Filling level measuring device

Abstract

The invention relates to a filling level measuring device (1) based on radar, which is used to determine the filling level (L) of a filling material (3) in a container (2). The filling level measuring device (1) comprises: at least one high-frequency unit (10), which is used to generate a high-frequency signal (S HF ) and is used based on the received high frequency signal (R HF ) to determine the filling level (L); a waveguide (11) for transmitting and / or receiving a high-frequency signal (S HF The waveguide is subdivided into a first part (11a) facing the high-frequency unit (10) and a second part (11b) facing the filling material (3), wherein the high-frequency signal (S HF , R HF ) A transparent insulator (12) fluidically isolates the first part (11a) from the second part (11b). The invention is characterized by a first cavity (13) which is fluidically connected to the second part (11b), wherein the first cavity (13) is arranged behind the insulator (12) relative to the second part (11b). This provides the advantage that the formation of condensate on the insulator (12) is prevented, thereby not impairing the function of the filling level measuring device (1).

Description

Technical Field

[0001] The invention relates to a radar-based filling level measuring device for measuring the filling level of a filling material located in a container. Background Art

[0002] In automation technology, in particular in process automation technology, field devices are often used, which are used to record and / or influence process variables. To record process variables, sensors are used, for example in fill level measuring devices, flow measuring devices, pressure and temperature measuring devices, pH redox potential measuring devices, conductivity measuring devices, etc. These devices record the corresponding process variables, such as fill level, flow, pressure, temperature, pH value, redox potential and conductivity. Endress+Hauser produces and sells a large number of such field devices.

[0003] In the case of filling of the container with material, non-contact measuring methods have proven to be reliable for filling level measurement, since such measuring methods are robust and require little maintenance. Another advantage of non-contact measuring methods is that they can measure the filling level almost continuously and thus can provide high resolution. For this reason, radar-based measuring methods are mainly applied. In this case, the established measuring principles are the FMCW measuring principle (“frequency modulated continuous wave”) and the pulse propagation time method. In both cases, the radar signal is sent from an antenna, which is fixed to the container and directed to the interior of the container. The radar signal propagates in the direction of the filling material and is received back after reflection. In this case, the frequency of the electromagnetic wave transmitted is in the region of the standardized center frequency. Therefore, within the scope of the present invention, the terms “radar” and “high-frequency signal” refer to frequencies between 0.3 GHz and 300 GHz. Due to government-mandated restrictions, frequencies of 6 GHz, 26 GHz or 79 GHz are usually used. The functional principle of the pulse propagation time method is described, for example, in DE 10 2010 063 430 A1, while the functional principle of the FMCW measuring method is covered, for example, in WO 2012 / 139852 A1. Independently of the measuring principle, higher frequencies are advantageous because the resolution of the fill level measurement may also increase with increasing frequency.

[0004] Depending on the intended use of the container, temperatures of up to 200°C and overpressures of several bars can prevail inside the container. A particular challenge in this context is to isolate the fill level measuring device from the fluid interior of the container, since, depending on the filling material, condensate can in a given case penetrate into the fill level measuring device via the antenna and subsequently the hollow conductor. In this way, the function of the fill level measuring device can in turn be deteriorated.

[0005] In order to prevent this, the hollow conductor is subdivided into two parts, of which the second part faces the antenna and thus faces the filling substance. In this case, a temperature- and pressure-blocking insulator transparent to high-frequency signals fluidly isolates the first part from the second part. In this way, the gaseous filling substance is prevented from penetrating into the filling level measuring device. However, in the case of this construction, it is not excluded that condensate is deposited on the insulator, as a result of which the transmission of the high-frequency signal in the hollow conductor is prevented. Summary of the invention

[0006] It is therefore an object of the present invention to provide a fill level measuring device which can be used at high temperatures and high pressures in the container.

[0007] The present invention achieves the object by a filling level measuring device, which is used to determine the filling level of a filling material in a container, and the filling level measuring device comprises:

[0008] a high-frequency unit, which is designed to generate and / or process a high-frequency signal for determining the fill level,

[0009] - a hollow conductor, which is coupled to the high-frequency unit for sending high-frequency signals to the filling substance and / or for receiving high-frequency signals reflected on the filling substance, wherein the hollow conductor is subdivided into:

[0010] o a first portion facing the high frequency unit, and

[0011] o a second part facing the filling material, the opening of which leading to the container can be expanded conically to form an antenna,

[0012] as well as

[0013] - an insulator which is transparent to high frequency signals and which fluidly isolates the first portion from the second portion.

[0014] According to the invention, the fill level measuring device is characterized by a first hollow space, which is fluidically connected to the second part. In this case, the first hollow space is arranged behind the insulation relative to the second part. In this connection, a fluid connection is defined as a material-permeable connection, via which a pressure balance is produced between the connected chambers. The fluidly connected hollow space preferably collects possible condensate from the interior of the container, since the first hollow space is located in a principally cooler region behind the insulation relative to the potentially hot interior of the container. In this way, the insulation remains free of condensate.

[0015] At high frequencies, due to the basically small dimensions of the hollow conductor, the invention is particularly advantageous for fill level measuring devices when the high-frequency unit is designed to generate high-frequency signals with frequencies greater than 75 GHz, in particular greater than 100 GHz.

[0016] Preferably, the thermal resistance of the hollow conductor between the insulator and the first hollow space is dimensioned such that a temperature difference of at least 30° C. between the temperature of at least 180° C. in the container and the temperature in the first hollow space is achieved. This increases the condensation rate in the first hollow space and reduces the risk of condensate forming on the insulator. In this regard, on the one hand, the thermal resistance of the hollow conductor between the insulator and the first hollow space can be set by correspondingly dimensioning the distance between the insulator and the first hollow space or by the cross-sectional area of ​​the hollow conductor in this region. On the other hand, the thermal resistance can be correspondingly influenced by the material of the hollow conductor, which material, for example, has a thermal conductivity λ of 15 W / (m*K) H of stainless steel.

[0017] In addition to setting the thermal resistance, the risk of condensate formation on the insulation can also be supplemented by a second hollow space that is fluidically connected to the second part. In this case, the second hollow space is arranged upstream of the insulation relative to the second part. Due to the increase in the second hollow space, the pressure rise in the entire hollow space is slowed down, and therefore the pressure required for the formation of condensate - the partial pressure of the gas at the hollow space temperature - is reached significantly later.

[0018] The fluid connection can be produced in a simple manner by fluidly connecting the first hollow space to the second part by means of a thread. This can be achieved, for example, by designing the hollow conductor such that the second part forms a holder for the first part. Alternatively, it is naturally also possible to choose to achieve a fluid connection that is structurally completely decoupled from the mechanical fixing of the two parts.

[0019] The present invention does not exclude that, as is known from the prior art, the filling level measuring device according to the invention comprises a process isolation as a supplement to the insulator in the hollow conductor. In this case, the process isolation separates the second part of the hollow conductor from the filling material. Since such process isolation is usually produced from a plastic material that is transparent to high-frequency signals, such as PMMA, PTFE or PS, it may happen that the shape of the process isolation and the hollow conductor is not sufficiently interlocked. In these cases, it may therefore be advantageous to arrange a fluid seal between the process isolation and the end region of the second part, in particular a fluid seal consisting of at least two sealing rings. When the filling level measuring device according to the invention comprises an additional process isolation, the first hollow space can be designed according to the permeability coefficient of the fluid seal so that the volume of the hollow space is sized to one per 10-12 kg / (s*bar) The permeability coefficient of the sealing part is at least 1.2cm 3 Such dimensioning ensures that the first hollow space is not completely filled with condensate during the sufficient operating time of the device. Thus, a sufficient service life of the filling level measuring device is ensured. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will now be explained in more detail based on the accompanying drawings, which show the following:

[0021] Figure 1 is the arrangement of the filling level measuring device according to the invention which is installed in principle on a container,

[0022] Figure 2 is a schematic diagram of components of the filling level measuring device of the present invention, and

[0023] Figure 3 A detailed view of the main components of a fill level measuring device. DETAILED DESCRIPTION

[0024] In order to provide a basic understanding of the present invention, Figure 1 The arrangement of the inventive radar-based fill level measuring device 1 on a container 2 is shown. A filling material 3 is located inside the container 2, the filling level L of which is to be determined.

[0025] In order to determine the filling level L, the filling level measuring device 1 is mounted on the container 2 above the filling material 3 at a pre-known installation height h relative to the container bottom. Depending on the container size, this installation height h can even be greater than 100 m. The filling level measuring device 1 is arranged on top of the container 2 so that the filling level measuring device 1 can send a high-frequency signal S to the filling material 3. HF This can occur using FMCW methods or pulse propagation time methods, for example at frequencies of 79 GHz or higher.

[0026] After reflection on the surface of the filling material, the filling level measuring device 1 receives the reflected high-frequency signal R HF , wherein the filling level measuring device 1 measures the high frequency signal S when sending and receiving HF , R HF The propagation time between θ and θ depends on the distance d of the filling level measuring device 1 from the filling material surface. The filling level measuring device 3 uses its installation height h and the propagation time, or the distance d from the filling material surface, for the subsequent calculation of the filling level L:

[0027] L=hd

[0028] Such as Figure 1As shown, the filling level measuring device 1 according to the invention can be connected to a superordinated unit 4, such as a process control system or a distributed database, by means of a bus system, such as a "PROFIBUS", "HART" or "WirelessHART" bus system. In this way, on the one hand, information about the filling level L can be transmitted in order to control the inflow and outflow into and out of the container 2 in a given case. In addition, information about the operating state of the filling level measuring device 1 can be communicated.

[0029] Depending on the application, temperatures T of up to 200° C. or even higher may occur, for example due to chemical reactions temporarily experienced by the filling substance 3 in the container 2 . C In the interior of the container 2, a positive pressure of several bars and / or a positive pressure of several bars may prevail. Furthermore, in given cases, due to the formation of gas bubbles or overfilling, the corrosive filling substance 3 may come into direct contact with the fill level measuring device 1. These influences may reduce the functionality of the fill level measuring device 1. In addition to the limited temperature stability of the electronic components of the fill level measuring device 1, condensate formation may in particular be experienced in the fill level measuring device 1.

[0030] exist Figure 2 The components of the fill level measuring device 1 which are relevant in this respect are shown in cross section:

[0031] - The high frequency unit 10 is used to generate the high frequency signal S to be transmitted HF And used to process the reflected high-frequency signal R HF The high-frequency unit 10 comprises the functional blocks required for a specific functional principle: In the case of an FMCW method, a high-frequency signal S can be generated with the aid of a PLL (“phase-locked loop”). HF ; The received high frequency signal R HF Can be used with the instantaneous high-frequency signal S HF A frequency mixing takes place, so that from the difference frequency of the mixed signal the distance d and the filling level L can be determined. An appropriately designed evaluation block can determine the difference frequency, for example, by means of an FFT (“Fast Fourier Transform”) of the mixed signal.

[0032] The hollow conductor 11 is connected to the high-frequency unit 10, in which case at least the inner wall of the hollow conductor 11 is electrically conductive with the signal line so as to transmit the high-frequency signal S HF The high frequency signal R is coupled into the container 2 and reflected in the container 2. HF Coupled back to the high frequency unit 10. Figure 2 In the case of the exemplary embodiment shown, the opening oriented toward the hollow conductor 11 of the filling mass 3 widens in a conical shape, so that a corresponding antenna is realized.

[0033] -For high frequency signal S HF , R HFThe process isolation part 15 made of a transparent material (eg, PMMA, PTFE or PC) seals the antenna and the hollow conductor 11 and blocks the filling substance 3 to prevent the filling substance 3 from penetrating into the hollow conductor 11 .

[0034] Figure 3 Shows Figure 2 Enlarged detail A of the fill level measuring device 1 of FIG. 1 . As can be seen from this detail view, in the area of ​​the antenna facing the process isolation 15, the hollow conductor 11 is supplementarily sealed by two outer sealing rings 16, since in a given case the process isolation 15 may not have the required accuracy for adapting the antenna. The pressure resistance of the fill level measuring device 1 is achieved primarily by the insulator 12 inside the hollow conductor 11. In the shown example of embodiment, the insulator 12 is disk-shaped and, depending on the expected positive pressure, can have a thickness between 200 μm and 5 mm. Ideally, this thickness is equal to the high-frequency signal S HF , R HF Moreover, the insulator 12 is less sensitive to the high-frequency signal S HF , R HF The material of the insulator 12 may be glass, for example.

[0035] The insulator 12 divides the hollow conductor 11 into a first part 11a facing toward the high-frequency unit 10 and a second part 11b, the second part 11b facing toward the filling substance 3 during operation. The insulator 12 seals the two parts 11a, 11b liquid-tightly from each other. In this case, the hollow conductor 11 is designed in the illustrated example of embodiment so that the second part 11b forms a holder for the first part 11a. In this regard, the first part 11a has an external thread and the second part 11b has a corresponding internal thread so that the first part 11a of the hollow conductor 11 can be screwed into the second part 11b forming the holder by means of the formed thread 14. Thus, such as Figure 3 As shown in FIG, the insulator 12 can be mounted on the first part 11a by means of welding before being screwed in.

[0036] Depending on the material, the sealing ring 16 has only -12A finitely small gas permeability coefficient in the order of magnitude of kg / (s*bar). As a result, despite the sealing ring 16 and despite the process isolation 15, with a prolonged operating duration, gases such as the gaseous part of the filling material 3 diffuse into the second part 11b of the hollow conductor. However, in order not to produce condensate, for example water condensate, on the insulator 12, the filling level measuring device 1 according to the invention comprises a first hollow space 13. This hollow space is arranged at a certain distance a behind the insulator 12 relative to the second part 11b and is therefore arranged in the region of the first part 11a. In this case, in the embodiment shown, the hollow space 13 is arranged rotationally symmetrically around the hollow conductor 11. In principle, however, a rotationally symmetrical design of the first hollow space is not necessary within the scope of the present invention. As Figure 3 As shown, the hollow space 13 is fluidically connected to the second part 11 b via a thread 14 , so that a gas supply or a pressure equalization can be produced through the hollow space 13 .

[0037] As a result of this, since according to the invention the first hollow space 13 is arranged relative to the insulator 12 so that the insulator 12 is further away from the filling material 3, the high temperature T in the container 2 C In this case, condensation does not occur on the insulation 12 , but rather occurs in the first hollow space 13 .

[0038] Therefore, by reducing the thermal resistance R of the hollow conductor 11 between the insulator 12 and the first hollow space 13 th,H , the formation of condensate on the insulator 12 can be avoided, wherein

[0039]

[0040] In this case, λ H is the thermal conductivity of the hollow conductor material used, in W / (m*K). Stainless steel, a potential material for the hollow conductor 11, for example has a thermal conductivity λ of 15 W / (m*K) H .

[0041] A H is the cross-sectional area of ​​the hollow conductor 11 in the region between the insulator 12 and the first hollow space 13. Cross-sectional area A H This is the case in the case of a rotationally symmetrical design of the hollow conductor 11 .

[0042]

[0043] In this case, D o is the outer diameter of the hollow conductor 11, and D i is the inner diameter of the hollow conductor 11. When the hollow conductor 11 is a square, the cross-sectional area A HFor another.

[0044]

[0045] In this case, D o and D i They are respectively the outer edge length and the inner edge length of the rectangular parallelepiped hollow conductor 11 .

[0046] When the thermal resistance R is established on the structure th,H So that the temperature T in the container 2 is at least 180°C C When the temperature difference to the temperature in the first hollow space 13 reaches at least 30° C., sufficient safety is provided to prevent condensate from forming on the insulating body 11. The volume of the first hollow space 13 is preferably dimensioned according to the permeability coefficient of the sealing ring 16 in order to prevent condensate from forming on the insulating body 12. A sufficient dimensioning rule is that the volume of the hollow space 13 should be 10 -12 kg / (s*bar) The permeability coefficient of the sealing part is at least 1.2cm 3 In this case, it is taken into account that the permeability coefficient of the sealing ring 16 depends in a given case on the temperature T in the container 2 C .

[0047] Figure 3 The filling level measuring device 1 shown comprises an optional extension of the invention in the form of a second hollow space 17. Compared to the first hollow space 13, the second hollow space 17 is positioned relative to the second part 11b of the hollow conductor 11 in front of the insulating layer 12. In this way, the risk of condensate formation on the insulation 12 can be additionally reduced, because by adding the second hollow space, the pressure rise in the total hollow space is slowed down and the pressure necessary for the formation of condensate is reached later.

[0048] This expansion is meaningful when, for example, for structural or production reasons, the first hollow space 13 cannot be dimensioned sufficiently large according to the above-mentioned volume specifications, or when, for example, for structural reasons, the thermal resistance R of the hollow conductor 11 is th,H When it cannot be reduced to the above maximum value.

[0049] Reference numerals list

[0050] 1Filling level measuring device

[0051] 2 Containers

[0052] 3 Filling material

[0053] 4 Parent Unit

[0054] 10 high frequency units

[0055] 11Hollow conductor

[0056] 11a First hollow conductor portion

[0057] 11b Second hollow conductor portion

[0058] 12 Insulator

[0059] 13 The First Hollow Space

[0060] 14 threads

[0061] 15Process Isolation Department

[0062] 16 Sealing ring

[0063] 17 The Second Hollow Space

[0064] A H Cross-sectional area of ​​hollow conductor

[0065] a Distance between the second hollow conductor portion and the hollow space

[0066] D O The outer diameter or edge length of the hollow conductor

[0067] D i The inner diameter or edge length of a hollow conductor

[0068] d Distance from the filling level measuring device to the filling material

[0069] h Installation height of filling level measuring device

[0070] L Filling level

[0071] S HF ,R HF High frequency signal

[0072] R th,H Thermal resistance of hollow conductor

[0073] T C Temperature inside the container

[0074] λ H Thermal conductivity

Claims

1. A filling level measuring device, the filling level measuring device being used to determine a filling level (L) of a filling material (3) located in a container (2), the filling level measuring device comprising: a high-frequency unit (10) which is designed to generate and / or process high-frequency signals in order to determine the filling level (L), - a hollow conductor (11) coupled to the high-frequency unit (10) in order to transmit the high-frequency signal to the filling substance (3) and / or in order to receive the high-frequency signal reflected on the filling substance (3), wherein the hollow conductor is subdivided into a first portion (11a) facing the high-frequency unit (10), and o a second portion (11b) capable of facing toward said filling substance (3), as well as - an insulator (12) which is transparent to the high frequency signal and fluidically isolates the first part (11a) from the second part (11b), It is characterized in that A first hollow space (13) which is in fluid connection with the second portion (11b), wherein the first hollow space (13) is arranged behind the insulator (12) relative to the second portion (11b).

2. The filling level measuring device according to claim 1, It is characterized in that A second hollow space (17) which is in fluid connection with the second portion (11b), wherein the second hollow space (17) is arranged upstream of the insulating body (12) relative to the second portion (11b).

3. The filling level measuring device according to claim 1 or 2, wherein: The high-frequency unit (10) is designed to generate a high-frequency signal having a frequency greater than 75 GHz.

4. The filling level measuring device according to claim 1 or 2, wherein: The high-frequency unit (10) is designed to generate a high-frequency signal having a frequency greater than 100 GHz.

5. The filling level measuring device according to claim 1 or 2, It is characterized in that The thermal resistance (R th,H ) is dimensioned so that the temperature (T C ) to the temperature in the first hollow space (13) reaches a temperature difference of at least 30°C.

6. The filling level measuring device according to claim 1 or 2, It is characterized in that The threads (14) fluidically connect the first hollow space (13) with the second portion (11b).

7. The filling level measuring device according to claim 1 or 2, comprising: A process partition (15) which blocks an end region of the second part (11b) which can face the filling material (3).

8. The filling level measuring device according to claim 7, wherein: A fluid seal is arranged between the process isolation (15) and an end region of the second part (11b).

9. The filling level measuring device according to claim 7, wherein: A fluid seal formed by at least two sealing rings (16) is arranged between the process isolation (15) and an end region of the second part (11b).

10. The filling level measuring device according to claim 8 or 9, It is characterized in that The first hollow space (13) is designed according to the permeability coefficient of the fluid sealing part, so that the volume of the first hollow space (13) is determined to be -12 kg / (s*bar) The permeability coefficient of the sealing part is at least 1.2cm 3 .

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