DE102010001273A1 - Device with coaxial construction - Google Patents

Abstract

The invention relates to a device for determining at least one process variable of a medium or for transporting electricity, with an at least partially coaxial structure, which consists of at least three elements (1, 2, 3), the second element (2) being the first element ( 1) surrounds coaxially and wherein the third element (3) surrounds the second element (2) coaxially. The invention is characterized in that the first element (1) is made of a first material (M1) with a first thermal expansion coefficient (α1), and that the second element (2) is made of a second material (M2) with a second thermal expansion coefficient Expansion coefficient (α2) is made, and that the third element (3) is made of a third material (M3) with a third thermal expansion coefficient (α3), the materials (M1, M2, M3) are selected such that the third thermal expansion coefficient (α3) is greater than the first (α1), and the second thermal expansion coefficient (α2) is greater than the third (α3).

Description

The present invention relates to a device for determining at least one process variable of a medium or for the transport of electricity, having an at least partially coaxial structure, which consists of at least three elements, wherein the second element coaxially surrounds the first element and wherein the third element, the second element Coaxially surrounds. The device is, for example, a capacitive or conductive fill level measuring device, in particular with a flush-mounted sensor. Furthermore, the invention relates to a method for producing a device with coaxial symmetry.

Various devices are known from the prior art, which have a coaxial construction. In addition to coaxial cables, these are, for example, devices for determining process variables of a medium in containers or pipelines. In particular, capacitive or conductive probes with coaxially arranged electrodes are frequently used in level measurement. In the case of a conductive probe, use is made of the fact that, in the case of an electrically conductive medium, an electrical connection is produced between a ground electrode and a sensor electrode subjected to an alternating voltage, as soon as both electrodes touch the medium, from which reference is made to reaching a predetermined fill level. In a capacitive probe, the sensor electrode and the container wall or a second electrode form a capacitor whose capacitance depends on the level of the intermediate medium.

In order to be able to use such probes also in batch-forming media, it is known to surround the sensor electrode with a guard electrode which lies at the same electrical potential as the sensor electrode and is separated therefrom by an insulating layer. In addition, if a ground electrode is provided for the probe, the ground electrode usually encloses the assembly of sensor electrode and guard electrode concentrically, so that a coaxial structure of three electrodes and at least two insulating layers is formed. Such a sensor structure is for example in the DE 3212434 C3 or the DE 10 2007 049 526 A1 described.

A problem which occurs with concentrically arranged layers of different materials is that over a wide temperature range leaks and gaps form in the boundary regions, in particular at interfaces between metals and plastics. When the temperature changes, the different materials expand to different degrees, or they contract to different degrees so that gaps can form in the boundary areas. In this column, liquid can penetrate, which can lead to probes to malfunction or even failure of the meter. Furthermore, bacteria and fungi can build up in the cracks. This is problematic, especially in the food or pharmaceutical industry, where stringent hygienic guidelines must be adhered to.

The invention is therefore based on the object to provide a device with a coaxial structure and a method for producing such, whose components have a radially sealed connection in a wide temperature range.

The object is achieved with respect to the device in that the first element is made of a first material having a first thermal expansion coefficient, that the second element is made of a second material having a second coefficient of thermal expansion, and that the third element of a third material is made with a third coefficient of thermal expansion, wherein the materials are selected such that the third thermal expansion coefficient is greater than the first and the second thermal expansion coefficient is greater than the third. Thus, α ₂ > α ₃ > α ₁ , where α _{i is} the thermal expansion coefficient of the ith element.

Coaxial abutments are typically made to be radially dense at room temperature. In the solution according to the invention, the structure retains its initial tightness. As a result, the device can be used over a wide temperature range.

In a first embodiment of the solution according to the invention, the radial dimensions of the first element, the second element and the third element are chosen such that an area orthogonal to the axis of the coaxial structure which is enclosed between the first element and the third element, and the Surface of the second element in the same plane with a change in ambient temperature have the same effective thermal expansion. This is preferably true for all surfaces which extend along the axis of the coaxial structure.

According to a particularly advantageous embodiment, the ratio of the inner radius (R ₂ ) of the third element to the outer radius (R ₁ ) of the first element satisfies the following equation:

In a further embodiment of the invention, the third element on the second element, and the second element on the first element in the radial direction are tight. In other words, it is a radially dense structure without gaps or cracks between the individual elements.

According to a further embodiment of the device according to the invention, this further comprises a fourth element having a fourth coefficient of thermal expansion and a fifth element having a fifth coefficient of thermal expansion, wherein the fifth element coaxially surrounds the fourth element and wherein the fourth element coaxially surrounds the third element. The materials of the fourth element and the fifth element are in this case selected such that the equation α ₄ > α ₅ > α _{3 is} satisfied, where α _{i is} the coefficient of thermal expansion of the ith element. It goes without saying that the relation α ₂ > α ₃ > α ₁ set up for the thermal expansion coefficients of the inner three elements is equally fulfilled so that there is a radially tight connection between all five elements.

According to one embodiment, the second material and / or the fourth material is a plastic. For example, these are PEEK (polyether ketone), PEI (polyetherimide), PI (polyimide), PTFE (polytetrafluoroethylene) or an epoxy resin.

In another embodiment, the first material and / or the third material and / or the fifth material is a steel grade or a metal alloy. For example, the materials are a combination of materials selected from the group consisting of Invar (36% Ni iron-nickel alloy), Kovar, titanium, tantalum, aluminum alloys, Monel, stainless steel (stainless steel), and stainless steel. Preferably, the outer element is made of stainless steel. Invar and Kovar have relatively low thermal expansion coefficients between about 0 and 5 ppm / K.

In an alternative embodiment, the first material is a nickel-chromium-molybdenum alloy or an iron-nickel-cobalt alloy and that the third material is stainless steel.

In another embodiment, the third material is a nickel-chromium-molybdenum alloy or an iron-nickel-cobalt alloy, and the fifth material is stainless steel.

According to a further embodiment, the first material and / or the third material and / or the fifth material is a glass, a ceramic, a metal ceramic or a glass ceramic.

According to an embodiment, the first element and the third element are electrodes and the second element represents an insulation.

According to one embodiment, the device is a capacitive or conductive probe or a microwave probe.

In an alternative embodiment, the device is a coaxial cable for carrying power.

With regard to the method, the object is achieved in that a first element is made of a first material having a first coefficient of thermal expansion, that a second element made of a second material having a second coefficient of thermal expansion, which surrounds the first element coaxially, that third element is made of a third material having a third coefficient of thermal expansion, which coaxially surrounds the second element, and that the materials are selected such that the third thermal expansion coefficient is greater than the first, and the second thermal expansion coefficient is greater than the third , Preferably, the elements are configured radially symmetrical. It goes without saying that the method is also applicable to structures which consist of more than three elements.

In a preferred embodiment of the method according to the invention, the radial dimensions of the first element, the second element and the third element are chosen such that an orthogonal to the axis of the coaxial structure extending surface which is enclosed between the first element and the third element, and extend the area of the second element in the same plane with a change in ambient temperature with equal effective thermal expansion.

The invention will be explained in more detail with reference to the following figure.

1 shows a coaxial construction of three elements.

In 1 is a coaxial structure formed of three radially symmetrical elements shown, which can be found for example in a capacitive probe of a level gauge. Preferably, it is the axial structure of a flush-mounted sensor. The first element 1 consists of a first material M1 with a first expansion coefficient α ₁ and has the shape of a solid cylinder with the radius R ₁ . The second element 2 is a hollow cylinder with outer radius R ₂ and inner radius R ₁ , encloses the first element 1 coaxial and consists of a second material M2 with a second expansion coefficient α ₂ , which is different from α ₁ . The interfaces between the first element 1 and the second element 2 lie close to each other. Likewise, the third element encloses 3 from a third material M3 with a third expansion coefficient α _3, the second element 2 radially and extending from the radius R ₂ to a radius R ₃ . The third element 3 also closes flush with the second element 2 at. The three elements are designed so to speak form-fitting. The expansion coefficient α of a material indicates how much a body consisting of the corresponding material expands when the temperature changes. For example, for stainless steel 1.4435: α = 17 · 10 ^-6 1 / ° C or 17 ppm / K. The length L of an element after a heating by ΔT = T - T ₀ is given by L = L ₀ (1 + α · ΔT) wherein L ₀ denotes the initial length of the element at an initial temperature T _0th

The materials of the elements are now matched to one another such that their coefficients of expansion satisfy the following inequality: α ₂ > α ₃ > α ₁

For example, M1 and M3 are metals and M2 is a plastic, for example PEEK, PEI or epoxy resin. Particularly preferred is the third element 3 made of stainless steel. The material M1 of the first element 1 may be a glass or a glass-ceramic, or a metal or a metal alloy with a low expansion coefficient, such as Invar or Kovar. Regardless of the radial dimensions of the three elements 1 . 2 . 3 has a structure of such materials over the known from the prior art constructions improved radial tightness even at temperatures not equal to the initial temperature to T ₀ .

wobei R₁ der Außenradius des ersten Elements 1 und gleichzeitig der Innenradius des zweiten Elements 2 ist, und wobei R₂ der Außenradius des zweiten Elements 2 und der Innenradius des dritten Elements 3 ist. Sind die beiden Radien R₁ und R₂ so gewählt, dass sie dieser Formel genügen, ist gewährleistet, dass die Fläche, welche durch das erste Element 1 und das dritte Element 3 in einer Ebene begrenzt wird, die gleiche effektive thermische Ausdehnung aufweist wie das zweite Element 2 in dieser Ebene. Hierdurch füllt das zweite Element 2 die zwischen dem ersten Element 1 und dem dritten Element 3 liegende Fläche nicht nur bei einer Initialtemperatur T₀ aus, sondern auch bei beliebigen von T₀ verschiedenen Temperaturen. Somit findet bei einer Temperaturänderung keine Spaltbildung mehr statt und die Sonde bleibt radial dicht.In this preferred embodiment, the radial dimensions of the first element meet 1 , second element 2 and third element beyond the equation

where R _{1 is} the outer radius of the first element 1 and at the same time the inner radius of the second element 2 and R _{2 is} the outer radius of the second element 2 and the inner radius of the third element 3 is. If the two radii R ₁ and R ₂ are chosen so as to satisfy this formula, it is ensured that the surface which passes through the first element 1 and the third element 3 is limited in a plane having the same effective thermal expansion as the second element 2 in this level. This fills the second element 2 between the first element 1 and the third element 3 lying surface not only at an initial temperature T ₀ , but also at any of T ₀ different temperatures. Thus, no more gap formation occurs at a temperature change and the probe remains radially dense.

Mathematically, the above conditions follow the following considerations: Assuming that the material M2 behaves much more elastic than the materials M1 and M3, then for the radii R ₁ and R ₂ : R ₁ = R ₀₁ (1 + α ₁ · ΔT) R ₂ = R ₀₂ (1 + α ₃ · ΔT)

The area S, which is the second element 2 between the first element 1 and the third element 3 is available S ₀ = π (R 2/02 -R 2/01) S = π [R 2 / O 2 (1 + α ₃ .ΔT) ² - R 2/01 (1 + α ₁ .ΔT) ² }

The thermal expansion coefficient β of the area S between the first element 1 and the third element 3 calculates too

Since α << 1, approximately ΔT (R 2/02 α 2/3 - R 2/01 α 2/1 ) = 0 and

On the other hand, the area S of the second element 2 be calculated with α _{2 as} follows: S = π (R 2 / O 2 - R 2/01) (1 + α ₂ · ΔT) ² ≈ ≈ π (R 2 / O 2 - R 2/01) + 2π (R 2 / O 2 - R 2/01) α ₂ ΔT = = S ₀ + 2π (R 2/02 - 2/01) α ₂ ΔT

The expansion coefficient β 'of the area S of the second element 2 arises too

So that at a temperature change ΔT no gap between material 1 and material 2 or material 2 and material 3 arises, which must be between the first element 1 and the third element 3 enclosed area and the area of the second element 2 expand equally, ie it must apply:

From this follows the equation

In addition to the relation of the radius R _{1 of} the first element 1 to the outer radius of the second element 2 or to the inner radius of the third element 3 , R ₂ , follow conditions for the coefficients of expansion of the three elements. Since it can be assumed that the expansion coefficient α _{2 of} the second material M2 is greatest, as a rule α ₁ <α ₂ and α ₃ <α ₂ since R ₁ <R ₂ is still followed by α ₂ - α ₁ > α ₂ - α ₃ or α ₃ > α ₁ . In other words, the materials are to be selected so that the expansion coefficient α _{3 of} the cladding material M3 is greater than the expansion coefficient α _{1 of} the core material M1. So is for a coaxial Construction of three materials satisfies the relation α ₂ > α ₃ > α ₁ , it has a denser connection between the three materials compared to a structure of conventionally selected materials.

For more complex structures of four and five elements analogous considerations can be made. For example, for a coaxial construction of five elements, the fourth element enclosing the third and the fifth element the fourth radially, the relation α ₄ > α ₅ > α ₃ .

Such radially sealed structures are particularly advantageous in front-flush sensors for level measurement in pipelines, d. H. in sensors, which protrude to avoid the formation of turbulent flows only with a small length in the pipeline.

An example of a radially dense probe of three concentrically arranged elements is given by the following features:
M1: Hastelloy C4 with α ₁ = 11 · 10 ^-6 1 / ° C
M2: Plastic with α ₂ = 45 · 10 ^-6 1 / ° C
M3: Stainless steel 1.4435 with α ₃ = 17 · 10 ^-6 1 / ° C
Radius of the cylindrical Hastelloy core: R ₁ = 5.0 mm
Inner radius of the stainless steel sheath: R ₂ = 5.51 mm
Thickness of insulation material: 0.51 mm.

LIST OF REFERENCE NUMBERS

1

First element

2

Second element

3

Third element

4

Fourth element

5

Fifth element

M1

first material

M2

second material

M3

third material

M4

fourth material

M5

fifth material

α ₁

first thermal expansion coefficient

α ₂

second thermal expansion coefficient

α ₃

third thermal expansion coefficient

α ₄

fourth thermal expansion coefficient

α ₅

fifth thermal expansion coefficient

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3212434 C3 [0003]
• DE 102007049526 A1 [0003]

Claims (14)

Device for determining at least one process variable of a medium or for the transport of electricity, having an at least partially coaxial structure, which consists of at least three elements ( 1 . 2 . 3 ), the second element ( 2 ) the first element ( 1 ) coaxially surrounds and wherein the third element ( 3 ) the second element ( 2 ) coaxially, characterized in that the first element ( 1 ) is made of a first material (M1) having a first thermal expansion coefficient (α ₁ ), that the second element ( 2 ) is made of a second material (M2) having a second coefficient of thermal expansion (α ₂ ), and in that the third element ( 3 ) is made of a third material (M3) with a third coefficient of thermal expansion (α ₃ ), wherein the materials (M1, M2, M3) are selected such that the third thermal expansion coefficient (α ₃ ) is greater than the first (α ₁ ), and the second thermal expansion coefficient (α ₂ ) is larger than the third (α ₃ ). Device according to claim 1, characterized in that the radial dimensions of the first element ( 1 ), the second element ( 2 ) and the third element ( 3 ) are selected such that an area lying orthogonal to the axis of the coaxial structure, which between the first element ( 1 ) and the third element ( 3 ) and the area of the second element ( 2 ) in the same plane have the same effective thermal expansion when the ambient temperature changes. Apparatus according to claim 2, characterized in that the ratio of the inner radius (R ₂ ) of the third element ( 3 ) to the outer radius (R ₁ ) of the first element ( 1 ) satisfies the following equation:

Device according to at least one of the preceding claims, characterized in that the third element ( 3 ) on the second element ( 2 ) in the radial direction is tight, and that the second element ( 2 ) on the first element ( 1 ) in the radial direction is tight. Device according to at least one of the preceding claims, characterized in that the device comprises a fourth element ( 4 ) having a fourth thermal expansion coefficient (α ₄ ) and a fifth element ( 5 ) having a fifth thermal expansion coefficient (α ₅ ), the fifth element ( 5 ) the fourth element ( 4 ) coaxially encloses and wherein the fourth element ( 4 ) the third element ( 3 ) coaxially encloses, and that the materials (M4, M5) of the fourth element ( 4 ) and the fifth element ( 5 ) are chosen such that the equation α ₄ > α ₅ > α _{3 is} satisfied. Device according to at least one of the preceding claims, characterized in that the second material (M2) and / or the fourth material (M4) is a plastic. Device according to at least one of claims 1-6, characterized in that the first material (M1) and / or the third material (M3) and / or the fifth material (M5) is a steel grade or a metal alloy. Device according to at least one of claims 1-7, characterized in that the first material (M1) is a nickel-chromium-molybdenum alloy or an iron-nickel-cobalt alloy and that the third material (M3) is stainless steel. Apparatus according to claim 5 or 6, characterized in that the third material (M3) is a nickel-chromium-molybdenum alloy or an iron-nickel-cobalt alloy and that the fifth material (M5) is stainless steel. Device according to at least one of the preceding claims, characterized in that the first material (M1) and / or the third material (M3) and / or the fifth material (M5) is a glass, a ceramic or a glass ceramic. Device according to at least one of the preceding claims, characterized in that the first element ( 1 ) and the third element ( 3 ) Electrodes and that the second element ( 2 ) represents an insulation. Device according to at least one of the preceding claims, characterized in that it is the device is a capacitive or conductive probe or a microwave probe. Method for producing a device with coaxial symmetry, characterized in that a first element ( 1 ) is made of a first material (M1) with a first coefficient of thermal expansion (α ₁ ) that a second element ( 2 ) is made of a second material (M2) having a second coefficient of thermal expansion (α ₂ ), which is the first element ( 1 ) coaxially surrounds a third element ( 3 ) is made of a third material (M3) having a third coefficient of thermal expansion (α ₃ ), which is the second element ( 2 ) and that the materials (M1, M2, M3) are selected such that the third thermal expansion coefficient (α ₃ ) is greater than the first (α ₁ ), and the second thermal expansion coefficient (α ₂ ) is greater than the third (α ₃ ). A method according to claim 13, characterized in that the radial dimensions of the first element ( 1 ), the second element ( 2 ) and the third element ( 3 ) are selected such that an area running orthogonally to the axis of the coaxial structure, which extends between the first element ( 1 ) and the third element ( 3 ) and the area of the second element ( 2 ) in the same plane when the ambient temperature changes with the same effective thermal expansion.

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