Method for determining the inner diameter of a surge pipe using a fill level measuring device

Abstract

The invention relates to a method for determining the inner diameter (D) of a surge pipe (11) using a radar-based fill level measuring device (1) in order to ascertain the fill level (L) in an error-free manner. In the process, the surge pipe (11) extends into the process chamber of the container (3) in which the corresponding filling material (2) is located in order to measure the fill level, or the surge pipe (11) is attached next to the container (3) and is connected to the process chamber (5) for this purpose: In accordance with the FMCW principle, the method is based on not only transmitting radar signals (S1, S2) with a first center frequency (f1) or in a first frequency band but also transmitting radar signals with a second center frequency (f2), wherein in both cases one and the same reference object is used in order to reflect the signals. Thus, two corresponding intermediate frequency signals (ZF1, ZF2) or the frequencies (fZF1,2) thereof can be determined according to the FMCW principle such that the pipe inner diameter (D) is calculated according to the invention using the two center frequencies (f1, f2) of the radar signals (S1, S2) and the two frequencies (fZF1, fZF2) of the intermediate frequency signals (ZF1, ZF2).

Description

[0001] Method for determining the inner diameter of a stilling pipe using a level measuring device

[0002] The invention relates to a method for determining a pipe inner diameter of a surge pipe which extends into the process chamber for measuring the fill level of a filling material located in a process chamber of a container or is mounted next to the container and connected to the process chamber.

[0003] In process automation technology, appropriate field device types are used to record relevant process parameters. For the purpose of recording such process parameters, the respective field device types implement suitable measuring principles with which the relevant process parameters, such as level, flow, pressure, temperature, pH value, redox potential, or conductivity, can be recorded. A wide variety of such field device types are manufactured and distributed by the Endress + Hauser group of companies.

[0004] Non-contact measurement methods have become established for level measurement of filling materials in containers because they are robust and low-maintenance. Another advantage is their ability to measure the level virtually continuously, i.e., with very high resolution. In the field of continuous level measurement, radar-based measurement methods are predominantly used. The FMCW principle is an established measurement principle.

[0005] The FMCW principle is based on the continuous transmission of a high-frequency radar signal. The signal frequency lies within a specified frequency band around a standardized center frequency. Frequency bands in the 6 GHz, 26 GHz, or 79 GHz bands are typically used. A characteristic of the FMCW method is that the transmission frequency is not constant, but changes periodically within the frequency band. The change can be linear and have a sawtooth or triangular shape; however, a nonlinear change, such as a sinusoidal change, can also be implemented depending on the application.

[0006] A particular challenge is ensuring that the received signal, which is received after the radar signal has been reflected off the product surface, is free of interference signals, or that the corresponding interference components are known. Otherwise, this can generate erroneous measured values, which impair the functionality of the level measuring device. A key cause of this is the reception of interference echo signal components that do not originate on the product surface, but are caused by reflection of the transmitted radar signal off interfering objects, such as agitators or internal components in the container. In the case of liquid products, wave formation can also lead to an inaccurate level measurement.

[0007] One way to avoid measurement errors caused by this is to use a stilling pipe or bypass pipe for liquid products. This ensures that the product surface within the stilling pipe is homogeneous and the resulting beam path of the radar signal within the stilling pipe is free of obstructions. The stilling pipe is mounted approximately vertically within the process chamber in the vessel. The stilling pipe is designed to be vented so that the fill level of the product within the stilling pipe matches the fill level in the rest of the process chamber. In the case of a bypass pipe, this is mounted next to the vessel and is also connected to the process chamber in such a way that the fill level in the bypass pipe is the same as the fill level in the process chamber.

[0008] When using a stilling or bypass pipe, the level gauge is not positioned, as is otherwise usual, with its antenna pointing directly into the process chamber of the vessel containing the medium. Instead, the level gauge is mounted at the top of the pipe so that its antenna transmits the microwave signal along the stilling pipe toward the medium. Level gauges mounted on stilling pipes with a round inner diameter are usually calibrated to the inner diameter of the stilling pipe subsequently used. For applications in the oil and gas sector, calibration is often performed to a reference inner diameter according to DIN EN ISO 6708, for example, DN 100.

[0009] A problem arises when the inner diameter of the stilling well, to which the level gauge is later mounted, does not exactly match the reference inner diameter of the calibration tube. This common situation is disadvantageous in that even a small deviation between the inner diameters causes a significant measurement error in the level measurement and thus leads to a very inaccurate level measurement. The reason for this is that the signal propagation speed of the radar signals in the stilling well deviates significantly from the free-field propagation speed, depending on the pipe's inner diameter.

[0010] Publication DE 102016105419 A1 describes a method by which the pipe's inner diameter can be determined using radar-based level gauges. However, this method requires that the pipe's inner diameter be known at least roughly. The invention is therefore based on the object of providing a method with which FMCW-based level gauges used on stilling pipes or bypass pipes can accurately determine the fill level even when the pipe's inner diameter is completely unknown. In the context of the invention, the term "stilling pipe" is also understood to include bypass pipes.

[0011] The invention solves this problem by a method for determining the inner diameter of a stilling tube, which, for measuring the fill level of a product in a process chamber of a container, either extends into the process chamber or is mounted next to the container and connected to the process chamber. The method comprises the following steps:

[0012] Emitting a first radar signal according to the FMCW principle with a first center frequency into the stilling tube and receiving a corresponding first received signal after reflection at a fixed reference, generating a first intermediate frequency signal according to the FMCW principle on the basis of the first signals and determining the frequency of the first intermediate frequency signal,

[0013] Emitting a second radar signal according to the FMCW principle into the stilling well with a second center frequency that differs from the first center frequency, and receiving a corresponding second received signal after reflection at the fixed reference, generating a second intermediate frequency signal according to the FMCW principle based on the second signals and determining the frequency of the second intermediate frequency signal,

[0014] Determination of the pipe inner diameter based on at least o the center frequencies of the radar signals, and o the frequencies of the intermediate frequency signals.

[0015] Depending on the situation, application and design of the stilling pipe, the surface of the filling material, a lower end area of ​​the stilling pipe or the bottom of the container can serve as a fixed local reference.

[0016] Thus, based on the determined pipe inner diameter and the frequency of at least one of the intermediate-frequency signals, the current signal propagation velocity in the stilling well can be precisely determined, even without a rough prior knowledge of the pipe diameter. Based on the true signal propagation velocity, the exact fill level value can then be determined. Furthermore, the object underlying the invention is achieved by a fill level measuring device for implementing the method described in at least one of the preceding variants.For this purpose, the level measuring device comprises: A signal generation unit for FMCW-based generation of the radar signals, a coupling element by means of which the radar signals can be coupled into the stilling pipe and the received signals can be coupled out of the stilling pipe after they have been reflected, a mixer unit for mixing the radar signals to be transmitted with the respective received signal so that the corresponding intermediate frequency signal is generated according to the FMCW principle, an evaluation unit which is designed o to determine the frequencies of the intermediate frequency signals, o to determine the inner pipe diameter based on the center frequencies and the frequencies of the intermediate frequency signals, and o to determine the signal propagation speed in the stilling pipe or, based on this, the fill level.

[0017] The determined pipe inner diameter as well as the fill level value or the underlying intermediate frequency value can be output either via a corresponding interface to a higher-level unit or a mobile device. However, it is also conceivable to use the interface as an output unit other than the interface. For example, a display on the level gauge can also be used to display the determined pipe inner diameter.

[0018] With appropriate design, the level measuring device can transmit the two different center frequencies or radar signals, for example, at a defined clock rate alternately in the two different frequency bands, whereby the different frequencies of the corresponding intermediate frequency signals are to be determined in a correspondingly clocked manner in this case. The further the two center frequencies are separated from each other, the more accurately the pipe inner diameter can be determined. Sufficient accuracy is achieved if the signal generation unit generates the radar signals with appropriate design in such a way that their center frequencies differ by at least a factor of 1.1, in particular a factor of 1.5. After the initial, inventive determination of the pipe inner diameter, or if the pipe inner diameter per se is already known with sufficient accuracy, the formula In addition, any changes in the free-field propagation velocity or, based on this, any changes in the gas composition of the atmosphere above the filling material in the process chamber can be determined. Any changes in the free-field propagation velocity can be continuously monitored, for example, if the level gauge transmits the radar signals alternately in the two different frequency bands at a defined rate.

[0019] To check the determined fill level value for plausibility, for example, the evaluation unit can be designed to optionally determine the fill level value based on the first intermediate frequency signal or its frequency, as well as the second intermediate frequency signal or its frequency. If a comparison of the two values ​​does not result in sufficient agreement, they can be classified as implausible, so that the evaluation unit, for example, outputs a corresponding error signal.

[0020] Since the exact value of the pipe's inner diameter is determined per se within the scope of the invention, the signal generation unit or the evaluation unit of the level measuring device according to the invention can be expanded in such a way that, depending on the determined pipe's inner diameter, the radar signals to be transmitted and / or the intermediate frequency signals are subjected to dispersion correction. Dispersion correction is described in more detail, for example, in the publication DE 102016105419 A1.

[0021] The invention is explained in more detail with reference to the following figures. Shown are:

[0022] Fig. 1 : A typical arrangement of a level gauge operating according to the FMCW principle on a stilling well, and

[0023] Fig. 2: The characteristic frequency response of the radar signal in the FMCW principle within the scope of the invention. To understand the invention, Fig. 1 shows a container 3 with a liquid filling material 2, whose fill level L is to be determined. Depending on the type of filling material 2 and the area of ​​application, the container 3 can be up to more than 100 m high. The filling material 2 can be, for example, water or a fuel such as crude oil or liquefied gas.

[0024] To determine the fill level L, a radar-based level measuring device 1 is provided. Typically, the level measuring device 1 is connected to a higher-level unit 4, such as a local process control center or a decentralized server system, via a suitable interface, such as 4-20 mA, PROFIBUS, HART, or Ethernet. The measured level values ​​L can be transmitted via this interface, for example, to control any inflows or outflows of the container 3. However, other information about the general operating status of the level measuring device 1 can also be communicated via this interface.If the level measuring device 1 only transmits the raw measurement data, such as the frequency values ​​fi ,2 of the intermediate frequency signal ZFI ,2 in the case of the FMCW method, to the higher-level unit 4, the final calculation of the level value L based thereon can also be carried out in the higher-level unit 4 if designed accordingly.

[0025] In order to fundamentally rule out such measurement errors, which can arise, for example, from wave formation, the container s shown in Fig. 1 comprises a stilling pipe 11, via which the fill level L is determined. For this purpose, the stilling pipe 11 projects approximately vertically from the top of the container 3 into the process chamber 5 of the container 3. The length of the stilling pipe 11 is dimensioned such that the lower end region of the stilling pipe 11 is at the height which corresponds to the minimum fill level value L. In the embodiment shown in Fig. 1, the length of the stilling pipe 11 is dimensioned such that its lower end region is not at the level of the container bottom, but slightly above at the level of a filling material outlet. In order to ensure that the filling level L of the filling material 2 inside the stilling pipe 11 is equal to the filling level L outside the stilling pipe 11, corresponding passages are radially inserted into the stilling pipe 11 distributed over its height.

[0026] The level measuring device 1 is attached to the upper end of the stilling pipe 11 and comprises, for example, a coupling element or a suitable antenna structure, by means of which radar signals S1,2, RI,2 can be transmitted into the stilling pipe 11 or into the container 3 or, after their reflection at the filling material surface, can be decoupled from the stilling pipe 11. After the transmitted radar signal Si,2 has been reflected at the filling material surface, the level measuring device 1 receives the reflected radar signals RHF via its coupling element. The resulting signal propagation time t between transmission and reception of the respective radar signal SHF, RHF is as follows: proportional to the distance d between the level gauge 1 and the surface of the medium 2. For the variable ,,c m(D) “ is in this context the media or pipe inner diameter dependent radar propagation speed, which depends on the radar propagation speed in the free field c m , o varies to varying degrees depending on the pipe's inner diameter D. The radar propagation speed in the free field c m ,o in turn depends on the gas composition or those properties which influence the propagation speed of the radar signals Si,2, RI ,2 before and after reflection.

[0027] To determine the signal propagation time t, the FMCW principle is implemented in the level measuring device 1. Accordingly, the level measuring device 1 can comprise, for example, a phase-locked loop (“Phase Locked Loop”) as a signal generation unit. This is controlled so that the frequency of the transmitted radar signal Si,2 changes in a sawtooth shape, as shown in Fig. 2. As can be seen, the frequency f of the radar signal Si,2 has a cyclically recurring, constant frequency increase rate f' within a defined frequency band (e.g., 79 GHz - 80 GHz or 25.5 GHz - 26.5 GHz). Instead of a sawtooth-shaped frequency change rate f', any other shape is also conceivable, for example, a triangular or sinusoidal change within the respective frequency band. The frequency fi,2 that forms the arithmetic mean between the upper and lower limits of the respective frequency band is referred to as the center frequency fi,2.

[0028] After being received from the stilling well 11, the reflected radar signal Ri,2 is mixed within the level measuring device 1 according to the FMCW principle with the transmitted radar signal Si,2, thereby generating a corresponding intermediate frequency signal ZFi,2. The frequency fzFi,2 of the intermediate frequency signal ZFi,2 represents the signal propagation time t of the radar signal Si ,2, RI,2 between transmission and reception. Here, f' is the frequency change rate of the radar signals. To determine the frequency fzFi,2 of the intermediate frequency signal ZFI ,2 and thus the signal propagation time t, a fast Fourier transformation of the intermediate frequency signal ZFI ,2 can be performed in an evaluation unit of the level measuring device 1, for example. As is usual when processing such data, this is usually done on the basis of a digitized intermediate frequency signal ZFI ,2.

[0029] By determining the signal propagation time t, the evaluation unit is able to calculate the distance d using formula (1). This allows the level measuring device 1 to

[0030] (3) d = h - L determine the fill level L at least at a specific point, provided that the installation height h of the level gauge 1 above the container bottom or the minimum fill level is known. For a known empty container s, the installation height h can be determined by the level gauge 1 itself. This can be done, for example, as part of a teach-in process by measuring the distance h of the level gauge 1 from the container bottom, analogous to a measurement of the fill level L.

[0031] As can be seen from formula (1), it is necessary to determine the signal propagation speed c m (D) of the radar signals S1 ,2, RI ,2 in the stilling well 11. However, for this purpose, in addition to the radar propagation speed c m,o in the free field, knowledge of the pipe inner diameter D is required. For k mO This is a mode-dependent constant that comes from the Bessel function and is therefore known.

[0032] Although the pipe inner diameter D of the stilling well 11 is generally nominally known, for example, if it is a DN100 pipe, in practice the exact or true value is not known or it deviates noticeably from its target value. The consequence is that the level measuring device 1 cannot determine the fill level L precisely, since the level measuring device 1 or its evaluation unit for level calculation according to formulas (1) to (3) is not based on the correct value of the pipe inner diameter D. Using the method according to the invention, it is possible to precisely determine the pipe inner diameter D of the stilling well 11 without knowing the approximate or nominal diameter. Based on the determined pipe inner diameter D, the level measuring device 1 is in turn enabled to calibrate itself accordingly based on the pipe inner diameter D or to determine the signal propagation speed c m(D) in the stilling pipe 11 in order to be able to accurately determine the fill level L in the subsequent, regular measuring operation according to formulas (1) and (2). For this purpose, the fill level measuring device 1 transmits radar signals Si, S2 according to the FMCW principle not only in a first frequency band or with a first center frequency fi, but also in a second frequency band or with a second center frequency f2. Correspondingly, the evaluation unit of the fill level measuring device 1 according to the invention determines, as previously explained in connection with the FMCW principle, for each frequency band or each center frequency fi, f2, a corresponding intermediate frequency signal ZF1, ZF2 and its frequency fzFi, fz2. If the signal generation unit generates both radar signals Si, S2 despite the different frequency bands with the same frequency change rate f', the pipe inner diameter D can be determined by the evaluation unit or the higher-level unit 4 according to Where fi and f2 are the center frequencies of the two different frequency bands.

[0033] fzFi, fzF2 are the frequencies of the corresponding intermediate frequency signals SZFI, SZ2.

[0034] If the frequency change rate f'1,2 of the two radar signals Si, S2 is not identical, formula (5) expands to

[0035] The inventive idea can be implemented, for example, in that the signal generation unit alternates between the two frequency bands or the radar signals Si, S2 to be transmitted in a clocked manner, so that the radar signals Si, S2 are transmitted accordingly with clocked alternating center frequencies fi, f2. The different received signals Ri, R2 are received in a correspondingly clocked manner after reflection from a common reference, and their different frequencies fzFi, fz2 of the resulting intermediate frequency signals ZF1, ZF2 are determined. Within the scope of the invention, it is conceivable for the two frequency bands to overlap as long as their boundaries are not identical and their center frequencies fi, f2 differ from one another accordingly. In the example shown in Fig. 2, the center frequencies fi, f2 are 26 GHz and 79 GHz respectively, differing by approximately a factor of 3.Within the scope of the invention, however, it is sufficient if the center frequencies fi, f2 differ by at least a factor of 1.1. In principle, the greater the difference between the two center frequencies fi, f2, the more pronounced the difference in the intermediate frequencies fzFi, fzF2. Accordingly, the pipe's inner diameter can be determined with greater precision.

[0036] Since the inventive calculation of the pipe inner diameter according to formula (5) or formula (6) is only valid if the radar signals Si, S2 emitted in different frequency bands reflect one and the same reference, the circumstances under which the two radar signals Si, S2 are emitted in different frequency bands must be selected accordingly: In the simple case, the lower end region of the surge well 11 can be defined as a common, spatially fixed reference, provided that this end region, for example, comprises a slightly inwardly curved collar which enables sufficient reflection of the radar signals Si, S2. If the lower end region of the surge well 11 is located close to the bottom of the container interior from a radar technology perspective, this can also serve as a common reference for the reflection of the radar signals Si,2, RI,2 within the scope of the inventive method.If the filling level L does not change at the time at which the frequency band of the emitted radar signal Si, S2 is changed for the purpose of determining the pipe inner diameter according to the invention, the filling material surface can also be used as a common reference.

[0037] When and at what rate the level gauge 1 switches between the frequency bands to (re)determine the pipe's inner diameter D must be adapted to the respective application. For example, the level gauge 1 according to the invention can be designed such that it switches the frequency band only upon request or corresponding input, and that the level gauge 1 updates the pipe's inner diameter D accordingly only upon corresponding request.

[0038] However, it is also conceivable that the fill level measuring device 1 according to the invention is designed to switch automatically between the frequency bands at a predefinable clock rate. In this case, the corresponding two time periods within the clock cycle in which the fill level measuring device transmits the radar signal Si, S2 in the respective frequency band or with the respective center frequency T, f2 do not have to be symmetrical. In this case, the pipe inner diameter D is simply recalculated using the method according to the invention at the corresponding clock rate, provided that it is ensured that the radar signals Si, S2 of both frequency bands are reflected at the same reference at this time. Between the clocked change of the frequency bands, i.e. when the radar signal S1 is transmitted, for example, within the first frequency band or with the first center frequency fi, the fill level L is determined on the basis of the frequency fzFi of the corresponding, first intermediate frequency signal ZF1.In this context, it is not relevant within the scope of the invention on the basis of which of the two frequencies fzFi,2 of the two intermediate frequency signals ZF1, ZF2 the level measuring device 1 determines the level value L. In this context, it is also conceivable for the level measuring device 1 to determine a level value L based on both frequencies fzFi,2 or both intermediate frequency signals ZF1, ZF2 and, if necessary, to compare the two values ​​for the purpose of a plausibility check. It is also possible for the level measuring device 1, after determining the pipe's inner diameter D according to the invention, to determine the level L based on a third radar signal or on its third intermediate frequency.

[0039] Depending on the application, the cycle rate at which the two radar signals or frequency bands are switched between for pipe inner diameter determination can, for example, be up to one month if no change in the pipe inner diameter D is expected in the meantime. If a more rapid change is expected, such as in the case of buildup, the cycle rate in the level gauge 1 can also be preset to one day or shorter.

[0040] If the level measuring device 1 has an optical output unit such as a display and the pipe inner diameter D is not calculated in the higher-level unit 4, it is also possible, with an appropriate design of the level measuring device 1, for the determined pipe inner diameter D to be displayed here. Overall, the inventive determination of the pipe inner diameter D ensures that the level measuring device 1 or the higher-level unit 4 can always determine the level value L based on the correct signal propagation speed c m (D) in the stilling pipe 11.

[0041] List of reference symbols and variables

[0042] 1 level gauge

[0043] 2 Filling material

[0044] 3 containers

[0045] 4 Superior unit

[0046] 5 Process room

[0047] 11 Stilling pipe c m ,o Radar propagation speed in the free field cm (D) Signal propagation speed in the stilling well

[0048] D Pipe inner diameter d Distance to the filling material

[0049] RI , 2 Reflected radar signals f' Frequency change rate of the radar signals fi,2 Center frequencies of the radar signals fzFi,2 Frequencies of the intermediate frequency signals h Installation height of the level measuring device kmode Mode-dependent constant

[0050] L Fill level

[0051] Si,2 radar signals

[0052] ZFI,2Intermediate frequency signals

Claims

Patent claims 1 . Method for determining a pipe inner diameter (D) of a surge pipe (11) which extends into the process chamber (5) for measuring the fill level (L) of a filling material (2) located in a process chamber (5) of a container (3), or which is mounted next to the container (3) and connected to the process chamber (5) for measuring the fill level (L), comprising the following method steps: Emitting a first radar signal (Si) according to the FMCW principle with a first center frequency (fi) into the surge tube (11) and receiving a corresponding first received signal (Ri) after reflection at a fixed reference, Generating a first intermediate frequency signal (ZFi) according to the FMCW principle based on the first signals (Si, Ri) and determining the frequency (fzFi) of the first intermediate frequency signal (ZFi), Emitting a second radar signal (S2) according to the FMCW principle into the surge tube (11) with a second center frequency (fz) that differs from the first center frequency (fi), and receiving a corresponding second received signal (R2) after reflection at the fixed reference, generating a second intermediate frequency signal (ZF2) according to the FMCW principle on the basis of the second signals (S2, R2) and determining the frequency (fzF2) of the second intermediate frequency signal (ZF2), Determination of the pipe inner diameter (D) based on at least o the center frequencies (fi, fz) of the radar signals (Si , S2), and o the frequencies (fzFi, fzrz) of the intermediate frequency signals (ZFi, ZF2).

2. Method according to claim 1, wherein the filling material surface, a lower end region of the surge pipe (11) or the bottom of the container (3) serves as the locally fixed reference.

3. Method according to claim 1 or 2, wherein the radar signals (Si, S2) are each transmitted with the same, in particular sawtooth-shaped, frequency change rate (f').

4. The method according to claim 3, wherein according to the pipe inner diameter (D), or if the pipe inner diameter (D) is known, a free-field propagation velocity (c m ,o) of the radar signals (Si,2, RI ,2) is determined.

5. Method according to at least one of the preceding claims, wherein the signal propagation speed c is determined on the basis of the determined pipe inner diameter (D) and on the basis of the frequency (fzFi, fzF2) of at least one of the intermediate frequency signals (ZFi, ZF2) or the frequency of a third intermediate frequency signal. m (D) in the stilling pipe (11) or the filling level (L) is determined based thereon.

6. Level measuring device (1) for carrying out the method described in claim 5, comprising: A signal generation unit for FMCW-based generation of the radar signals (Si, S2), a coupling element by means of which the radar signals (Si, S2) can be coupled into the stilling pipe (11) and the received signals (Ri, R2) can be coupled out of the stilling pipe (11) after their reflection, a mixer unit for mixing the radar signals (Si, S2) to be transmitted with the respective received signal (Ri, R2) so that the corresponding intermediate frequency signal (ZF1, ZF2) is generated according to the FMCW principle, an evaluation unit which is designed to o determine the frequencies (fzFi, fzrz) of the intermediate frequency signals (ZF1, ZF2), o based on the center frequencies (fi, fz) and the frequencies (fzFi, fzrz) of the intermediate frequency signals (ZF1, ZF2) pipe inner diameter (D), and o to determine the pipe inner diameter (D) based on the determined pipe inner diameter (D) and the frequency (fzFi, fzrz) of at least one of the intermediate frequency signals (ZF1,IF2) or the frequency of a third intermediate frequency signal, the signal propagation speed (c, m (D)) in the stilling pipe (11) or to determine the filling level (L) based thereon.

7. Level measuring device according to claim 6, which is designed to transmit the radar signals (Si, S2) with the two different center frequencies (fi, fz) alternately and at a defined clock rate and to determine the different frequencies (fzFi, fzF2) of the corresponding intermediate frequency signals (ZF1, ZF2) in a correspondingly clocked manner.

8. Level measuring device according to claim 6 or 7, which is designed, in particular after determining the pipe inner diameter (D), the free-field propagation velocity (c m ,o) or to determine any change in the gas composition of the atmosphere above the filling material (2) in the process chamber (5).

9. Level measuring device according to at least one of claims 6 to 8, comprising: An output unit used specifically to display the pipe inner diameter (D).

10. Level measuring device according to at least one of claims 6 to 9, wherein the signal generating unit is designed to generate the radar signals (Si, S2) such that their center frequencies (fi, f2) differ by at least a factor of 1.1, in particular a factor of 1.

5.

11. Level measuring device according to at least one of claims 6 to 10, wherein the evaluation unit is designed to determine the level (L) based on the first intermediate frequency signal (ZF1) or its frequency (fzFi) and based on the second intermediate frequency signal (ZF2) or its frequency (fzF2).

12. Level measuring device according to one of claims 6 to 11, wherein the signal generating unit or the evaluation unit is / are designed to subject the radar signals (Si, S2) to be emitted and / or the intermediate frequency signals (ZF1, ZFFZ) to a dispersion correction depending on the determined pipe inner diameter (D).

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