System and method for determining level and density distribution

Abstract

A radar level gauge system comprises: a first transceiver; a second transceiver; an antenna arranged to radiate a first transmit signal generated by the first transceiver and a second transmit signal generated by the second transceiver, and to receive a first reflected signal and a second reflected signal; a tubular waveguide that guides the transmit signal toward an interface and guides the reflected signal back to the antenna; a plurality of signal interaction structures arranged at different levels along the tubular waveguide to selectively interact with the second transmit signal to facilitate the second reflected signal; and a processing circuit that determines the level of the interface between a liquid-phase product and a gas-phase product based on a relationship between the first transmit signal and the first reflected signal, and determines a density distribution based on a relationship between the second transmit signal and the second reflected signal.

Description

Technical Field

[0001] The present invention relates to a radar level gauge system and method for determining the level of an interface between a liquid phase product and a gas phase product in a tank and the density distribution in the liquid phase product. Background Art

[0002] Radar level gauge (RLG) systems are widely used to determine the fill level of products in tanks. Radar level gauging is typically performed by means of non-contact measurement, in which an electromagnetic signal is radiated toward the product contained in the tank, or by means of contact measurement, commonly known as guided wave radar (GWR). In the latter case, an electromagnetic signal is directed toward and into the product via a probe acting as a waveguide. The probe is typically arranged to extend vertically from the top of the tank toward the bottom.

[0003] An electromagnetic transmit signal is generated by the transceiver and propagates towards the surface of the product in the tank, and an electromagnetic reflect signal resulting from reflection of the transmit signal at the surface is propagated back to the transceiver.

[0004] Based on the relationship between the transmitted and reflected signals, the distance to the surface of the product can be determined.

[0005] In some applications, non-contact radar level gauge systems are used in conjunction with so-called still-pipes for measuring the fill level in tanks containing liquefied natural gas (LNG).Such tanks can be on land or on ships.

[0006] Depending on the conditions in the LNG tank, the density of the liquid product (primarily methane) can vary. Density can also vary at different levels in the tank. Therefore, density is an important parameter in addition to volume when determining the amount of product in the tank.

[0007] Furthermore, incidents involving a phenomenon known as "rollover" have occurred in LNG tanks. "Rollover" refers to the rapid release of LNG vapor, which can occur due to the spontaneous mixing of different density layers of LNG within a storage or cargo tank. Rollover presupposes that stratification has occurred, meaning that two separate layers of LNG with different densities exist within the tank. Naturally, the release of LNG vapor should be avoided whenever possible.

[0008] It would therefore be desirable to provide an improved determination of the amount of product in an LNG tank. In particular, it would be desirable to improve the determination of the density distribution in the liquid product in the tank, and the level of the interface between the liquid and gaseous product. Summary of the Invention

[0009] In view of the foregoing, a general object of the present invention is to provide an improved radar level gauge system, in particular a radar level gauge system that provides improved determination of the density distribution in a liquid product in a tank and of the level of the interface between the liquid product and the gaseous product.

[0010] According to a first aspect of the present invention, there is provided a radar level gauge system for determining the level of an interface between a liquid product and a gaseous product in a tank and a density distribution in the liquid product, the radar level gauge system comprising: a first transceiver for generating, transmitting and receiving electromagnetic signals having a first set of signal characteristics; a second transceiver for generating, transmitting and receiving electromagnetic signals having a second set of signal characteristics different from the first set of signal characteristics; an antenna coupled to the first transceiver and the second transceiver, the antenna being arranged to radiate an electromagnetic first transmit signal having the first set of signal characteristics and an electromagnetic second transmit signal having the second set of signal characteristics, and to receive a first reflected signal generated by reflection of the first transmit signal and a second reflected signal generated by reflection of the second transmit signal; and a tube extending from the top of the tank toward the bottom of the tank. The tubular waveguide is arranged and configured to guide the first transmission signal and the second transmission signal radiated by the antenna toward the interface between the liquid product and the gaseous product and into the liquid product, and to guide the first reflection signal and the second reflection signal back to the antenna; at least a first plurality of signal interaction structures are arranged at different levels along the first section of the tubular waveguide, each signal interaction structure in the first plurality of signal interaction structures is configured to selectively interact with the second transmission signal to facilitate the second reflection signal; and a processing circuit connected to the first transceiver and the second transceiver, the processing circuit being configured to determine the level of the interface between the liquid product and the gaseous product based on the relationship between the first transmission signal and the first reflection signal, and to determine the density distribution in the liquid product based on the relationship between the second transmission signal and the second reflection signal.

[0011] Each of the “first transceiver” and the “second transceiver” may be one functional unit capable of transmitting and receiving electromagnetic signals, or may be a system including a separate transmitter unit and a receiver unit.

[0012] The first transceiver and the second transceiver can advantageously constitute two separate and independent measurement channels that share the same antenna. Thus, the first reflected signal can effectively only include reflections of the first transmitted signal at impedance discontinuities experienced by the first transmitted signal, and the second reflected signal can effectively only include reflections of the second transmitted signal at impedance discontinuities experienced by the second transmitted signal.

[0013] An exemplary way of providing such mutually independent measurement channels is described in US 7701385, the entire content of which is incorporated herein by reference.

[0014] Each of the first transmit signal and the second transmit signal may advantageously be a microwave signal.For example, each transmit signal may be frequency and / or amplitude modulated on a carrier wave in the microwave frequency range.

[0015] The first and second sets of signal characteristics may include any characteristics of the first transmitted signal and the second transmitted signal, respectively, that can be used to separate the first reflected signal and the second reflected signal between the first transceiver and the second transceiver and provide desired selective interaction with the signal interaction structure. Such signal characteristics may include, for example, propagation mode and polarization.

[0016] Examples of suitable antennas may include horn antennas, array antennas, solid dielectric antennas, parabolic antennas, and the like.

[0017] The tubular waveguide can be configured as a so-called static tube, a metal tube with a circular cross-section extending from the top to the bottom of the tank. A microwave absorber can be positioned below the lower end of the tubular waveguide to at least reduce the amplitude of the bottom-reflected signal. The tubular waveguide can be provided with openings distributed along its length to enable the liquid product to move between the inside and outside of the tubular waveguide, thereby ensuring that the interface level between the liquid product and the vapor product is the same on both the inside and outside of the tubular waveguide.

[0018] Each of the first plurality of signal interaction structures is arranged and configured to selectively interact with the second transmit signal to contribute to the second reflected signal, which should be understood to mean that the interaction between each of the first plurality of signal interaction structures and the second transmit signal is significantly greater than any interaction between each of the first plurality of signal interaction structures and the first transmit signal. Furthermore, the signal interaction structures can advantageously be attached to a tubular waveguide.

[0019] It is known per se that the propagation of an electromagnetic signal through a dielectric material depends on the dielectric constant of the material. In particular, it is known that the propagation speed of a signal through a dielectric material is at least approximately inversely proportional to the square root of the relative dielectric constant of the material.

[0020] It is known per se that there is a correlation between the density of a dielectric material and the dielectric constant of the material and that this correlation is given by the Clausius-Mossotti equation.

[0021] Thus, the density of a dielectric material can be determined based on an estimate of the propagation of an electromagnetic signal through the material, either directly through correlation or indirectly by determining the dielectric constant and then through a correlation between the dielectric constant and density.

[0022] It should be noted that the processing circuitry may be provided as one device or as several devices working together.

[0023] The present invention is based on the recognition that different transceivers / measurement channels for transmitting and receiving electromagnetic signals having different sets of signal characteristics, sharing the same antenna and tubular waveguide, can be used to determine the level of the interface between a liquid product and a vapor product, as well as the density distribution in the liquid product. The inventors further recognized that the determination of the interface level can be improved by arranging signal interaction structures at different levels along the tubular mounting structure, wherein each signal interaction structure is arranged and configured to interact substantially with the signal in only one of the measurement channels. Thus, the density distribution can be determined using the signal that interacts with the signal interaction structure, and the level of the interface can be determined using the signal that does not interact, or interacts only relatively weakly, with the signal interaction structure. This can facilitate more accurate determination of the level of the interface, particularly in situations where the level of the interface is close to the level of one of the signal interaction structures.

[0024] According to embodiments, the first plurality of signal interaction structures may include at least three signal interaction structures spaced apart from one another along the first section of the tubular waveguide. In principle, a density distribution measurement in the liquid product phase can be determined based on signal interaction with two signal interaction structures positioned below the interface between the liquid and vapor product phases. Providing additional signal interaction structures below the maximum permissible interface level in the tank can provide a more accurate density distribution determination.

[0025] According to an embodiment, the first set of signal characteristics may include a first polarization and the second set of signal characteristics may include a second polarization different from the first polarization.

[0026] Advantageously, the first polarization may be a first linear polarization and the second polarization may be a second linear polarization orthogonal to the first linear polarization.

[0027] According to an embodiment, the first set of signal characteristics may include a first propagation mode, and the second set of signal characteristics may include a second propagation mode different from the first propagation mode. Examples of suitable propagation modes may include TE 11 ,TM 01 TE 21 and TE 31 .

[0028] According to various embodiments, each signal interaction structure in the first plurality of signal interaction structures may be a reflective structure arranged and configured to exhibit a first reflection coefficient for a first transmit signal and a second reflection coefficient for a second transmit signal, the ratio between the first reflection coefficient and the second reflection coefficient being less than 1:2. In other words, the first reflection coefficient may be less than half the second reflection coefficient. Advantageously, this ratio may be less than 1:3, and even more advantageously, less than 1:4.

[0029] In embodiments where the first transmit signal and the second transmit signal exhibit different linear polarizations, the reflective structure may be provided, for example, in the form of a pin, which may be made of metal or dielectric material, or a composite material of conductive and dielectric materials. Such a pin may have any cross-section.

[0030] In embodiments where the first transmit signal and the second transmit signal propagate in different propagation modes, the reflective structure may be provided, for example, in the form of one or more rings arranged inside the tubular waveguide. Examples of such reflective structures are described in US Pat. No. 7,345,622, which is incorporated herein by reference in its entirety.

[0031] When a reflected signal interaction structure is used, the processing circuit can be configured in an embodiment to: determine a first measurement value based on the relationship between the second transmitted signal and the second reflected signal, the first measurement value indicating the distance between the first signal interaction structure and the second signal interaction structure in the first plurality of signal interaction structures; determine a second measurement value based on the relationship between the second transmitted signal and the second reflected signal, the second measurement value indicating the distance between the third signal interaction structure and the fourth signal interaction structure in the first plurality of signal interaction structures; determine a first average density of a first level range in the tank based on the first measurement value; and determine a second average density of a second level range in the tank based on the second measurement value. The above-mentioned second signal interaction structure and third signal interaction structure can be the same signal interaction structure.

[0032] The density (average density) can be determined based on a direct and previously determined correlation between the density and the measured electrical distance. Alternatively, the density (average density) can be determined based on a direct and previously determined correlation between the dielectric constant and the measured electrical distance, as well as based on a known relationship between the dielectric constant and density. Furthermore, the (electrical) distance between two reflectors can be measured when the reflectors are not immersed in the liquid product (e.g., when the tank has been emptied) and when the reflectors are immersed in the liquid product. The relationship between these measurements is sufficient to determine the change in the dielectric constant, which can be used to determine the average density in the level segment between the two reflectors.

[0033] Furthermore, it may be beneficial to measure the temperature in the tank and / or the change in the cross-sectional dimensions of the tubular waveguide (eg the diameter of the circular cross-section) and additionally determine the density distribution based on such measurement data.

[0034] According to various embodiments, each signal interaction structure in the first plurality of signal interaction structures may be a resonator arranged and configured to be excited only by the second transmitted signal, with the resonant frequency of the resonator indicating the density of the liquid product filling the resonator. The density distribution in the liquid product may be determined based on an evaluation of the spectrum of the second reflected signal for different filling levels to identify the resonant frequencies of different resonators. In particular, the processing circuitry may be configured to determine a first density at a first filling level in the tank based on the resonant frequency of a first resonator arranged at a first filling level; and to determine a second density at a second filling level in the tank based on the resonant frequency of a second resonator arranged at a second filling level.

[0035] The resonator may advantageously have a high Q value, for example above 100.

[0036] In an embodiment, the resonator may comprise a resonant cavity arranged to contain the liquid product when immersed in the liquid product. The resonant frequency of the resonant cavity will depend on the dielectric constant and thus the density of the liquid product filling the resonant cavity. As an alternative to the resonant cavity, a dipole resonator may be used.

[0037] According to an embodiment, the tubular waveguide may include at least a first plurality of openings to selectively enable an electromagnetic field generated by a second transmit signal to be present on the outside of the tubular waveguide at each opening in the first plurality of openings; and each signal interaction structure in the first plurality of signal interaction structures may be arranged on the outside of the tubular waveguide at each opening in the first plurality of openings to be able to be excited by the second transmit signal.

[0038] According to an embodiment, the radar level gauge system may further include a second plurality of signal interaction structures arranged at different material levels along a second section of the tubular waveguide that is different from the first section, each signal interaction structure in the second plurality of signal interaction structures being configured to selectively interact with the first transmission signal to facilitate the first reflection signal; and the processing circuit may be configured to: for the material level along the first section of the tubular waveguide, determine the material level of the interface between the liquid product and the gaseous product based on the relationship between the first transmission signal and the first reflection signal, and determine the density distribution in the liquid product based on the second reflection signal along the first section of the tubular waveguide and based on the first reflection signal along the second section of the tubular waveguide; and for the material level along the second section of the tubular waveguide, determine the material level of the interface between the liquid product and the gaseous product based on the relationship between the second transmission signal and the second reflection signal, and determine the density distribution in the liquid product based on the first reflection signal along the second section of the tubular waveguide.

[0039] In these embodiments, a greater number of signal interaction structures can be used without excessively interfering with the determination of the interface level. This enables more accurate determination of the density distribution in the liquid product and / or determination of the density distribution in larger (higher) tanks.

[0040] According to a second aspect of the present invention, there is provided a method for determining an interface level between a liquid product and a gaseous product in a tank and a density distribution in the liquid product using a radar level gauge system, the radar level gauge system comprising: a first transceiver; a second transceiver; an antenna coupled to the first transceiver and the second transceiver; a tubular waveguide extending from the top of the tank toward the bottom of the tank; at least a first plurality of signal interaction structures arranged at different levels along a first section of the tubular waveguide, each of the first plurality of signal interaction structures being configured to selectively interact with an electromagnetic signal having a given set of signal characteristics; and a processing circuit connected to the first transceiver and the second transceiver, the method comprising: The following steps include: transmitting, by a first transceiver, an electromagnetic first transmission signal having a first set of signal characteristics different from a given set of signal characteristics; transmitting, by a second transceiver, an electromagnetic second transmission signal having a second set of signal characteristics constituting the given set of signal characteristics; radiating, by an antenna, the first transmission signal and the second transmission signal into a tubular waveguide; receiving, by the antenna, an electromagnetic first reflection signal generated by reflection of the first transmission signal and an electromagnetic second reflection signal generated by reflection of the second transmission signal; determining, by a processing circuit, an interface level between a liquid-phase product and a gas-phase product based on a relationship between the first transmission signal and the first reflection signal; and determining, by the processing circuit, a density distribution in the liquid-phase product based on the second reflection signal.

[0041] It should be noted that the above steps do not necessarily have to be performed in any particular order, and at least some of the steps may be performed simultaneously.

[0042] Further effects and variations of the second aspect of the invention are generally similar to those described above with reference to the first aspect of the invention.

[0043] In summary, embodiments of the present invention therefore relate to a radar level gauge system comprising: a first transceiver; a second transceiver; an antenna arranged to radiate a first transmit signal generated by the first transceiver and a second transmit signal generated by the second transceiver, and to receive a first reflected signal and a second reflected signal; a tubular waveguide that guides the transmit signal toward an interface and guides the reflected signal back to the antenna; a plurality of signal interaction structures arranged at different levels along the tubular waveguide to selectively interact with the second transmit signal to contribute to the second reflected signal; and a processing circuit that determines an interface level between a liquid product and a gaseous product based on a relationship between the first transmit signal and the first reflected signal, and determines a density distribution based on a relationship between the second transmit signal and the second reflected signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] These and other aspects of the present invention will now be described in more detail with reference to the accompanying drawings, which show exemplary embodiments of the invention, and in which:

[0045] Figure 1 schematically illustrates an exemplary tank monitoring device including a radar level gauge system according to an embodiment of the present invention;

[0046] Figure 2 is included Figure 1 Schematic diagram of the measuring unit in the radar level gauge system;

[0047] Figure 3 is a partial schematic block diagram of a radar level gauge system according to an embodiment of the present invention;

[0048] Figure 4 Schematically shows an example of a transmission signal transmitted by a transceiver in a radar level gauge system according to an embodiment of the present invention;

[0049] Figure 5 is a diagram showing an intermediate frequency signal formed based on a transmitted signal and a reflected signal;

[0050] Figure 6 schematically illustrates a first exemplary configuration of signal interaction structures comprising a first plurality of signal interaction structures in the form of reflectors arranged at different levels along a tubular waveguide;

[0051] Figure 7A schematically illustrates a second exemplary configuration of signal interaction structures comprising a first plurality of signal interaction structures in the form of resonators arranged at different levels along a tubular waveguide;

[0052] Figure 7B yes Figure 7A Schematic diagram of an exemplary configuration of a resonator in;

[0053] Figure 8 schematically illustrates a third exemplary configuration of signal interaction structures comprising a first plurality of signal interaction structures and a second plurality of signal interaction structures in the form of reflectors arranged at different levels along a first section and a second section, respectively, of the tubular waveguide;

[0054] Figure 9 is a flow chart schematically illustrating an exemplary embodiment of the method according to the present invention;

[0055] Figure 10A is a diagram showing an exemplary echo curve based on a first transmission signal and a first reflection signal;

[0056] Figure 10B is a diagram showing an exemplary echo curve based on a second transmission signal and a second reflection signal; and

[0057] Figure 11 are graphs illustrating measurement errors introduced by a signal interaction structure in the form of an exemplary reflector in the first and second measurement channels, respectively. DETAILED DESCRIPTION

[0058] In this detailed description, various embodiments of the present invention are discussed primarily with reference to an FMCW type radar level gauge system.

[0059] It should be noted that this in no way limits the scope of the present invention, which also covers pulsed radar level gauge systems using electromagnetic signals within a suitable frequency range.

[0060] Figure 1 Schematically shown is a tank monitoring device 1 comprising a radar level gauge system 3 according to an exemplary embodiment of the present invention and a host system 5 which is shown as a control room.

[0061] The radar level gauge system 3 is installed at the tank 7 together with an additional gauging device, and together they form a so-called inventory tank gauging system.

[0062] The tank 7 , which may be an LNG tank, contains the liquid phase product 9 and the gas phase product 11 .

[0063] The radar level gauge system 3 is installed to measure the level of the interface 13 between the liquid product 9 and the liquid product 11, and the density distribution in the liquid product 9. The radar level gauge system 3 includes: a measuring unit 15; an antenna ( Figure 1 tubular waveguide 17; and at least a first plurality of signal interaction structures ( Figure 1 ), which are arranged at different filling levels along the first section of the tubular waveguide 17.

[0064] Reference Figure 2As shown in the schematic diagram in FIG. 1 , the radar level gauge system 3 includes an antenna 19, which is arranged in the form of an array antenna to radiate electromagnetic signals into the tubular waveguide 17. Figure 2 As schematically shown in , in the form of a high-level block diagram in the physical outline of the measurement unit 15 , the radar level gauge system 3 comprises: a first transceiver 21 ; a second transceiver 23 ; processing circuitry including a first measurement processor 25 and a second measurement processor 26 ; a communication interface 27 ; and a communication antenna 29 .

[0065] like Figure 2 As schematically shown in FIG, the first measurement processor 25 controls the first transceiver 21 to generate, transmit, and receive electromagnetic signals having a first set of signal characteristics, and the second measurement processor 26 controls the second transceiver 23 to generate, transmit, and receive electromagnetic signals having a second set of signal characteristics. The antenna 19 is coupled to the first transceiver 21 and the second transceiver 23 and is configured to radiate the first transmit signal S from the first transceiver 21. T1 and the first reflected signal S generated by the reflection of the first transmitted signal R1 , and radiates a second transmission signal S from the second transceiver 23 T2 and a second reflected signal S generated by the reflection of the second transmitted signal. R2 .

[0066] The first measurement processor 25 is based on the first transmission signal S T1 and the first reflected signal S R1 The level of the interface 13 is determined, and the second measurement processor 26 determines the level of the interface 13 based on the second transmission signal S T2 and the second reflected signal S R2 Determine the density distribution in the liquid product. The measured value indicating the level of the interface 13 and the measured value indicating the density distribution are provided from the first measurement processor 25 and the second measurement processor 26 to an external device, such as a control center, via the communication interface 27 and the communication antenna 29. The radar level gauge system 3 can advantageously be configured according to the so-called WirelessHART communication protocol (IEC 62591).

[0067] Using the radar level gauge system 3 according to various embodiments of the present invention, it is possible to obtain a first signal S which is phase modulated based on the first transmitted signal S. T1 With the first reflected signal S R1 The level of the interface 13 can be determined based on the phase difference between the second transmitted signal S and the second transmitted signal S. T2 and the second reflected signal S R2 The density distribution in the liquid product 9 is determined by the phase difference between them. This type of measurement scheme is generally referred to as FMCW (Frequency Modulated Continuous Wave), which is itself well known to those skilled in the art.

[0068] Now refer to Figure 3 , showing the inclusion of Figure 2 FIG. 2 is a block diagram of an exemplary configuration of a first measurement channel of the first transceiver 21 and the first measurement processor 25. It should be understood that the second measurement channel including the second transceiver 23 and the second measurement processor 26 may have substantially the same configuration.

[0069] The first transceiver 21 is shown here as comprising a microwave source 31 driven by a step generator 33, which in turn is controlled by a timing circuit 35 included in the first measurement processor 25. The microwave source 31 is connected to the antenna 19 via a power splitter 37. The power splitter 37 is arranged to connect the reflected signal from the antenna 19 to a mixer 39, which is also connected to receive the signal from the microwave source 31. The mixer output is connected to a low pass filter 41 and an amplifier 43.

[0070] In addition to the timing circuit 35 mentioned above, the first measurement processor 25 here also includes a sampler 45, which is configured to sample the intermediate frequency signal S output by the mixer 39, low-pass filtered by the low-pass filter 41 and amplified by the amplifier 43. IF1 The sampler 45 may, for example, comprise a sample-and-hold circuit combined with an A / D converter, or be implemented as a sigma-delta converter. The sampler 45 is controlled by the timing circuit 35 to be in phase with the transmit signal S T1 The first measurement processor 25 further comprises a signal processor 47 , a memory 49 and a level determiner 51 .

[0071] While the elements of the first transceiver 21 and the second transceiver 23 are typically implemented in hardware and form part of an integrated unit (often referred to as a microwave unit), at least some portions of the first measurement processor 25 and the second measurement processor 26 may typically be implemented by software modules executed by an embedded processor. The present invention is not limited to this particular implementation, and any implementation suitable for implementing the functionality described herein is contemplated.

[0072] Reference Figure 3 The timing circuit 35 controls the microwave source 31 via the step signal generator 33 to form a transmission signal S T1 . Reference Figure 4 , transmit signal S T1 At multiple discrete and mutually different frequencies f n The time series f0 to f N Provided in the form of. Figure 4 As schematically shown in FIG, discrete and mutually different frequencies f0 to f NDefine a bandwidth B. In this particular example, frequencies that are adjacent in frequency are also adjacent in the time series, but this is not necessarily the case. Alternatively, discrete and mutually different frequencies may be output in an arbitrary but known order.

[0073] Reflected signal S R1 By the transmission signal S T1 At impedance discontinuities (including Figure 1 Due to the time-of-flight from the radar level gauge system to the different impedance discontinuities and back, the reflected signal S R1 will be the transmitted signal S T1 delayed replicas of the reflected signal S from different impedance discontinuities R1 The various parts of the transmission signal S T1 In addition, the phase difference will vary with the discrete frequency f n changes in a step-by-step manner.

[0074] By combining the transmit signal S in the mixer 39 T1 and the reflected signal S R1 To form the intermediate frequency signal S IF1 .exist Figure 5 The intermediate frequency signal S is schematically shown in FIG. IF1 .

[0075] If a transmit signal with continuously varying frequency had been used, the intermediate frequency signal would be a continuous signal comprising one frequency component for each time of flight corresponding to a different impedance discontinuity encountered by the transmit signal.

[0076] Since in this particular example the transmitted signal S T1 Instead it is a series of discrete frequencies f n , so the phase difference will change in a step-by-step manner, which means that the intermediate frequency signal S IF1 The duration of the constant part is the same as the emission signal S T1 Different frequencies f n The emission duration is the same. Figure 5 It is schematically shown in FIG.

[0077] Already by Figure 3 The intermediate frequency signal S sampled by the sampler 45 IF1 The signal is processed by the signal processor 47 to determine a data set indicative of the echo from the impedance discontinuity. Figure 5 The intermediate frequency signal S IF1 Transform from time domain to frequency domain. IF1After transformation into the frequency domain, the obtained spectrum is converted into an echo curve, which is used by the level determiner 51 to determine the level of the interface 13 between the liquid phase product 9 and the gas phase product 11 .

[0078] In the second measurement channel, the second measurement processor 26 may include a density distribution determiner instead of the level determiner 51 of the first measurement channel described above.

[0079] In the first measurement channel, the first transceiver 21 generates, transmits, and receives electromagnetic signals having a first set of signal characteristics, and in the second measurement channel, the second transceiver 23 generates, transmits, and receives electromagnetic signals having a second set of signal characteristics. As mentioned above, the signal characteristics may include, for example, polarization state and / or microwave propagation mode. The signal interaction structures arranged at different levels along the tubular waveguide 17 are configured to selectively interact with the transmitted signal based on the set of signal characteristics exhibited by the transmitted signal.

[0080] Figure 6 A first exemplary configuration of signal interaction structures is schematically shown, comprising a first plurality of signal interaction structures in the form of reflectors 53a to 53d arranged at different filling levels along the tubular waveguide 17. Figure 6 As shown schematically in , the tubular waveguide 17 (often called a still tube) comprises a plurality of openings 55 (only one of which is indicated with a reference numeral to avoid cluttering the drawing) to enable the level of the interface 13 to equalize inside and outside the tubular waveguide 17 .

[0081] exist Figure 6 In FIG. 5 , each reflector 53 a to 53 d is provided in the form of a pin oriented in the “x” direction, as shown in FIG. Figure 6 The pin, which may be made of metal or a suitable dielectric material, will exhibit a much higher reflection coefficient for an electromagnetic signal linearly polarized along the "x" direction than for an electromagnetic signal linearly polarized along the "y" direction. Figure 6 The configuration of the signal interaction structure 53a to 53d in the first transmission signal S T1 It should preferably be linearly polarized along the "y" direction, and the second transmit signal S T2 It should preferably be linearly polarized along the "x" direction. This means that the first transmitted signal S T1 will propagate through the tubular waveguide with very little interaction with the reflectors 53a to 53d, and due to the first transmitted signal S T1 The reflection at the interface 13 between the liquid product 9 and the gaseous product 11 produces a relatively undisturbed first reflection signal S having a strong reflection signal component. R1 On the other hand, the second transmission signal S T2The linear polarization of the transmission signal will strongly interact with the reflectors 53a to 53d, thereby generating a strong reflected signal component due to reflection at the reflectors 53a to 53d. A detailed description of how to provide a linearly polarized transmission signal and how to appropriately configure the reflectors 53a to 53d is omitted, as this is well known to those skilled in the art of radar level measurement. For example, reference may be made to US Pat. No. 5,136,299. The entire content of this document is incorporated herein by reference.

[0082] Figure 7A A second exemplary configuration of signal interaction structures is schematically shown, comprising a first plurality of signal interaction structures in the form of resonators 57a to 57d arranged at different filling levels along the tubular waveguide 17 .

[0083] exist Figure 7A In the embodiment, each resonator 57a to 57d is provided in the form of a horizontally aligned microwave resonator attached to the outside of the tubular waveguide 17 and aligned with a corresponding opening in the wall of the tubular waveguide 17. Figure 7B Exemplary microwave resonator configurations are described in more detail.

[0084] Electromagnetic signals linearly polarized along the "x" direction will exhibit some leakage through the openings, and the leaked signals will interact with the microwave resonators 57a to 57d. Electromagnetic signals linearly polarized along the "y" direction will not leak through the openings or will leak only very little, and therefore will not be affected by the openings and the microwave resonators 57a to 57d. Figure 7A The configuration of the signal interaction structure 57a to 57d in the first transmission signal S T1 It should preferably be linearly polarized along the "y" direction, and the second transmit signal S T2 It should preferably be linearly polarized along the "x" direction. This means that the first transmitted signal S T1 will propagate through the tubular waveguide 17 with very little interaction with the microwave resonators 57a to 57d, and due to the first transmission signal S T1 The reflection at the interface 13 between the liquid product 9 and the gaseous product 11 produces a relatively undisturbed first reflection signal S having a strong reflection signal component. R1 On the other hand, the second transmission signal S T2 will strongly interact with the microwave resonators 57a to 57d, so that for the corresponding resonant frequencies of the microwave resonators 57a to 57d, in the second reflected signal S R2 The resonant frequency of a particular microwave resonator 57a to 57d will depend on the dielectric constant of the liquid product 9 filling and / or surrounding the microwave resonator 57a to 57d. Based on the dielectric constant at different levels, the density at different levels can be determined, as described in more detail in US Pat. No. 9,410,904, which is incorporated herein by reference in its entirety.

[0085] Reference Figure 7B , the horizontally aligned microwave resonator 57 comprises a shell 61 having a round cylindrical shape. The shell 61 has an open end 63 and a closed end 65. In order to further simplify the discharge and filling of the liquid product 9 in the microwave resonator 57, holes 67 are provided in the shell 61. These holes 67 can advantageously be relatively small, preferably having a diameter of less than 4 mm. The appropriate diameter for a given application is determined by the diameter and the resonant frequency of the resonator. The holes 67 should be small enough so that they do not affect the characteristics of the resonator, in particular smaller than the wavelength of the resonant frequency. In addition, the microwave resonator 57 comprises an inner rod 69, which is fixed to the closed end 63 and extends inside the shell 61 from the closed end 63 towards the open end 65. Described in US 9410904 Figure 7B The functions of the microwave resonator 57 in FIG. 5 and other feasible microwave resonator designs.

[0086] Figure 8 A third exemplary configuration of signal interaction structures is schematically shown, comprising first and second pluralities of signal interaction structures in the form of reflectors arranged at different levels along the first and second sections 71 , 73 of the tubular waveguide 17 , respectively.

[0087] exist Figure 8 In the embodiment, each reflector 53a to 53h is provided in the form of a pin. Each pin 53a to 53d in the first plurality of signal interaction structures is as follows Figure 8 oriented in the "x" direction as schematically shown in FIG, and will therefore exhibit a much higher reflection coefficient for electromagnetic signals linearly polarized in the "x" direction than for electromagnetic signals linearly polarized in the "y" direction. Each pin 53e to 53h in the second plurality of signal interaction structures is as shown in FIG. Figure 8 oriented along the "y" direction as schematically shown in FIG, and will therefore exhibit a much higher reflection coefficient for electromagnetic signals linearly polarized along the "y" direction as compared to electromagnetic signals linearly polarized along the "x" direction.

[0088] Since in this example, the first transmission signal S T1 Linearly polarized along the "y" direction, and the second transmitted signal S T2 is linearly polarized along the “x” direction, so the first transmitted signal S T1 In practice, only reflectors 53e to 53h of the second plurality of reflectors will be interacted with, and the second transmit signal S T2 In practice, only reflectors 53a to 53d of the first plurality of reflectors will interact. T1 and the first reflected signal S R1It can be advantageously used to determine the level of the interface 13 when the level of the interface 13 is located in the first section 71 of the tubular waveguide 17, and to determine the density distribution along the second section 73 of the tubular waveguide 17. When the level of the interface 13 is located in the first section 71 of the tubular waveguide 17, as described above with reference to Figure 6 The second transmission signal S T2 and the second reflected signal S R2 The density distribution along the (immersed part of) the first section 71 of the tubular waveguide 17 is determined. When the level of the interface 13 is located in the second section 73 of the tubular waveguide 17, the second transmission signal S T2 and the second reflected signal S R2 It can be used advantageously to determine the level of the interface 13 .

[0089] Figure 9 is a flow chart schematically illustrating an exemplary embodiment of the method according to the present invention.

[0090] In a first step S1, an electromagnetic first transmission signal S having a first set of signal characteristics is transmitted via a first measurement channel comprising a first transceiver 21 and a first measurement processor 25. T1 As mentioned above, the first transmission signal S T1 For example, along Figure 6 The “y” direction in the graph shows linear polarization.

[0091] In a second step S2 which may occur simultaneously with the first step S1 , an electromagnetic second transmit signal S having a second set of signal characteristics is transmitted via a second measurement channel comprising a second transceiver 23 and a second measurement processor 26 . T2 As mentioned above, the second transmission signal S T2 For example, along Figure 6 The "x" direction in the graph shows linear polarization.

[0092] In the third step S3, the first transmission signal S is transmitted via the antenna 19. T1 and the second transmission signal S T2 radiated into the tubular waveguide 17, which guides the first transmission signal S toward the liquid product 9. T1 and the second transmission signal S T2 And the first transmission signal S T1 and the second transmission signal S T2 is guided into the liquid product 9, and the first reflected signal S R1 and the second reflected signal S R2 This leads back to the first transceiver 21 and the second transceiver 23 .

[0093] In the following step S4, a first measurement channel including a first transceiver 21 and a first measurement processor 25 is used to receive a first reflected signal SR1 , and in step S5 which may occur simultaneously with S4, a second measurement channel comprising a second transceiver 23 and a second measurement processor 26 is used to receive a second reflected signal S R2 .

[0094] In the next step S6, based on the first transmission signal S T1 and the first reflected signal S R1 The level of the interface 13 between the liquid product phase 9 and the gaseous product phase 11 is determined.

[0095] In step S7, based on the second transmission signal S T2 and the second reflected signal S R2 Determine the density distribution in the liquid product.

[0096] Will refer to Figure 10A and Figure 10B The echo curve in describes in more detail the determination of the level of the interface 13 and the determination of the density distribution in the liquid product 9.

[0097] Figure 10A is based on the first transmitted signal S for two different distances from the interface 13 T1 and the first reflected signal S R1 A diagram of an exemplary echo curve of Figure 10B is based on the second transmission signal S for two different distances from the interface 13 T2 and the second reflected signal S R2 Figure 1 shows an exemplary echo curve for generating Figure 10A and Figure 10B The signal interaction structure configuration of the measured echo curve in the Figure 6 , except that only two of the reflector pins 53a to 53d are installed to simplify the echo curve and description.

[0098] Figure 10A The graph in FIG includes a first echo curve 75 of a low interface level and a second echo curve 77 of a high interface level. As described above, the first measurement channel including the first transceiver 21 and the first measurement processor 25 has been used to determine Figure 10A The echo curve in Figure 6 As explained, the first transmission signal S is linearly polarized in the "y" direction. T1 The first echo curve 75 therefore comprises only one significant peak 79, based on which the (relatively low) filling level of the interface 13 can be determined, and the second echo curve 77 comprises only one significant peak 81, based on which the (relatively high) filling level of the interface 13 can be determined.

[0099] for Figure 10A In the picture, Figure 10B The diagram in FIG. 8 includes a first echo curve 83 for a low interface level and a second echo curve 85 for a high interface level. As further described above, the second measurement channel comprising the second transceiver 23 and the second measurement processor 26 has been used to determine Figure 10B The echo curve in Figure 6 As explained, the second transmission signal S is linearly polarized in the "x" direction. T2 The first echo curve 83 is relatively strongly reflected by the reflectors 53a to 53d. Therefore, the first echo curve 83 includes a first peak 87 indicating the electrical distance from the first reflector 53a, a second peak 89 indicating the electrical distance from the second reflector 53b, and a third peak 91 indicating the (relatively low) level of the interface 13. The second echo curve 85 includes a first peak 93 indicating the electrical distance from the first reflector 53a, a second peak 95 indicating the electrical distance from the second reflector 53b, and a third peak 97 indicating the (relatively high) level of the interface 13.

[0100] Based on the first peak 87 and the second peak 89 of the first echo curve 83, a first electrical distance d1 between the first reflector 53a and the second reflector 53b can be determined for a case where the first reflector 53a and the second reflector 53b are located above the material level of the interface 13. Based on the first peak 93 and the second peak 95 of the second echo curve 85, a second electrical distance d2 between the first reflector 53a and the second reflector 53b can be determined for a case where the first reflector 53a and the second reflector 53b are located below the material level of the interface 13.

[0101] The first electrical distance d1 and the second electrical distance d2 mentioned above are proportional to the time period it takes for the electromagnetic signal to travel from the first reflector 53a to the second reflector 53b in both cases (two reflectors above the interface 13 and two reflectors below the interface 13, respectively). As is well known to those skilled in the art, the propagation speed of the electromagnetic signal from the level of the first reflector 53a to the level of the second reflector 53b is inversely proportional to the square root of the (average) relative dielectric constant of the material between the first reflector 53a and the second reflector 53b. Based on this knowledge, the relative dielectric constant of the gaseous product, and the fact that the actual distance between the two reflectors 53a and 53b is the same in both measurements, the average relative dielectric constant of the liquid product 9 between the level of the first reflector 53a and the level of the second reflector 53b in the tank can be determined (second echo curve 85).

[0102] Knowing this dielectric constant will enable the Clausius-Mosotti equation (or equivalently the Lorentz-Lorentz equation) to be used to derive the average density between the levels of the first and second reflectors 53a, 53b in the tank.

[0103]

[0104] The ratio constant κ is an intrinsic characteristic constant representing the electronic polarization rate of liquid molecules. When the molecules retain a permanent dipole moment, this characteristic constant depends on temperature.

[0105] In a typical volume of liquefied natural gas, the characteristic constant κ representing the polarization rate of electrons varies depending on the composition of the liquid.

[0106] A typical value may be κ = 4 × 10 -4 m 3 / kg. This value indicates the dielectric constant ε r =1.69 LNG volume has a value close to ρ = 467 kg / m 3 density.

[0107] Alternatively, the density may be determined based on an empirical relationship between dielectric constant and density, wherein additional parameters such as temperature and / or pressure may also be taken into account to improve the determination of the density distribution.

[0108] In the above example, for simplicity, the (average) density was determined for only one level (range). In order to determine the density distribution in the liquid product, the density should be determined for at least two different levels or level ranges.

[0109] from Figure 10B As can be clearly seen from the echo curves in , the same measurement channel can be used to determine the density distribution of the liquid material 9 and to measure the level of the interface 13. However, the inventors have realised that doing so would result in a less reliable and accurate determination of the interface level than is obtainable by embodiments of the present invention.

[0110] Figure 11 is a diagram illustrating measurement errors introduced by a signal interaction structure in the form of an exemplary reflector, and includes: a first curve 99 indicating the measurement error as a function of the distance from the interface 13 for a first measurement channel including the first transceiver 21; and a second curve 101 indicating the measurement error as a function of the distance from the interface 13 for a second measurement channel including the second transceiver 23.

[0111] The first curve 99 shows that the measurement error is relatively small throughout the measurement range, and the variation in the measurement error is also relatively small. In contrast, the second curve 101 shows that the measurement error is quite large throughout the measurement range and exhibits two distinct peaks in the measurement error for distances corresponding to the positions of the reflectors 53a-53b along the tubular waveguide 17.

[0112] A person skilled in the art realizes that the present invention is by no means limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

1. A radar level gauge system for determining the level of an interface between a liquid product and a gaseous product in a tank and the density distribution in the liquid product, the radar level gauge system comprising: a first transceiver for generating, transmitting, and receiving electromagnetic signals having a first set of signal characteristics; a second transceiver for generating, transmitting, and receiving electromagnetic signals having a second set of signal characteristics different from the first set of signal characteristics; an antenna coupled to the first transceiver and the second transceiver, the antenna being arranged to radiate an electromagnetic first transmit signal having the first set of signal characteristics and an electromagnetic second transmit signal having the second set of signal characteristics, and to receive a first reflected signal resulting from a reflection of the first transmit signal and a second reflected signal resulting from a reflection of the second transmit signal; a tubular waveguide extending from the top of the tank toward the bottom of the tank, the tubular waveguide being arranged and configured to guide the first transmission signal and the second transmission signal radiated by the antenna toward an interface between the liquid-phase product and the gas-phase product and into the liquid-phase product, and to guide the first reflection signal and the second reflection signal back to the antenna; at least a first plurality of signal interaction structures disposed at different levels along a first section of the tubular waveguide, each signal interaction structure of the first plurality of signal interaction structures being configured to selectively interact with the second transmit signal to contribute to the second reflected signal; as well as a processing circuit connected to the first transceiver and the second transceiver, the processing circuit being configured to determine a level of an interface between the liquid-phase product and the gas-phase product based on a relationship between the first transmission signal and the first reflection signal, and to determine a density distribution in the liquid-phase product based on a relationship between the second transmission signal and the second reflection signal, Wherein, the processing circuit is configured to: determining a first measurement value based on a relationship between the second transmitted signal and the second reflected signal, the first measurement value indicating a distance between a first signal interacting structure and a second signal interacting structure of the first plurality of signal interacting structures; determining a second measurement value based on a relationship between the second transmitted signal and the second reflected signal, the second measurement value indicating a distance between a third signal interaction structure and a fourth signal interaction structure of the first plurality of signal interaction structures; determining a first average density for a first level range in the tank based on the first measurement value; and A second average density for a second fill level range in the tank is determined based on the second measured value.

2. The radar level gauge system according to claim 1, wherein: The first plurality of signal interacting structures includes at least three signal interacting structures spaced apart from one another along a first segment of the tubular waveguide.

3. The radar level gauge system according to claim 1 or 2, wherein: The first set of signal characteristics includes a first polarization, and the second set of signal characteristics includes a second polarization different from the first polarization.

4. The radar level gauge system according to claim 3, wherein: The first polarization is a first linear polarization, and the second polarization is a second linear polarization orthogonal to the first linear polarization.

5. The radar level gauge system according to claim 1 or 2, wherein: The first set of signal characteristics includes a first propagation mode, and the second set of signal characteristics includes a second propagation mode different from the first propagation mode.

6. The radar level gauge system according to claim 1 or 2, wherein: Each of the first plurality of signal interaction structures is a reflection structure, which is arranged and configured to present a first reflection coefficient for the first transmission signal and a second reflection coefficient for the second transmission signal, and the ratio between the first reflection coefficient and the second reflection coefficient is less than 1:

2.

7. A radar level gauge system for determining the level of an interface between a liquid product and a gaseous product in a tank and the density distribution in the liquid product, the radar level gauge system comprising: a first transceiver for generating, transmitting, and receiving electromagnetic signals having a first set of signal characteristics; a second transceiver for generating, transmitting, and receiving electromagnetic signals having a second set of signal characteristics different from the first set of signal characteristics; an antenna coupled to the first transceiver and the second transceiver, the antenna being arranged to radiate an electromagnetic first transmit signal having the first set of signal characteristics and an electromagnetic second transmit signal having the second set of signal characteristics, and to receive a first reflected signal resulting from a reflection of the first transmit signal and a second reflected signal resulting from a reflection of the second transmit signal; a tubular waveguide extending from the top of the tank toward the bottom of the tank, the tubular waveguide being arranged and configured to guide the first transmission signal and the second transmission signal radiated by the antenna toward an interface between the liquid-phase product and the gas-phase product and into the liquid-phase product, and to guide the first reflection signal and the second reflection signal back to the antenna; at least a first plurality of signal interaction structures disposed at different levels along a first section of the tubular waveguide, each signal interaction structure of the first plurality of signal interaction structures being configured to selectively interact with the second transmit signal to contribute to the second reflected signal; as well as a processing circuit connected to the first transceiver and the second transceiver, the processing circuit being configured to determine a level of an interface between the liquid-phase product and the gas-phase product based on a relationship between the first transmission signal and the first reflection signal, and to determine a density distribution in the liquid-phase product based on a relationship between the second transmission signal and the second reflection signal, in: the tubular waveguide comprising at least a first plurality of openings to selectively enable an electromagnetic field generated by the second transmit signal to be present on an outside of the tubular waveguide at each opening of the first plurality of openings; and each signal interaction structure of the first plurality of signal interaction structures being arranged on an outer side of the tubular waveguide at each opening of the first plurality of openings so as to be stimulable by the second transmit signal, Wherein, the processing circuit is configured to: determining a first measurement value based on a relationship between the second transmitted signal and the second reflected signal, the first measurement value indicating a distance between a first signal interacting structure and a second signal interacting structure of the first plurality of signal interacting structures; determining a second measurement value based on a relationship between the second transmitted signal and the second reflected signal, the second measurement value indicating a distance between a third signal interaction structure and a fourth signal interaction structure of the first plurality of signal interaction structures; determining a first average density for a first level range in the tank based on the first measurement value; and A second average density for a second fill level range in the tank is determined based on the second measured value.

8. The radar level gauge system according to claim 7, wherein: Each signal interaction structure of the first plurality of signal interaction structures is a resonator arranged and configured to be excitable only by the second transmit signal, the resonant frequency of the resonator being indicative of a density of a liquid product filling the resonator.

9. The radar level gauge system according to claim 8, wherein: The resonator comprises a resonant cavity arranged to contain the liquid product when immersed in the liquid product.

10. The radar level gauge system according to claim 8, wherein: The processing circuit is configured to: determining a first density of the first level in the tank based on a resonant frequency of a first resonator arranged at the first level; as well as A second density of the second level in the tank is determined based on a resonant frequency of a second resonator arranged at the second level.

11. A radar level gauge system for determining the level of an interface between a liquid product and a gaseous product in a tank and the density distribution in the liquid product, the radar level gauge system comprising: a first transceiver for generating, transmitting, and receiving electromagnetic signals having a first set of signal characteristics; a second transceiver for generating, transmitting, and receiving electromagnetic signals having a second set of signal characteristics different from the first set of signal characteristics; an antenna coupled to the first transceiver and the second transceiver, the antenna being arranged to radiate an electromagnetic first transmit signal having the first set of signal characteristics and an electromagnetic second transmit signal having the second set of signal characteristics, and to receive a first reflected signal resulting from a reflection of the first transmit signal and a second reflected signal resulting from a reflection of the second transmit signal; a tubular waveguide extending from the top of the tank toward the bottom of the tank, the tubular waveguide being arranged and configured to guide the first transmission signal and the second transmission signal radiated by the antenna toward an interface between the liquid-phase product and the gas-phase product and into the liquid-phase product, and to guide the first reflection signal and the second reflection signal back to the antenna; at least a first plurality of signal interaction structures disposed at different levels along a first section of the tubular waveguide, each signal interaction structure of the first plurality of signal interaction structures being configured to selectively interact with the second transmit signal to contribute to the second reflected signal; as well as a processing circuit connected to the first transceiver and the second transceiver, wherein: The radar level gauge system further comprises a second plurality of signal interaction structures arranged at different levels along a second section of the tubular waveguide different from the first section, each signal interaction structure of the second plurality of signal interaction structures being configured to selectively interact with the first transmit signal to contribute to the first reflected signal; and The processing circuit is configured to: For a level along a first section of the tubular waveguide, determine a level of an interface between the liquid-phase product and the gas-phase product based on a relationship between the first transmitted signal and the first reflected signal, determine a density distribution in the liquid-phase product along the first section of the tubular waveguide based on a relationship between the second transmitted signal and the second reflected signal, and determine a density distribution in the liquid-phase product along a second section of the tubular waveguide based on a relationship between the first transmitted signal and the first reflected signal; and For the material level along the second section of the tubular waveguide, the material level of the interface between the liquid-phase product and the gas-phase product is determined based on the relationship between the second transmitted signal and the second reflected signal, and the density distribution in the liquid-phase product along the second section of the tubular waveguide is determined based on the relationship between the first transmitted signal and the first reflected signal.

12. A method for determining the level of an interface between a liquid product and a gaseous product in a tank and the density distribution in the liquid product using a radar level gauge system, the radar level gauge system comprising: a first transceiver; a second transceiver; an antenna coupled to the first transceiver and the second transceiver; a tubular waveguide extending from a top portion of the tank toward a bottom portion of the tank; at least a first plurality of signal interaction structures arranged at different levels along a first segment of the tubular waveguide, each signal interaction structure of the first plurality of signal interaction structures being configured to selectively interact with an electromagnetic signal having a given set of signal characteristics; and a processing circuit connected to the first transceiver and the second transceiver, The method comprises the following steps: transmitting, by the first transceiver, an electromagnetic first transmit signal having a first set of signal characteristics different from the given set of signal characteristics; transmitting, by the second transceiver, an electromagnetic second transmit signal having a second set of signal characteristics constituting the given set of signal characteristics; radiating the first transmit signal and the second transmit signal into the tubular waveguide by the antenna; receiving, by the antenna, an electromagnetic first reflected signal generated by reflection of the first transmitted signal and an electromagnetic second reflected signal generated by reflection of the second transmitted signal; determining, by the processing circuit, a level of an interface between the liquid-phase product and the gas-phase product based on a relationship between the first transmission signal and the first reflection signal; determining a first measurement value based on a relationship between the second transmitted signal and the second reflected signal, the first measurement value indicating a distance between a first signal interacting structure and a second signal interacting structure of the first plurality of signal interacting structures; determining a second measurement value based on a relationship between the second transmitted signal and the second reflected signal, the second measurement value indicating a distance between a third signal interaction structure and a fourth signal interaction structure of the first plurality of signal interaction structures; determining a first average density for a first level range in the tank based on the first measurement value; and A second average density for a second fill level range in the tank is determined based on the second measured value.

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