Leakage gauge with at least one measuring tube, as well as a method for operating and a method for adjusting such a leakage gauge.

Description

[0001] The present invention relates to a density measuring instrument with at least one measuring tube, as well as a method for operating and a method for adjusting such a density measuring instrument, wherein the density measuring instrument serves to determine a density measurement value of a medium, in particular a liquid, guided in the at least one measuring tube. Such density measuring instruments exhibit excellent measuring accuracy under ideal conditions with a homogeneous, single-phase medium and a measuring tube in its original condition. On the other hand, deviations from these ideal conditions, such as a multi-phase medium, fouling, or material loss from the measuring tube due to abrasion or corrosion, can impair the measuring accuracy. This fact is known per se, and models are known for each of these deviations to correct the associated measurement errors or at least to warn that the measuring accuracy is impaired.

[0002] To compensate for the influence of the so-called resonator effect in the density measurement of media containing gas-laden liquids, multi-frequency technology can be used, for example. In this process, the first symmetric bending vibration mode, the so-called f1 mode, and the second symmetric bending vibration mode, the so-called f3 mode, are excited in order to determine the speed of sound in the medium and a density measurement corrected for the influence of the gas loading, based on their two corresponding natural frequencies, as described in DE 10 2015 122 661 A1. If the f3 mode can no longer be excited due to the medium's resonance frequency being too close to the natural frequency of the f3 mode, the first antisymmetric vibration mode (f2 mode) can be used, whose natural frequency lies between the natural frequencies of the f1 and f3 modes.An advantageous design for a vibration exciter is described in the still unpublished application DE 10 2020 129 999.8.

[0003] To detect a coating, for example, the damping of a vibration mode can be evaluated, as described in DE 10 2018 101 923 A1. However, this only works reliably if gas loading of a liquid medium can be ruled out. As soon as gas loading is present, the method described there can no longer reliably differentiate between gas loading and coating.

[0004] By monitoring the vibration amplitude of the fundamental bending mode (f1 mode) outside of resonance, a change in modal bending stiffness can be determined, thus allowing the detection of the effects of abrasion or corrosion, as described, for example, in the still unpublished application DE 10 2019 124 709.8 and in WO 2012 / 062 551 A1. However, this procedure is very complex, as amplitudes outside of resonance are very difficult to measure. Furthermore, amplitude measurement exhibits cross-sensitivities to influencing factors in the measurement section that are unrelated to the modal bending stiffness.

[0005] The as yet unpublished application DE 10 2020 111 127.4 describes a method for updating a calibration factor calf, which depends on a modal bending stiffness of the f2 mode, by monitoring this bending stiffness using amplitude measurement, and for classifying damage to the measuring tube based on a relationship between changes in the calibration factor and the modal bending stiffness of the f1 mode. However, as mentioned previously, the informative value of amplitude measurement is limited by cross-sensitivities to other influencing factors in the measuring section.

[0006] WO2020 / 239319 describes a measuring system for measuring at least one measured quantity, for example a flow parameter such as a mass flow rate or a volume flow rate, or a material parameter such as a density or a viscosity, of a flowing fluid.

[0007] The discussed state of the art makes valuable contributions to the operation of Coriolis mass flow meters and their use as density meters. However, it essentially addresses only partial aspects under the assumption of specific media properties. The object of the present invention is to provide a more robust method for operating a density meter.

[0008] The problem is solved according to the invention by the method according to claim 1 for operating a density measuring device, the method according to claim 13 for adjusting a density measuring device and the density measuring device according to claim 14.

[0009] The method according to the invention serves to operate a density measuring device, which comprises at least one vibrating measuring tube for guiding a medium and an exciter arranged on the measuring tube for exciting vibration modes of the at least one measuring tube, at least one vibration sensor and a carrier body, wherein the measuring tube has inlet-side and outlet-side end sections which are connected to the carrier body, wherein the method comprises the following steps: Excitation of at least three vibration modes of the measuring tube; determination of natural frequency measurements of the at least three vibration modes; determination of a density measurement of the medium, taking into account any gas loading that may be present; and determination of a characteristic property of the at least three measuring tubes based on the natural frequencies of the three vibration modes.

[0010] In a further development of the invention, the characteristic property of the measuring tube comprises an effective wall thickness value or an effective material removal value of the measuring tube or an effective mass coating value for a mass coating on the wall of the measuring tube.

[0011] In a further development of the invention, the effective mass coating value comprises a coating thickness value, which is determined as a function of a density value (ρ B ) for a material of the mass coating.

[0012] In a further development of the invention, the method further comprises the acquisition of at least one parameter measurement value, of at least one parameter to which at least one of the natural frequencies has a cross-sensitivity.

[0013] In a further development of the invention, the at least one parameter is selected from a list which includes a temperature of the at least one measuring tube, a temperature of a carrier body, and a pressure of the medium guided in the measuring tube.

[0014] The support body can comprise a body, in particular a metallic one, by which the at least one measuring tube is held in an inlet-side section and an outlet-side section, in particular to suppress or minimize relative movements between the end sections of the measuring tube.

[0015] In a further development of the invention, the at least three vibration modes are selected from a list comprising the first symmetric vibration mode (f1 mode), the first antisymmetric vibration mode (f2 mode), the second symmetric vibration mode (f3 mode), the second antisymmetric vibration mode (f4 mode), the third symmetric vibration mode (f5 mode), and the third antisymmetric vibration mode (f6 mode). Currently, it is preferred to select the modes with the lowest modal bending stiffnesses. These are, in most cases, the f1 mode, the f2 mode, and the f3 mode.

[0016] In a further development of the invention, the excitation of bending vibration modes is achieved by scanning suitable frequency ranges and adjusting the phase relationship between the vibration amplitude and the excitation signal. The exciter can, in particular, be a symmetrically arranged exciter (except for manufacturing tolerances), with which all symmetrical bending vibration modes can be efficiently excited. However, antisymmetrical vibration modes cannot be reliably excited with this method in homogeneous media within the measuring tube. If, however, a medium with inhomogeneities is present in the measuring tube, for example due to gas content or a solid phase, a symmetry breaking in the mass distribution can occur, making excitation of the antisymmetrical modes, especially the f² mode, possible.The excitation of bending vibration modes can also be achieved with a slightly eccentrically arranged exciter, as described, for example, in the still unpublished patent applications with the file numbers DE 10 2020 111 127.4 and DE 10 2020 123 999.8. Furthermore, it is possible to apply an antisymmetric excitation signal to electrodynamic vibration sensors that are arranged symmetrically to each other with respect to the center of the measuring tube in the longitudinal direction of the measuring tube, in order to excite the antisymmetric modes.

[0017] In a further development of the invention, the method comprises solving a system of equations consisting of at least three equations of the type: ρ M = K c , ​ ​ f n A n + B ′ n f n where ρ M is the media density, K describes a correction function for the resonator effect, and c is the speed of sound of the medium guided in the pipe.

[0018] The resonator effect describes the effect of a forced vibration of the medium against the measuring tube, which shifts the observed natural frequency fn of a bending vibration mode of the measuring tube. The strength of the resonator effect depends in particular on the ratio of the considered natural frequency of the measuring tube and the resonance frequency of the medium, the latter being proportional to the speed of sound in the medium, which decreases with increasing gas loading of the medium. The resonance frequency of the medium is usually many times higher than the observed natural frequency of the measuring tube. In this case, the natural frequency is lower than it would be without the resonator effect. Without correction for the resonator effect, an excessively high density reading would be obtained. In these cases, according to a further development of the invention, the correction function K for the resonator effect is K ≤ 1, where the value 1 is approached with increasing speed of sound in the medium.The correction function K approaches zero when the resonant frequency of the medium approaches the natural frequency of the vibration mode under consideration. In this case, however, excitation of the vibration mode is not possible.

[0019] In equation I, A n is a mode-specific function that depends on a characteristic property of the measuring tube and is independent of the respective natural frequency of a bending vibration mode. Furthermore, in equation IB' n is a mode-specific function that depends on at least one characteristic property of the measuring tube and the respective natural frequency fn of a bending vibration mode, where B' n in particular comprises a quotient of a mode-specific, frequency-independent function B n and the square of the respective natural frequency fn.

[0020] According to a further embodiment, the at least one characteristic property comprises an effective mass coating or coating thickness b and / or an effective material removal a of the pipe wall. According to a further embodiment of the invention, a known value, in particular zero, is assigned to one of the two characteristic quantities.

[0021] In a further development of the invention, the functions A n and B' n or B n have linear functions of the characteristic quantities.

[0022] In a further development of the invention, the function A n includes a term that depends on the arrangement of at least one mass element on the measuring tube, wherein the mass element particularly comprises the exciter. Since the exciter has a non-negligible mass and is usually positioned at the location of the largest displacement of the measuring tube for symmetrical vibration modes, which coincides with a node of the antisymmetric vibration modes, taking into account the mode-specific influence of the vibrations by the exciter mass leads to a significantly more accurate determination of the characteristic parameters of the measuring tube. The masses of the vibration sensors, on the other hand, can be neglected.

[0023] In a further development of the invention, at least one of the functions B' n or B n exhibits dependencies on parameters to which the density measurement has cross-sensitivities. According to a further development of the invention, these parameters are selected from a list that includes a media pressure, a media temperature, and an ambient temperature.

[0024] In a further development of the invention, the functions B' n and B n have linear functions of the parameters.

[0025] In a further development of the invention, the functions A n and B' n or B n have linear functions of mode-specific coefficients, the latter being obtained using the inventive method for adjusting a density measuring instrument, wherein adjustment measurements are determined with at least two media of different densities, wherein the density of the first of the media differs from the density of the second of the media by at least a factor of 100, in particular by at least a factor of 400 and preferably by at least a factor of 800 under normal conditions, wherein the media particularly comprise air and water, and wherein the natural frequencies of at least three vibration modes of the measuring tube were determined during the adjustment.

[0026] In a further development of the invention, the adjustment measurements are carried out at several values ​​of the parameters to which the density measurement has a cross-sensitivity.

[0027] The density measuring device according to the invention comprises: at least one oscillating measuring tube for guiding a medium; at least one support body, wherein the at least one measuring tube is mechanically coupled to the support tube at the inlet and outlet sides in order to suppress relative movements between an inlet-side end section and an outlet-side end section of the measuring tube; at least one exciter by which the at least one measuring tube can be excited to bending vibrations; at least one vibration sensor for detecting bending vibrations of the at least one measuring tube; and an operating and evaluation circuit for supplying the exciter with an excitation current and for acquiring and evaluating measurement signals from the at least one vibration sensor;wherein the operating and evaluation circuit is configured to carry out the inventive method for operating a density measuring device, wherein in particular the operating and evaluation circuit has a data storage device in which coefficients determinable with the method according to claim are stored.

[0028] In a further development of the invention, the measuring and operating circuit is configured to receive at least one parameter measurement value from at least one parameter to which the measuring tube vibrations have cross-sensitivities, wherein the at least one parameter is selected from a list which includes the pressure of the medium guided in the measuring tube, a temperature of the measuring tube and a temperature of the support body.

[0029] The invention will now be explained in more detail with reference to the embodiment shown in the drawings. It shows: Fig. 1a: a side view of an embodiment of a density measuring device according to the invention for carrying out the method according to the invention; Fig. 1b : a spatial representation of the embodiment of the density measuring device according to the invention made of Fig. 1a ; Fig. 1c : a schematic representation of a cross-section of a measuring tube of a density measuring device according to the invention with various characteristic parameters influencing the vibration behavior of the measuring tube; Fig. 2 : a schematic representation of the deflection curves of three vibration modes of a measuring tube; Fig. 3 : a flowchart of an embodiment of a method according to the invention for operating a density measuring device; and Fig. 4 : a flowchart of an embodiment of a method according to the invention for adjusting a density measuring device.

[0030] In the Figures 1a and 1bFigure 2 shows an embodiment of a density measuring device 2 according to the invention, which is configured to carry out the method according to the invention. The density measuring device 2 is also configured as a mass flow meter operating according to the Coriolis principle, in order to perform a mass flow measurement in addition to the density measurement. The density measuring device 2 has two oscillatively mounted measuring tubes A and B, each of which is arc-shaped and runs parallel to each other. The density measuring device 2 can be inserted into a pipeline (not shown) such that the fluid flowing in the pipeline flows through both measuring tubes A and B. On the inlet and outlet sides, the measuring tubes A and B are each held in flow dividers or collectors 4 and 6, the latter being rigidly connected to each other by a support tube T.The inlet and outlet end sections of the measuring tubes are thus coupled to the support tube T, effectively suppressing relative movements between these sections. An electrodynamic exciter 8 is arranged between the two measuring tubes A and B, causing them to bend relative to each other. A free oscillation length for each measuring tube A and B is defined by coupling elements 10 and 11, which mechanically couple the tubes at the inlet and outlet. Electrodynamic vibration sensors 14 and 16 are arranged between the two measuring tubes A and B, one at the inlet and one at the outlet. The density meter 2 further comprises an operating and evaluation circuit 18 for supplying the exciter 8 with an excitation current and for acquiring and evaluating measurement signals from the electrodynamic vibration sensors 14 and 16.The density measuring device 2 further comprises a first temperature sensor (not shown here), which is arranged, for example, on the first coupling element 10 to determine a first temperature measurement representative of the temperature of the measuring tubes A, B. Positioning the first temperature sensor on the coupling element 10 is appropriate because the coupling element is only connected to the measuring tubes A, B, so that the temperature of the coupling element is largely defined by the temperature of the measuring tubes. Likewise, the first temperature sensor can also be arranged on one of the measuring tubes, particularly outside the oscillating section defined by the coupling elements, which results in a shorter response time for the first temperature sensor.The density measuring device 2 further comprises a second temperature sensor (not shown here) which is arranged on the support tube T, in particular on its inner surface, to provide a second temperature measurement representative of the temperature of the support tube. Differences between the temperature of the measuring tubes and the temperature of the support tube cause axial stresses that influence the vibration behavior of the measuring tubes A and B. Therefore, the measuring and operating circuit 18 is configured to acquire measurement signals from the temperature sensors, which represent the first and second temperature measurements, and to take them into account in the determination of the media density according to the invention. Furthermore, the first temperature measurement is included in the calculation of the temperature-dependent modulus of elasticity.

[0031] To carry out the method according to the invention, it is advantageous if the measuring and operating circuit also has an input for a pressure measurement value p in order to be able to take the media pressure into account when carrying out the method according to the invention for operating the density measuring device.

[0032] Although Figs. 1a and 1b While an embodiment of a density measuring device with a pair of measuring tubes bent in their rest position is shown, the invention is equally applicable to density measuring devices with a single measuring tube or with several pairs of measuring tubes. Likewise, instead of the illustrated measuring tubes bent in their rest position with mirror symmetry about a transverse plane of the measuring tubes, S-shaped or straight measuring tubes can also be used to implement the invention.

[0033] Fig 1cFigure 1 shows a schematic cross-section through a measuring tube A of a density measuring device according to the invention. A measuring tube is generally made of a metallic material, in particular stainless steel or titanium. However, other materials are also suitable, for example, glass, ceramics, semiconductor materials such as silicon, or polymers. The measuring tube can have a straight or curved shape in its rest position. The measuring tube A has a substantially circular cross-section with an outer radius ra and an initial inner radius ri under reference conditions, the difference between the two radii defining a wall thickness w of the measuring tube. The initial area moment of inertia I 0 of the measuring tube cross-section in the delivered state of the measuring device is given by these two radii. I 0 = π 4 ⋅ r a 4 − r i 4

[0034] All modal stiffnesses of the measuring tube are proportional to the modulus of elasticity E and the area moment of inertia I. The former describes a temperature-dependent material property, and the latter can change if material a is removed from the wall of the measuring tube by abrasion or corrosion caused by the medium flowing through it. The remaining area moment of inertia I can then be described as: I = π 4 ⋅ r a 4 − r i + a 4

[0035] As material is removed, the cross-sectional area AM of the measuring tube decreases, which is given as: A M = π ⋅ r a 2 − r i + a 2

[0036] The mass, and therefore the inertial forces, of the measuring tube are proportional to its cross-sectional area, as is any axial force acting on the measuring tube. These quantities therefore vary depending on the amount of material removed (a).

[0037] Depending on the medium, a deposit with a layer thickness b can form on the measuring tube wall. The mass of this deposit is proportional to the deposit cross-sectional area A b , which is given by: A b = π ⋅ r i + a 2 − r i + a − b 2

[0038] To the extent that the coating has a different density than the medium, it can distort the density measurement.

[0039] The mass of the medium, or the mass on which the density measurement is based, is proportional to the clear pipe cross-section or the fluid cross-sectional area A f : A f = π ⋅ r i + a − b 2

[0040] Although the equations above, for the sake of general validity, include both a contribution a for material removal and a contribution b for deposit formation, in practice, i.e., at a specific measuring point, only one of the phenomena will usually occur: either material removal or deposit formation. By selecting the phenomenon, for example during commissioning, the irrelevant contribution can be set to zero.

[0041] Assuming that the wall thickness w, the material removal a and the coating formation b are small in relation to the radius, the following approximations result.

[0042] For the area moment of inertia: I = π ⋅ r 3 ⋅ w ⋅ 1 − a w

[0043] Here, r is the inner radius of the measuring tube and w is its wall thickness.

[0044] For the measuring tube cross-section: A M = 2 ⋅ π ⋅ r ⋅ w ⋅ 1 − a w

[0045] For the fluid cross-sectional area: A f = π ⋅ r 2 ⋅ 1 + 2 a r − 2 b r

[0046] For the cross-sectional area of ​​the pavement: A b = 2 ⋅ π ⋅ r ⋅ b

[0047] The terms above illustrate how geometric quantities relevant to the vibration behavior of a measuring tube change due to deposit formation and material removal.

[0048] Another aspect to consider when modeling an oscillator with an oscillating measuring tube concerns its components, as their mass influences the vibration characteristics depending on the mode. The exciter mass mE is particularly important here, since the exciter is usually positioned at an antinode for symmetrical vibration modes and at a node for antisymmetrical vibration modes. The inhomogeneous mass distribution along the measuring tube facilitates the identification of the various changes in the measuring tube and / or the medium, which would not be readily distinguishable with a homogeneous mass distribution. Since the mode-specific kinetic energy of the exciter contributes to the mode-specific kinetic energy of the oscillator, it is advantageous to consider this in the oscillator model. The mode-dependent ratio of the kinetic energies of the exciter mass and the measuring tube mass is given by: r c 0 n = m E ⋅ a n 2 0 ρ T ⋅ π ⋅ r ⋅ w ⋅ l

[0049] Here, I is the length of the measuring tube, ρT is the density of the measuring tube material, mE is the excitation mass, and an(0) is the mode-dependent displacement at the center of the measuring tube, where the displacement is normalized with a normalization divider. The mode-specific normalization divider is given as the square root of the integral dζ of the square of the displacement (an(ζ)) 2< , the mode-specific deflection curve from the center of the measuring tube to one end of the measuring tube, i.e., from ζ=0 to ζ=I / 2. Examples of the deflection curves of the first three bending vibration modes of a straight measuring tube are shown in Fig 2 depicted.

[0050] In addition to the above-mentioned properties of the oscillator, the properties of the medium must also be considered, in particular its compressibility due to gas loading. As is known, the density value ρM of a compressible medium can be obtained by multiplying a preliminary density value ρprel, determined under the assumption of an incompressible medium, by a correction factor K, i.e.: ρ M = ρ prel ⋅ K f 0 f n , where the correction factor K is given as: K f 0 f n = 1 K 0 + K 1 1 − f n f 0 2 , where fn is the natural frequency of the nth mode of oscillation of the oscillator, where fo is the resonance frequency of the medium oscillating against the measuring tube, where K 0 and K 1 are weighting factors with Ko = 0.163 and K 1 = 1 - Ko.

[0051] The resonance frequency fo of the medium is given to a first approximation as: f 0 = c ⋅ λ 2 π ⋅ r , where c is the speed of sound in the medium, and λ is an eigenvalue with the value λ = 1.842.

[0052] Thus, the correction factor K can also be specified as a function of the natural frequency of the nth oscillation mode of the oscillator and the speed of sound of the medium oscillating against the measuring tube, i.e.: K c f n = 1 K 0 + K 1 1 − f n c ⋅ λ 2 π ⋅ r 2

[0053] To determine the media density, the natural frequencies fn of at least three vibration modes, in particular f 1 , f 2 and f 3, must be excited, and the following system of equations must be solved: ρ M = K c f n ⋅ A n + B ′ n f n

[0054] The following applies: A n = c 0 n ⋅ 1 − 2 r + 1 w ⋅ a + 2 r ⋅ b + r c 0 n ⋅ 1 − 2 r ⋅ a + 2 r ⋅ b − ρ b ⋅ 2 r ⋅ b where ρ b denotes the coating density, where: B ′ n f n = B n ⋅ 1 f n 2 with: B n = c 1 n + c 2 n ⋅ T M + c 3 n ⋅ T T ⋅ 1 − 2 r + 1 w ⋅ a + 2 r ⋅ b + c 4 n ⋅ p where TM and TT are the temperatures of the measuring tube and a support body, respectively, with the ends of the measuring tube fixed to the support body, and where p denotes the pressure of the medium. These temperatures and the medium pressure are measured quantities. The radius r and the wall thickness w are the initial values ​​for the inner radius and wall thickness of the measuring tube. The density of the coating ρb is a medium-dependent material parameter that must be provided when the method is applied to a medium in which coating formation is expected. The coefficients c0n, c1n, c2n, c3n, and c4n are to be determined during an initial adjustment of the measuring instrument, as explained below. In the system of equations L2, in addition to the desired density value ρM, the speed of sound c, the material removal a, and the coating thickness b are unknown quantities, where a or b can be set to zero depending on the medium.

[0055] This leaves only three unknown quantities: firstly, the density value ρ M; secondly, the speed of sound c; and thirdly, either the material removal a or the coating thickness b, with which the quantities can be determined on the basis of the three natural frequencies fn.

[0056] If material loss is expected at a measuring point, it can now be precisely determined during ongoing measurement, even with media containing gas, while density measurement is in progress. The resulting change in the measuring tube due to material loss, and the associated decrease in pressure resistance, can be signaled if a limit value is exceeded, for example, by an alarm.

[0057] If the density meter is also designed as a Coriolis mass flow meter, the effect of material removal on a calibration factor calf for the mass flow measurement can be taken into account. This factor relates a mass flow rate to be determined to a measured time difference between the zero crossings of two vibration sensors. In conventional Coriolis mass flow meters, the excitation is the first symmetric bending vibration mode, the so-called f1 mode. In this case, the calibration factor depends in particular on the modal stiffness of the first antisymmetric bending vibration mode, the so-called f2 mode. Using the method according to the invention, a change in the calibration factor dependent on material removal can now be detected. In the simplest case, this can signal a decrease in measurement accuracy, or the calibration factor calf can be adjusted depending on the material removal.

[0058] If, on the other hand, deposit formation is expected, this can also be signaled when a limit value is exceeded, for example to initiate cleaning of the measuring tube. Cleaning can now also be carried out with the density meter installed, as the deposit thickness can be determined even during the cleaning process.

[0059] By determining the speed of sound, the invention thus provides the basis for determining a gas volume fraction in the medium, regardless of coating or material removal.

[0060] The steps of the inventive method 100 for operating a density measuring device in connection with Figures 1a and 1b The types described are summarized in Fig. 3As shown, in a first step, at least three vibration modes of the measuring tube are excited. This is followed by the determination of natural frequency measurements fn of the at least three vibration modes. Based on these natural frequency measurements fn, a density measurement ρ M of the medium can be determined (130), taking into account any gas loading that may be present; and a characteristic property a, b of the measuring tube can be determined based on the natural frequencies of the three vibration modes by solving the above system of equations L2.

[0061] The coefficients c0n, c1n, c2n, c3n, and c4n of the system of equations are to be determined during an initial adjustment of the measuring instrument by subjecting the density meter to air and water as the medium, recording the natural frequencies fn of at least three vibration modes at several values ​​for the temperatures TM and TT and the pressure p. Since neither material removal nor deposit formation occurs at this point, the system of equations L2 simplifies to: ρ M = K c f n ⋅ c 0 n ⋅ 1 + r c 0 n + c 1 n + c 2 n ⋅ T M + c 3 n ⋅ T T + c 4 n ⋅ p ⋅ 1 f n 2

[0062] Provided that the calibration measurements are performed with water at a sufficiently high operating pressure that significantly exceeds the respective vapor pressure, the correction factor K(c, fn) = 1 can be assumed. For calibration measurements with air, K(c, fn) = 1 can also be used as a general rule. In fact, assuming a constant density of 1.2 kg / m³ for air at standard pressure yields sufficiently accurate results.

[0063] For the density of water, temperature- and pressure-dependent reference values ​​must be used according to the required accuracy.

[0064] To calculate the mode-dependent ratio r c0n, the normalized vibration amplitude at (0) at the position of the exciter can be determined based on numerical modal analysis of the eigenmodes of the bending vibration modes in order to calculate the mode-dependent ratio r c0n according to: r c 0 n = m E ⋅ a n 2 0 ρ M ⋅ π ⋅ r ⋅ w ⋅ l .

[0065] This leaves only the four coefficients c 0n , c 1n , c 2n , c 3n and c 4n as unknowns, which are to be determined by solving the system of equations L6.

[0066] The adjustment method 200 according to the invention is in Fig. 4 The process begins with the setting (205) of a set of measurement conditions from a sequence of different sets of measurement conditions, wherein the measurement conditions comprise a medium density (ρ M ) and a set of values ​​of the parameters to which the measuring tube exhibits a cross-sensitivity. These parameters are, in particular, the pressure and temperature of the medium. This is followed by the excitation (210) of at least three vibration modes of the at least one measuring tube of the density measuring device and the determination (220) of the natural frequencies (fn ) of the vibration modes.

[0067] Exciting symmetrical vibration modes is straightforward with a symmetrically arranged exciter. To also excite antisymmetrical vibration modes, the exciter can be positioned slightly eccentrically, or an antisymmetrical excitation signal can be fed in via two symmetrically arranged vibration sensors.

[0068] The next step (225) is to check whether the measurement was performed under a sufficient number of sets of measurement conditions. If the check is unsuccessful, the preceding procedure steps are repeated with a new set of different sets of measurement conditions. If, on the other hand, a sufficient number of measurements were performed under different measurement conditions, the mode-specific coefficients (c0n, c1n, c2n, c3n, c4n) are calculated (230). These are stored in a data memory of the measurement and operating circuit of the density measuring device to enable the execution of the density measurement method according to the invention.

Claims

1. A method (100) for operating a density meter, which comprises at least one measuring tube that can oscillate for conducting a medium, and an exciter arranged on the measuring tube for initiating oscillation modes of the at least one measuring tube, and a support body, wherein the measuring tube has inlet-side and outlet-side end sections which are connected to the support body, wherein the method includes the following steps: Initiating (110) at least three oscillation modes of the measuring tube; Determining (120) natural frequency measured values (fn) of the at least three oscillation modes; Determining (130) a density measured value (ρM) of the medium, taking any gas load present into account; and determining a characteristic property (a, b, w) of the at least one measuring tube based on the natural frequencies of the three oscillation modes, wherein the characteristic property of the measuring tube comprises an effective wall thickness value (w) or an effective material removal value (a) of the wall of the measuring tube, or an effective mass coating value for a mass coating on the wall of the measuring tube.

2. The method as claimed in claim 1, wherein the effective mass coating value comprises a coating thickness value (b), which is determined as a function of a density value (ρB) for a material of the mass coating.

3. The method as claimed in one of the preceding claims, further comprising: Detecting at least one parameter measured value of at least one parameter to which at least one of the natural frequencies has a cross-sensitivity.

4. The method as claimed in claim 3, wherein the at least one parameter is selected from a list which comprises a temperature (TM) of the at least one measuring tube, a temperature (TT) of the support body, and a pressure (p) of the medium conducted in the measuring tube.

5. The method as claimed in one of the preceding claims, wherein the at least three oscillation modes are selected from a list which comprises the first symmetrical oscillation mode (f1 mode), the first antisymmetric oscillation mode (f2 mode), the second symmetrical oscillation mode (f3 mode), the second antisymmetric oscillation mode (f4 mode), the third symmetrical oscillation mode (f5 mode), and the third antisymmetric oscillation mode (f6 mode).

6. The method as claimed in claim 5, wherein the modes with the smallest modal bending stiffnesses are selected, in particular the f1 mode, the f2 mode and the f3 mode.

7. The method as claimed in one of the preceding claims, wherein the bending oscillation modes are initiated by scanning suitable frequency ranges and adjusting to a defined phase relation between the oscillation amplitude and the exciter signal.

8. The method as claimed in one of the preceding claims, wherein the determination of the density measured value involves solving a system of equations made up of at least three equations of the type: ρ M = K c f n A n + B ′ n f n wherein ρM is the media density, K describes a correction function for the resonator effect, and c is the sound velocity of the medium conducted in the tube, wherein An is a mode-specific function which depends on a characteristic property hi of the measuring tube, wherein An is independent of the respective natural frequency of a bending oscillation mode, and B'n is a mode-specific function which depends on the at least one characteristic property hi of the measuring tube and the respective natural frequency fn of a bending oscillation mode, wherein B'n in particular comprises a quotient of a mode-specific, non-frequency-dependent function Bn and the square of the respective natural frequency fn.

9. The method as claimed in claim 8, wherein the functions An and B'n or Bn have linear functions of at least one of the characteristic properties of material removal (a) or coating thickness (b).

10. The method as claimed in claim 8 or 9, wherein a specified value, in particular zero, is applied to one of the two characteristic properties of material removal (a) and coating thickness (b).

11. The method as claimed in one of claims 8 to 10, wherein the function An has a term which depends on an arrangement of at least one solid body on the measuring tube, wherein the solid body comprises in particular the exciter.

12. The method as claimed in claim 4 and one of claims 8 to 11, wherein the function Bn is dependent on at least one of the parameters to which the density measurement has cross-sensitivities, wherein the function Bn in particular has a linear function of at least one of the parameters.

13. A method (200) for adjusting a density meter with an operating and evaluation circuit (18), which is configured to carry out the method as claimed in one of claims 1 to 12, wherein the functions An and B'n or Bn comprise linear functions of mode-specific coefficients (C0n, C1n, C2n, C3n, C4n), wherein the mode-specific coefficients (C0n, C1n, C2n, C3n, C4n) are determined based on adjustment measurements with at least two media with different densities, wherein each adjustment measurement is performed with multiple values of the parameters to which the density measurement has a cross-sensitivity, and wherein the natural frequencies of at least three oscillation modes of the measuring tube are determined during the adjustment measurements, wherein the method (200) comprises: Configuring (205) a set of measuring conditions from a sequence of different sets of measuring conditions, wherein the measuring conditions comprise: A media density (ρM), and a set of values for the parameters to which the measuring tube has a cross-sensitivity; Initiating (210) at least three oscillation modes of the at least one measuring tube of the density meter; Determining (220) the natural frequencies (fn) of the oscillation modes; Checking (225) whether the measurement was performed with a sufficient number of sets of measuring conditions; Repeating the preceding process steps with the next set from the sequence of different sets of measuring conditions if the check is negative; and Calculating (230) the mode-specific coefficients (C0n, C1n, C2n, C3n, C4n) if the check is positive (225).

14. A density meter (2) comprising: At least one measuring tube (A, B) that can oscillate for conducting a medium; At least one support body (T), wherein the at least one measuring tube (A, B) is mechanically coupled with the support tube (T) on the inlet side and on the outlet side in order to suppress relative movements between an inlet-side end section and an outlet-side end section of the measuring tube; At least one exciter (8), which can be used to cause bending oscillations of the at least one measuring tube (A, B); At least one oscillation sensor (14, 16) for detecting bending oscillations of the at least one measuring tube (A, B); and An operating and evaluation circuit (18) for supplying the exciter (8) with an exciting current and for detecting and evaluating measurement signals from the at least one oscillation sensor (14, 16); wherein the operating and evaluation circuit (18) is configured to carry out the method as claimed in one of claims 1 to 12, wherein the operating and evaluation circuit (18) in particular has a memory in which coefficients which can be determined using the method as claimed in claim 13 are saved.

15. The density meter (2) as claimed in claim 14, wherein the measuring and operating circuit is configured to receive at least one parameter measured value of at least one parameter to which the measuring tube oscillations have cross-sensitivities, wherein the at least one parameter is selected from a list which comprises the pressure of the medium conducted in the measuring tube, a temperature of the measuring tube, and a temperature of the support body.