract

The present invention relates to a method and device for measuring the dielectric properties of a fluidum comprising a function generator and a spectrum analyzer, both operatively connected to a coaxial stub resonator, characterized by a first outer conductor surface surrounding a helical wound or elsewise bended first inner conductor surface, whereby the first inner conductor has two endpoints respectively marking the beginning and the end of said inner conductor and whereby the first inner conductor is more than 20% longer than the total length of said first outer conductor. The fluidum e.g., a fluid, a gas or a fluid - gas mixture under investigation, is applied as deielectric of the coaxial stub resonator. The present invention also relates to the 2D analogue of a coaxial stub resonator i.e., a so called stripline resonator, whereby the inner conductor of the stripline resonator is more than 20% longer than the outer conductor of the stripline resonator.

Description

Method and device for measuring dielectric properties of a fluidum in a modified coaxial stub resonator

Introduction

The present invention relates to a method and device for inline measurement of the properties of a fluidum, without the use of chemicals, using the physical principles of transmission line technology. More specifically, the present invention relates to a method and device to measure the dielectric properties of fluids, fluid - solid suspensions, fluid -gas suspensions and solid - gas suspensions. Even more specifically, the present invention relates to a method and device to assess the properties of water, such as drinking water, waste water and industrial process water.

Many prior art methods to determine the quality of drinking water, process water and industrial water are labor intensive, require the use of chemicals for chemical and / or biological analysis and are offline. As a result, many prior art water analysis techniques are expensive and introduce a time delay before measurement information is available. A promising technique to track changes in the quality of a water stream is to asses its dielectric properties. Existing prior art methods to measure the dielectric properties of a solution are based on classic capacitance measurements in discrete elements such as a plate capacitor. These methods are offline and suffer from relatively large parasitic capacitance and / or parasitic inductance, thereby limiting the sensitivity of the measurements, and / or require a relatively complex measurement set-up, involving relatively high investment cost. A recently developed technique for inline measurement of the dielectric properties of fluids is based on transmission line technology. A fluid sample is applied as dielectric in a transmission line resonator, such as a coaxial stub resonator or a stripline resonator. Subsequently, the electric properties of the transmission line resonator, further on referred to as stub resonator, are characterized by an amplitude versus frequency plot, further on referred to as A-f plot. The shape of the A-f plot is determined by the dielectric properties of the fluid under investigation. By the use of transmission line theory i.e., by solving the telegrapher's equations, the dielectric properties of a fluid i.e, its dielectric permittivity and loss tangent as a function of frequency, can be derived directly from the A-f plot. More qualitatively, an A-f plot provides a fingerprint of the fluid and can be used to track changes of the fluid composition and to use these changes in an early warning system.

In this document, a stub resonator is defined as a resonator based on any type of transmission line such as, but not limited to, a coaxial transmission line resonator, a stripline or any combination of striplines and / or coaxial transmission line resonators.

In this document, a stripline is defined as any type of transmission line or transmission line resonator present on a PCB or other platform to which metal etching techniques can be applied.

The sensitivity and the reliability of a sensor based on a transmission line resonator depends to a large extend on the geometrical construction of the connections of the function generator and the spectrum analyzer with the resonator and on the geometry of the transmission line resonator itself. In a long transmission line resonator, the relative contribution of the connectors, for the transmission lines towards the function generator and spectrum analyzer, to effective length of the resonator is small since it is determined by the ratio of the connector length and the total length of the resonator. Additionally, a long resonator results in a low base frequency of the transmission line resonator since the resonant frequency is inversely proportional to the total resonator length. This brings along the advantage that relatively simple and commercially available electronic components can be applied to realize function generator and spectrum analyzer analogies. Besides the advantage of a low base resontant frequency, a long resonator produces a large number of harmonics with a relatively low frequency distance between each harmonic (typically the distance is 2 times the base frequency). This opens possibilities for accurate dielectric spectroscopy.

In practical applications related to the present invention, a long resonator brings along the disadvantage that it has limited applicability since it takes a lot of space if installed in a process stream. Also the relatively large fluid volume in the resonator as compared to the fluid volume in the process concerned, e.g. biotechnological applications, can be a serious disadvantage thereby limiting the application of the sensor. Also in batch applications, a long sensor would require large fluid volumes to asses the properties of the dielectric under investigation.

In open literature, see Proceedings of the IRE issue December 1959, pages 2099 - 2105 by the authors W.W. Macalpine and R.O. Schildknecht, it is known that the apparent length of a coaxial transmission line resonator can be increased efficiently by applying a helical wound metal rod as inner conductor. By doing so, the total length of the inner conductor becomes larger than that of the outer conductor, resulting in a lower resonant frequency than expected from the length of the outer conductor. In the same paper it is also pointed out that this type of resonators have a high quality factor despite the geometrical deviations from coaxial stub resonators with a straight cylindrical inner conductor.

In literature, it is also known that a coaxial stub resonator can be applied for dielectric spectroscopy i.e., as a sensor for assessing the dielectric properties of a gas, a fluid or a gas-fluid mixture.

The inventor of the present invention has discovered that a coaxial stub resonator and / or a stripline resonator with a bended and / or helical inner conductor that is at least 20% longer than the outer conductor of that resonator performs much better as a sensor to determine the dielectric properties of a fluid, a gas or a gas-fluid mixture than the stub resonators that are known in prior art. Additionally, the inventor of the present invention has discovered that this better sensor performance i.e., higher sensitivity of the sensor for small changes in dielectric properties, can be even much further increased by attaching geometrical structures to the inner conductor. It appears that a stub resonator design with the right balance between both a helical and / or bended structure of an inner conductor on one hand and geometrical structures attached to the inner conductor on the other hand results in increased dielectric losses in the fuild under investigation and thus in a lower quality factor of the resonator. Additionally, the dielectric losses are strongly frequency dependent i.e., more frequency dependent as compared to the frequency dependence observed in priori art resonators. These operating conditions, which are highly undesired in radio engineering applications of stub resonators e.g., in rf filters, result in a very sensitive sensor to asses the properties of a dielectric under investigation. Since the resonant frequency of the resonator according to the present invention is determined by the length of the inner conductor and not by the length of its outer conductor, it is also possible to assess the properties of a relatively small fluid sample in a frequency range with a low base frequency range. Reason for this is that the resonator according to the present invention can be made relatively short (containing less fluid) as compared to its equivalent with a straight cylindrical inner conductor. Finally, a resonator according to the present invention has much more inner conductor surface per unit volume of fluid sample in the resonator, which also increases the sensitivity of the sensor in those applications where changes take place on the surface of the inner conductor. This is the case in applications where the sensor is applied as biofilm sensor, corrosion sensor or sensor with a polymer layer on the inner conductor that adsorbs or absorbs chemicals compounds present in the fluid sample under investigation.

Summarizing, the present invention relates to a method and device for measuring the properties of a dielectric combining the advantage of a long resonator with those of a small fluid volume in the sensor. At the same time, the technology according to the present invention relates to a method and device to drastically increase the sensitivity of the new transmission line based sensor by deliberately increasing dielectric losses in the sensor.

Description of the technology according to the present invention

According to a first aspect, the present invention relates to a first function generator FG. This function generator preferably produces a sinus or square wave electrical signal with a frequency that can be adjusted in the range between 1 kHz and 50 GHz.

According to a second aspect, the present invention relates to a first spectrum analyzer or a hf (high frequency) rectifier SA, preferably able to measure the amplitude of a sinus or square wave electrical signal with a frequency in the range between 1 kHz and 50 GHz. According to a third aspect, the present invention relates to at least a first resonator RE that is operatively connected to the first function generator FG and the first spectrum analyzer SA by the use of transmission lines.

According to a fourth aspect, the present invention relates to at least a microcontroller or microprocessor with software to automatically produce A-f plots through producing a predefined number of electrical signals with the first function generator FG in a defined frequency range and reading the amplitude response of the resonator using first spectrum analyzer or rectifier SA.

According to a fifth aspect, the present invention relates to at least a first resonator RE with at least a first inner conductor and at least a first outer conductor whereby said first inner conductor is at least 20% longer than said first outer conductor. Preferably, the first inner conductor is equipped with sensitivity increasing geometrical elements that will be further detailed in the preferred embodiments of the present invention. Since the inner conductor is relatively long, the geometrical constructions can be attached to the inner conductor without disturbing the operating principle of the the resonator (TEM mode electromagnatic waves): for a long resonator, the characteristic length of any geometrical structures attached to the inner conductor can be kept short as compared tot the total inner conductor length thereby ensuring the operating principle of the resonator.

Now that the technology of the present invention has been defined, a number of preferred embodiments of the present invention will be defined, a number of examples will be given and explanatory figures will be shown.

Figure 1 gives a schematic representation of the sensor set-up according to the present invention. As grapically expressed by figure 1, the sensor according to the present invention can be a coaxial resonator or its "2D equivalent" i.e., a stripline resonator. In figure 1, the numbers have the following meaning: 1. Function generator 2. Transmission line between the function generator and a connector 3. First connector 4. Transmission line between the first connector and the resonator 5. Dielectric under investigation between inner conductor and outer conductor of the resonator 6. Transmission line between the second connector and the resonator 7. Second connector 8. Tranmission line between the second connector and the spectrum analyzer 9. Spectrum analyzer 10. Inner conductor of the resonator

As a person skilled in the art will understand, connectors 3 and 7 in figure 1 are especially useful to connect coaxial transmission line to their stripline analogues with a minimum of dielectric losses.

Figure 2 shows a schematic representation of a first practical implementation of a resonator according to the present invention.

In figure 2, the numbers have the following meaning: 1. The resonator according to the present invention 2. Spirally wound or elsewise wound or bended inner conductor that is more than 20% longer than outer conductor 3 of the resonator 3. Outer conductor of the resonator 4. Inner conductor of transmission line to the spectrum analyzer and / or function generator 5. Outer conductor of transmission line to the spectrum analyzer and / or function generator 6. Transmission line to the spectrum analyzer and / or function generator

Figure 3 shows a schematic respresentation of a second practical implementation of a resonator according to the present invention.

In figure 3, the numbers have the following meaning: 1. The resonator according to the present invention 2. Spirally wound or elsewise wound or bended inner conductor that is more than 20% longer that outer conductor 3 of the resonator characterized by geometrical conductive or non conductive elements attacted to the inner conductor 3. Outer conductor of the resonator 4. Inner conductor of transmission line to the spectrum analyzer and / or function generator 5. Outer conductor of transmission line to the spectrum analyzer and / or function generator 6. Transmission line to the spectrum analyzer and / or function generator

Now the technology according to the present invention has been further elucidated through figures 1 to 3, a number of preferred embodiments according to the present invention will be discussed.

According to a first preferred embodiment, the present invention relates to a coaxial resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is at least wound or bended and at least 20% longer than the outer conductor. Obviously, the sensors according to the technology of the present invention can be applied as inline flow through sensor. Further it is noted that the sensors according to the technology of the present invention are especially feasible to assess the dielectric properties of a fluidum with more than 10 volume percent of a fluid. According to a second preferred embodiment, the present invention relates to a coaxial resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is spirally wounded and at least 20% longer than the outer conductor.

According to a third preferred embodiment, the present invention relates to a stripline resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is at least wound or bended and at least 20% longer than the outer conductor.

According to a fourth preferred embodiment, the present invention relates to a stripline resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is constructed according to a 2D projection of spirally wound inner conductor and at least 20% longer than the outer conductor. According to a fifth preferred embodiment, the present invention relates to a coaxial resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is at least wound or bended and at least 20% longer than the outer conductor. Further, the inner conductor according to fifth embodiment is characterized by conductive geometrical constructions attached to the inner conductor with a total characteristic length of each conductive geometrical construction that is smaller than 30% of the total inner conductor length.

According to a sixth preferred embodiment, the present invention relates to a coaxial resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is spirally wound and at least 20% longer than the outer conductor. Further, the inner conductor according to the sixth embodiment is characterized by conductive geometrical constructions attached to the inner conductor with a total characteristic length of each conductive geometrical construction that is smaller than 30% of the total inner conductor length. It is noted that a straight inner conductor (so a cylinder or other non bended structure) with geometrical structures attached to it, also makes part of the present invention.

According to a seventh preferred embodiment, the present invention relates to a stripline resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is at least wound or bended and at least 20% longer than the outer conductor. Further, the inner conductor according to seventh preferred embodiment is characterized by conductive geometrical constructions attached to the inner conductor with a total characteristic length of each conductive geometrical construction that is smaller than 30% of the total inner conductor length.

According to an eighth preferred embodiment, the present invention relates to a stripline resonator shown in figures 1 and 2 whereby the outer conductor length is in the range from 1 cm to 30 cm and whereby the inner conductor is constructed according to a 2D projection of spirally wound inner conductor and at least 20% longer than the outer conductor. Further, the inner conductor according to the eighth preferred embodiment is characterized by conductive geometrical constructions attached to the inner conductor with a total characteristic length of each conductive geometrical construction that is smaller than 30% of the total inner conductor length.

According to a nineth preferred embodiment, a resonator according to the present invention is applied with dimensions equal to or smaller than 1 cm. Preferably, such resonator is constructed by the use of stripline technology. The resonator can be realized on a PCB (printed circuit board) or by the use of other surface etching technologies.

According to a tenth preferred embodiment, an inner conductor of the resonator according to the present invention is equipped with a coating that specifically adsorbs or absorbs components present in the fluidum under investigation thereby increasing the sensitivity of the sensor.

According to an eleventh preferred embodiment, a resonator according to the present invention is filled with or embedded in a package of particles, such as, but not limited to, glass beads. These particles may adsorb or absorb chemicals in the dielectric under investigation, thereby increasing the sensitivity of the sensor.

According to a twelfth preferred embodiment, a resonator according to the present invention is equal to or longer than 30 cm. Preferably, such resonator is realized through building it into existing process equipment. Non limiting examples are resonators built into fluid transportation tubes, gas transportation tubes, oil tanks and sand filters.

According to a thirteenth preferred embodiment, a resonator according to the present invention is applied as corrosion sensor.

According to a fourteenth preferred embodiment, a resonator according to the present invention is applied as biofilm sensor

According to a fifteenth preferred embodiment, a resonator according to the present invention is applied as sensor to measure the loading degree of an ion exchange resin (IOX).

Now a number of preferred embodiments have been explained, a brief explanation of the working principle of the resonator according to the present invention will be given.

As explained in the introduction section of this document, the apperent length of a resonator can be increased by applying a stub resonator with longer inner conductor than the length of its outer conductor housing. To achieve this, the inner conductor has a helical or elsewise bended geometry. If a resonator with, for example, a helical inner conductor is filled with a fluid under investigation, the total surface area per unit volume of fluid in the sensor is relatively large. This makes the sensor more sensitive to dielectric losses. Also losses in the dielectric between the "different windings" contribute significantly to the sensitivity of the sensor. This effect can be further increased by attaching conducting geometrical structures to the inner conductor (thereby combining the constructions in figures 2 and 3). In this case, both the capacitance, surface and conductance added to the inner conductor result in a higher sensitivity of the sensor. The geometrical structures attached to the inner conductor can be small metal cylinders, spheres, helical cylinders or any other structures. It was found that the geometrical structures attached to the inner conductor should preferably have a characterisitic length that is typically less than 15% of the total length of the inner conductor applied. It was also found that application of non conductive geometrical structures to the conductive inner conductor may result in a very sensitive sensor. Especially biofilm formation on the surface of these non conductive elements can be measured with high sensitivity. Another very effective structure attached to the inner conductor was realized by sand blasting the surface of an inner conductor. By sandblasting the inner conductor, the effective surface area of the inner conductor drastically increased (high roughness of the surface), resulting in a higher capacitance and inductance of the inner conductor and, as a result, a lower resonant frequency of the inner conductor. In case corrosion of the inner conductor occurs or in case (conductive) biofilm formation occurs on the sand blasted inner conductor, the sensitivity of the sensor increases.

Example 1 A cylindrical copper tube with a diameter of 15 mm and a length of 190 mm was used as an outer conductor for a stub resonator and operatively connected to an SMA connector (with a total length of 15 mm). The stub resonator was equipped with an inner conductor consisting of a copper wire with a total length of 200 mm and a diameter of 1mm. The inner conductor was also operatively connected to the SMA connector. The SMA connector was operatively connected to a SMA T connector that was in its turn operatively connected to a HAMEG HMS3010 spectrum analyzer with tracking generator by the use of two SMA transmission lines with a characteristic impedance of 50 Ohm.

An A-f plot was made by the use of the spectrum analyzer with air in the resonator as dielectric. The dotted plot in figure 4, marked as plot #1 shows the A-f plot obtained for the experiment in example 1.

Example 2

The same experiment as described in example 1 was executed but now an inner conductor with a diameter of 1 mm and a total length of 1 m was used. This inner conductor was spiral wound as depicted in figure 2 so that it fits into the 190 mm copper tube.

The plot in figure 4 with a continuous line and marked as plot #2 shows the A-f plot obtained for the experiment in example 2. A comparision of both plots in figure 4 reveals that the resonator with the 1 meter length spiral wound inner conductor (curve #2) has a much lower base resonant frequency as compared to the resonator with the inner conductor with a length of 20 cm (curve #1). It is clear to a person skilled in the art that curve number 2, will provide much more information on the dielectric properties of the dielectric between inner and outer conductor than curve number 1.

Example 3

The same experiment as described in example 1 was executed but now an inner conductor with a length of 20 cm was applied to which 6 geometrical constructions were welded. These geometrical constructions consisted of pieces of copper wire with a diameter of 1 mm and a length of 15 mm each. Each construction was welded onto the inner conductor at a length coordinate of 7.5 mm (so at its center) thereby placing each wire piece perpendicular to the cylindrical axis of the inner conductor. The dotted plot in figure 4, marked as plot #1 shows the A-f plot obtained for the experiment in example 1. The plot in figure 5 with a continuous line and marked as plot #2 shows the A-f plot obtained for the experiment in example 3. Figure reveals that placing the geometric constructions onto the inner conductor results in a lower base resonant frequency of the resonator. This is caused by a higher capacitance and inductance of the inner conductor as compared to the inner conductor described in example 1. The experiment in example 3 reveals that a resonator with geometric constructions onto the inner conductor opens possibilities to effectively increase the surface area of the inner conductor and to reduce the base resonant frequency, resulting in more resonant frequencies in a specified frequency range.

Example 4

The same experiment as described in example 2 but now the resonator is filled with milliQ water (deionized water). The dotted A-f plot in figure 6 shows the results for the inner conductor with a diameter of 1 mm and a total length of 200 mm. The continuous line in figure 6 shows the A-f plot for the spiral wound inner conductor with a total length of 1 meter. Example 4 shows that the resonator also performs rather well with a relatively lossy medium as water as dielectric and a spiral wound inner conductor.

Example 5.

The same experiment as in example 3 but now the resonator is filled with milliQ water (deionized water). The dotted A-f plot in figure 7 shows the results for the inner conductor with a diameter of 1 mm and a total length of 200 mm. The continuous line in figure 7 shows the A-f plot for the inner conductor with a total length of 20 cm equipped with the 6 geometrical constructions as described in example 3. The experiment in example 5 shows that the resonator also performs rather well with a relatively lossy medium as water as dielectric and an inner conductor with attached geometrical structures.

Example 6. A cylindrical 316 L stainless steel cylindrical tube with an inner diameter of 25 mm and a total effective length of 33 cm was used as an outer conductor for a resonator. As first inner conductor, a 6 mm diameter galvanized iron tube with a very smooth surface (reflecting visible light very well) was applied. As second inner conductor, a 6 mm galvanized iron tube with a rough surface (sand blasted surface with a visibly different (rough) surface structure) and exactly the same length as the first inner conductor was applied. The resonator was operatively connected to the HAMEG HMS 3010 spectrum analyzer according to the procedure in example 1.

An A-f plot with the first inner conductor (smooth surface, air as dielectric between inner and outer conductor) results in an A-f plot with a first resonant frequency at about 218 MHz and a second resonant frequency at about 646 MHz.

An A-f plot with the second inner conductor (sand blasted rough inner conductor) results in an A-f plot with a first resonant frequency at about 216 MHz and a second resonant frequency at about 645 MHz. As expected, the roughness of the inner conductor surface resulted in a higher inner conductor capacitance and inner conductor inductance and hence in a lower base resonant frequency. The experiment in example 6 proves that introducing roughness to the inner conductor of a resonator is equivalent to attaching a geometrical structure to the inner conductor.

Clauses 1. Sensor for measuring the dielectric properties of a fluidum with at least 10 volume percent of fluid in a modified coaxial stub resonator characterized by • a first function generator and • a first spectrum analyzer or rectifier both operatively connected to • a first resonator characterized by • at least a first inner conductor that is • spiral wound or elsewise bended so that its axial length is smaller than its total length and • at least a first outer conductor whereby • the total inner conductor length exceeds the length of the outer conductor by at least 20% • at least one microcontroller operatively connected to the function generator and to the spectrum analyzer or rectifier, equipped with software for automated measurement of A-f plots. 2. Sensor according to clause 1 whereby the resonator is a stripline resonator 3. Sensor according to one of the previous clauses 1 or 2 with a rough surface structure of the inner conductor 4. Sensor according to one of the previous clauses 1 to 3 whereby additional geometrical structures are operatively connected to the inner conductor 5. Corrosion sensor according to one of the previous clauses 1 to 4 with the inner conductor surface as sensitive element 6. Biofilm sensor according to one of the previous clauses 1 to 4 with the inner conductor surface as sensitive element 7. Biofilm sensor according to one of the previous clauses 1 to 4 with both inner conductor surface and glass beads between inner and outer conductor as sensitive element 8. Sensor to measure the loading degree of ion exchange resin according to one of the previous clauses 1 to 4 whereby the ion exchange resin is placed in the resonator. 9. Flow through sensor according to the one of the previous clauses 1 to 8. 10. Method for the production of a sensor according to claim 3 by sand blasting the inner conductor of that sensor. 11. Method for measuring the dielectric properties of a fluidum characterized by a sensor accordiing to one of the previous clauses 1 to 10.

Claims

A sensor for measuring the dielectric properties of a fluid consisting of fluid for more than 10 volume percent characterized by • a first function generator and • a first spectrum analyzer or rectifier both of which are operatively connected to • a first resonator characterized by • at least one first inner conductor which is • spiral wound or otherwise bent so that the axial length of this first inner conductor is smaller than its total length and • at least a first outer conductor wherein • the total length of the first inner conductor is at least 20% greater than that of the first outer conductor • at least one microcontroller or microprocessor operatively connected to the first function generator and to the first spectrum analyzer or rectifier wherein the microcontroller or microprocessor is provided with software to automatically generate amplitude versus frequency plots (Af plots).

The sensor of claim 1 wherein the resonator is a stripline resonator.

Sensor according to one of the preceding claims 1 and 2, wherein the inner conductor is provided with a rough surface structure.

Sensor according to any of the preceding claims 1 to 3, wherein additional geometric structures are operatively connected to the inner conductor.

Corrosion sensor according to one of the preceding claims 1 to 4, with the surface of the inner conductor as the sensitive element of the sensor.

Biofilm sensor according to one of the preceding claims 1 to 4, with the surface of the inner conductor as the sensitive element of the sensor.

Biofilm sensor according to one of the preceding claims 1 to 4, with both the surface of the inner conductor and packing particles between inner conductor and outer conductor as sensitive elements.

Sensor for measuring the degree of loading of ion exchanger according to one of the preceding claims 1 to 4, wherein the ion exchanger is placed between the inner conductor and the outer conductor.

Sensor according to one of the preceding claims 1 to 8, wherein this sensor is flowed through with liquid.

A method for the production of a sensor according to claim 3, wherein the roughness is applied to the inner conductor by means of sandblasting.

A method for measuring the dielectric properties of a fluid characterized by a sensor according to one of the preceding claims 1 to 10.