WO2011080000A1

WO2011080000A1 - Device having a coaxial design

Info

Publication number: WO2011080000A1
Application number: PCT/EP2010/067259
Authority: WO
Inventors: Sergej Lopatin; Robert Schmidt
Original assignee: Endress+Hauser Gmbh+Co.Kg
Priority date: 2009-12-30
Filing date: 2010-11-11
Publication date: 2011-07-07
Also published as: DE102010001273A1

Abstract

The invention relates to a device for determining at least one process variable of a medium, or for transporting current, having a coaxial design at least in sections, which consists of at least three elements (1, 2, 3), wherein the second element (2) surrounds the first element (1) coaxially and wherein the third element (3) surrounds the second element (2) coaxially. The invention is characterised in that the first element (1) is made of a first material (M1) having a first thermal expansion coefficient (α₁), the second element (2) is made of a second material (M2) having a second thermal expansion coefficient (α₂), and the third element (3) is made of a third material (M3) having a third thermal expansion coefficient (α₃), wherein 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

Device with coaxial construction

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 a conductive probe is exploited that in an electrically conductive medium between a ground electrode and one with an AC voltage

acted upon sensor electrode, an electrical connection is formed as soon as both electrodes touch the medium, from which it is concluded that a predetermined level has been reached. In a capacitive probe, the sensor electrode and the container wall or a second electrode form a capacitor whose

Capacity 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, when a ground electrode is provided for the probe, the ground electrode usually encloses the structure

Sensor electrode and guard electrode concentric, so that a coaxial structure of three electrodes and at least two insulation layers is formed. Such a sensor structure is described for example in DE 3212434 C3 or DE 102007049526 A1. A problem with concentrically arranged layers

different materials occurs is that spread over a large

Temperature range form leaks and gaps 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 made with a third coefficient of thermal expansion,

wherein the materials are chosen such that the third thermal

Coefficient of expansion is greater than the first and the second thermal

Expansion coefficient is greater than the third. So it applies

, in which

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 has a large

Temperature range can be used. In a first embodiment of the solution according to the invention, the radial dimensions of the first element, of the second element and of the third element are chosen such that an area lying orthogonal to the axis of the coaxial structure which is between the first element and the third element

is included, and the area of the second element in the same plane at a change in ambient temperature the same effective thermal

Have 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 (R2) of the third element to the outer radius (R1) 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 radial

Direction close. 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 with a fourth thermal

Expansion coefficient 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 chosen such that the equation

is satisfied, where

the thermal

Expansion coefficient of the ith element is.

It goes without saying that the relation established for the thermal expansion coefficients of the inner three elements

is equally satisfied, 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, this is 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 formed by Invar (iron-nickel alloy with 36% Ni), 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 on. In an alternative embodiment, the first material is a nickel-chromium-molybdenum alloy or an iron-nickel-cobalt alloy, and the third material is stainless steel.

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

Fig. 1 shows a coaxial construction of three elements. In Fig. 1, a coaxial structure is formed formed from three radially symmetric elements, which, for example, in a capacitive probe of a

Level gauge is to be found. Preferably, it is the axial structure of a flush-mounted sensor. The first element 1 consists of a first material M1 having a first coefficient of expansion

and has the shape of a solid cylinder with the radius Ri. The second element 2 is a hollow cylinder with outer radius R2 and inner radius R1, coaxially surrounds the first element 1 and consists of a second material M2 having a second coefficient of expansion which is different. The interfaces between the first element 1

and the second element 2 are close to each other. Likewise, the third member 3 of a third material M3 having a third expansion coefficient c (3 radially surrounds the second member 2 and extends from the radius R2 to a radius R3 The third member 3 is also flush with the second member 2. 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 heating by ΔΤ = T-T ₀ is given by

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:

For example, M1 and M3 are metals and M2 is a plastic, for example PEEK, PEI or epoxy resin. Particularly preferably, the third element 3 is made of stainless steel. The material M1 of the first element 1 may be a glass or a glass

Be 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, 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 . Moreover, in this preferred embodiment, the radial dimensions of the first element 1, second element 2, and third element satisfy the equation

wherein Ri is the outer radius of the first element 1 and simultaneously the inner radius of the second element 2, and wherein R2 is the outer radius of the second element 2 and the inner radius of the third element 3.

If the two radii R1 and R2 are chosen to satisfy this formula, it is ensured that the area bounded by the first element 1 and the third element 3 in a plane has the same effective thermal

Has expansion as the second element 2 in this plane. As a result, the second element 2 fills the area lying between the first element 1 and the third element 3 not only at an initial temperature T ₀ , but also at any temperatures other than T ₀ . Thus, one finds

Temperature change no more gap formation takes place and the probe remains radially tight.

Mathematically, follow the above conditions for the following reasons: For example, suppose the material M2 behaves significantly more elastic than the materials M1 and M3, then for the radii R 1 and R _2:

The area S which is available to the second element 2 between the first element 1 and the third element 3 is

The thermal expansion coefficient β of the area S between the first

Element 1 and the third element 3 are calculated to

Since a «1, approximately and

On the other hand, the area S of the second element 2 with a ₂ can be calculated as follows:

The expansion coefficient β 'of the surface S of the second element 2 is given by

In order to avoid a gap between material 1 and material 2 or material 2 and material 3 in the event of a temperature change ΔΤ, the area enclosed between the first element 1 and the third element 3 and the surface of the second element 2 must equally expand, i. it must apply:

From this follows the equation

In addition to the relation of the radius Ri of the first element 1 to the outer radius of the second element 2 and to the inner radius of the third element 3, R ₂ , conditions for the coefficients of expansion of the three elements follow from this.

Since it can be assumed that the coefficient of expansion O _{2 of} the second material M2 is the greatest, as a rule

Since Ri <R2 continues to follow a ₂ -a _l> a ₂ - a ₃ or a _3> a _x. In other words, those are

Select materials so that the expansion coefficient of the jacket material M3 is greater than the coefficient of expansion ai of the core material M1.

Thus, if the relation a ₂ >03> ai is satisfied for a coaxial construction of three materials, 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 are made. For example, for a coaxial structure of five elements, wherein the fourth element encloses the third and the fifth element, the fourth radially, the relation α ₄ > a ₅ > 03.

Such radially sealed structures are particularly advantageous in flush-mount sensors for level measurement in pipelines, i. 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 Oi = 11 10 ⁶ 1 / ° C

M2: plastic with a ₂ = 45 - 10 ⁶ 1 / ° C

M3: Stainless steel 1 .4435 with a ₃ = 17 - KT ⁶ 1 / ° C

Radius of the cylindrical Hastelloy core: R1 = 5.0 mm

Inner radius of the stainless steel sheath: R2 = 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

αι 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

Claims

claims

1 . Device for determining at least one process variable of a medium or for the transport of electricity, having an at least partially coaxial construction, which consists of at least three elements (1, 2, 3), wherein the second element (2) coaxially surrounds the first element (1) and wherein the third element (3) coaxially surrounds the second element (2),

characterized,

the first element (1) is made of a first material (M1) having a first thermal expansion coefficient (ai),

the second element (2) is made of a second material (M2) having a second coefficient of thermal expansion (0 ₂ ),

and that the third element (3) is made of a third material (M3) having a third thermal expansion coefficient (0 ₃ ),

wherein the materials (M1, M2, M3) are selected such that the third thermal expansion coefficient (0 ₃ ) is greater than the first (ai), and the second thermal expansion coefficient (0 ₂ ) is greater than the third (03).

2. Apparatus according to claim 1,

characterized,

that the radial dimensions of the first element (1), the second

Elements (2) and the third element (3) are chosen such that a surface orthogonal to the axis of the coaxial structure, which is enclosed between the first element (1) and the third element (3), and the surface of the second element (2) in the same plane with a change in ambient temperature the same effective thermal

Have expansion.

3. Apparatus according to claim 2,

characterized,

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:

4. Device according to at least one of the preceding claims,

characterized,

the third element (3) bears tightly against the second element (2) in the radial direction, and that the second element (2) bears tightly against the first element (1) in the radial direction.

5. Device according to at least one of the preceding claims,

characterized,

in that the device comprises a fourth element (4) with a fourth coefficient of thermal expansion (a ₄ ) and a fifth element (5) with a fifth coefficient of thermal expansion (a ₅ ), the fifth

Element (5) encloses the fourth element (4) coaxially and wherein the fourth element (4) surrounds the third element (3) coaxially,

and that the materials (M4, M5) of the fourth element (4) and the fifth element (5) are chosen such that the equation is satisfied.

6. Device according to at least one of the preceding claims,

characterized,

the second material (M2) and / or the fourth material (M4) is a plastic.

7. Device according to at least one of claims 1 -6,

characterized,

the first material (M1) and / or the third material (M3) and / or the fifth material (M5) is a steel grade or a metal alloy.

8. Device according to at least one of claims 1 -7,

characterized, 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.

9. Apparatus according to claim 5 or 6,

characterized,

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.

10.Vorrichtung according to at least one of the preceding claims,

characterized,

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.

1 1 .Vorrichtung according to at least one of the preceding claims,

characterized,

in that the first element (1) and the third element (3) are electrodes and that the second element (2) is an insulation.

12. Device according to at least one of the preceding claims,

characterized,

that the device is a capacitive or conductive probe or a microwave probe.

13. Method for producing a device with coaxial symmetry,

characterized,

a first element (1) is produced from a first material (M1) having a first coefficient of thermal expansion (ai),

a second element (2) is made of a second material (M2) having a second coefficient of thermal expansion (0 ₂ ) coaxially surrounding the first element (1), a third element (3) is produced from a third material (M3) having a third coefficient of thermal expansion (03) which coaxially surrounds the second element (2),

and that the materials (M1, M2, M3) are selected such that the third thermal expansion coefficient (03) is greater than the first (αι), and the second thermal expansion coefficient (02) is greater than the third (03).

14. The method according to claim 13,

characterized,

that the radial dimensions of the first element (1), the second

Element (2) and the third element (3) are chosen so that an orthogonal to the axis of the coaxial structure extending surface which is enclosed between the first element (1) and the third element (3) and the surface of the second Expand elements (2) in the same plane with a change in ambient temperature with the same effective thermal expansion.