KR100724682B1 - Bottom reflector for a radar-based level gauge - Google Patents

Abstract

A method of calculating the level of liquid stored in the tank from the transmission of a microwave signal from the top of the tank toward the surface of the liquid, the reception of the microwave signal reflected against the surface of the liquid, and the propagation time of the transmitted and received microwave signal In the bottom reflector 15 for a radar based level meter 11 which measures the level 71 of liquid stored in the tank 12 by means of which the bottom reflector 15 is close to the bottom of the tank. And the first reflection coefficient of the microwave signal when the liquid level is higher than the reflective structure and the second reflection coefficient of the microwave signal when the liquid level is lower than the reflective structure, wherein the first reflection coefficient is Reflective structure lower than the second reflection coefficient (23 to 24; 25; 23, 26; 27; 29; 32; 33; 32, 35; 36; 37; 41, 42; 43,44; 51 to 52; 55 to 56 ; 61,72) Floor reflector, characterized in that made.

Water Level Meters, Radar, Floor Reflectors, Resonant Structures, Polarization Tuning Structures, Absorption Structures

Description

Bottom reflector for a radar-based level gauge

FIELD OF THE INVENTION The present invention relates generally to water level measurements, and more particularly to floor reflectors for radar based water level meters.

Radar-based methods have been widely used for several years to measure water levels in various tanks. In such a level measurement, the distance from the top of the tank to the liquid surface stored in the tank transmits microwaves toward the liquid surface, receives the microwaves reflected from the liquid surface, and calculates the liquid level in the tank from the propagation time of the transmitted and reflected microwaves. It was measured by the method.

The inherent limitation of radar-based water level measurement is that many liquids must be at least somewhat transparent in order for microwaves to be used, and when measuring low liquid levels, microwaves reflected from the bottom of the tank and microwaves reflected from the liquid surface may interfere. Is there. In other words, this distance determination method may be insufficient to distinguish microwaves reflected from the tank bottom from those reflected from the liquid surface. Thus, microwaves from the bottom of the tank tend to interfere with measurements at low liquid levels. With this type of radar level measurement, this problem can occur at levels below tens of centimeters. This is a particularly common problem in tanks that have flat bottoms and contain liquids such as petroleum materials that are almost transparent.

In some tanks the bottom may be inclined. There are no typical problems that interfere with radar echo in these tanks. However, on the other hand, if the tank is empty, the radar beam is deflected from the receiver of the water level measurement so that no microwaves reflected from the bottom of the tank are received. If the tank is empty, it is desirable to have a fairly strong radar echo from the bottom to confirm that condition. In other cases, there may be an unknown layer of sediment at the bottom of the tank that, in spite of the flat bottom, either creates an ambiguous radar echo or no radar echo at all.

The problem of not receiving bottom radar echo in an empty tank can be solved by having a simple reflective structure welded to the bottom of the tank. However, such reflective structures will cause interference at the liquid level slightly above the reflective structure if the liquid in the tank passes at least some microwaves.

 An object of the present invention is to provide a bottom reflector for radar-based water level measurement for measuring the level of liquid in a tank, the microwave being reflected from the bottom of the tank when the meter is used for measuring water at low liquid level In addition to reducing interference from, it is to provide a bottom reflector that can provide sufficient microwave reflection even when the tank is empty.

In this respect, a specific object of the present invention is to provide a bottom reflector suitable for use where the water level is measured through a pipe or otherwise.

It is another object of the present invention to provide a bottom reflector suitable for use in which radar-based water level measurement is used regardless of frequency modulated continuous wave (FMCW) radar devices, wave radar devices, or any other form of ranging radar.

Another object of the present invention is to provide a low cost bottom reflector that is simple, reliable, efficient, accurate, precise, and easy to install and measure.

This object is achieved by the bottom reflector claimed in the appended patent claims.

The inventors are concerned with providing a bottom reflector, preferably close to the bottom of the tank, which can be mounted at a predetermined height in the tank for reflection of microwave signals if the tank is empty or only very low levels of liquid are present in the tank. And a first reflection coefficient of the microwave signal when the liquid level is higher than the reflective structure and a second reflection coefficient of the microwave signal when the liquid level is lower than the reflective structure, wherein the first reflection coefficient is the second reflection It has been found that the above objects can be achieved if it is smaller than the coefficient.

Preferably, the liquid is an oily substance such as, for example, crude oil, liquid petroleum gas (LPG), liquid natural gas (LNG), other liquid hydrocarbons, or a liquid that passes at least some microwaves. The material typically has a dielectric constant in the range of 1.6 to 3, whereas the atmosphere above the water level typically has a dielectric constant in the range of 1 to 1.03 by its composition and pressure.

More preferably, when the liquid level is higher than the reflective structure, the microwave signal reflected from the reflective structure is weak compared to the microwave signal reflected from the liquid level, and more preferably very weak.

When the liquid level is lower than the reflecting structure, the microwave signal reflected from the reflecting structure is preferably similar or slightly stronger than the microwave signal reflected from the liquid level.

Various reflective structures can be used to achieve proper functionality based on the difference in permittivity of the liquid and the atmosphere thereon. Such reflective structures are specifically described in the detailed description of the present invention and are preferred embodiments-blocking gratings, resonant structures such as dipoles, dielectric reflectors, polarization rotating structures, small gaps for guiding transmitted and reflected microwave signals. Particularly applicable structures-where microwave guide structures and free space propagation of microwaves are used-can be grouped into different groups.

With respect to the group of polarization rotating structures, it can be understood that the microwave signal used for level guidance has a specific polarization state and that the first and second reflection coefficients are given in that specific polarization state. Thus, the effect of polarization rotation of a linearly polarized microwave signal in reflection in liquid is equivalent to reducing the reflection coefficient of the microwave signal in its particular polarization state.

The main advantage of the present invention is that the water level measurement can be performed without any interference from the microwaves reflected from the bottom of the tank. When the tank is empty or almost empty, i.e. when the reflective structure of the bottom reflector is exposed above the level of liquid to be measured, a separate reflection is obtained from the bottom reflector, which is an indication of an empty tank or a low liquid level.

The radar-based water level meter is used for the purpose of the present invention as well as in large containers, for example, in reactors, centrifuges, mixers, planters, graders, heat treatment furnaces and food chemistry, preparation chemistry, biochemistry, oilfield chemistry, petrochemical It is used to measure the water level in tanks containing processing devices such as similar devices.

Other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments of the present invention and the accompanying Figures 1-7, which are given by way of illustration only and are not intended to limit the invention. no.

1 is a schematic side view of a radar based level meter including a bottom reflector in accordance with the general principles of the present invention.

2A-E are plan views (FIGS. 2A-B) and side cross-sectional views (FIGS. 2C-E) of bottom reflectors according to a first preferred embodiment group of the present invention.

Figure 2f is a schematic diagram showing the correlation between the magnitude and the frequency of the transmitted and reflected microwave signal by the bottom reflector according to the first preferred embodiment group of the present invention.

3A-E are schematic side views (FIG. 3A), a perspective view (FIG. 3B), and top views 3C-E of a bottom reflector according to a second preferred embodiment group of the invention.

3F is a schematic diagram showing the correlation between the magnitude and the frequency of the microwave signal transmitted and reflected by the bottom reflector according to the second preferred embodiment group of the present invention.

4A-B are schematic perspective views (FIG. 4A) and side views (FIG. 4B) of a bottom reflector according to a third preferred embodiment group of the invention.

4C is a schematic diagram showing the correlation between the magnitude and the frequency of the transmitted and reflected microwave signal by the bottom reflector according to the third preferred embodiment group of the present invention.

5A-C are schematic top (FIG. 5A) and side cross-sectional views (FIG. 5B) of a bottom reflector according to a fourth preferred embodiment group of the invention.

6 is a schematic side cross-sectional view of a bottom reflector according to a fifth group of preferred embodiments of the present invention.

7 is a schematic perspective view of a bottom reflector in accordance with a sixth preferred embodiment group of the invention.

A preferred embodiment of the present invention will be described with reference to FIG. 1, which schematically illustrates a device for radar-based water level measurement. The device may be a frequency modulated continuous wave (FMCW) radar device, a wave radar device or any other form of ranging radar.

Many frequencies can be used for radar level measurements, but bands close to 5.8, 10 and 25 GHz are widely used. This frequency is most commonly used because the microwave signal is least sensitive to such contamination at the lowest of these frequencies in tanks where foaming or contamination occurs frequently.

The radar-based level gauge shown in FIG. 1 to 11 is installed above the opening of the roof of the tank or container 12 filled with liquid at the level to be measured. Preferably the liquid is an oily material such as, for example, crude oil, liquid petroleum gas (LPG), liquid natural gas (LNG), other liquid hydrocarbons or a liquid that permeates at least some microwaves. Such materials typically have a dielectric constant in the range of 1.6 to 3, while the atmosphere above the water level typically has a dielectric constant in the range of 1 to 1.03, depending on its gas composition and pressure.

The radar based water level meter 11 transmits a microwave signal towards the surface of the liquid in the tank 12 and receives the microwave signal reflected against the surface of the liquid in the tank 12. In addition, the radar-based water level meter 11 is provided with or connected to a signal processing device (not shown) for calculating the number of liquids in the tank 12 from the propagation time of the transmitted and reflected microwave signal. Typically, a vertical pipe 13 is provided primarily to guide the transmitted and reflected microwave signal. This pipe 13 is attached to the tank 12 by a support 14 and penetrates inside and outside thereof to obtain the same liquid level, regardless of the density layer possible. However, the level gauge may operate in a free space propagation mode, in which case the pipe 13 may be unnecessary.

The bottom reflector 15 is provided near the bottom of the tank 12 for the reflection of the microwave signal in the absence of liquid in the tank 12. If a pipe such as 13 of FIG. 1 is used for propagation guidance, the bottom reflector 15 is typically installed at the bottom of the pipe. Optionally, the bottom reflector can be installed at the bottom of the tank.

According to the present invention, the bottom reflector 15 has a first reflection coefficient for the microwave signal when the liquid level is higher than the reflection structure and a second reflection coefficient for the microwave signal when the liquid level is lower than the reflection structure. The coefficient is composed of a reflective structure (not shown in FIG. 1) which is sufficiently low compared to the second reflection coefficient.

Such a facility can ensure proper operation without interference of microwaves reflected from the floor. This is particularly important when measuring low liquid levels in the tank 12. When the liquid level falls below the reflecting structure of the bottom reflector 15, a fairly strong reflection is obtained by indicating a low level.

It should be noted that the first and second reflection coefficients are conveniently the so-called in-polarization reflection coefficients. In-polar reflection coefficient is defined as the ratio of the incident microwave signal (i.e. its magnitude) to the microwave signal (i.e. its magnitude) by reflection at the surface of the bottom reflector in a particular polarization state, where a particular polarization state is It is given by the polarization of the microwave signal according to its reflection in. Thus, given a linear polarization of the incident microwave signal, a particular polarization is a linear polarization state because the linearly polarized microwave signal when reflected by the fluid surface does not change its polarization state. Given a left hand circularly polarized incident microwave signal, the specific polarization is right hand circular polarization because the rotation of the electric field is reversed as the microwave signal is reflected by the surface of the fluid.

Preferably, when the liquid level is higher than the reflective structure, the microwave signal reflected from the reflective structure is weak compared to the microwave signal reflected from the liquid level, and more preferably very weak. If the liquid level is lower than the reflecting structure, the microwave signal reflected from the reflecting structure is preferably similar or slightly stronger than the microwave signal reflected from the liquid level. Too strong reflections should be avoided.

Preferably, the first reflection coefficient is higher than 0 and lower than 0.2, more preferably lower than 0.1, and more preferably lower than 0.05. The second reflection coefficient is preferably higher than 0.1 and lower than 1, more preferably higher than 0.2 and lower than 1. In addition, the second reflection coefficient is preferably higher than zero and lower than 0.5, more preferably lower than 0.4 and most preferably lower than 0.3.

In most cases, it is desirable that the reflected microwave signal upon reflection from the bottom of the tank 12 be deflected or absorbed by structures 16 such as inclined plates, conical surfaces, pieces of radar absorbent material, and the like.

Hereinafter, various embodiments of the bottom reflector according to the present invention will be described in detail.

desirable Example

2A-E illustrate various bottom reflectors based on waveguide cutoff frequency theory. Each bottom reflector prevents the microwave signal from being transmitted when the grating structure is above the liquid level, and instead gratings slightly smaller than λ / 2 (where λ is the vacuum wavelength of the microwave signal) in order to reflect the microwave signal It is a grid structure with a thickness. As the liquid level rises above the lattice structure, the wavelength of the microwave signal decreases because of the higher dielectric constant of the liquid. Thus the grating appears wider for the microwave signal and the microwave signal can pass through the grating structure.

In FIG. 2A a bottom reflector is shown which has a lattice structure arranged inside the pipe 13. In this case the pipe 13 is arranged to support the propagation of microwaves mainly in the direction of the electric field in the H ₁₁ mode as indicated by the arrow 22. The lattice structure consists of a plurality of conductive metal ribbons 23 with a spacing slightly smaller than λ / 2 to block propagation when the liquid level is below the lattice structure.

For example at 10 ms the spacing may be 14 mm and the ribbon 23 may have a height of 28 mm, which corresponds to a distance of λ _m / 2 in the waveguide formed by the ribbon, where λ _m is The wavelength of the microwave signal in the liquid in pipe 13 (ie, when the liquid level is below the lattice structure). A ribbon height of λ _m / 2 allows propagation without reflection within a defined frequency band, as is standard radar radome. λ _m may be used as the local wavelength in the pipe 13 and may be recognized as different in different contexts of the present description.

A support 24 made of metal or some dielectric material is used to support the lattice structure. FIG. 2A shows a lattice structure of straight ribbons, but may be bent to suit the shape of the electric field of waveguide mode H ₁₁ . In addition, the grating structure can be modified to suit other waveguide propagation modes.

In one embodiment, the bottom reflector of the present invention may be of a net lattice structure. Another change is that parts of the ribbon can be made of attenuating material to reduce transmission and reflection and to extend the bandwidth used.

In another embodiment, a grating structure comprising a plurality of circular conductive strips arranged concentrically in pipe 13 is used to obtain an operation corresponding to H ₀₁ propagation mode, as shown in FIG. 2B. do. The electric field direction 22 of the H ₀₁ waveguide mode is completely enclosed and arranged such that a plurality of centrally short cylindrical pipes or rings 25 are parallel to the electric field direction 22. Referring to the example of 10 ms, the spacing between the rings changed according to the cylindrical waveguide shape of the H ₀₁ mode is about 14 mm and the height is about 28 mm. In practical implementations, support elements (not shown) of dielectric or metal materials are included. Similar waveguide mode structures are possible, similar to the H ₁₁ and H ₀₁ cases.

For example, the ribbon may be arranged radially for the microwave signal in E ₀₁ mode.

Figure 2c shows a part of the bottom reflector in a side sectional view. This embodiment is similar to the embodiment of FIG. 2A-a straight ribbon 23 is shown. However, this embodiment is further provided with an antireflective structure consisting of pins 26 arranged horizontally in parallel with the ribbon 23. The pins 26 are located about and lambda / 4 from above and below the ribbon. Similar to the descriptions corresponding to radar radome, there are a number of choices to achieve similar functions that increase the range of use of frequency and liquid dielectric constants, which is obvious to those skilled in the art.

To prevent reflections at the tank bottom from recombining to the pipe 13 and interfering with reflections from the liquid surface-if the tank bottom is flat or close to flat-the opacity of the bottom reflector when submerged in liquid The measurements to be measured may appear in the opposite direction. Various alternatives are possible: the microwaves transmitted through the bottom reflector can be deflected or absorbed.

2d shows a 45 ° metal reflector 27 installed below the bottom reflector 15 in the pipe. The reflector is attached to the bottom of the pipe 13 or tank (not shown) by means of the support structure 28.

Depending on the shape of the tank, a similar deflector (hereinafter referred to as deflector) may be used with all bottom reflector embodiments of the present invention. The angle can be very different from 45 ° and very different types of obstructions can be used to disperse the radar wave to prevent it from entering the pipe 13 again.

Cone parts that are attached to the floor or pipe are embodiments of other useful deflectors. Such a part is indicated by reference numeral 16 in FIG. 1.

FIG. 2E shows an appropriately shaped element of damping material, such as a carbon-based material filled with Teflon (PTFE), for example, arranged at the bottom of the tank under the bottom reflector 15 in the pipe 13 for low reflection at the bottom. 29 is shown. The element 29 is shown by dashed lines for illustrative purposes, but the actual shape may be very different from the box shape as shown. For example, the device may be in the form of a standard anechoic absorber.

2E shows a bottom reflector 15 installed in the pipe 13, the bottom reflector may alternatively be integrated with a deflection or absorbing element 29. The bottom reflector design will give substantial reflections from the bottom of the tank rather than from the bottom of the pipe 13. Some of the bottom reflectors described here are very small (one or two λ / 2 dipole antennas), obviously very simple and direct.

FIG. 2F is a reflectivity showing reflection with liquid at different frequencies and different dielectric constants (1.7 and 2.5) in the dry and submerged state, ie when the liquid level is lower and higher than the reflective structure of the bottom reflector. The transmission is also shown in the dry state.

The solid line shows almost total reflection due to the blocking state when the grating is in air and the dashed and dashed lines show the corresponding transmission through the λ / 2 grating. The dashed line shows the small reflection at the lowest dielectric constant within the range of use of the dielectric constant value and the dashed line shows the reflection corresponding to the highest dielectric constant. The reflection is lower at the dielectric constant in the middle of the range. In addition, the reflections above the high frequency range of 9.5-10.5 kHz are completely constant.

3A-3E show various bottom reflectors based on resonant structural theory. Each bottom reflector consists of a resonant structure comprising a plurality of dipole antennas tuned to resonate in a dry state, for example, to distinguish reflections when the tank is empty. As the resonant structure is submerged in liquid, the dipoles deviate from resonance and the reflection decreases considerably.

3A shows a bottom reflector consisting of two pole antennas 32 stacked in two layers. The two dipole antennas 32 are tuned for maximum reflection when the dielectric constant of the surrounding medium is close to one (i.e., air or gas). While the vertical distance between them is chosen to be close to one quarter of the wavelength (λ _m / 4) in a typical liquid with a dielectric constant of 2.1. This design limitation will provide the desired reflection state of the radar wave 31 as shown in the diagram of FIG. 3F. The dipole antenna 32 is attached to a vertical support pin 33 made of metal or dielectric material. If the vertical support pin 33 is metal, the finished structures 32, 33 can be stamped out on the same piece of plate (as indicated in FIG. 3A). The structures 32, 33 are attached to a predetermined support 34 on or in contact with the floor. The dipole antenna length is typically slightly shorter than 14-15 mm at 10 Hz.

3B shows a bottom reflector comprising dual cross dipole antennas 32, 35 elements to achieve polarization independent function. In addition to the dipole antenna 32 shown in Fig. 3a, two cross dipole antennas 35 of the same length are provided. The cross dipole antenna is preferably not connected and FIG. 3B shows that it is installed in the dielectric pin 33.

Referring to FIG. 3C, a reflective structure such as the embodiment of FIG. 3A or 3B is installed in the middle of the pipe 13. In the H ₁₁ mode, the resonant reflector may be installed in the middle of the waveguide pipe 32. If the polarization is well known, the short-polarization model 32 may be used, and an optional dipole antenna 35 may be added to make the reflector independent of polarization.

In many waveguide modes the transverse electric field is low in the middle. This case can be overcome by the use of a number of typical resonant structures such as shown in FIG. 3A. 3d shows such an embodiment in a pipe 13. Depending on the two to four resonant structures located in accordance with the mode electric field pattern of the microwave signal, mode specific reflection can be achieved. The embodiment of FIG. 3D is particularly suitable for using microwave signals in the H ₀₁ propagation mode.

Another way to obtain a resonant structure in H ₀₁ mode is to place the ring 37 in the middle of the pipe 13 as arranged in FIG. 3E. The ring 37 resonates when the circumference is one wavelength (or integer multiple of the wavelength) in the dry state. This resonance changes under wet conditions. Two or more rings may be stacked like the dipole antenna in FIGS. 3A-B. H ₀₁ Because of the small electric field in the middle of the pipe with mode, the reflection is much smaller than that obtained by the embodiment shown in FIG. 3D, which can be very useful for liquids with low reflections, such as some liquid gases. .

This embodiment is also useful for propagation of microwaves in modes other than H ₀₁ . In microwaves in H ₁₁ mode, the shape of this ring can be made independent of reflected polarization, for example, and can optionally be used in the form of a cross dipole antenna.

Figure 3f shows the reflection of a dipole antenna structure consisting of two dipole antennas stacked vertically at a distance corresponding to λ _m / 4 when immersed in a typical liquid (ε = 2.1 in this case). The length of the dipole antenna corresponds to near maximum reflection when the dipole antenna is dry (i.e. slightly shorter than [lambda] / 2). In a typical band, the submerged reflection is at least 20 dB weaker than in air (or surrounded by gas). This form is compared to the strongest reflection (under drying conditions), and the reflection on the metal surface is typically 10-20 dB. The reduction in reflection under the locked condition is the result of the untuned and vertical space of the dipole antennas to cancel the two dipole antennas from each other. Thus, this dipole antenna combination provides a reflection similar to the oil surface in the dry state and considerably lower reflection in the locked state. In addition, two dipole antennas can be stacked for more wideband suppression of radar echo in the locked state, and other resonant structures other than the dipole antenna can be used.

4A-B show various bottom reflectors based on the theory relating to dielectric radar radome.

Referring to FIG. 4A, a dielectric plate 41 is shown, which has a plurality of holes 42, preferably in contrast to the typical radome. The volume of the hole compared to the plate 41 and the volume of the plate is chosen to have an effective thickness of λ _m / 2 when the plate 41 is below the liquid surface to minimize reflection. Where λ _m is the wavelength of the microwave signal in the dielectric when the holes in the plate are filled with liquid. In the dry state the effective dielectric constant of the plate 41 is changed and its effective thickness tested by the microwave signal is also changed. Thus, the reflection increases as the half-wave state can no longer be maintained.

Another suitable arrangement for achieving similar heterogeneous behavior is a plate made of horizontally placed dielectric bars or pins.

With another reflective structure still dependent on the plane, a thick plate 43 with a small dielectric element 44 on its lower side is shown in FIG. 4B. The flat top facilitates accurate reference measurements by traditional mechanical means.

FIG. 4C is a reflection plot of the plate shown in FIG. 4A when the dielectric plate is partially formed in the reference frequency range. In this example the dielectric plate is a 50% perforated PTFE plate. That is, half of the volume is PTFE and half is empty. In the dry state, the average dielectric constant is the average of the PTFE and air (gas) dielectric constants and in the locked state the average dielectric constant is the average of the PTFE and liquid dielectric constants. In the locked state the thickness is very close to λ _m / 2 in the middle of the band where the reflection is considerably reduced, but in the dry state a certain reflection is expected, which is achieved by the effective electrical thickness being significantly deviated from λ _m / 2.

In all cases described with reference to FIGS. 4A-C, the precise form of the void filled with liquid is not particularly important. As will be apparent to those skilled in the art, some of the dielectric material may be composed of attenuating materials that attenuate both reflection and transmission (such as carbon series of PTFE).

5A-C show various bottom reflectors based on polarization tuning theory.

5A-B show reflectors known as twist reflectors in antenna engineering. The text citations referenced herein are the first and second editions of the Antenna Design Guide, A. Double U. Rouge et al., Peter Peregrinus, 1986, pages 184–185, and the Antenna Engineering Guide, Third Edition, R.C. Johnson, McGraw-Hill, 1993, pages 17-28-17-29.

The twist structure 51 is composed of a plurality of straight parallel valleys 52 and a conductor material. Preferably it is a metal plate capable of casting a plurality of valleys. The valleys may have a height of approximately λ _m / 4 and an interval of λ _m / 4 to λ _m / 2 when the liquid is above the valleys 52 of the reflector 51. The reflector 51 may be located at the lower end of the pipe 13, in which the thin valleys 52 are 45 ° from the average electric field 53 of the incident microwave signal propagating in the H ₁₁ waveguide.

To understand the twist function, the incident electric field can be understood as the superposition of two electric fields in the -45 ° and + 45 ° directions with respect to the electric field line 53. One of these electric fields will be parallel to the valleys 52 and reflected from the top of the valleys 52, while the other polarization is hardly affected by the valleys, but will be reflected from the reflective structure as they are exposed between the valleys 52. Because of the height of the valley 52 of λ _m / 4, a relative phase change of 2 × 90 ° = 180 ° will be induced, resulting in 90 ° twisting of the reflected microwave electric field 54 with respect to the electric field 53 of the incident microwave. will be. This behavior is similar to that of standard twist reflectors in antenna engineering. The slight difference, however, is that the dielectric constant of the material that fills the spaces between the bones depends on whether it is a liquid, air or gas. Once the tank is empty the twist function will only partially occur and it will be possible to obtain reflection from the reflector 51.

An important feature of this embodiment is that the reflector is very thin and will be placed very close to the bottom of the tank to allow the water level measurement to be performed close to the bottom.

It will be apparent to those skilled in the art that more complex structures will allow wider tolerances in liquids of different frequencies and different dielectric constants.

It is also known to those skilled in the art that the microwave signal used in the water level measurement of the above-described embodiments has a specific polarization state, and that the first and second reflection coefficients are given in the specific polarization state as described above and disclosed in the claims below. It is obvious. Thus, the rotational effect of the polarization of the linearly polarized microwave signal in reflection in liquid is equivalent to reducing the reflection coefficient of the microwave signal in its specific polarization state.

The polarization rotation theory is useful for changing the propagation mode of the reflected microwave to another as shown in FIG. 5C. The twist structure can be implemented by a spiral structure, which is disclosed in U.S. Patent No. 4,641,139, which changes the propagation mode of the reflected microwave from H ₀₁ to E ₀₁ and is filed by KOEvarvarsson.

Similar functionality may be achieved by a number of suitably positioned antennas 56 on the plate 56 made of conductive or damping material. This arrangement is similar to the arrangement shown in FIG. 3D, but there is a plate 56 and more and other antennas 55 in the other direction. In addition, the antennas 55 in FIG. 5C are monolayer and not stacked.

The antenna forms a frequency sensing surface that generates a reflection similar to that of the bottom plate 55 but generates other polarizations and phases. If the incident microwave propagates in the H ₀₁ mode in the pipe 13, the reflected microwave will propagate mainly in the E ₀₁ mode if the reflector is submerged in liquid and properly designed. In contrast to the known frequency sensitive surface which is non-synchronized, a large amount of reflected microwaves H ₀₁ when the reflector is in dry condition Will propagate in mode.

Thus, the proper design of the reflector of FIG. 5C provides very little reflection in the incident waveguide mode when the reflector is submerged in liquid and considerably strong reflection in the incident waveguide mode when the reflector is in air or gas.

Depending on the material of the bottom plate, the reflection in the incident waveguide mode may be very strong, i.e., full reflection (if plate 55 is metal), or similar to reflection from the oil surface (if plate 55). Is a suitable damping material). As will be apparent to those skilled in the art, the attenuation material can be included in any polarization tuning structure that can control reflection in the dry state.

6 shows a bottom reflector based on the theory of gradually reducing the cross section of the pipe. In order to reduce or eliminate radar reverberation from the bottom of the tank under pipe 13, a device is used which can be bent in small spaces to reduce the pipe diameter and move the radar wave away from the pipe 13. In one embodiment, only materials suitable for installation in close proximity to the bottom 60 of the tank (ie, steel) are used and the diameter is reduced close to the diameter of the short-mode waveguide based on the funnel shape. Such waveguide 62 may be easily provided with a 90 ° bend near the bottom 60 to divert the microwaves through the waveguide portion 64.

The waveguide portion 64 can be easily designed to produce reflections as low as the waveguide is filled with a liquid (including hydrocarbon range) of well known properties. The distance from the tank bottom 60 and the end of the pipe 13 can be made very short. Funnel 61 and waveguide may be attached to tank bottom 60 or pipe 13.

If waveguide 62 is narrow to limit propagation to one mode when it is submerged in liquid, the propagation will be blocked in the dry state and the empty tank will have reflection at the bottom of funnel 61.

Optionally, the funnel 61 is a conical conductive or resistive structure, installed at the bottom of the tank such that its top is up in the pipe, preferably with at least one pipe diameter at the bottom of the pipe (not shown). do. The space formed between the bottom of the pipe and the face of the conical structure needs to obtain a smooth change from the pipe on the conical structure to the coaxial waveguide formed by the pipe and the conical structure. Microwaves deflect away from the pipe as it propagates out of the bottom of the pipe. The coaxial waveguide formed may be provided with an attenuation material. By means of suitable resonant structures (such as half-wave slots in coaxial waveguides) the bottom reflector can be formed with very small reflections when submerged in liquid and very strong reflections in a dry state.

Finally, FIG. 7 shows a bottom reflector applicable when free space propagation of microwaves is used, ie without a pipe for guiding the microwaves. The radar level meter 11 is installed on top of the tank 12 with the main direction 77 of the antenna beam 76 close to vertical so as to receive reflections from the liquid surface 71. The dotted line 76 indicates the width of the antenna beam. Interference problems are common when the liquid surface 71 is close to the bottom 75 and a similar problem occurs if water is near the bottom 75. The water level is initially unrecognizable, and stronger radar reverberation may occur than the bottom.

When the present invention is applied in this case, the bottom reflector 72 is installed close to the floor above the support element 73. The bottom reflector 72 may be similar to FIG. 2A but should be much larger. As the π × [square root (hλ)] ^2/4 is generated in the reflection region close to, the bottom reflector should have an area including at least a major portion of the active reflection area. Where h is the distance from the level gauge 11 to the bottom reflector 72. In addition, when the bottom reflector 72 has a structure as shown in FIG. 2A, there is an advantage in that no water or precipitate accumulates therein.

In a specific embodiment of the invention the bottom reflector 72 may have a smaller size. If it is possible to control the phase change of the transmission through the bottom reflector 72, the result is that some of the bottom or water reflections passing through the bottom reflector 72 will cancel some of the reflections outside the bottom reflector 72. This arrangement will reduce reflections from the bottom or water surface as well as reflections from the bottom reflector. The support pin 73 should be as low as possible in view of the expected water level.

It will be apparent to those skilled in the art that the bottom reflection theory as described with reference to FIGS. 2-5 is also useful for floor reflectors arranged to transmit and receive microwaves without guidance by a pipe.

Claims (44)

By calculating the level of liquid stored in the tank from the transmission of the microwave signal from the top of the tank towards the surface of the liquid, the reception of the microwave signal reflected off the surface of the liquid, and the propagation time of the transmitted and received microwave signal; In the bottom reflector 15 for a radar based water level meter 11 which measures the water level 71 of the liquid stored in the tank 12, The bottom reflector 15 may be installed at the specific height in the tank to reflect the microwave signal when there is no liquid at the specific height in the tank, The bottom reflector 15 has a first reflection coefficient of the microwave signal when the level of the liquid is higher than the reflection structure and a second reflection coefficient of the microwave signal when the level of the liquid is lower than the reflection structure. Reflective structures lower than the second reflection coefficient (23 to 24; 25; 23, 26; 27; 29; 32; 33; 32, 35; 36; 37; 41, 42; 43, 44; 51 to 52) 55 ~ 56; 61,72) floor reflector, characterized in that. The bottom reflector of claim 1, wherein a first reflection coefficient due to the reflection structure of the microwave signal is smaller than a second reflection coefficient due to the liquid of the microwave signal. The bottom reflector according to claim 1 or 2, wherein the second reflection coefficient due to the reflection structure of the microwave signal is equal to or larger than the first reflection coefficient due to the liquid of the microwave signal. The bottom reflector of claim 1, wherein the liquid has a dielectric constant in the range of 1.6-3. The bottom reflector of claim 1, wherein the liquid is a group of liquids consisting of crude oil, liquid petroleum gas (LPG), liquid natural gas (LNG), other liquid hydrocarbons, or a liquid that passes at least some microwaves. . The bottom reflector of claim 1, wherein the microwave signal has a specific polarization state and the first and second reflection coefficients are given to the microwave signal in the specific polarization state. The floor reflector of claim 1 wherein the microwave signal is within a predetermined frequency range. The floor reflector of claim 1 wherein the first reflection coefficient is greater than zero and less than 0.2. The floor reflector of claim 1 wherein said second reflection coefficient is greater than 0.1 and less than one. The floor reflector of claim 1 wherein the second reflection coefficient is greater than zero and less than 0.5. The floor reflector of claim 1 wherein the reflecting structure is manually installed for use in manual water level measurement. A floor reflector as claimed in claim 1, wherein the level gauge consists of a substantially vertical tube (13) for guiding the transmitted and reflected microwave signal. 13. The bottom reflector according to claim 12, wherein the bottom reflector (15) is installed at the lower end of the substantially vertical tube (13). 14. The bottom reflector of claim 12 or 13, wherein the microwave signal has a specific polarization mode and the first and second reflection coefficients are given to the microwave signal in the specific polarization mode. 13. The bottom reflector of claim 12, wherein said reflective structure is a blocking grating (23-24; 25; 26; 27; 29). 16. The bottom reflector of claim 15, wherein said microwave signal propagates in H ₁₁ mode and said blocking grating consists of a plurality of balanced strips (23) arranged parallel to the electric field of said microwave signal. 16. The bottom reflector of claim 15, wherein said microwave signal propagates in H ₀₁ mode and said blocking grating consists of a plurality of coaxial circular strips (25) arranged parallel to the electric field of said microwave signal. The bottom reflector of claim 1, further comprising another reflecting structure (27) disposed below said reflecting structure. A bottom reflector according to claim 1, further comprising an absorbing structure (29) disposed below said reflecting structure. 13. The bottom reflector of claim 12, wherein the reflective structure is a resonant structure, the resonant structure comprising a dipole antenna (32, 33, 32, 35, 36, 37). 21. A bottom reflector as claimed in claim 20, wherein said resonant structure is a stacked bipolar antenna (32). 21. A bottom reflector as claimed in claim 20, wherein said reflective structure consists of a double cross dipole antenna (32, 35). 21. The method of claim 20, wherein the microwave signal is propagated in H ₁₁ mode and the resonant structure is a single dipole antenna element (32, 35) disposed in the central axis of the substantially vertical tube (13). Floor reflector. 21. The microwave signal according to claim 20, wherein the microwave signal is propagated in H ₀₁ mode, and the resonant structure is a plurality of two-pole antenna elements 36 arranged to be spaced apart from a central axis in the substantially vertical tube 13. Floor reflector, characterized in that. 21. The bottom reflector according to claim 20, wherein said microwave signal propagates in H ₀₁ mode and said resonant structure is a ring (37) having an axis of symmetry coinciding with the central axis of said substantially vertical tube (13). . 13. The bottom reflector of claim 12, wherein the reflecting structure is a dielectric reflector (41, 42; 43, 44). 27. The bottom reflector of claim 26, wherein the dielectric reflector is a through plate (41, 42). 27. The bottom reflector of claim 26, wherein the dielectric reflector is comprised of horizontally arranged plates (43) formed with small dielectric elements (44) facing downwards. 13. The bottom reflector of claim 12 wherein the reflecting structure is a polarization tuning structure (51-52; 55-56). 30. The bottom reflector of claim 29, wherein the polarization tuning structure is a twist reflector structure (51) comprising a valley (52). 30. The bottom reflector of claim 29, wherein the polarization tuning structure includes a reflector (55) and a plurality of differently directed antennas (56). 13. The reflecting structure of claim 12, wherein the reflecting structure is a microwave guiding structure disposed within the substantially vertical tube (13) and guided so that the transmitted and received microwave signal is away from the interior of the substantially vertical tube (13). Floor reflector, characterized in that. 33. The bottom reflector of claim 32, wherein said microwave guide structure is a funnel (61). The floor reflector of claim 1, wherein the level gauge is installed for free space propagation of microwave signals. 35. The bottom reflector of claim 34, wherein said reflective structure is a blocking grating, a resonant structure, a resonant structure comprising a dipole, a dielectric reflector or a polarization rotating structure. Of claim 34 or claim 35, wherein the reflective structure 72 includes at least under the level gauge based on the radar π × [square root (hλ)] region ^2/4, h is the water level based on the radar A distance from a meter to the bottom reflector and λ is the wavelength of the microwave signal. 36. The method of claim 34 or 35, wherein the area of the bottom reflector 72 is smaller than the area of the microwave signal at the height of the bottom reflector; When the level of the liquid is higher than the bottom reflector, the phase change of the microwave signal transmitted through the bottom reflector 72 is controlled to control the portion of the microwave signal reflected from the tank bottom outside the bottom reflector 72 and the bottom reflector 72. Bottom reflector characterized in that part of the microwave signals reflected from the bottom of the tank cancel each other after passing through). The method of claim 1, And the bottom reflector is installed close to the bottom of the tank. A radar based water level meter (11) system comprising the bottom reflector (15) of claim 1. The floor reflector of claim 1 wherein the first reflection coefficient is greater than zero and less than 0.1. The floor reflector of claim 1 wherein the first reflection coefficient is greater than zero and less than 0.05. The floor reflector of claim 1 wherein said second reflection coefficient is greater than 0.2 and less than one. The floor reflector of claim 1 wherein said second reflection coefficient is greater than zero and less than 0.4. The floor reflector of claim 1 wherein the second reflection coefficient is greater than zero and less than 0.3.

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