MASS FLOW METER ACCORDING TO THE CORIOLIS PRINCIPLE AND METHOD FOR DETERMINING A MASS FLOW

Abstract

The present disclosure relates to a Coriolis mass flow meter including: a measuring tube; an one exciter for generating vibrations in the measuring tube; two sensors for detecting the vibrations in the measuring tube and for outputting associated sensor signals; and an operating and evaluating unit for determining a mass flow value of a medium in the measuring tube based on a phase difference or time difference between the sensor signals, wherein for Reynolds numbers below a Reynolds number threshold a cross-sensitivity to a viscosity of the medium correlates with a Stokes number, wherein the operating and evaluating unit is configured to determine a current value of the Stokes number for Reynolds numbers below the lower Reynolds number threshold and to compensate for the influence of the cross-sensitivity in the determining of the mass flow.

Claims

1-13. (canceled)

14. A mass flow meter operating according to the Coriolis principle, the mass flow meter comprising: a measuring tube having a diameter and configured to guide a medium flowing through the measuring tube at a flow rate to be measured; an exciter configured to generate vibrations in the measuring tube; at least two sensors configured to detect the vibrations in the measuring tube and to output first and second sensor signals, respectively, that depend upon the vibrations; and an operating and evaluating unit configured to drive the exciter, to detect the first and second sensor signals and to determine a mass flow value based on a phase difference or a time difference between the first and second sensor signals, wherein the phase difference or the time difference is proportional to the mass flow on a first approximation, wherein the vibrations of the measuring tube have a cross-sensitivity to a viscosity of the medium, wherein for Reynolds numbers at least below a lower Reynolds number threshold, the cross-sensitivity correlates with a Stokes number, and wherein the operating and evaluating unit is configured to determine, at least for Reynolds numbers below the lower Reynolds number threshold, a current value of the Stokes number as a function of: the diameter of the measuring tube; a characteristic oscillation frequency of the measuring tube; and the viscosity of the medium flowing in the medium, wherein the Stokes number is a gauge for a depth of penetration of the vibrations from the measuring tube into the medium, and wherein the operating and evaluating unit is further configured to compensate for an influence of the cross-sensitivity as a function of the current value of the Stokes number in determining the mass flow value, wherein the vibrations of the measuring tube, which have the cross-sensitivity correlating with the Stokes number, includes the phase difference or time difference of the first and second sensor signals.

15. The mass flow meter of claim 14, wherein the operating and evaluating unit is further configured to determine the Stokes number as a function of a kinematic viscosity of the medium flowing in the measuring tube.

16. The mass flow meter of claim 15, wherein the operating and evaluating unit is configured to determine the Stokes number as a monotonic function of the kinematic viscosity of the medium flowing in the measuring tube.

17. The mass flow meter of claim 14, wherein the characteristic oscillation frequency is equal to a current excitation frequency.

18. The mass flow meter of claim 17, wherein the characteristic oscillation frequency is a natural frequency of a measuring tube oscillation for a medium having a characteristic density, wherein the characteristic density is selected from a current density of the medium in the measuring tube, a density of the medium in the measuring tube averaged over a temperature range and/or a pressure range, or a reference density.

19. The mass flow meter of claim 14, wherein the characteristic oscillation frequency is equal to a flexural vibration useful mode.

20. The mass flow meter of claim 14, wherein the operating and evaluating unit is configured to determine a viscosity measured value for the medium flowing in the measuring tube based on a damping of a measuring tube oscillation.

21. The mass flow meter of claim 20, wherein the operating and evaluating unit is configured to determine a viscosity measured value for the medium flowing in the measuring tube based on a ratio between a signal representing an exciter power and a signal representing an oscillation amplitude.

22. The mass flow meter of claim 14, wherein the operating and evaluating unit is configured to compensate for the influence of the cross-sensitivity using a polynomial, a linear function, a logarithmic function, or another monotonic function of the Stokes number.

23. A method for determining a mass flow rate with a mass flow meter operating according to the Coriolis principle, the method comprising: providing a mass flow meter comprising: a measuring tube having a diameter and configured to guide a medium flowing through the measuring tube at a mass flow rate to be measured; an exciter configured to generate vibrations in the measuring tube; at least two sensors configured to detect the vibrations in the measuring tube and to output first and second sensor signals, respectively, that depend upon the vibrations; and an operating and evaluating unit configured to drive the exciter, to detect the first and second sensor signals and to determine a mass flow value based on a phase difference or a time difference between the first and second sensor signals, wherein the phase difference or the time difference is proportional to the mass flow on a first approximation, wherein the vibrations of the measuring tube have a cross-sensitivity to a viscosity of the medium, wherein for Reynolds numbers at least below a lower Reynolds number threshold, the cross-sensitivity correlates with a Stokes number; driving the exciter to generate the vibrations in the measuring tube; detecting the first and second sensor signals; determining the phase difference or time difference between the first and second sensor signals; and determining the mass flow value based on the phase difference or the time difference, which is, on a first approximation, proportional to the mass flow, wherein, at least for Reynolds numbers below the lower Reynolds number threshold, a current value of the Stokes number is determined as a function of the diameter of the measuring tube, a characteristic oscillation frequency of the measuring tube, and a viscosity of the medium flowing in the measuring tube, wherein the Stokes number is a gauge for a depth of penetration of the oscillating vibrations from the measuring tube into the medium, and wherein the influence of the cross-sensitivity is compensated for as a function of the current value of the Stokes number in determining the mass flow, wherein the vibration behavior of the measuring tube, which has the cross-sensitivity correlating with the Stokes number, includes the phase difference or time difference of the first and second sensor signals.

24. The method of claim 23, wherein the Stokes number is determined as a monotonic function of a kinematic viscosity of the medium.

25. The method of claim 23, wherein the characteristic oscillation frequency is equal to a current excitation frequency.

26. The method of claim 23, wherein the characteristic oscillation frequency is a natural frequency of a measuring tube oscillation for a medium having a characteristic density, wherein the characteristic density is selected from a current density of the medium in the measuring tube, a density of the medium in the measuring tube averaged over a temperature range and/or a pressure range, or a reference density.

27. The method of claim 23, wherein a viscosity measured value for the medium flowing in the measuring tube is determined based on a damping of a measuring tube oscillation.

28. The method of claim 27, wherein a viscosity measured value for the medium flowing in the measuring tube is determined based on a ratio between a signal representing an exciter current and a signal representing an oscillation amplitude.

29. The method of claim 23, wherein the influence of the cross-sensitivity is compensated for by a polynomial, a linear function, a logarithmic function, or another monotonic function of the Stokes number.

30. The method of claim 23, wherein the medium flowing in the measuring tube is a homogeneous medium free of solids or free bubbles, which are accelerated by the measuring tube vibrations relative to a liquid phase of the medium.

Description

[0034] The invention will now be disclosed on the basis of the exemplary embodiments shown in the drawings. The following are shown:

[0035] FIG. 1a: a sketch of vibrations of plane-parallel plates, between which a medium is located;

[0036] FIG. 1b: a diagram showing a depth of penetration of vibrations into a medium as a function of the kinematic viscosity;

[0037] FIG. 1c: a sketch of shear movements in a measuring tube;

[0038] FIG. 2: a diagram showing a relative mass flow measurement error for various flow velocities above the Reynolds number;

[0039] FIG. 3: a diagram showing a relative mass flow measurement error for various flow velocities as a function of the Stokes number;

[0040] FIG. 4a: a representation of an exemplary embodiment of a mass flow meter according to the invention;

[0041] FIG. 4b: a detailed view of the exemplary embodiment from FIG. 4a;

[0042] FIG. 5a: a flow chart of an exemplary embodiment of a method according to the invention;

[0043] FIG. 6a: a flow chart of a first detail of an exemplary embodiment of a method according to the invention;

[0044] FIG. 6b: a flow chart of a second detail of an exemplary embodiment of a method according to the invention; and

[0045] FIG. 6c: a flow chart of a second detail of an exemplary embodiment of a method according to the invention.

[0046] In order to motivate the problem underlying the invention and the solution approach according to the invention, reference is initially made to FIGS. 1a through 1c. FIG. 1a shows two plane-parallel plates P that enclose a non-flowing medium M and carry out vibrations with a maximum velocity V.sub.max in opposite phase to each other at a constant distance. As a function of the viscosity of the medium, the vibrations are transmitted into the medium at different distances, as shown in FIG. 1b for a plate distance of a few centimeters and a maximum velocity of the vibrations on the order of 1 m/s. The solid line shows the velocity distribution for a medium with a kinematic viscosity of approximately 1 cSt. Here, only a thin boundary layer of the medium is moved. The dotted line and the dot-dashed line apply to kinematic viscosities of approximately 100 cSt and approximately 1,000 cSt respectively. It can be clearly seen how, with increasing viscosity, the vibrations penetrate further and further into the medium as shear oscillations, and thus release vibration energy to the medium, thereby damping the oscillations of the plates. A gauge of a depth of penetration of the vibrations is a Stokes number St, which is given, for example, as

[00001]$\mathrm{St}=\frac{\sqrt{v/f}}{D}$

[0047] wherein v is the kinematic viscosity of the medium, f is the frequency of the vibrations, and D indicates the plate spacing.

[0048] FIG. 1c schematically shows a measuring tube of a mass flow meter, which is excited in the X-direction to flexural vibrations in the so-called f.sub.1 mode or useful mode. The arrows at .sub.in and .sub.out indicate, by way of example, a resulting shear rate distribution as it might occur in a medium contained in the measuring tube. In the case of a stationary medium, this leads only to a damping of the symmetrical flexural vibration useful mode. However, if the medium flows, the antisymmetric f.sub.2 mode or Coriolis mode is excited out of resonance at the useful frequency f.sub.1. The superposition of the antisymmetric with the symmetrical flexural vibration useful mode leads to symmetry breaks in the velocity distribution in the medium, and thus to cross-sensitivities of the vibration behavior that go beyond uniform damping, in particular, to a cross-sensitivity of the phase relationship between typical vibration sensor signals of a mass flow meter according to the Coriolis principle, on the basis of which the mass flow is determined. While this effect may be negligible for larger Reynolds numbers, such as Re>1,000, it is significant for smaller Reynolds numbers, such as Re<100, and can cause relative measurement errors of more than 1%.

[0049] FIG. 2 shows typical relative measurement errors {dot over (m)}/{dot over (m)} above the Reynolds number Re for a mass flow meter neglecting the above effect. The crosses represent data for a first flow velocity of media in the measuring tube. The triangles represent data for a second flow velocity of media in the measuring tube that is four times the first flow velocity. The circles represent data for a third flow velocity of media in the measuring tube that is four times the second flow velocity. The first flow velocity is less than 1 m/s, while the second flow velocity is greater than 1 m/s. The Reynolds number, for this effect, is unsuitable as a basis for correction.

[0050] FIG. 3 shows typical relative mass flow measurement errors {dot over (m)}/{dot over (m)} above the Stokes number for the flow velocities from FIG. 2, wherein the symbols have corresponding meanings with respect to the velocities. The relative measurement error increases monotonically with the Stokes number, and the correlation between the relative measurement error and the Stokes number appears essentially independent of the flow velocity or Reynolds number of the medium. Thus, an approach for compensating for the cross-sensitivity of the vibrations has been found. The method according to the invention is further explained below.

[0051] The exemplary embodiment of a mass flow meter 1 according to the invention shown in FIGS. 4a and 4b includes a first connecting flange 2 and a second connecting flange 3 for installation in a pipeline. Each of the flanges 2, 3 has a collector, which are connected to each other by a rigid carrier pipe 6. From the carrier pipe 6 extends to the side a meter housing 10 in which two curved measuring tubes 21, 22 run, the ends of which are combined on both sides in one collector. The carrier pipe 6 has two lateral openings 61, 62, through which the measuring tubes 21, 22 are guided out of the carrier pipe 6 and back in again. The measuring tubes 21, 22 are coupled near the two collectors, each with some node plates 217a, 217b, 217c, 218a, 218b, 218c, which determine the vibration behavior of the pair of measuring tubes. To bring about flexural vibrations, an electrodynamic exciter 30 is arranged centrally between the two measuring tubes 21, 22 in the area of vertex arcs 41, 42 of the measuring tubes. Between the vertex arcs 41, 42 and the next node plates 217a, 218a respectively, a first electrodynamic vibration sensor 31 and a second electrodynamic vibration sensor 32 are arranged, symmetrically to the vertex arcs, between the two measuring tubes 21, 22, in order to detect the measuring tube vibrations. The carrier pipe 6 has, in a mounting surface 63, an electrical feedthrough 64, through which signal lines (not shown here) are guided. A connection adapter 8 with a transducer 9, which contains an operating and evaluating circuit 90, is connected to the mounting surface 63. The operating and evaluating circuit 90 is connected via the signal lines to the exciter 30 and the vibration sensors 31, in order to drive the exciter 30 and to detect and evaluate the signals of the latter, in order to determine a mass flow measured value by means of the method according to the invention, which can be output via signal and supply lines of the transducer 9.

[0052] As shown in FIG. 5a, in a preferred exemplary embodiment of the method 100 according to the invention, in a first step 110, a preliminary mass flow measurement value is first generated, which can include the compensation for other interferences if necessary.

[0053] In a second step 120, a test is performed to determine whether a critical Reynold number Re.sub.crit of, for example, Re.sub.crit=100 has been undershot.

[0054] If yes, in a third step 130, compensation for the cross-sensitivity to the Stokes number then takes place; otherwise, the method starts again with the first step 110.

[0055] The first step 110 can contain, for example, the substeps shown in FIG. 5b. The first step begins with driving the exciter 111 to bring about vibrations. The sensor signals 112 of the two vibration sensors are then detected. This is followed by determining a phase difference or time difference 113 between the sensor signals. On the basis of the determined phase difference or time difference, a preliminary mass flow measurement value 114, which, to a first approximation, is proportional to the phase difference or time difference, is finally determined.

[0056] The second step 120 requires (if not yet available) the determination of the current Reynolds number. For this purpose, first, a current viscosity measured value 121 is determined from the ratio of an excitation current signal with which the exciter is fed and the amplitude of the signals of the vibration sensors. A current value Re of the Reynolds number 122 based upon the provisional mass flow measurement value m, and the viscosity measurement value q according to Re={dot over (m)}/(R), where R is the radius of a measuring tube, are then calculated. This is followed by the check 123 of whether the current value of the Reynolds number Re falls below the critical Reynolds number Re.sub.crit.

[0057] The third step 130 comprises in detail, as shown in FIG. 5d, determining a current natural frequency value f of a flexural vibration mode 131, determining a density measurement value of the medium 132 contained in the measuring tube from the natural frequency value, calculating a value for kinematic viscosity v 133 on the basis of the viscosity measurement value and of the density measurement value according to v=/. This is followed by the calculation of the Stokes number St 134 according to

[00002]$\mathrm{St}=\frac{\sqrt{v/f}}{R}.$

[0058] In an optional step 135, there can be a check of whether the Stokes number St exceeds at least one of a critical value St.sub.crit of, for example, St.sub.crit=0.05. If not, compensation may be dispensed with, because its effect is negligible.

[0059] The measurement error 136, which depends upon the Stokes number, is determined according to:

[00003]$\frac{.Math..Math.{\stackrel{.}{m}}_{\mathrm{St}}}{\stackrel{.}{m}}=C_1.Math.\mathrm{St}+C_0,$

[0060] where C.sub.1 and C.sub.2 are device-specific constants.

[0061] The method concludes with the correction 137 of the provisional mass flow measurement value by the measurement error {dot over (m)}.sub.St, which depends upon the Stokes number.