WO2002041025A2

WO2002041025A2 - Sensor apparatus

Info

Publication number: WO2002041025A2
Application number: PCT/EP2001/013113
Authority: WO
Inventors: Don Eason
Original assignee: Endress + Hauser Gmbh + Co. Kg
Priority date: 2000-11-20
Filing date: 2001-11-13
Publication date: 2002-05-23
Also published as: EP1340095A2; AU2002219097A1; WO2002041025A3

Abstract

A sensor apparatus for transmitting electromagnetic waves from a signal line (12) into a vessel or vice versa to measure a process variable is provided, wherein impedance variations and/or geometric variations of the parts thereof are minimized while at the same time high mechanical stability of the apparatus is ensured, the sensor apparatus (1) comprising: a mounting section (1) configured to be coupled to the vessel, a dielectric element (5, 5a, 5b, 5c, 5d) located inside the mounting section (1), and a conductive probe (9, 9a, 9b, 9c, 9d), which extends through the dielectric element (5, 5a, 5b, 5c, 5d) into the vessel, which comprises radially outwardly extending protrusions (19, 23, 25, 33) that extend into conjoining cavities (21, 32, 39) in the dielectric element (5, 5a, 5b, 5c, 5d) thus forming an interlocking structure (17) that is holding the probe (9, 9a, 9b, 9c, 9d) inside the dielectric element (5, 5a, 5b, 5c, 5d).

Description

Sensor Apparatus

Background and Summary of the Invention

The present invention relates to a sensor apparatus for transmitting electromagnetic waves from a signal line into and out of a vessel to measure a process variable. In particular a single conductor surface wave transmission line (Goubau line) is adapted as a sensor for industrial process variable measurement, in particular for level measurement. Such devices are intended for use for example in the process and storage industry.

An electrical pulse sent down a probe is affected by any change of the electrical properties of the surroundings of the probe. A material located ins^'ide the vessel for example causes a change in electrical impedance at the material surface. At least part of the pulse will thus be reflected at the surface. The level of the material inside the vessel can be determined from the time required for the pulse to propagate to the surface and back.

Other process variables can be determined. The amplitude of a reflected pulse for example is a measure of the change in impedance at the reflecting surface and can be used to determine the dielectric constant of the material. Also it is feasible to measure thickness and/or dielectric constants of layers of different materials stored in a vessel from the amplitude and the time-of-flight of the respective number of reflected pulses.

Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and time their return with very inexpensive circuits. See, for example, U.S. Patent No. 5,345,471 and U.S. Patent No. 5,361,070 assigned to The Regent of the University of California. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher and have an average power level of about 1 nano Watt or lower. These factors present new problems that must be overcome to transmit the pulse down and back and to process and interpret the returned pulses .

It is of essential importance to provide a design for the sensor apparatus which ensures a high mechanical stability suitable for industrial applications while at the same time maintaining the electrical operation of a Goubau line. This includes ensuring a smooth impedance transition of the electromagnetic waves from the signal line and transmission through the mounting to the probe and vice versa. Changes in electrical impedance throughout the apparatus, i.e. the signal line, the mounting area and the probe inside and outside the mounting section are to be avoided. Every electrical impedance discontinuity causes a partial reflection of energy of the pulse and thus reduces the signal to noise ratio.

In copending U.S. Patent Application Serial No. 08/574,818 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT filed on December 19,1996 and a related Continuation in Part Application U.S. Patent Application Serial No. 08/735,736 with the same title filed on October 23, 1996 sensor apparati for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable are described.

A sensor apparatus for transmitting electromagnetic waves from a signal line into a vessel or vice versa to measure a process variable is described, the sensor apparatus comprising:

- a mounting section configured to be coupled to the vessel,

- a dielectric element located inside the mounting section, and

- a conductive probe,

-- which extends through the dielectric element into the vessel.

The conductive probe is mounted inside the mounting section. It includes a head portion and an elongated conductive portion extending away from the head portion. The elongated portion extends through the dielectric element into the vessel. The head portion of the probe comprises an outwardly tapered section facing towards the vessel and an inwardly tapered section facing away from the vessel. The dielectric element comprises inner tapered surfaces that closely engage the outer tapered surfaces of the head portion of the probe, thus preventing axial movement of the probe.

The bi-conical structure of the head portion of the probe and the correspondingly shaped dielectric element are an effective method to obtain structural integrity and rigidity. Just as well as any other geometric discontinuity though, this shape can cause discontinuity reflections. Any change in diameter along the probe will as a physical consequence result in a change of impedance in that region. In order to maintain a specified characteristic impedance along the probe it is thus necessary, for the inner diameter of the mounting section to vary in such a way as to compensate for the variation of the outer diameter of the probe, which again might cause discontinuity reflections. It is an object of the invention to provide a sensor apparatus wherein impedance variations and/or geometric variations of the parts thereof are minimized while at the same time high mechanical stability of the apparatus is ensured.

To this end the invention comprises a sensor apparatus for transmitting electromagnetic waves from a signal line into a vessel or vice versa to measure a process variable, the sensor apparatus comprising:

- a mounting section configured to be coupled to the vessel,

- a dielectric element located inside the mounting section, and

- a conductive probe,

- which extends through the dielectric element into the vessel,

- which comprises radially outwardly extending protrusions that extend into conjoining cavities in the dielectric element thus forming an interlocking structure that is holding the probe inside the dielectric element.

Furthermore the invention comprises a sensor apparatus for for transmitting electromagnetic waves from a signal line into a vessel or vice versa to measure a process variable, the sensor apparatus comprising:

- a mounting section configured to be coupled to the vessel,

- a dielectric element located inside the mounting section, and

- a conductive probe,

- which extends through the dielectric element into the vessel,

- wherein the dielectric element comprises radially inwardly extending protrusions that extend into conjoining cavities in the probe thus forming an interlocking structure that is holding the probe inside the dielectric element.

According to a refinement of the invention, the axial and radial dimensions of the protrusions and the cavities are small compared to a wavelength of the electromagnetic waves transmitted via the probe.

According to a refinement of the invention, the protrusions are helical ridges and the probe is screwed into the dielectric element.

According to a refinement of the invention, the protrusions form a thread.

According to a refinement of the invention, the protrusions are knurls or extensions.

According to a refinement of the invention, the cavities are grooves .

Furthermore, the invention comprises a sensor apparatus for transmitting electromagnetic waves from a signal line into a vessel or vice versa to measure a process variable, the sensor apparatus comprising:

- a mounting section configured to be coupled to the vessel,

- a dielectric element located inside the mounting section,

- a conductive probe,

- which extends through the dielectric element into the vessel,

- which is held inside the dielectric element by a mechanically interlocking structure, and

- an impedance transition zone adjacent to an end of the mounting section facing towards the vessel, — which is formed by a continuous change of the inner and/or outer diameter of the dielectric element and/or a continuous increase of the inner diameter of the mounting section.

According to a refinement of the invention the inner diameter of the mounting section increases and/or the outer diameter of the dielectric element changes linearly, exponentially or hyperbolically in a direction towards the vessel.

The invention and its advantages are explained in more detail using the figures of the drawing, in which several exemplary embodiments are shown. The same reference numerals refer to the same elements throughout the figures.

Brief Description of the Drawings

Fig. 1 shows a longitudinal section of a sensor apparatus;

Fig. 2 shows an embodiment of a probe element of Fig. 1 comprising protrusions consisting of a continuos helical winding;

Fig. 3 shows a longitudinal section of a dielectric element of Fig. 1, comprising conjoining cavities for a continuos helical winding;

Fig. 4 shows an embodiment of a probe element of Fig. 1 comprising protrusions consisting of several helical winding segments;

Fig. 5 shows an embodiment of a probe element of Fig. 1 comprising protrusions consisting of knurls;

Fig. 6 shows a cross-section of a dielectric element of Fig. 1, comprising conjoining cavities for knurls;

Fig. 7 shows an embodiment of a probe element of Fig. 1 wherein protrusions are formed by extensions;

Fig. 8 shows a cross-section of a dielectric element of Fig. 1, comprising conjoining cavities for the extensions;

Fig. 9 shows a cross-section of a probe of Fig. 1 comprising ringcylindrical cavities;

Fig. 10 shows a cross-section of a dielectric element of Fig. 1, comprising conjoining ringcylindrical protrusions;

Fig. 11 shows a cross-section of a sensor apparatus comprising an impedance transition zone; and

Fig. 12 to Fig. 26 show further embodiments of impedance transition zones.

Detailed Description of the Drawings

Fig. 1 shows a longitudinal section of a sensor apparatus for transmitting electromagnetic waves from a signal line into a vessel or vice versa to measure a process variable. The vessel is not shown. It could be a tank or a storage basin or any other type of container. Depending on the application the sensor apparatus can be employed to send electromagnetic waves from the signal line onto the probe extending into the vessel or to receive electromagnetic waves from the probe or to send electromagnetic waves down the probe into the vessel and to receive for example their reflection.

The sensor apparatus comprises an essentially cylindrical mounting section 1 which is configured to be coupled to the vessel. In the embodiment shown in Fig. 1 the mounting section 1 comprises a thread 3 located at the end of the mounting section 1 facing towards the vessel. It is designed to be screwed into a threaded opening located on the vessel. Other types of mounting a sensor apparatus to a vessel, e.g. by welding the mounting section 1 to the vessel or by providing a flange connection, are also possible.

The mounting section 1 has a central aperture extending through the same. A dielectric element 5 is located inside the mounting section 1. It consists of a dielectric material. One preferable example of such a dielectric material is Polyetherimide with fiber glass loading. It fullfills the dielectric requirements and in addition, it has a very high tensile strength, which is advantageous for load bearing. Other dielectric materials, for example

Polytetraflourethylen (PTFE) which ensures a high chemical resistivity, can also be used.

Means are provided for preventing a movement of the dielectric element 5 in a direction toward the vessel. In the embodiment shown in Fig. 1 these means comprise an outwardly tapered inner surface 7 of the mounting section 1 located near the end of the mounting section 1 facing towards the vessel. The dielectric element 5 has an identically shaped outwardly tapered outer surface which abuts on the outwardly tapered inner surface 7 of the mounting section 1.

For the scope of this application the term outwardly tapered surface shall mean a surface defining a cone which has one end with a small diameter and another end with a larger diameter and the end with the larger diameter is facing away from the vessel. Accordingly the term inwardly tapered surface shall mean a surface defining a cone which has one end with a small diameter and another end with a larger diameter and the end with the larger diameter is facing toward the vessel.

It is also possible to provide the dielectric element 5 with an external thread and to fasten it inside the mounting section 1 by screwing it into a corresponding thread inside the mounting section 1.

A conductive probe element 9 is mounted inside the mounting section 1. It consists of a metal, preferably of stainless steel, which provides for high mechanical stability. Depending on a specific application of the apparatus, other materials can also be used as long as the probe element 9 or at least ist outer surface remain electrically conductive. The probe element 9 is cylindrical and comprises a elongated portion that extends through the dielectric element 5 into the vessel. The elongated portion may extend further into the vessel forming a single rigid rod element or it may comprise a solid or a flexible probe extension 11, in particular a rod, a rope or a wire. In the embodiment shown in Fig. 1, the elongated portion the probe element 9 has a coupling section 13 facing toward the vessel and a flexible probe extension 11 is mounted inside the coupling section 13. It is fastened inside the hollow section 13 for example by swaging, by one or more set screws, by a tapered couling clamp, by bimetal alloying, by welding or by fusing. To keep the flexible extension 11 taught and straight, it is either to be fixed to the bottom of the vessel, or a weight is to be connected to the free end of the flexible extension 11. A section 15 of the probe element 9 located inside the dielectric element 5 comprises radially outwardly extending protrusions that extend into conjoining cavities in the dielectric element 5 thus forming an interlocking structure that is holding the probe 9 inside the dielectric element 5. In Fig. 1 protrusions and cavities are only indicated schematically by a dotted line 17.

A dielectric insert 6 is located inside the mounting section 1 on top of the dielectric element 5. It has approximately the shape of a hollow cylinder and closely engages an inwardly tapered end portion of the probe element 9 and an end region of the dielectric element 5 facing away from the vessel .

A metallic insert 8 is located inside the mounting section 1 abutting on the dielectric insert 6. A central bore extends through the metallic insert 8 and the end of the probe 9 facing away from the vessel extends into the bore. A high frequency electrical connector 10 is fastened to the metallic insert 8 inside the bore. One side of the connector 10 facing away from the vessel is coupled to the signal line 12. Since high frequency pulses are to be transmitted the signal line 12 is preferably a coaxial cable. An inner conductor of the connector 10 is connected to the inner conductor of the coaxial cable and the outer conductor of the connector 10 is connected to the metallic insert 8.

One end of the inner conductor of the connector 10 forms a male pin which extends into a hollow cylinder located on the end of the probe element 9 extending in a direction away from the vessel toward the connector 10. The signal line 12 is coupled directly to the probe element 9. No further impedance transition elements or intermediate connecting elements, e.g. conical nuts, are needed. Preferably the metallic insert 8 is stainless steel. It is electrically connected to ground potential. The metallic insert 8 comprises a thread and is screwed into the mounting section 1 in a direction towards the vessel. Any other way of mounting the metallic insert 8 is also feasible as long as the metallic insert 8 is securely pressing the dielectric insert 6 and the dielectric element 5 in a direction towards the vessel.

Fig. 2 shows a view of a part of an embodiment of a probe 9a. The protrusions 19 shown in the embodiment of Fig. 2 are helical ridges. They have the shape of helical windings that run along the outside of the probe 9a. Fig. 3 shows a cross- section of the correspondingly shaped dielectric element 5a surrounding it. It comprises conjoining cavities 21 which have the shape of helical grooves. Because of the helical form of the protrusions 19 it is possible to mount the probe 9a inside the dielectric element 5a by screwing the probe 9a into the dielectric element 5a. In the embodiment shown in Fig. 1 the helical windings are continuously linked to form a thread.

Fig. 4 shows an embodiment of a probe 9b with protrusions 23, which are also helical ridges. They have the shape of helical winding-segments that run along the outside of the probe 9b. They are not continuously linked but they are preferably equidistantly spaced in order to allow mounting of the probe 9b by screwing the probe 9b into a dielectric element comprising conjoining cavities. These cavities have the shape of helical grooves .

Fig. 5 shows a view of another probe 9c. The protrusions 25 shown in this embodiment are knurls covering an outer cylindrical surface of the probe 9c. Fig. 6 shows a cross- section of a corresponding dielectric element 5c. The dielectric element 5c is preferably molded around and onto the probe element 9c by insertion molding. During insertion molding the probe element 9c is inserted into a mold cavity and a dielectric polymer, for example Polyetherimid, is molded around and onto it. This technique yields intimately conjoining shapes and very high cured radial strength of the molded part. In this embodiment the dielectric element 5c forms a cylinder with a central bore 31. The dielectric element 5c comprises conjoining cavities 33 for the knurls 25 covering an inside wall of the dielectric element 5c. Fig. 7 shows a view of another probe 9d. The protrusions 33 shown in this embodiment are extensions extending from an outer cylindrical surface of the probe 9d. In the embodiment shown, the extensions are aligned along four equidistant axial straight lines running on the outside of the probe 9d. Fig. 8 shows a cross-section of a corresponding dielectric element 5d. Again the dielectric element 5d forms a cylinder with a central bore 37. The dielectric element 5d comprises conjoining cavities 39 for the extensions covering an inside wall of the dielectric element 5d. As described before with respect to the embodiment shown in Fig. 5 and Fig. 6 the dielectric element 5d is preferably molded around and onto the probe element 9d.

Fig. 9 shows a longitudinal section of yet another embodiment of a probe 9e and Fig. 10 shows a cross-section of the corresponding dielectric element 5e. Whereas in all the other embodiments described before, the protrusions 19, 23, 25, 33 belonged to the respective probe 9a, 9b, 9c, 9d- and the conjoining cavities 21, 31, 39 where located in the corresponding dielectric elements 5a, 5b, 5c, 5d in this embodiment, the inverse case is shown. The dielectric element 5e comprises radially inwardly extending protrusions 41 that extend into matching cavities 43 in the probe 9e, thus forming an interlocking structure that is holding the probe 9e inside the dielectric element 9e. The protrusions 41 are ringcylinders and the cavities 43 are identically shaped ringylindrical grooves cut into an outer cylindrical surface of the probe 9e.

Again, the dielectric element 5e forms a cylinder with a central bore 45 and is preferably molded around and onto the probe element 9e.

For all geometries described above for protrusions and cavities, it applies that the corresponding inverse geometry is also feasible.

With respect to all embodiments described above, axial and radial dimensions of the protrusions 19, 23, 25, 33, 41 and the cavities 21, 32, 39, 43 are preferably small compared to a wavelength of the electromagnetic waves transmitted via the probes 9, 9a, 9b, 9c, 9d, 9e. The wavelength is the wavelength that the electromagnetic waves have inside the mounting section. The wavelength can be approximated by assuming that probe, dielectric element and mounting section form a coaxial conductor. If broad band electromagnetic signals are transmitted the wavelength should be calculated based on an upper limit of the frequency spectrum used. Preferably the dimensions of the protrusions and the cavities are about one hundred times smaller than the relevant wavelength. This measure does not completely exclude the occurrence of extraneous modes but it does ensure, that especially in the frequency region of microwaves energy losses due to protrusions and cavities are negligible. For a sensor apparatus with a mounting section with a diameter of approximately a decimeter and electromagnetic waves ranging from 500 MHz up to 1 GHz protrusions and cavities preferably have axial and radial dimensions of no more than several millimeters. This is sufficient to provide for secure mounting of the probe 9 inside the dielectric insert 5 inside the mounting section 1. When conical surfaces are used to secure the probe, as is described above with respect to the prior art, only one surface needs to take up the entire load exerted on the probe by pulling forces or by pressurized vessels. This supporting surface need to be fairly large compared to the dimensions of the protrusions and cavities according to the invention. According to the invention the probe is held inside the dielectric element by a mechanically interlocking structure. Therefore the load is distributed onto many surfaces which, since they take only a small fraction of the load, can have small dimensions and at the same time ensure high mechanical stability.

The small dimensions which are made possible by holding the probe inside the dielectric element by a mechanically interlocking structure ensure that very little energy losses occur.

For a sensor apparatus, an embodiment of which is shown in Fig. 11, for transmitting electromagnetic waves from a signal line 12 into a vessel or vice versa to measure a process variable, with a mounting section 1 configured to be coupled to the vessel, a dielectric element 5 located inside the mounting section 1 and a conductive probe 9, which extends through the dielectric element 5 into the vessel, which is held inside the dielectric element 5 by a mechanically interlocking structure 17, the transmission of electromagnetic waves between the signal line 12 and an end of the apparatus facing towards the vessel is very energy- efficient .

Due to the fact that these apparati have very low energy losses inside the sensor apparatus energy losses occurring on the end of the sensor apparatus facing towards the vessel become important. The reason for these energy losses is the difference in impedance occurring at the transition from a coaxial wave guide to a surface wave guide. Preferably the impedance throughout the sensor apparatus is equal to the impedance of the signal line 12, for a commercially available coaxial cable this is typically 50 Ω. The impedance along a section of the probe extending into the vessel depends on the frequency used. For the example given above of a frequency range of 500 GHz to 1 MHz, the impedance amounts to several hundred Ohms .

According to an aspect of the invention, the sensor apparatus comprises an impedance transition zone 47 adjacent to the end of the mounting section 1 facing towards the vessel. The impedance of this zone increases gradually in a direction towards the vessel. By means of the impedance transition zone 47 the amount of unwanted energy reflection at this end of the sensor apparatus is dramatically reduced.

The impedance transition zone 47 according to the invention is formed by a continuous change of the inner and/or outer diameter of the dielectric element 5 and/or a continuous increase of the inner diameter of the mounting section 1.

The sensor apparatus shown in Fig. 11 is to a large extend identical to that shown in Fig. 1. The sensor apparatus comprises a flange 46 for mounting the sensor apparatus on a corresponding counterflange on a vessel whereas the sensor apparatus as shown in Fig. 1 comprises a thread 3. The main difference is that the sensor apparatus shown in Fig. 11 comprises the impedance transition zone 47.

In this embodiment, the impedance transition zone 47 comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel which is filled by a ringcylindrical extension 50 of the dielectric element 5. The probe 9 is held inside the dielectric element 5 by the mechanically interlocking structure 17 and extends through the ringcylindrical extension 50 of the dielectric element 5 into the vessel.

On an end facing towards the vessel the inner diameter of the mounting section 1 increases gradually thus forming an air filled horn 51.

Fig. 12 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone. It comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel which is filled by a ringcylindrical extension 50 of the dielectric element 5. On an end facing towards the vessel the inner diameter of the mounting section 1 increases gradually thus forming a horn 51. The ringcylindrical extension 50 of the dielectric element 5 extends into the horn 51 and its outer diameter decreases inside the horn 51 in a direction facing towards the vessel thus forming a conical tip 53 pointing into the vessel .

Fig. 13 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone. In this embodiment the mounting section 1 comprises an end section 55 with a gradually increasing inner diameter. The end section 55 is filled with an extension 56 of the dielectric element 5.

Fig. 14 shows another embodiment of an impedance transition zone. Again the mounting section 1 comprises the cylindrical extension 49 facing towards the vessel. A ringcylindrical extension of the dielectric element 5 extends into the cylindrical extension 49 and its outer diameter decreases inside the extension 49 in a direction facing towards the vessel thus forming a conical tip 57 pointing into the vessel.

Fig. 15 shows a further embodiment of an impedance transition zone. Again the mounting section 1 comprises the cylindrical extension 49 facing towards the vessel. A ringcylindrical extension 50 of the dielectric element 5 extends into the cylindrical extension 49 and fills it completely. Outside the mounting section 1 the outer diameter of the dielectric element 5 decreases in a direction facing towards the vessel thus forming a conical tip 59 pointing into the vessel.

Fig. 16 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone. In this embodiment the mounting section 1 comprises the end section 55 with a gradually increasing inner diameter. The end section 50 is filled with an extension 56 of the dielectric element 5. Outside the mounting section 1 the outer diameter of the dielectric element 5 decreases in a direction facing towards the vessel thus forming a conical tip 61 pointing into the vessel.

Fig. 17 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone. In this embodiment the mounting section 1 comprises the end section 55 with a gradually increasing inner diameter. The dielectric element 5 comprises a conical tip 63 inside this endsection 55. An outer diameter of the tip 63 decreases in a direction facing towards the vessel.

Fig. 18 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone, which is to a large extent identical to the embodiment shown in Fig. 12. It also comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel which is filled by a ringcylindrical extension 50 of the dielectric element 5. On an end facing towards the vessel the inner diameter of the mounting section 1 increases gradually thus forming a horn 51. The ringcylindrical extension 50 of the dielectric element 5 extends throu the horn 51 and its outer diameter decreases in a direction facing towards the vessel thus forming a conical tip 65 pointing into the vessel that ends outside the horn 51.

Fig. 19 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone. It comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel which is patially filled by an extension 67 of the dielectric element 5. On an end facing towards the vessel the inner diameter of the extension 67 increases gradually thus forming a horn inside the ringcylindrical extension 49 of the mounting section. Fig. 20 shows a longitudinal section of one of two identical halves of a further embodiment of an impedance transition zone. It comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel. The inner diameter of an end section 69 of the mounting section 1 facing towards the vessel increases in a direction towards the vessel. The cylindrical extension 49 and the end section 69 are partially filled by an extension 71 of the dielectric element 5. On an end facing towards the vessel the inner diameter of the extension 71 increases gradually thus forming a horn inside the end section 69 of the mounting section 1.

In the embodiment shown in Fig. 21, the impedance transition zone comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel. An extension 50 of the dielectric element 5 extends through the cylindrical extension 49 and fills it completely. On an end facing towards the vessel the inner diameter of the mounting section 1 increases gradually thus forming a horn 51. The extension 50 of the dielectric element 5 extends into the horn 51 and its inner diameter increases inside the horn 51 in a direction facing towards the vessel thus forming a horn 73 inside the horn 51.

In Fig. 22 a further embodiment of an impedance transition zone is shown. It comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel. An extension 50 of the dielectric element 5 extends through the cylindrical extension 49 and fills it completely. On an end facing towards the vessel outside the extension 49 the inner diameter of the extension 50 increases gradually thus forming a horn 75. The outer diameter of the horn 75 remains constant .

Fig. 23 shows a longitudinal section of one of two identical halves of another embodiment of an impedance transition zone. In this embodiment the mounting section 1 comprises the end section 55 with a gradually increasing inner diameter. The end section 50 is filled with an extension 56 of the dielectric element 5. Outside the mounting section 1 the inner diameter of the dielectric element 5 increases in a direction facing towards the vessel thus forming a horn 77 opening towards the vessel.

Fig. 24 shows another embodiment of an impedance transition zone. It is almost identical to the embodiment shown in Fig. 21. The only difference is that the extension 50 not only extends into the horn 51 but through the horn 51, thus forming an inner horn 79 that opens in a direction towards the vessel and ends outside the horn 51.

In the embodiment shown in Fig. 25 the impedance transition zone comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel. An extension 50 of the dielectric element 5 extends through the cylindrical extension 49 and fills it completely. On an end facing towards the vessel the inner diameter of the mounting section 1 increases gradually thus forming a horn 51. The extension 50 of the dielectric element 5 fills the horn 51 completely and in front of the horn 51, its outer diameter decreases thus forming a tip 81 pointing towards the vessel.

Fig. 26 shows a further embodiment of an impedance transition zone. It comprises a cylindrical extension 49 of the mounting section 1 facing towards the vessel. An extension 50 of the dielectric element 5 extends through the cylindrical extension 49 and fills it completely. On an end facing towards the vessel the inner diameter of the mounting section 1 increases gradually thus forming a horn 51. The extension 50 of the dielectric element 5 extends through the horn 51 and its inner diameter increases in a direction facing towards the vessel whereas its outer diameter remains constant. It forms a horn 83 extending through the horn 51. A gap 85 exists between the outer horn 51 and the inner horn 83.

With respect to all the embodiments of impedance transition zones, the increase of the inner diameter of the mounting section 1 and/or the change of the inner and/or outer diameter of the dielectric element 5 can for example be linear, exponential or hyperbolical in a direction towards the vessel depending on the frequency range, the change in impedance required and the space available.

Claims

WHAT IS CLAIMED IS:

1. A sensor apparatus for transmitting electromagnetic waves from a signal line (12) into a vessel or vice versa to measure a process variable, the sensor apparatus (1) comprising:

- a mounting section (1) configured to be coupled to the vessel,

- a dielectric element (5, 5a, 5b, 5c, 5d) located inside the mounting section (1) , and

- a conductive probe (9, 9a, 9b, 9c, 9d) ,

- which extends through the dielectric element (5, 5a, 5b, 5c, 5d) into the vessel,

- which comprises radially outwardly extending protrusions (19, 23, 25, 33) that extend into conjoining cavities (21, 32, 39) in the dielectric element (5, 5a, 5b, 5c, 5d) thus forming an interlocking structure (17) that is holding the probe (9, 9a, 9b, 9c, 9d) inside the dielectric element (5, 5a, 5b, 5c, 5d) .

2. A sensor apparatus for transmitting electromagnetic waves from a signal line (12) into a vessel or vice versa to measure a process variable, the sensor apparatus comprising:

- a mounting section (1) configured to be coupled to the vessel,

- a dielectric element (5, 5e) located inside the mounting section (1) , and

- a conductive probe (9, 9e) ,

- which extends through the dielectric element (5, 5e) into the vessel,

- wherein the dielectric (5, 5e) element comprises radially inwardly extending protrusions (41) that extend into conjoining cavities (43) in the probe (9, 9e) thus forming an interlocking structure (17) that is holding the probe (9, 9e) inside the dielectric element (5, 5e) .

3. A sensor apparatus according to one of the previous claims, wherein axial and radial dimensions of the protrusions (19, 23, 25, 33, 41) and the cavities (21, 32, 39, 43) are small compared to a wavelength of the electromagnetic waves transmitted via the probe (9, 9a, 9b, 9c, 9d, 9e) .

4. A sensor apparatus according to one of the previous claims, wherein the protrusions (19, 23) are helical ridges and the probe (9a, 9b) is screwed into the dielectric element (5a) .

5. A sensor apparatus according to claim 4, wherein the protrusions (19) form a thread.

6. A sensor apparatus according to claims 1 or 2, wherein the protrusions (25, 33) are knurls or extensions.

7. A sensor apparatus according to claims 1 or 2, wherein the cavities (43) are grooves.

8. A sensor apparatus for transmitting electromagnetic waves from a signal line (12) into a vessel or vice versa to measure a process variable, the sensor apparatus comprising:

- a mounting section (1) configured to be coupled to the vessel,

- a dielectric element (5) located inside the mounting section (1) ,

- a conductive probe (9),

- which extends through the dielectric element (5) into the vessel,

- which is held inside the dielectric element (5) by a mechanically interlocking (17) structure, and

- an impedance transition zone (47) adjacent to an end of the mounting section (5) facing towards the vessel, -- which is formed by a continuous change of the inner and/or outer diameter of the dielectric element (5) and/or a continuos increase of the inner diameter of the mounting section (1) .

9. A sensor apparatus according to claim 8, wherein the inner diameter of the mounting section (1) increases and/or the outer diameter of the dielectric element (5) changes linearly, exponentially or hyperbolically in a direction towards the vessel.