JP4695394B2 - Liquid level measurement apparatus and measurement method using radar - Google Patents

Description

  The present invention relates generally to radar-based liquid level measurement, and more particularly, the present invention provides high accuracy using radar via a waveguide without prior knowledge of the exact gas composition and / or pressure on the liquid surface. The present invention relates to an apparatus and a method for performing liquid level measurement.

  The device for measuring the liquid level in the container is transmitted and reflected by a transmitter for transmitting the microwave signal in the direction of the liquid level and a receiver for receiving the microwave signal reflected by the liquid level. And a signal processing device for calculating the liquid level in the container from the propagation time of the microwave signal.

  Such devices are becoming increasingly important, especially for petroleum products such as crude oil and products produced therefrom. Here, the container means a large container that forms a part of the total loading capacity of the tanker, or a larger, usually cylindrical, ground tank having a capacity of tens of cubic meters or thousands of cubic meters.

  For one particular type of device that utilizes a radar to measure the liquid level in a container, the microwave signal passes through a vertical steel tube mounted in the container that functions as a waveguide for the microwave. Transmitted, reflected, and received. An example of liquid level measurement utilizing such a tube is disclosed in US Pat. No. 5,136,299 to Edvardsson. The speed of the microwave in the waveguide is lower than in free wave propagation, but when calculating the liquid level in the container from the propagation time, either a calculation based on knowledge of the waveguide dimensions or a calibration procedure is used. This can be taken into account.

  Furthermore, the speed of the microwave is reduced by the gas on the liquid surface. This deceleration can be accurately evaluated, but only if the gas composition, temperature and pressure are known and such is rare.

  When normal, ie, petroleum products that are fluid at ambient temperatures, are used, the gas in the tube is typically air. The nominal dielectric constant in air is typically 1.0006 with a variation of ± 0.0001. However, for steam such as hydrocarbons, the dielectric constant of the tank contents will be higher than for air. Such an increase may be significant.

  Furthermore, when measuring the liquid level in a container filled with liquefied gas under overpressure, the change in speed is very significant. Of the common gases, propane has the highest dielectric constant and a deceleration of about 1% occurs at a pressure of 10 bar (corresponding to ε = 1.02). Such large discrepancies are unacceptable for many applications, such as storage and delivery applications.

  Therefore, there is often a need for higher accuracy, defined as storage and delivery accuracy. The expression storage accuracy here means sufficient accuracy to permit approval for storage and delivery, which is the official requirement for many commercial uses of liquid level measurement. From the viewpoint of propagation speed, the accuracy of storage and delivery means the liquid level determination accuracy in the range of about 0.005 to 0.05%. However, although storage and delivery requirements vary greatly from country to country and from organization to organization, the above example clearly does not fit any storage and delivery accuracy.

  Therefore, a main object of the present invention is to provide an apparatus and method for performing high-accuracy liquid level measurement using a radar via a pipe without prior knowledge of the exact gas component and / or pressure on the liquid surface. It is to provide.

  A particular object of the present invention is that 0.4%, preferably 0.1%, and preferably 0.1% for a gas or gas mixture on a liquid surface where the dielectric constant is anywhere between 1 ≦ ε ≦ 1.03. It is most preferable to provide the above-described apparatus and the above-described method that provide a measurement liquid level accuracy better than 0.01%. This width is selected to include propane, butane, methane, and other common gases with some margin of error.

  In this regard, a specific object of the present invention is to provide the above apparatus and the above method capable of measuring the liquid level with storage and delivery accuracy.

  Another object of the present invention is to provide the above apparatus and method for measuring liquid level through a tube which also provides an accurate measurement of the internal dimensions of the tube.

  Yet another object of the present invention is, for example, the cross-sectional dimension of the tube, the change in cross-sectional dimension along the length of the tube, the degree of concentricity of the tube, the presence of impurities, especially solid or liquid hydrocarbons, on the inner wall of the tube, Or measuring the liquid level through a tube, such as the presence of mist in the gas, in particular the presence of oil mist, or reducing the error by evaluating one or more characteristics of the environment in the container. It is to provide an apparatus and a method as described above.

  The above objective is accomplished by the apparatus and method particularly pointed out in the appended claims.

  Radar level gauges use a fairly wide bandwidth (the width can be 10 to 15% of the center frequency), and the propagation characteristics are indicated by the group velocity at the center of this band. The inventor has made it possible over the range of dielectric constants of interest, preferably between 1 and 1.03, by appropriate selection of the frequency band and propagation mode of the transmitted and received microwave signals and the internal dimensions of the tube. It has been found that it is possible to obtain a constant group velocity of microwave signals. Analysis shows that the group velocity fluctuates by only ± 0.005% over a dielectric constant range of 1 to 1.03, while ± 0.75% for conventional devices utilizing free space propagation, for example. Fluctuations will occur.

The center frequency of the frequency band of the microwave signal is preferably about (2 / ε) ^1/2 times or close to the cutoff frequency in vacuum at the selected mode and tube internal dimensions, where ε Is the value of the center frequency of the range of dielectric constants of interest, for example about 1.015 in the preferred range above. Thus, for a gas having a dielectric constant that is in the middle of the dielectric constant range of interest, the optimum center frequency is approximately ^21/2 times the actual cutoff frequency.

In a frequency band that includes frequencies that deviate by less than 7%, preferably less than 5%, more preferably less than 3%, even more preferably less than 2%, and even more preferably less than 1% from the optimal frequency f _opt Since a microwave signal is transmitted, the present invention can be defined quantitatively. However, the optimum frequency is calculated as follows.

ただし、ｆ_ｃ０は前記管内での前記第１伝播モードの遮断周波数であり、εは関心対象の誘電率範囲の中心の誘電率である。これらの周波数は単一モードの伝播が保証されなければならない場合に用いられる周波数よりも高いが、双方ともＥｄｖａｒｄｓｓｏｎに付与された米国特許第４，６４１，１３９号及び米国特許第５，１３６，２９９号に記載されているように、過大サイズの管及びモード抑制が適用されている場合に典型的に用いられる周波数よりもずっと低い。したがって、本発明で用いられている周波数は少なくとも部分的に、管と液位測定の双方に関して先行技術で使用されている周波数範囲の外にある。

Where f _c0 is the cutoff frequency of the first propagation mode in the tube and ε is the dielectric constant at the center of the dielectric constant range of interest. These frequencies are higher than those used when single mode propagation must be guaranteed, but both are US Pat. No. 4,641,139 and US Pat. No. 5,136,299 issued to Edwardsson. As described in the issue, it is much lower than the frequencies typically used when oversized tubes and mode suppression are applied. Thus, the frequencies used in the present invention are at least partly outside the frequency range used in the prior art for both tube and liquid level measurements.

Most advantageously, however, the frequency band is the optimal frequency f _opt or has a center frequency that _deviates from the optimal frequency f _{opt by} less than 1 to 7%.

Preferably, a circular tube and the mode H ₁₁ are used for measurement. The microwave signal can also be propagated in the E ₀₁ mode by selecting a frequency about (2 / ε) ^½ times the cutoff frequency in the case of mode H _{11 in} vacuum. The microwave signal can be measured separately from each other in these two modes, the measurement of the E ₀₁ mode microwave signal provides information on the dimensions of the tube and / or the gas or gas mixture on the surface of the liquefied gas. It can be used to infer information about dielectric properties.

  More generally, microwave signals can be measured in at least two different modes that are distinct from each other. Such dual-mode measurements infer and use information about tube conditions, such as tube dimensions, or atmospheric conditions of the tube, such as the presence of an oil reservoir or mist on the inner wall. Thus, it can be used to reduce errors in the measured liquid level induced by tube conditions.

  The main advantage of the present invention is that high-precision liquid level measurement can be performed through a tube without prior knowledge of the composition and pressure of the gas present on the surface to be measured.

  Another advantage of the present invention is that errors induced by tube conditions can be reduced by dual mode measurements.

  Yet another advantage of the present invention is that by selecting a frequency close to the optimum frequency as described above for a dielectric constant range of 1 to 1.03, variations in the amount of hydrocarbon droplets in the tube and on the inner wall of the tube The effect of the change in the thickness of the thin hydrocarbon layer is minimized.

  Other features of the present invention and its advantages will be described in the following detailed description of the preferred embodiments of the invention and the accompanying drawings, which are given solely for the purpose of illustration and are therefore not intended to limit the invention. It is clarified from FIGS. 1 to 13.

In the following description, the waveguide names H ₁₁ , E ₀₁ , H ₀₁ and the like correspond to the names TE ₁₁ , TM ₀₁ , TE _{01 and the} like, and are used as a completely similar system.

  A preferred embodiment of the present invention will be described with reference to FIG. 1, which schematically shows a liquid level measurement device using a radar in a perspective view. This device may be a frequency modulated continuous wave (FMCW) radar device, or a pulse radar device or other type of distance measuring radar device, but the former is preferred. The radar device may have the ability to transmit a microwave signal at an adjustable variable frequency.

  In FIG. 1, 1 indicates a substantially vertical tube, or a tube fixedly attached to a container, the upper limit of which is indicated by 3. The container contains a liquid that may be a petroleum product, such as crude oil or a product produced therefrom, or condensed gas that is stored under pressure and / or cooled and stored in the container. Propane and butane are two typical gases stored as liquids.

  The tube 1 is preferably made of a metal material that can function as a microwave waveguide, and may have an arbitrary cross-sectional shape. However, it is preferably a circular, square or super elliptical cross section. The total length of the tube is not shown, only the top and bottom. The wall of the tube is provided with a number of relatively small openings 2 so that liquid can communicate from the container to the interior of the tube, so that the liquid level in the tube and in the container is the same. It has been found that the size and position of the holes can be selected so that the wave propagation is not disturbed and the liquid levels inside and outside can be made sufficiently quick and even.

  A unit 4 is fixedly attached to the tube. This unit 4 is for transmitting a microwave signal (not explicitly shown), a receiver for receiving the reflected microwave signal, and for determining the reflected portion of the reflected microwave signal. And a signal processing device.

  The transmitter comprises a waveguide, indicated by 5 in FIG. 1, which is surrounded by a protective tube 8. The waveguide 5 passes through the conical intermediate member 9 to the tube 1.

  In operation, the transmitter generates a microwave signal which is sent into the tube 1 via the waveguide 5 and the conical intermediate member 9. The microwave signal propagates in the direction of the surface to be leveled in the tube 1, is reflected by the surface and re-propagates in the direction of the receiver. The reflected signal passes through the conical intermediate member 9 and the waveguide 5 and is received by the receiver. The signal processing device calculates the liquid level from the round trip time of the microwave signal.

In accordance with the present invention, the transmitter is adapted to transmit a microwave signal in a frequency band that includes a frequency that deviates by less than 7% from the optimal frequency f _opt , where the optimal frequency is:

で計算され、ただしｆ_ｃ０は管１内での真空中での伝播モードの遮断周波数であり、εはそれに限定されるものではないが好適には１から１．０３、又はそのサブ範囲に設定された関心対象の誘電率範囲の中心の誘電率である。

Where f _c0 is the cut-off frequency of the propagation mode in vacuum in tube 1 and ε is preferably, but not limited to, 1 to 1.03 or a sub-range thereof. The dielectric constant at the center of the specified dielectric constant range of interest.

  By so selecting the frequency, the change in the group velocity of the microwave signal when the dielectric constant of the gas in the tube 1 above the liquid level is changed from 1 to 1.03 is very small. Accurate liquid level measurement can be performed without knowledge of the above components and pressure.

Preferably, the frequency _{deviates by} less than 5%, more preferably less than 3%, more preferably less than 2%, and even more preferably less than 1% from the optimum frequency f _opt , more preferably the frequency is It is the same as the optimum frequency f _opt . Optionally, the frequency band has a center frequency that deviates by less than 7%, 5%, 3%, 2%, or 1% from the optimal frequency f _opt .

  These numbers result in a slightly greater speed change than can be obtained using the optimal frequency, but the change is still much smaller than that obtained using the frequency used in prior art devices.

  The theory behind the present invention and the deviation from the above optimal frequency will be described.

  Propagation in any homogeneous (ie, filled with a single material having a dielectric constant ε) hollow waveguide can be explained by a change in phase constant β resulting in a phase change in radius of 1 meter.

ただし、ｋは波数（ｋ＝２πｆ／ｃ、ただしｆは周波数、ｃは真空中の光速である）、またｋ_ｃ０は真空中の遮断周波数の数（ｋ＝２πｆ_ｃ０／ｃ、ただしｆ_ｃ０は真空中の遮断周波数）であり、これは導波管内での伝播の下限値である。上記の公式は導波管の断面に関わりなくどの単一の伝播モードにも当てはまる。

Where k is the wave number (k = 2πf / c, where f is the frequency, c is the speed of light in vacuum), and k _c0 is the number of cutoff frequencies in the vacuum (k = 2πf _c0 / c, where f _c0 is Cutoff frequency in vacuum), which is the lower limit of propagation in the waveguide. The above formula applies to any single propagation mode regardless of the waveguide cross section.

The cutoff wave k _c0 is related to the cross-sectional shape of the waveguide. In the case of a circular cross section having a radius a:
k _c0 = X / a (Formula 2)
Where X is an applicable root in the Bessel function (J ₀ (x), J ₁ (x), etc.), and 0 in k _c0 is inserted to emphasize that k _c0 is applied to a vacuum. Is done. Some lowest modes within a circular waveguide (diameter D = 2a) are listed in Table 1 below.

  For comparison, the number of cutoff waves in a rectangular waveguide having a cross-sectional size of b times a (where a> b) can be expressed as follows.

ただし、ｎ及びｍは別の制約ｎｍ＞０（Ｅモード）又はｎ＋ｍ＞０（Ｈモード）を伴う非負整数である。

However, n and m are non-negative integers with another constraint nm> 0 (E mode) or n + m> 0 (H mode).

Returning now to the propagation constant β, it should be noted that this depends at least slightly on the non-linear frequency compared to the propagation constant of the free propagation wave. Conventionally, the band-limited signal propagation is expressed as a group velocity v _g is calculated as follows.

ただし、ｃは真空内での光速（２９９７９２４５８ｍ／秒）であり、係数ｃ／ｖ_ｇは１よりも少なくともわずかに大きい。（自由空間の場合に近づく）断面積が極めて大きい導波管の場合、Ｋ_ｃ０は無視してもよく、そこで係数は単に誘電率εの平方根である。

However, c is the velocity of light in a vacuum (299792458m / sec), the coefficient c / _{v g} is at least slightly greater than 1. For a waveguide with a very large cross-sectional area (approaching free space), K _c0 may be ignored, where the coefficient is simply the square root of the dielectric constant ε.

Table 1. Mode in a circular waveguide. A common notation, X, λ _c0 / D (where λ _c0 is a cutoff wavelength in vacuum, D is a diameter, D = 2a) is given to each propagation mode.

A closer examination of Equation 4 reveals that it has a minimum value whenever the dielectric constant ε can vary over all positive values. This, epsilon cases slightly larger than a value constituting the denominator zero, the value of c / v _g is very large, it can easily be seen clearly epsilon is the same as that very large. This minimum appears when ε is a physically unrealistic value, but whenever the waveguide diameter is 2a, the frequency at which this minimum occurs for possible values of ε (or Wave number k) can be recommended (since K _c0 is related to the diameter 2a based on equation 2).

This minimum appears when ε has a physically unrealistic value, but whenever the tube diameter is 2a, the frequency at which this minimum occurs for a possible value of ε (or the wave's Several k) can be recommended. To find the minimum of c / v _g, the second derivative is formed according to the following.

minimum value of c / v _g is obtained when the derivative is zero. Therefore, the optimum wave number called wave number k _opt that satisfies such conditions is as follows.

With this choice, changing ε slightly (around the assumed center of ε, which can be 1 to 1.03) will give a very slight change in vg that the following numerical evaluation quantifies I can expect. This phenomenon can be described as a combination of two factors that contribute to v _g, but if ε increases the speed of the microwave decreases, thereby also become waveguide appear larger, as a result, guide Wave tube propagation speed is increased. The derivative equation shows that these two conflicting effects can be canceled out.

To explain the characteristics, a diameter of 2a = 100 mm, width of ε is a small change in v _g is given cases from 1 obtained waveguide in the range of 1.03. That is, the optimum wave number k _opt is found when ε = 1.015. When the lowest propagation mode H ₁₁ is used, the optimal wave number k _opt 51.5m ⁻¹ is obtained using Equations 2 and 6. This optimum wave number corresponds to the optimum frequency f _opt which is 2.46 GHz.

FIG. 2 is a graph of the group velocity normalized to the speed of light in vacuum as a function of dielectric constant for H ₁₁ mode microwave radiation in a 100 mm waveguide at 2.46 GHz at 2.46 GHz. Is shown.

  If the dielectric constant ε varies from 1 to 1.03 (± 1.5%) or contains air (ε = 1.0006) relative to propane (ε = 1.03), the speed change is ± 0.005%. If the range of permittivity values is limited to a width less than 1 to less than 1.03, the speed change improvement is 150 times and more.

Figure 3 is normalized with respect to the speed of light in vacuum as a function of the dielectric constant in the case of the _{H 11} mode microwave radiation within the waveguide of 100mm in the case of 2.46 GHz, 10 GHz, and 2GHz for comparison The graph of the generalized group velocity is shown. Note that the vertical scale is magnified 200 times that of FIG. The curve at the optimum frequency appears as a horizontal straight line. That is, there is no dependence of ε on the group velocity, while the group velocities at 2 GHz and 10 GHz greatly depend on ε in the illustrated width.

FIG. 3 shows the effect of waveguide diameter on the resulting group velocity. The graph is a function of the dielectric constant for a waveguide with a diameter of 100 mm, a waveguide with a diameter larger by 0.005%, and a microwave radiation of H ₁₁ mode in a waveguide with a diameter smaller by 0.005%. The graph of the group velocity normalized with respect to the speed of light in the vacuum is shown.

  The position of the maximum group velocity does not change significantly when the diameters are slightly different. However, the group velocity is highly dependent on the diameter, so the waveguide diameter needs to be measured or calibrated very closely. This point will be further described later. But first, the concept of the present invention is further illustrated in FIGS.

FIG. 5 shows the case where the values of the dielectric constant ε are different, that is, when ε = 1.0006 (air) and ε = 1.03 (1.02 corresponds to propane under a pressure of 10 atm, which seems to be the worst case. FIG. 6 shows a graph of group velocity normalized to the speed of light in vacuum as a function of the wave number of the H ₁₁ mode in a 100 mm waveguide. Note that the two curves intersect at a particular wave number, which is actually the optimal wave number k _opt .

FIG. 6 shows a graph of the group velocity normalized against the speed of light in vacuum as a function of the wave number of the H ₁₁ mode in a 100 mm waveguide for different values of dielectric constant ε. Is clearly shown. The speed is shown for the following cases: ε = 1.03, 1.02, 1.01 and 1.0006. The intersection is found at k = 51.5 m ⁻¹ , which is the optimum wave number as described above.

  Calibration and measurement of waveguide diameter that must be performed more closely than when using oversized waveguides and mode suppression disclosed in US Pat. No. 4,641,139 to Edwardsson Several methods are available.

  One method is to determine the effective diameter for one or several liquid levels by in-situ calibration for one or several known heights. In FIG. 1, a relatively thin metal pin 10 is mounted on the lower part of the tube 1 that is perpendicular to the longitudinal direction of the tube 1 in the radial direction. The metal pin 10 has a reactance that induces a defined reflection of the radiated microwave signal received by the receiver in the unit 4 and calibrates the liquid level measurement function via the electronic unit. Such field calibration is further disclosed in US Pat. No. 5,136,299 to Edvardson, which is incorporated herein by reference. Of course, it is preferable to perform the same calibration many times using an accurate measurement of the actual liquid level.

  Another method is to receive a portion of the microwave signal that is received in the second propagation mode, re-propagated through the tube, and received in one of the first and second propagation modes. In order to distinguish, the microwave signal is transmitted through the gas in the tube 1 in the direction of the liquid level in the second propagation mode using the supply device.

Figures 7a to 7b show an example of waveguide supply in two modes. The tube 1 is closed by a cover 10 sealed by sealings 11 and 12. Under sealing, a λ / 2 dipole 13 supplies the H ₁₁ mode in the tube 1 and then it is supplied via two wires 15. Below the dipole 13, a member 14 having a shape suitable for supplying an _E01 mode to the tube 1 is mounted symmetrically. Member 14 is then fed by line 16. Lines 15 and 16 pass through the pressure sealing 12 and are connected to lines and cables (not shown) of the measuring device. Since the two lines 15 are sent to the balun, they are sent in opposite phases, so the line 15 and a single line 16 that is sent as a coaxial line with a part of the cover 10 as other parts automatically. Insulated. It is clear that with the proper shape for matching etc., basically the same outline will work for both of the two modes (H ₁₁ and E ₀₁ ) and for either one of these two modes. The antenna and supply lines 13 to 16 can be formed on a printed circuit board indicated by a dotted line 17.

  In this way, two independent measurements can be made and not only the liquid level but also the diameter of the tube 1 such as the effective diameter or the average diameter can be inferred from the measurements. See Equation 4. One way to achieve this is to use a waveguide connection that imparts two modes, and if the modes are very different, the group velocities in the two modes have a reverberation in time for pulsed systems, and In the case of FMCW systems, it is to take advantage of the fact that it may be different enough to separate the reverberations in frequency.

  The receiver of unit 4 in FIG. 1 can be adapted to distinguish the part of the microwave signal received in one of the first and second propagation modes based on the part with different arrival times at the receiver. The currently supplied waveguide has two (or more) connection lines adapted to couple different modes and the RF switch connects the modes sequentially or two (or more) Part of the receiver or transmitter chain is doubled so that measurement of the mode is possible.

  The portions of the microwave signal can have very different propagation times so that they can be detected sequentially. If this is not the case, the transmitter of unit 4 may transmit the microwave signal sequentially in the first and second propagation modes.

  Alternatively, the transmitter of unit 4 is adapted to transmit microwave signals in first and second propagation modes that are spectrally separated. Thus, the waveguide supply has a different function for different frequencies giving one frequency width in one mode and the other frequency width in the other mode.

  Or (if the diameter is known) the signal processing device above the liquid level based on the received and differentiated portion of the microwave signal received in one of the first and second propagation modes The dielectric constant of the gas may be calculated.

Figures 8a to 8b show another waveguide supply suitable for supplying microwave signals in _H01 and _E01 modes. The tube 1, cover 10 and sealing 11 are the same as in the embodiment of FIG. The antenna device typically formed by the printed circuit board 20 is a critical component of supply. The printed circuit board is supplied by a coaxial line, the outside of which is shown at 21. The printed circuit board 20 has four λ / 2 dipoles 25 mounted thereon, which are supplied in phase (combined efficiently in parallel by radial wires not shown) and as indicated by arrows. the electric field direction grant, thus dipoles is capable of being efficiently bound to H ₀₁ waveguide propagation mode. The distance from the printed circuit board 20 to the cover 10 is about λ / 4. The outside of the supply coaxial line 21 is also the inside of another coaxial line having an insulator 23 and a shield 24. This coaxial line provides the E ₀₁ mode generated by the part 24 and part of the pattern on the printed circuit board 20. The insulators 23 and 22 are pressure sealing. The mechanical fixtures 22 and 23 are not shown.

FIGS. 9a to 9c show yet another method of configuring a waveguide supply for generating microwave signals in modes H ₀₁ and H ₁₁ . For example, the antenna element 30 in the form of a printed circuit board has four dipoles 33 that are supplied by four coaxial cables 32 through a ceiling 31. Outside the container, the four cables are fed through a coupling network, indicated by 34 in FIG. 9c, which is located outside the container or possibly on the antenna element 30. Yes. The supply network consists of four standard hybrid circuits 34 that can generate three different waveguide modes. The top input gives the H ₀₁ mode with four dipoles oriented as solid arrows (see FIG. 9b). The other two inputs give the H ₁₁ mode supplied with right-handed circularly polarized light and left-handed circularly polarized light used for transmission and reception of the H11 mode.

  Each supply device as shown in FIGS. 7 to 9 may be provided with a funnel (not shown) in the tube 1 to accommodate the diameter of the tube 1. The funnel can be suspended within the tube 1 as described in US Pat. No. 4,641,139, which is incorporated herein by reference.

In Table 2 below are shown the attenuation of a suitable combination with a center frequency and tube diameter for the four waveguide modes _{H 11 / E 01 / H 01} / H 02. Attenuation on a 25m tube (ie 2x25m transmission) is shown in the given order for these four modes and in dB separated by a slash.

  Note that the numbers in Table 2 only identify different examples. Theoretically, any mode may be used as the main propagation mode. Different modes are shown in Equations 2 and 3 and Table 1. However, two of the combinations in Table 2 appear to have particularly favorable properties.

The combination of H ₀₂ / E ₀₁ in a 100 mm tube using a frequency of about 10 GHz utilizes two rotationally symmetric modes, where the H ₀₂ mode is (like the more well known H ₀₁ mode) It is useful because it is completely independent of wall conditions and E ₀₁ has a propagation similar to conventional radar liquid level measurements through the tube, far from its blockage.

The H ₁₁ / E ₀₁ combination in a 100 mm tube utilizing a frequency range close to 2.5 GHz (eg in the 2.4 to 2.5 GHz ISM band) is a mechanical such as tube holes, fittings, etc. It is a method that uses low frequencies that are not sensitive to details and that the cost of microwave hardware is not so high.

  Finally, in the table below, the use of shorter tubes (the height of many LPG spheres is at most 10 to 15 m) allows a smaller size without excessive attenuation (proportional to the length of the tube). Tubes and other modes can be used.

Table 2. Attenuation on a 2 × 25 m stainless steel tube (0.5 Ω / square at 10 GHz) in four waveguide modes H ₁₁ / E ₀₁ / H ₀₁ / H ₀₂ with different frequency and tube diameter choices. NP indicates no propagation (blocking). NA indicates that none of the modes can propagate, 1.41-modes that satisfy the condition are underlined, and the combination of the two modes that are most likely to be preferred is underlined in one of them. A line is attached to indicate that the mode satisfies the 1.41-condition. The frequencies shown are for display purposes only and need to be slightly different to meet the 1.41- condition.

  In these embodiments, it is assumed that the same frequency is utilized in both modes, which typically means separation by switches, separate transmitter or receiver channels, and the like. Obviously, different frequencies can be used to bring the system closer to two separate microwave units (or widely tunable units) connected to the same tube and having common signal processing components. In that case, a filtering function can be used to separate the signals and the mode generator can generate different modes for different frequencies.

  By measuring the microwave signal in two independent propagation modes, it is possible to detect and compensate for the characteristics of the environment in the tube or container. To obtain good results, the mode can be selected such that the microwave signal in one mode is severely disturbed while the microwave signal in the other mode is disturbed very little.

  The signal processing device of the unit 4 preferably calculates the liquid level in the container from the propagation time of the microwave signal transmitted and reflected in each propagation mode, and the tube based on the calculated liquid level in the container. Or, evaluate one or more characteristics of the environment within the container.

  Alternatively, the signal processing device of unit 4 calculates the attenuation of the differentiated portion of the received microwave signal in one of the first and second propagation modes, and calculates the differentiated portion of the microwave signal. It is adapted to evaluate one or more characteristics of the environment in the tube or container based on the attenuation.

  One or more characteristics of the environment within the tube or container include the cross-sectional dimension of the tube, the change in cross-sectional dimension along the length of the tube, the degree of concentricity of the tube, impurities on the inner wall of the tube, particularly solid or liquid This includes the presence of hydrocarbons or the presence of mist in the gas. Modes with different characteristics can be used to account for different parameters.

10 to 12 show modes H ₁₁ (FIG. 10), E ₀₁ (FIG. 10) of microwave radiation in a gas-filled waveguide with different dielectric constants ε and dielectric layers of different thickness t on the inner wall. 11) and a schematic graph of the inverted group velocity normalized to the group velocity in vacuum as a function of wave number for each of H ₀₁ (FIG. 12). The dielectric constant of the dielectric layer is set to 2.5, which is a typical value in the case of the oil layer.

Note that the behavior for gas filling and dielectric layer is similar (a gas with ε = 1.03 draws approximately the same curve as a 1 mm thick oil layer). For mode H _11, the behavior of the thin dielectric layer is very similar to the gas, the thicker layer moves in the direction of fewer wavenumber zero crossing. In the case of mode E _01, the influence by the dielectric layer is somewhat strong, while in the case of mode H ₀₁ , the dielectric layer has a very small influence.

  In this way, it is possible to evaluate and correct the oil layer (eg, average thickness or dielectric constant) by the difference in influence due to the dielectric layer.

Finally, the reflective reactance 10 arranged in the tube 1 may be designed such that the reflection of the microwave signal is significantly stronger in one propagation mode than in the other propagation mode. Reflecting reactance 10 may be implemented as a (non-reflective and comprising shape in the case of H ₁₁₎ short metallic pin supported coaxially in the tube 10 by PTFE strip. This can be used to cause a reference reflection at a mechanically known position in the E ₀₁ mode, but only a very weak reflection in the H ₁₁ mode.

1 is a schematic perspective view of a liquid level measuring device using a radar according to a preferred embodiment of the present invention. FIG. 5 is a schematic graph of group velocity as a function of dielectric constant for H ₁₁ mode microwave radiation in a waveguide at an optimal frequency. H ₁₁ modes in a waveguide at three different frequencies, one at the optimum frequency, one at a much lower frequency, and one at a much higher frequency, illustrating the principles of the present invention FIG. 6 is a schematic graph of group velocity as a function of dielectric constant for microwave radiation of FIG. At different diameters of the waveguide is a schematic graph of the group velocity as a function of the dielectric constant in the case of the H ₁₁ mode microwave radiation in the waveguide at an optimum frequency. FIG. 6 is a schematic graph of group velocity as a function of wave number for H ₁₁ mode microwave radiation in waveguides filled with gases having different dielectric constants. FIG. 5 is a schematic graph of group velocity normalized to group velocity in vacuum as a function of wave number for H ₁₁ mode microwave radiation in waveguides filled with gases having different dielectric constants. FIG. 2 is a schematic cross-sectional side view of a device for supplying separate waveguides in mode H ₁₁ or E ₀₁ , or both waveguide supplies utilizing separate supply points. FIG. 6 is a schematic cross-sectional bottom view of a device for supplying separate waveguides in mode H ₁₁ or E ₀₁ , or both waveguide supplies utilizing separate supply points. FIG. 4 is a schematic cross-sectional side view of a device for mode H ₀₁ or E ₀₁ waveguide delivery with a separate feed point. FIG. 8b is a schematic diagram of an antenna device provided in the device of FIG. 8a. FIG. 3 is a schematic cross-sectional side view of a device for waveguide delivery in modes H ₁₁ and H ₀₁ with separate delivery points. FIG. 8b is a schematic diagram of an antenna device provided in the device of FIG. 8a. FIG. 9b is a schematic diagram of a combined network for supplying to the antenna device of FIG. 9b. The group velocity in vacuum as a function of the wave number in the H ₁₁ mode of microwave radiation in a gas-filled waveguide with different dielectric constants and different thicknesses on the inner wall. It is the schematic of the normalization inversion group velocity. Different has a dielectric constant, a dielectric layer having a thickness different on the inner wall, as a function of wavenumber in the E ₀₁ mode of microwave radiation within the waveguide filled with gas, the group velocity in vacuum It is the schematic of the normalization inversion group velocity. Different has a dielectric constant, a dielectric layer having a thickness different on the inner wall, as a function of wavenumber in the H ₀₁ mode of microwave radiation within the waveguide filled with gas, the group velocity in vacuum It is the schematic of the normalization inversion group velocity.

Claims (23)

A device for measuring the liquid level in a container in which a gas having a dielectric constant within a predetermined dielectric constant range is present above the liquid level,
A pipe (1) having a pipe wall arranged in the container is provided with a plurality of holes (2) in the pipe wall, and the liquid flows through the holes (2), so that the pipe (2 ) A tube (1) adapted to keep the liquid level inside and outside equal,
A transmitter (4, 5, 9) for transmitting a microwave signal in the first propagation mode through the gas in the tube (1) in the direction of the liquid level;
Receivers (4, 5, 9) for receiving microwave signals reflected at the liquid level and propagating back through the tube (1);
In an apparatus for measuring a liquid level in a container, comprising: a signal processing device for calculating the liquid level of the liquid in the container from the propagation time of the transmitted and reflected microwave signal ,
The transmitter (4, 5, 9) is configured to transmit the microwave in a frequency band such that a group velocity of the microwave signal in the first propagation mode is slightly dependent on the dielectric constant within the predetermined dielectric constant range. A wave signal is transmitted into the tube (1), the frequency band comprising a frequency deviated by less than 7% from an optimal frequency f _opt , wherein the optimal frequency is:

Where f _c0 is the cutoff frequency of the first propagation mode in the tube, and ε is the dielectric constant at the center of the dielectric constant range. The frequency is less than 5% from the optimum frequency f _opt, more preferably less than 3%, more preferably less than 2%, and most preferred device according to claim 1, shifted by less than 1% in. 3. An apparatus according to claim 1 or 2, wherein the frequency band has a center frequency deviated by less than 7% from the optimum frequency _fopt . The apparatus according to claim 1, wherein the predetermined dielectric constant range is 1 to 1.03 . The apparatus according to any one of claims 1 to 4, wherein the liquid is composed of a condensed gas, the gas is composed of the condensed gas in a gas phase, and the condensed gas is stored in the container with overpressure. Claim is either the transmitter (4, 5, 9) said tube (1) the first propagation mode which is adapted to transmit the microwave signal in the _{H 11, H 01,} and _{H 02} 5. The apparatus according to 5 . The transmitter is adapted to transmit the microwave signal through the gas in the second propagation mode in the tube (1) in the direction of the liquid level;
The receiver (4, 5, 9) receives the microwave signal reflected from the surface of the liquid in the second propagation mode, re-propagates through the tube, and the first and second propagations. 7. A device according to any one of the preceding claims, adapted to distinguish portions of the microwave signal received in another one of the modes . The tube (1) has a circular cross-section, Apparatus according to claim 7 is any one of the second propagation mode E ₀₁ or H _11. 8. The receiver is adapted to distinguish a portion of the microwave signal received in one of the first and second propagation modes based on different arrival times of the portion to the receiver. Or the apparatus of 8 . The apparatus according to claim 7 or 8, wherein the transmitter transmits the microwave signal after being spectrally separated in the first and second propagation modes . The signal processing device is configured to allow the dielectric constant of the gas above the liquid level based on the received and differentiated portion of the microwave signal received in one of the first and second propagation modes. Device according to any of claims 7 to 10, adapted to calculate . The signal processing device is adapted to calculate a cross-sectional dimension of the tube based on the received and differentiated portion of the microwave signal received in one of the first and second propagation modes. An apparatus according to any one of claims 7 to 10 . The signal processing device is adapted to calculate a liquid level in the container from a propagation time of the transmitted and reflected microwave signal in the second propagation mode;
The signal processing device evaluates one or more characteristics of the tube or the environment in the container based on the liquid level of the liquid in the container, preferably the liquid in the container. 13. Apparatus according to any of claims 7 to 12, adapted to utilize the evaluation of the one or more characteristics to calculate a corrected liquid level . The signal processing device is adapted to calculate an attenuation of the differentiated portion of the microwave signal received in one of the first and second propagation modes;
The signal processing device evaluates one or more characteristics of the environment within the tube or container based on the calculated attenuation of the differentiated portion of the microwave signal, preferably the container 12. Apparatus according to any one of claims 7 to 11 adapted to utilize the evaluation of the one or more characteristics to calculate a modified liquid level of the liquid within . 15. An apparatus according to any preceding claim, wherein the microwave signal is a frequency modulated continuous wave (FMCW) signal . The apparatus according to claim 1, wherein the microwave signal is a pulse radar signal . According to a method for measuring the liquid level of a liquid in a container, a gas having a dielectric constant within a predetermined dielectric constant range is present above the liquid level of the liquid in the container, and a pipe is provided in the container. A tube having a wall is provided, and a plurality of holes are provided in the tube wall so that the liquid flows through the holes and the liquid level of the liquid inside and outside the tube is maintained at the same level. A method for measuring a liquid level in a container, comprising:
Determining a first quantity representing an internal dimension of the tube;
Based on the first quantity, in a frequency band in which the group velocity of the microwave signal of the first propagation mode in the tube is only slightly dependent on the dielectric constant within the predetermined dielectric constant range, The frequency band includes frequencies that deviate by less than 7% from the optimal frequency f _opt , the optimal frequency being

Where f _c0 is a cutoff frequency of the first propagation mode in the tube, and ε is a dielectric constant at the center of the dielectric constant range, and determining a frequency band;
Tuning the transmitter to operate in the frequency band;
Transmitting a microwave signal in the tube in the first propagation mode through the gas in the frequency band toward the surface of the liquid;
Receiving in the frequency band a microwave signal that is reflected off the surface of the liquid and propagates back through the tube;
Determining a second quantity representative of the propagation time of the transmitted and reflected microwave signal;
Calculating the liquid level in the container based on the first and second quantities;
A method comprising the steps of: The frequency _deviates from the optimum frequency f _{opt by} less than 7%, more preferably less than 5%, even more preferably less than 3%, even more preferably less than 2% and most preferably less than 1%. 18. The method of claim 17, wherein the frequency is the same as the optimal frequency f _opt . The frequency band is centered less than 7%, preferably less than 5%, more preferably less than 3%, more preferably less than 2%, and most preferably less than 1% from the optimum frequency f _opt. 19. A method according to claim 17 or 18 having a frequency . The method according to any one of claims 17 to 19, wherein the predetermined dielectric constant range is 1 to 1.03 . 21. A method according to any one of claims 17 to 20, wherein the liquid comprises a condensed gas, the gas comprises the vapor-phase condensed gas, and the condensed gas is stored in the container with overpressure . Is either the transmitter (4, 5, 9) is adapted first propagation mode to transmit the microwave signal in said tube (1) in the _{H 11, H 01,} and _{H 02} A method according to any of claims 17 to 21 . The microwave signal is transmitted through the gas in the second propagation mode in the tube (1) in the direction of the liquid level;
Receiving the microwave signal reflected from the surface of the liquid and re-propagating in the tube in the second propagation mode;
23. A method according to any one of claims 17 to 22, wherein portions of the microwave signal received in one of the first and second propagation modes are distinguished .

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