Coriolis mass flowmeter

Abstract: A Coriolis mass flowmeter includes a housing (8) which contains at least one measuring pipe (2), a supporting mechanism (13,14) connected to both ends of the measuring pipe (2), a vibration generator (5) which gives a vibration to the measuring pipe (2), at least two sensors (6a,6b) for detecting the vibration of the measuring pipe (2), an inlet conduit (7) which introduces a fluid to be measured into the measuring pipe (2) and an outlet conduit (8) which discharges the fluid out of the measuring pipe (2). A Coriolis vibration is applied to the measuring pipe (2) which generates a Coriolis force in a fluid flowing in the measuring pipe (2). The Coriolis force is utilized for the measurement of a flow rate of the fluid. The Coriolis vibration has a frequency higher than a principal natural frequency of the supporting mechanism (13,14), i.e., in an inertial control region. The mass flowmeter is of a simple structure and lightweight, minimizes leakage of the energy of vibration to the outside, and has a high mechanical Q value.

Description: The present invention relates to a mass flowmeter which measures mass flow utilizing Coriolis' force generated in proportion to the mass flow of a fluid flowing in at least one pipe to be measured which is vibrated and more particularly to a mass flowmeter which is lightweight and simplified in structure and which enables improvement in handiness, reliability, and precision of measurement. As is well known, a Coriolis mass flowmeter is a direct measurement type mass flowmeter which measures mass flow utilizing Coriolis' force generated in proportion to the mass flow of a fluid flowing in a vibrating pipe to be measured. Accordingly, it enables high precision measurement of mass flow. Generally, mass flowmeters of this kind are roughly grouped into two types. One is a mass flowmeter which has a curved measuring pipe such as U-shaped or S-shaped one and the other is a mass flowmeter which has a straight measuring pipe. Also, there are a mass flowmeter of the type in which two vibrating measuring pipes are provided and a fluid to be measured is separated into two streams which are led to two measuring pipes, a mass flowmeter of the type which has a two measuring pipes that are continuously connected to each other and that are resonant with each other, and a mass flowmeter of the type which has a single measuring pipe. Of the above-descried mass flowmeters, the one which includes a single straight pipe mass flowmeter, i.e., the passage for the fluid to be measured in the measurement device inclusive of its inlet portion and outlet portions for the fluid is in the form of a straight pipe as the entire structure of the mass flowmeter not only has a low fluid resistance, that is, shows a small loss in pressure, but also forms no fluid pool inside the detector so that cleaning of the inside of the measuring pipe can be performed with ease. Further, as compared with the mass flowmeters which have a curved measuring pipe or a plurality of measuring pipes, the single measuring pipe mass flowmeter has a simplified structure and, hence, manufacturing costs can be reduced while it has the following problems. Generally, in Coriolis mass flowmeters, there is given a vibration to the measuring pipe which vibration is such that Coriolis' force is generated when a fluid flows in the measuring pipe and which allows measurement of a fluid by means of Coriolis' force (hereafter, referred to as "Coriolis' vibration") in order to measure the flow rate of the fluid which flows in the measuring pipe. In this case, it is generally the case that in order to stabilize the Coriolis vibration, i.e., to stabilize the measurement, the measuring pipe is vibrated at its natural frequency (or principal frequency). When a single measuring pipe is used, there arise two problems. Firstly, in the case where a plurality of measuring pipes are used, they are arranged in a symmetrical structure such as a tuning fork so that the vibration or oscillation energy of the measuring pipe will not leak to the external system easily, thus giving high vibrational Q. On the other hand, with a single measuring pipe, the vibrational energy of the measuring pipe will leak to the external system through a fixation portion of the measuring pipe. Accordingly, high vibrational Q is difficult to obtain. This leads to unstable Coriolis vibration and, hence, unstable measurements of mass flow. Secondly, a plurality of measuring pipe to be used can be arranged in a differential structure to cancel the effect of external vibrations. In contrast, with a single measuring pipe, no differential structure can be adopted and, hence, the mass flowmeter with a single measuring pipe tend to be affected by external vibrations. Various efforts have been made to solve the above-described problems. For example, U. S. Patent 4,831,885 (hereafter, first conventional technique) discloses a mass flowmeter in which a measuring pipe is held by a strong, rigid member called "housing". This structure reduces variation of the portion of the measuring pipe where it is held and achieves a state resembling a "node", so that the measuring pipe is allowed to generate Coriolis vibration having a relatively high mechanical Q. However, this approach involves suppression of the vibration of the measuring pipe by the rigidity of the housing and, hence, raises a problem that the housing must be large and heavy. In addition, various problems occur. Although the position of the measuring pipe where it is held resembles a state of a node, the energy of vibration leaks. As a result, loss of vibrational energy increases to an undesirable extent to decrease the mechanical Q of the vibration when the mass flowmeter is connected to a flange or the like means for connecting it to external piping through a relatively short conduit as in the first conventional technique described above. As a result, no stable vibration can be obtained or conditions of vibration change due to a stress applied by an external pipe arrangement. In order to solve these problems, further efforts have been made. For example, Japanese Patent Application Laying-open No. 178217/1983 (hereafter, referred to "second conventional technique") discloses a mass flowmeter having the following construction. That is, in the mass flowmeter according to the second conventional technique, a vibrating conduit in which a fluid to be measured flows is fixed at both ends thereof to a first base so that the vibrating conduit can be fixed rigidly in the axial direction. On the other hand, the first base and a second base to which an external conduit is fitted are connected to inlet and outlet pipe couplings each of which is held at both ends thereof with respective O-rings softly. Thus, the pie arrangement and the vibrating conduit are separated from each other with respect to vibration, so that not only leakage of the vibrational energy of the vibrating conduit from the fixation portion to the external system, as was observed in the mass flowmeter according to the first conventional technique, can be prevented but also strains or vibrations which are transmitted from the outer pipe arrangement can be isolated to achieve vibrational characteristics which are less affected by vibrations of external systems. However, in the second conventional technique described above, stabilization of the vibration of the measuring pipe was effected by fixing the measuring pipe to the first base which is rigid as in the mass flowmeter according to the first conventional technique. Accordingly, the attempts to increase the Q value of the Coriolis vibration of the measuring pipe result in increase in the weight of the base as in the case of the housing as described above. In the second conventional technique, the measuring pipe is supported softly with an organic material such as O-ring and thus the elasticity and viscosity tend to change with time so that it is difficult to insure stable characteristics for a long period of time and it is necessary to detach the mass flowmeter from the pipe arrangement when exchanging deteriorated O-rings. Since, in the second conventional technique, the vibrating conduit or tube and the first base fixing thereto the vibrating tube, the first base being heavy as described above, are held softly by the second base fixing thereto the external piping, a strong force is urged onto the pipe couplings or O-rings to cause deformations thereto when external forces are applied to the device as by shocks, for example, during transportation of the device. Therefore, the mass flowmeter according to the second conventional technique uses the structure in which the first base is supported by the second base through another O-ring. However, even with this structure, the characteristics of the elastic material change with time so that the material becomes harder for the reasons described above, with the result that the characteristics of the O-ring and, hence, of the mass flowmeter is deteriorated. As the third type of conventional techniques, there can be cited those mass flowmeters which are disclosed U. S. Patent 5, 287, 754 and Japanese Patent Application Laying-open No. 248912/1993. In the examples of these publications, the measuring pipes used are curved pipes. However, the techniques disclosed in the publications are applicable to straight measuring pipes. In the mass flowmeters according to the third conventional technique, the Coriolis conduit in which a fluid to be measured flows is supported at its both ends by a supporter called a "carrier system" which is considered to have a rigidity higher than the Coriolis conduit as in the mass flowmeters according to first and second conventional techniques so that the natural frequency of the carrier system is higher than the natural frequency of the Coriolis conduit and the frequency of Coriolis vibration. At the same time, the support is connected to the housing through the inlet and outlet conduits themselves to statically support the weight of the support and the natural frequency of the support is made higher than that of the entire mass flowmeter consisting of the Coriolis conduit, carrier system, and inlet and outlet conduits so that on one hand the vibrational energy of the Coriolis conduit does not leak to the external system due to difference in natural frequency and on the other hand the mass flowmeter is isolated from external vibrations of the pipe arrangement to realize the separation of vibrations. With the above arrangement, the effects of deterioration with time of the elastic properties of the O-rings as in the mass flowmeter according to the second conventional technique can be avoided. However, the supporting of the both ends of the Coriolis conduit by the carrier system having a natural frequency higher than that the Coriolis vibration has the same effect as the supporting of the measuring pipe by the housing and bases shown in the mass flowmeters according to the first and second conventional techniques. Therefore, obviously the problem also arises that the carrier system must inevitably be sufficiently heavy. In the above-described measure according to the third conventional technique, the use of the heavy carrier system which is kept by the inlet and outlet conduits themselves in contrast to the mass flowmeter according to the first conventional technique in which the inlet and outlet conduits are held softly through the O-rings. Naturally, the softness in supporting achieved in the third conventional technique is different from those attained according to the first and second conventional techniques. Further, when external forces as by shocks are applied the mass flowmeter during it transportation, there is an increased danger that the inlet and/or outlet conduit(s) are or is damaged. In order to prevent such a damage, a possible countermeasure will be to increase the rigidity of the inlet and outlet conduits or add an auxiliary support as shown in one of the examples of the third conventional technique as described above. However, this countermeasure leads to an increased natural frequency of the entire mass flowmeter so that separation of vibrations due to difference in natural frequency is insufficient. The present invention has been accomplished with view to obviating the above-described problems of the conventional mass flowmeters. An object of the present invention is to provide a mass flowmeter which is of a simple construction, is lightweight, easy to handle, and does not allow the energy of Coriolis vibration to leak to an external system so that a vibration system having a high mechanical Q value can be realized. Another object of the present invention is to provide a mass flowmeter which is lightweight in the entire structure of a vibration system so that there can be avoided the occurrence of large displacements of the measuring pipe due to an impact force during transportation or due to external vibrations of low frequencies to prevent damages of the measuring tube. Still another object of the present invention is to provide a mass flowmeter which has a measuring pipe with as high as a natural frequency so that the natural frequency of the measuring pipe can be isolated from the frequencies of noise external vibrations and avoid influences of noise external vibrations by the frequency filtering effect of the vibration system itself that has a high mechanical Q value. In a first aspect of the present invention, there is provided a Coriolis mass flowmeter comprising: a housing; at least one measuring pipe contained in the housing, the at least one measuring pipe enabling measurement of flow rate of a fluid which flows in the pipe under application of a vibration which generates a Coriolis force in the flowing fluid; a supporting mechanism connected to both ends of the measuring pipe; a vibration generator which is arranged to apply said vibration to the measuring pipe; at least two sensors for detecting Coriolis vibration generated in the measuring pipe; an inlet conduit arranged to introduce said fluid to be measured into the measuring pipe; and an outlet conduit arranged to discharge the fluid out of the measuring pipe; characterised in that said Coriolis vibration has a frequency which is at least 2 times the principal natural frequency of the supporting mechanism. Preferably, the supporting mechanism further comprises a mass. The supporting mechanism and the mass may be integral to each other. The Coriolis vibration applied to the measuring pipe may be of a higher order mode than a principal natural vibration of the measuring pipe. The mass flowmeter may further comprise a signal processor for measuring the flow rate of the fluid flowing through the measuring pipe based on a phase difference between signals from the at least two vibration sensors. The at least two vibration sensors may comprise each a speed sensor. At least two vibration sensors may comprise each an acceleration sensor. The measuring pipe may comprise a straight pipe and the signal processor may be arranged to improve the accuracy of the flow rate value based on any two natural frequencies of the measuring pipe. The two natural frequencies used for the improvement in accuracy of the flow rate may comprise natural frequencies of the Coriolis vibration in first and third order modes. The two natural frequencies used for the improvement in accuracy of the flow rate may comprise natural frequencies of the Coriolis vibration in first and fifth order modes. The signal processor may be arranged to make a signal amplitude of the frequency of the first order mode vibration detected by the vibration sensors smaller than a signal amplitude of the frequency of the Coriolis vibration detected by the vibration sensors. The signal processor may be arranged to make a amplitude of the frequency of the first order mode vibration detected by the vibration sensors no higher than -40 dB. According to the present invention, the frequency of Coriolis vibration of the measuring pipe is made higher than the principal natural frequency of the supporting mechanism which supports the measuring pipe, i.e., a so-called inertial control is used. This construction enables to realize a mass flowmeter in a construction of a simple structure, lightweight, and easy to handle, for example, in a single straight measuring pipe, which shows a decreased displacement of the supporting mechanism without enlarging it or making it heavier and which also minimises, if any, leakage of the energy of Coriolis vibration to the outside so that stable vibration can occur, thus allowing measurements of mass flow under stabilised conditions. By setting the frequency of Coriolis vibration of the measuring pipe to above the value by 2 time the principal natural frequency of the supporting mechanism, a structure of which the energy of vibration is more difficult to leak to the outside than the conventional apparatuses which use a so-called elastic control mechanism can be realised. Further, an additional mass added to the supporting mechanism decreases the principal natural frequency of the supporting mechanism and allows an increase in its inertial effect. In addition, fabrication of the supporting mechanism and additional mass as one piece, for example, by casting or die casting results in a simplified structure. Coriolis vibration of the measuring pipe in a higher order mode enhances the effect of inertial control and use of detection of phase difference increases the sensitivity of the measuring apparatus to the mass flow rate. An increased frequency of Coriolis vibration of the measuring pipe enables separation thereof from noise frequencies so that S/N ratio increases. Provision of a speed sensor or an acceleration sensor further improves S/N ratio. According to the present invention, the support mechanism can be made compact and the rigidities of the inlet and outlet conduits can be made high so that the measuring device is resistant to an impact force applied thereto during transportation or on some other occasions and increase in the sensitivity allows to decrease the amplitude of vibration of the measuring pipe to reduce the stress of the measuring pipe. As a result, the mass flowmeter of the present invention can have an improved reliability. Further, use of a higher order mode, as compared with use of a primary or principal mode, decreases adverse effect of an axial force generated in the straight measuring pipe on the sensitivity of measurement of mass flow rate. An influence of axial force on the sensitivity of measurement of mass flow rate can be corrected by measuring two natural frequencies of different orders and obtaining a ratio of one to the other. In particular, this can be advantageously carried out by choosing Coriolis vibration as a third order mode of the measuring pipe and determining an axial force by the use of a ratio of the third order natural frequency of the Coriolis vibration to the first or fifth order natural frequency. Here, in the signal components detected by a vibration sensor, controlling the signal component of the first or fifth order natural vibration to a level sufficiently small, desirably no higher than -40 dB, as compared with the signal component of the Coriolis vibration, enables to suppress an adverse influence of the first or fifth order natural vibration on the Coriolis vibration. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiment thereof taken in conjunction with the accompanying drawings. Fig. 1 is a schematic view illustrating the principle cf the movement of a vibrational system; Fig. 2 is a graph illustrating a relationship between angular frequency of an external force and displacement a mass; Figs. 3A to 3D are each a graph illustrating the wave form of a flexural vibration; Fig. 4 is a cross sectional view showing a mass flowmeter according to the first embodiment of the present invention; Fig. 5 is a cross sectional view showing a mass flowmeter according to the second embodiment of the present invention; Fig. 6 is a cross sectional view showing a mass flowmeter according to the third embodiment of the present invention; Fig. 7 is a cross sectional view showing a mass flowmeter according to the fourth embodiment of the present invention; Fig. 8 is a graph illustrating a phase difference generated in accordance with the position of the sensor; Fig. 9 is a graph illustrating the relationship between the position of the sensor and the vibration speed when the stress generated at a fixed end of the measuring pipe is constant; Fig. 10 is a graph illustrating the relationship between the position of the sensor and vibration acceleration when the stress generated at a fixed end of the measuring pipe is constant; Fig. 11 is a graph illustrating the relationship between the position of the sensor and the span temperature difference coefficient of the measuring pipe; Fig. 12 is a schematic diagram showing an arrangement of a mass flowmeter according to an embodiment of the present invention; and Figs. 13A to 13F are each a graph illustrating the fluctuation of a zero point of Coriclis vibration. The Coriolis mass flowmeter of the present invention, in contrast to the above-described conventional mass flowmetcrs, utilizes the phenomenon called "inertial control" or "mass control" (hereafter, "inertial control") in vibration science, and suppresses vibrational displacement of a fixed end of the measuring pipe by making the principal natural frequency of the supporting mechanism lower than the frequency of Coriolis vibration of the measuring pipe. The above-described conventional mass flowmeters generally comprise a measuring pipe which is called a vibrating pipe vibrating tube, Coriolis conduit or the like, in which a fluid to be measured flows, and adopts a configuration in which both ends of the measuring pipe are fixedly supported on a supporting mechanism, called housing, base or carrier system, having a sufficiently high rigidity not to be deformed by a force statically determined which deforms and vibrates the measuring pipe in order to prevent leakage of the energy of Coriolis vibration given to the measuring pipe to the external system and realize a stable Coriolis vibration having a high vibrational Q value. Particularly, in the first and third conventional techniques described above reference is made to the natural frequency of the supporting mechanism and, hence, it would seem that consideration has given to dynamic deformations including the vibration of the supporting mechanism. Actually, however, it was presupposed that the natural frequency of the supporting mechanism was higher than the frequency of the measuring pipe. This means that the displacement of the member is suppressed predominantly by its rigidity as called elastic control in the vibration science. In other words, the conventional measuring devices resembles a static system in substance. On the other hand, in a Coriolis mass flowmeter which vibrates and relatively displaces two or more symmetrical measuring pipes, not only static forces but also inertial forces balance so that no force leaks to the external system. As a result, if the connecting point of the two measuring pipes is connected to external piping, no substantial decrease in vibrational Q value is observed. However, when a supporting mechanism having a high rigidity is used tor connection as in a Coriolis mass flowmeter with a single measuring pipe, the inertial force of the vibrating measuring pipe does not balance and an external force is generated. In this case, if the connecting point of the measuring pipe with the supporting mechanism is connected to external piping, the connection point displaces with respect to the external piping, with the result that the energy of the Coriolis vibration leaks to the external system. This leakage of the energy decreases the Q value of Coriolis vibration so that not only stability of the resonance frequency aggravates but also the measuring pipe vulnerable to influences such as stresses from the external piping. In order to suppress this phenomenon, according to the present invention, is used a so-called inertial control region in which the frequency of Coriolis vibration of the measuring pipe is made higher than the principal natural frequency of the supporting mechanism in contrast to the conventional mass flowmeters in which the static rigidity of the supporting mechanism is made high as described above. Here, an explanation will be made of elastic control and inertial control with reference to an example of a simple vibration system as shown in Fig. 1. Fig. 1 shows an example of a non-attenuated vibration system which comprises a mass point 30 with a mass of m is attached to one end of a spring 31 another end of which in turn is attached to a solid wall 32 which does not displace. The mass point 30 displaces only in the direction X or up and down in Fig. 1. The amount of displacement of the mass point 30 is defined as x. Then, the natural frequency, ωn, of the vibration system is known to be expressed by equation (1) below: ωn = k/m wherein k is a constant. Assuming a dynamic external force F·sinωt (where F is amplitude of the external force ω is angular frequency of the external force, and t is time) is applied to the mass point 30, then the displacement x is expressed by equation (2) below: x = F·sinωt/(k-mω 2 ) Fig. 2 is a graphical representation of the equation (2) above, in which the horizontal axis indicates the angular frequency ω of an external force and the vertical axis indicates the amplitude of displacement x. In Fig. 2, Curve A represents a boundary of an elastic control region which corresponds to a condition where the external force has an angular frequency of ω<ωn. In the elastic control region, the smaller the angular frequency ω of the external force than the natural frequency ωn of the vibration system, the smaller the amplitude of displacement x. However, the minimum value of the amplitude of displacement x is F/k, which is not zero. Curve B in Fig. 2 represents a boundary of an inertial control region where the external force has an angular frequency of ω>ωn. In the inertial control region, the amplitude of displacement x is assigned a negative sign (-), which indicates that the external force and the displacement x are in inverted phases to each other. In the inertial control region, the higher the angular frequency ω of the external force than the natural frequency ωn of the vibrating system, the smaller the displacement x. However, when the following condition is met: ω = 2k/m = 2 ·ωn then, the amplitude of displacement x equals to - F/k, which is the same value as the minimum value in the elastic control region. Furthermore, the displacement x approaches zero (0) according as the angular frequency ω increases, so that the displacement x becomes smaller than in the elastic region. Thus, in the inertial control region, the displacement x can be made smaller than in the elastic region when the condition ω> 2 ·ωn is met. Applying the theory about the difference between the elastic region and inertial region as described above Lc a mass flowmeter, the mass point 30 and the spring 31 in Fig. 1 constitute a supporting mechanism of a measuring pipe and the natural frequency ωn of the vibrating system may be deemed as a principal natural frequency of the supporting mechanism. The external force F·sinωt is ascribable to the Coriolis vibration of the measuring pipe applied to the supporting mechanism and thus the displacement x may be deemed as the displacement of the supporting mechanism accompanied by the Coriolis vibration of the measuring pipe. To set the principal natural frequency ωn at high levels as in the conventional techniques results in setting the condition ω<ωn for the frequency ω of an external force, i.e., the Coriolis vibration of the measuring pipe. This means that the vibration occurs in the elastic control region. Increased rigidity of the supporting mechanism to increase the principal natural frequency ωn amounts to make ω relatively small as compared with ωn in the elastic control region, so that the displacement of the supporting mechanism is made small to minimize leakage of the energy of the vibrating system to an external system. In this conventional approach, not only the supporting mechanism must be large and heavy in order to increase the rigidity of the supporting mechanism but also the amplitude of displacement x is not smaller than F/k. In contrast to the above, in the present invention, the frequency ω of the Coriolis vibration of the measuring pipe is made higher than the principal natural frequency ωn of the supporting mechanism so that the inertial control region can be used. In the inertial control region too, the condition that ω is higher than ωn (ω>ωn) allows to make displacement x of the supporting mechanism in the same manner as in the elastic control region so that leakage of the energy of the vibration system to the outside can be minimized. In this case, the rigidity of the supporting mechanism can be decreased to the minimum value that is required so that the supporting mechanism can be of as lightweight as possible. This is one of the advantages of the mass flowmeter of the present invention over the conventional techniques. Another advantage of the present invention is that when the inertial control region is used, use of an increased frequency ω of the Coriolis vibration of the measuring pipe to a level of 2 times as high as the principal natural frequency ωn of the supporting mechanism allows the displacement x of the supporting mechanism to decrease to a level smaller than that attained by the conventional approach in which the principal natural frequency ωn of the supporting mechanism is increased in the elastic region. Further, the mass flowmeter can be configured so that an additional mass is added as far as the supporting mechanism is not too heavy. In this construction, m can be increased without increasing k in equation (1) above. This feature is effective in decreasing the principal natural frequency ωn of the supporting mechanism and the effect of the inertial control can be enhanced. In this case, the structure of the mass flowmeter can be simplified when the supporting mechanism and the additional mass are fabricated solidly by casting, die casting or the like method. In addition, according to the present invention, excellent characteristics mentioned below can be realized by allowing the Coriolis vibration of the measuring pipe to occur at natural frequencies higher than those of the principal mode of the measuring pipe. As is well known, the natural frequencies of the measuring pipe include in addition to that of a principal mode (first order mode), those of innumerable higher order vibration modes. Figs. 3A to 3D are graphical illustrations of wave forms in various vibration modes of a flexural vibration or Coriolis vibration of a straight beam fixed at both ends thereof having a uniform cross section along longitudinal axis thereof. Fig. 3A illustrates a wave form of a first order mode. Fig. 3B illustrates a wave form of a second order mode. Fig. 3C illustrates a wave form of a third order mode. Fig. 3D illustrates axes of coordinates for the graphs in Figs. 3A to 3C. As illustrated in Figs. 3B and 3C, Coriolis vibrations at higher order natural frequencies have increased natural frequencies than the first order mode vibration. For example, the natural frequency of the second order mode vibration is 2.8 times as high as the frequency of the first order vibration while the third order mode vibration has a natural frequency by 5.4 time as high as the frequency of the first order mode vibration. As a result, the use of the higher order mode vibrations is effective in the inertial control. As will be understood from the above-described explanation, use of a higher frequency of Coriolis vibration of the measuring pipe has been considered disadvantageous conventionally. For example, U. S. Patent 5,287,754 cited above describes to the effect that the increase in the frequency of the Coriolis vibration of the measuring pipe is a disadvantage of the mass flowmeter of a straight pipe type. However, the investigation by the present inventors revealed that the increase in the frequency of the Coriolis vibration of the measuring pipe is rather advantageous if the following modifications are made: (1) A phase difference is between two vibration sensors provided upstream and downstream is detected; and (2) Vibration of the straight pipe is detected by a speed sensor such as a electromagnetic sensor or an acceleration sensor. This is because according to the conventional approaches which detect time differences, it has been theoretically clarified that a time difference becomes shorter at higher frequencies of the Coriolis vibration while a phase difference generated by the respective Coriolis forces of the two vibration sensors is greater at higher frequencies of the Coriolis vibration and because speed sensors and acceleration sensors are more sensitive at higher frequencies or for some other reasons. Furthermore, as described hereinafter, the method in which the frequency of Coriolis vibration is increased by allowing the Coriolis vibration of the measuring pipe to occur in a higher order mode is more advantageous in many respects than the method in which the measuring pipe is allowed to vibrate at the principal natural frequency. For example, influences of external vibrations transmitted from piping or fluid to be measured on the characteristics of the measuring is determined depending on S/N ratio, i.e., ratio of the frequency of the Coriolis vibration to the frequencies of the external vibrations. According to YOKOGAWA GIHO, Vol. 34, No. 1, 1990, pp. 49-52, the frequencies of external vibrations generated by the piping are at most 200 Hz. On the other hand, according to the measurement performed by the present inventors, the noises of the external vibrations generated by the piping and the pressure of the fluid to be measured are in total at most about 600 Hz and, hence, the frequencies of the external vibrations would seem to be at most on the order of several hundreds Hz. As in the present invention, higher frequencies of the Coriolis vibration of the measuring pipe give rise to much less noises generated in this region, which in combination with a frequency filtering effect of the vibration system itself having a high mechanical Q, is effective to give high S/N ratios. Use of a speed sensor or acceleration sensor results in an increase in S/N ratio since such a sensor has a higher sensitivity to vibrations at higher frequencies. Actually, it has been confirmed experimentally that influences of the external vibrations from the piping and the fluctuation of the zero point due to the pressure noise by the fluid to be measured can be suppressed because of the high S/N ratios. Since the frequency of the Coriolis vibration of the measuring pipe and the frequencies of external vibration noises are separated from each other, the rigidity of the inlet and outlet conduits which support the vibration system including the measuring pipe and the supporting mechanism does not have to be so small as in the case of the conventional mass flowmeters so that the mass flowmeter of the present invention is fully resistant to impact forces. Further, as described above, increasing the frequency of the Coriolis vibration of the measuring pipe using a speed sensor and an acceleration sensor results in an increased sensitivity of the sensor to the Coriolis vibration so that the amplitude of the Coriolis vibration can be decreased. As a result, stresses applied to the measuring pipe can be decreased and thus the reliability of the mass flowmeter can be increased against damages of the measuring pipe which is the most important problem to be solved in respect of Coriolis mass flowmeters. Furthermore, as will be apparent from the basic knowledge of vibration science, the measuring pipe vibrated in a higher order mode shows a portion where vibration occurs in an inverted phase so that movement of the center of the gravity of the entire system is canceled at that portion and, hence, is decreased accordingly so that the energy of the Coriolis vibration is difficult to leak to the outside. In addition, use of vibration in a higher order mode decreases changes in sensitivity to mass flow rate due to an axial force exerted on the measuring pipe. This effect is observed when a straight measuring pipe is used. For example, a change in the temperature of the fluid to be measured a time constant of heat conduction give rise to a difference or change in temperature between the measuring pipe and the supporting mechanism, which will generate or change an axial force applied to the measuring pipe due to its thermal expansion. As a result, the wave form of a flexural vibration shown in one of Figs. 3A to 3C changes and the sensitivity to mass flow rate changes. The investigation by the present inventors revealed that use of a vibration in a higher order mode decreases the change in sensitivity as described above. Further, measurement of a ratio of one to the other of any two natural frequencies of the measuring pipe can be used as a basic data for calculating an axial force exerted on the measuring pipe. Therefore, the precision of mass flow rate measurement can be increased by correcting the change in sensitivity based on the axial force thus obtained. In this case, it is practically, preferred that vibration in a third order mode of the measuring pipe be used as Coriolis vibration and that a ratio of the natural frequency of the vibration in a third order mode to the natural frequency of the vibration in a first or fifth order mode be obtained. When the measuring pipe is vibrated in a first or fifth order mode in order to measure such a ratio as described above, of the signal components detected by the vibration sensor, the signal component corresponding to the frequency of the vibration in a first or fifth order mode is made sufficiently small as compared with the signal component according to the frequency of the Coriolis vibration so that the secondary vibration does not give an adverse influence on the Coriolis vibration, major vibration. Preferably, the signal component corresponding to the frequency of the vibration in a first or fifth order mode is no higher than -40 dB. Hereinafter, the present invention will be described in greater detail by embodiments with reference to the attached drawings. However, the present invention should not be construed as being limited thereto. EMBODIMENT 1 Fig. 4 is a vertical cross sectional view showing a mass flowmeter according to the first embodiment of the present invention. A mass flowmeter or measuring device 1 has the following structure. That is, the measuring device 1 includes a linear (straight pipe) measuring pipe 2, fixing members 3a and 3b fixed to both ends of the straight measuring pipe 2, for example, by brazing or welding, a cylindrical beam 4 fixed to the fixing members 3a and 3b, for example, by welding in order to cancel the vibration of the fixing members 3a and 3b in the direction of the vibration of the straight measuring pipe 2. The fixing members 3a and 3b and the cylindrical beam 4 constitute a supporting mechanism 13 which connects the both ends of the straight measuring pipe 2. A vibration generator 5 for vibrating or oscillating the measuring pipe 2 is provided as intervening the cylinder beam 4. The vibration generator 5 includes a magnet 5a and a coil 5b fixed to the cylindrical beam 4. The vibration generator 5 are sandwiched by speed sensors (electromagnetic pickups) 6a and 6b for detecting vibration of the measuring pipe 2 and arranged so that they are positioned symmetrically with respect to the vibration generator 5. Each of the speed sensors 6a and 6b includes a magnet 6c and a coil 6d fixed to the cylindrical beam 4. In the present embodiment, Coriolis vibration of the measuring pipe 2 is allowed to occur in a third order mode as described hereinafter. Accordingly, the speed sensor of the type which has a good sensitivity to high frequency vibrations is suited rather than a displacement sensor such as a sensor for detecting change in an amount of light passing between slits. In the present embodiment, two vibration sensors are used. However, more than two vibration sensors may be provided as necessary. An inlet conduit 7a and an outlet conduit 7b are provided which are connected to the measuring pipe 2 so that the inlet and outlet conduits 7a and 7b communicate with the measuring pipe 2. In the present embodiment, the inlet and outlet conduits 7a and 7b are fabricated solidly to the measuring pipe 2. Of course, they may be fabricated separately from the measuring pipe 2 and then connected thereto as by welding. The inlet and outlet conduits 7a and 7b, which are connected to end portions of a housing 8, serve to introduce and discharge a fluid to be measured into and from the measuring pipe 2, respectively. The housing 8 include the above-described various elements therein and is connected to flanges 9a and 9b by screws or by welding or the like means. The flanges 9a and 9b comprise each discs 9c and 9d parallel to each other spaced apart and connected each other via a cylinder 9e through which the fluid to be measured flows. The plate 9d is fixed to one of an longitudinal end of the housing 8 by means of screws 9f. With the above-described construction, according to the present embodiment, the straight measuring pipe 2 is driven by the vibration generator 5 and a drive circuit 20 to realize a Coriolis vibration at its natural frequency in a third order mode. For example, the frequency of the Coriolis vibration of the straight measuring pipe 2 is about 4,500 Hz and the principal natural frequency of the supporting mechanism 13 is about 3,000 Hz. Thus, the displacement of the supporting mechanism 13 is suppressed by an inertial control so that leakage of the energy of vibration to the outside is minimized. In particular, the ratio of the frequency of the Ccriolis vibration to the principal natural frequency is 1.5, which is more than 2 described above. Therefore, in the present embodiment, the effect of suppressing the displacement of the supporting mechanism is more efficient than the conventional elastic control. The frequency of the Coriolis vibration of the measuring pipe 2 at a level of 4,500 Hz is sufficiently higher than the frequency of the above-described external vibration at a level of several hundreds Hz and, hence, the S/N ratio can be increased by separation of the frequencies one from another. Further, the S/N ratio is further increased by the use of speed sensors 6a and 6b having good sensitivities to high frequency vibrations. The speed sensors 6a and 6b which detect Coriolis vibration of the measuring pipe 2 outputs signals representing the Coriolis vibration and the output signals are forwarded to a phase difference detector and signal processor (or phase difference detection type signal processing circuit) 21, in which a phase difference between the output signals from the speed sensors 6a and 6b are obtained by a predetermined calculation whose program is stored therein to give a mass flow rate of the fluid which flows in the measuring pipe 2. The results are converted into a predetermined signal or signals and then outputted. A program for this procedure is stored in the circuit 21 or in a memory connected thereto. As described detail hereinafter, when a mass flow rate is to be obtained from the phase difference, Coriolis vibration of the measuring pipe 2 in a third order mode is more advantageous than Coriolis vibration in a first order (principal) mode since sensitivity of measuring a mass flow rate of a fluid higher in the former so that an increase in precision of measurement can be attained. Further, as described in detail hereinafter, Coriolis vibration of the measuring pipe 2 in a third order mode is more advantageous than Coriolis vibration in a first order (principal) mode since change in sensitivity to a mass flow rate is decreased due to an axial force exerted on the measuring pipe 2. EMBODIMENT 2 Fig. 5 is a cross sectional view showing a mass flowmeter according to the second embodiment of the present invention. The mass flowmeter according to the present embodiment differs from the mass flowmeter according to the first embodiment in that an additional mass 10 is attached to the cylindrical beam 4 in a central part thereof in the axial direction. The additional mass 10 coaxially surrounds the cylindrical beam 4. The lower parts of the additional mass 10 sandwich the vibration generator 5 to form a symmetrical structure. The fixing member 3a and 3b and the cylindrical beam 4 as well as the additional mass 10 constitute a supporting mechanism 14. Although the additional mass 10 makes the entire supporting mechanism 14 heavy, the weight of the cylindrical beam 4 can be increased without increasing the rigidity of the cylindrical beam 4 so that use of the additional mass 10 is effective in reducing the principal natural frequency of the supporting mechanism 14 and, hence, in increasing the effect of the inertial control. In the present embodiment, the additional mass 10 are attached to the cylindrical beam 4 in the central part thereof. However, they may be attached to both ends, respectively, of the cylindrical beam 4. What is important is to attach the additional mass 10 in appropriate positions so that the natural frequency of the supporting mechanism 14 can be decreased effectively. Particularly, the additional mass 10 attached to the central part in the axial direction of the cylindrical beam 4 are effective in simplifying the structure of a supporting mechanism and in decreasing the principal natural frequency of the supporting mechanism. Further simplification in structure is realized by fabricating the supporting mechanism 13 and the additional mass 10 by casting, die casting or the like integrally or as one piece. EMBODIMENT 3 Fig. 6 is a cross sectional view showing a mass flowmeter according to the third embodiment of the present invention. The mass flowmeter according to the present embodiment differs from the mass flowmeter according to the first embodiment in that the vibration sensors 6a and 6b were replaced by acceleration sensors 11a and 11b. This is because use of acceleration sensors further increases the sensitivity and S/N ratios at higher frequencies since speed and acceleration are proportional to ω and ω 2 , respectively. The increase in sensitivity enables to decrease the amplitude of the Coriolis vibration of the measuring pipe 2 so that stresses exerted on the measuring pipe 2 can be decreased, with the result that reliability of the measuring pipe against damages which is most important for Coriolis mass flowmeters can be improved. EMBODIMENT 4 Fig. 7 is a cross sectional view showing a mass flowmeter according to the fourth embodiment of the present invention. The mass flowmeter according to the present embodiment differs from the mass flowmeter according to the first embodiment in that in the former, two natural frequencies of the measuring pipe 2 are measured, ratios of one to the other are obtained, phase differences measured are corrected to give final mass flow rates, and these are converted into predetermined signals and outputted. In a mass flowmeter having two straight measuring pipes 2 as in the present embodiment, an axial force is exerted to the measuring pipe 2 due to, for example, temperature difference between the measuring pipe 2 and the supporting mechanism 13. The axial force causes changes in the wave form of flexural vibration of the measuring pipe 2 as shown in Figs. 3A to 3C and, hence, the sensitivity of measurement to a mass flow rate. This is a cause of errors in measurement. Accordingly, high precision measurement of a mass flow rate is realized according to the present embodiment by correcting the measured value of a mass flow rate with the axial force which is obtained by measuring the two natural frequencies of the measuring pipe 2 and obtaining ratios of one to the other. Here, in the same manner as in the first embodiment, Coriolis vibration in a third order vibration mode of the measuring pipe 2 is allowed to occur by means of the drive circuit 20 and the vibration generator 5. In order to obtain ratios of natural frequencies, the measuring pipe 2 is also vibrated in a first or fifth order mode. In this case, in the signal components detected by the vibration sensors 6a and 6b, the signal component for the first or fifth order mode is made sufficiently low, preferably as low as no higher than -40 dB as compared with the signal component for the Coriolis vibration so that the vibration in a first or fifth order mode gives no adverse influence on the Coriolis vibration. The phase difference detector and signal processor 21 includes a frequency ratio calculator 51, a phase difference calculator 52, a temperature calculator 53, and a correction calculator 54. The frequency ratio calculator 51 calculates a ratio of the natural frequency of the third order mode vibration to the natural frequency of the first or fifth order mode vibration from signals from the vibration sensors 6a and 6b driven by the drive circuit 20 and forwards the results to the correction calculator (or correction calculating portion) 54. The phase difference calculator (or phase difference calculating portion) 52 calculates a phase difference between the output signals from the two vibration sensors 6a and 6b according to a predetermined well known calculation procedure and forwards the results to the correction calculator 54. Further, in the present embodiment, a temperature sensor 12 such as a temperature indicating resistor or a thermocouple is provided on an outer surface of the inlet conduit 7a or an outlet conduit 7b for temperature correction. The sensor 12 measures the temperature of the measuring pipe 2 and correct temperature dependent changes in the mechanical properties such as Young's modulus of the measuring pipe 2. Generally, in the case of the mass flowmeter according to the present embodiment, the temperature dependent changes of the mechanical properties such as Young's modulus of the measuring pipe 2 causes changes in phase difference generated corresponding to the mass flow rate (cf. equations (3) to (13), particularly equation (8) in which yc(x) is a function of E). Accordingly, temperature correction is necessary. The temperature sensor 12 is fabricated integrally to the measuring pipe 2 in order not to give an adverse influence on the Coriolis vibration of the measuring pipe 2 so that it is attached to the inlet conduit 7a or the outlet conduit 7b. The temperature calculator (temperature calculating portion) 53 receives temperarure data signals from the temperature sensor 12 and obtains temperature and forwards the results to the correction calculator 54. The correction calculator 54 makes a correction to the phase difference data from the phase difference calculator 52 for changes in the sensitivity of phase difference to mass flow rate due to axial force based on the ratios of natural frequencies from the frequency ratio calculator 51 and/or for Young's modulus E or the like mechanical properties of the measuring pipe 2 based on the temperature data from the temperature calculator 53. Further, the correction calculator 54 makes a correction to phase difference based on the frequency of the Coriolis vibration of the measuring pipe calculated from the signals outputted by the vibration sensor 6a (cf. equations (3) to (13) hereinbelow for the frequency dependence of the phase difference, particularly equation (8) in which yc(x) contain ω 0 ). By the above-described corrections, precision of the measurement of mass flow rate is increased and the data are converted into signals and outputted as Qm by the correction calculator 54. EMBODIMENT 5 This embodiment is to further explain various effects described above based on the measurements with the measuring pipe vibrated in first and third order modes, respectively. 1) Phase difference generated The phase difference generated between two vibration sensors arranged in symmetrical positions with respect to the center of the measuring pipe 2 which is of an ideal straight measuring pipe with its both ends fixed and having a uniform cross section when a fluid flows through such a measuring pipe, as shown in Figs. 3A to 3C, is as follows. First, the displacement Y(x,t) by a natural vibration of the measuring pipe is expressed by the following equation (3) : Y(x,t) = y(x) · sin(ω 0 t) y(x) = D{cos(λx/L) + αsin(λx/L) - cosh(λx/L) - αsinh(λx/L)} α = (coshλ - cosλ)/(sinλ -sinhλ) ω 0 = λ 2 EI/ρL 4 where various symbols have the following meanings: L: length of the measuring pipe; E: Young's modulus E of the measuring pipe; I: moment of inertia of area of the measuring pipe; ρ: linear density of the measuring pipe (inclusive of the fluid to be measured; D: constant proportional to amplitude of vibration; λ: constant varying depending on the mode of vibration; first order mode 4.7300 second order mode 7.8532 third order mode 10.9956 ω 0 : natural angular frequency for each mode. Next, displacement Yc(x,t) of the flexure of the measuring pipe due to Coriolis force generated when a fluid flows in the measuring pipe is expressed by the following equation (7) in which Qm is a mass flow rate: Yc(x,t) = yc(x) · cos(ω 0 t) yc(x) = 2Qmω 0 DL 3 / (EIλ 3 ) · {sin(λx/L) - αcos(λx/L) + sinh(λx/L) + αcosh(λx/L) - C 1 (x/L) 3 - C 2 (x/L) 2 - 2λx/L} C 1 = λβ 1 - 2β 2 +2λ C 2 = -λβ 1 + 3β 2 - 4λ β 1 = cosλ + αsinλ + coshλ + αsinhλ β 2 = sinλ - αcosλ + sinhλ + αcoshλ The displacement of the measuring pipe is the sum of Y(x,t) + Yc(x,t) and the phase difference α generated between the outputs from the two vibration sensors arranged in symmetrical positions with respect to the center of the measuring pipe is expressed by the equation (13): α = 2tan -1 {yc(x) /y(x)} As will be understood from the above explanation, the generated phase difference α varies depending on the vibration mode and the positions of the sensors. The phase difference α was calculated under the conditions of the same flow rate for first and third order modes, respectively, and plotted in a graph with the sensor position x/L being horizontal axis (Fig. 8). The constants used are: L 0.252 (m) E 0.1932E + 12 (Pa) I 531E - 12 (m 4 ) ρ 0.321 (Kg/m) Qm 100 (Kg/min) Fig. 8 shows that the magnitude of the phase difference for different modes varies depending upon the position of the sensors. In Fig. 8, 30 and 31 are sensitivity curves for first and third order mode vibrations, respectively. Meaningful measurements cannot be performed unless the positions of the sensors is determined at appropriate positions so that the phase difference is large and that the sensitivity of the sensors for detecting vibration is sufficiently high, i.e., the amplitude of the vibration of Lhe measuring pipe is sufficiently high. In order to satisfy the both requirements, usually the sensors are attached at the position of x/L = about 0.2 for both first and third order mode vibrations. In this region, the third order mode vibration (31) gives by 2 to 5 times as large as the phase difference obtained by the first order mode vibration (30) (Fig. 8). 2) Sensitivity of detecting vibration (stress generated in the measuring pipe upon vibration) The stress generated by the natural vibration of an ideal straight measuring pipe fixed at both ends thereof and having a uniform cross section, as illustrated in Figs. 3A to 3C, is proportional to twice differentiated y(x) described above as equation (4) and reaches maximum at x = 0, i.e., at the fixed end for both first and third order modes. Then, calculation is made of the sensitivity of the sensor when the maximum stress at the fixed end is made constant, and the results are plotted in graphs with the position x/L of the sensor being horizontal axis as shown in Figs. 9 and 10 illustrating first and third order mode vibrations, respectively. In Fig. 9, 32 and 33 are sensitivity curves for first and fifth order mode vibrations respectively. In Fig. 10, 34 and 35 are sensitivity curves for first and third order mode vibrations, respectively. Fig. 9 indicates the sensitivity of the speed sensor in terms of speed of vibration (m/sec) in vertical axis and illustrates variation of the sensitivity (output) of the sensor depending on the position x/L of the sensor and the mode of vibration when a speed sensor is used and the measuring pipe is vibrated so that the stress at the fixed end is at a constant level. Fig. 10 is a graph similar to the graph illustrated in Fig. 9 except that the vertical axis indicates acceleration of the vibration of the measuring pipe and acceleration sensors are used. As illustrated in Figs. 9 and 10, at an ordinary position of the sensors x/L = about 0.2, use of a third order mode vibration gives a sensitivity about twice as high as that obtained by using a first order mode vibration in the case where speed sensors are used while with acceleration sensors, use of a third order mode vibration gives a sensitivity about ten times as high as that obtained by using a first order mode vibration. If the amplitude of the vibration is decreased by the amount of the increment in sensitivity, then the stress generated in the measuring pipe is decreased accordingly. 3) Change in sensitivity to mass flow rate measurement (generated phase difference) due to axial force exerted on the measuring pipe. In the case where a straight measuring pipe is used, a problem arises. That is, when a difference in temperature is generated or temperature changes between the measuring pipe and the supporting mechanism due to, for example, temperature variation of the fluid to be measured and time constant heat conduction, an axial force is generated in the measuring pipe or such a force changes due to thermal expansion, resulting in a change in the wave form of the flexural vibration so that the sensitivity of measurement of mass flow rate, i.e., change in the generated phase difference described above. Fig. 11 illustrates results of calculation of the change in the generated phase difference for an ideal straight measuring pipe fixed at both ends thereof and having a uniform cross section as illustrated in Figs. 3A to 3C. In Fig. 11, the horizontal axis indicates the position x/L, of the sensor while the vertical axis indicates % change in span (generated phase difference) due to an axial force generated when the temperature difference between the measuring pipe and the supporting mechanism changes by 1°C. Fig. 11 illustrates results of calculation of data for first, second and third order mode vibrations. In Fig. 11, 36, 37 and 38 are sensitivity curves for first, second and third order mode vibrations, respectively. The degree of change in span or generated phase difference due to the axial force varies depending on the position of the sensor and vibration mode. Comparing at the position of the sensor x/L = about 0.2, the third order mode vibration can decrease the change to about 1/2 to 1/5 time as great as that obtained with the first order mode while the second order mode vibration can decrease the change further. By obtaining generated axial force from the ratio of natural frequencies and utilizing coefficients which are obtained from Fig. 11, the change in the sensitivity of the measurement of mass flow rate can be corrected. 4) Experimental Data In order to estimate the degree of an improvement in the S/N ratio, for example, the fluctuation of the zero point with respect to external vibrations in the mass flowmeter as arranged as shown in Fig. 12. In Fig. 12, a mass flowmeter 42 is connected to upstream and downstream valves 41 and 43 at the ends thereof, respectively. A pump 40 is connected to the mass flowmeter 42 through the upstream valve 42. The fluid to be measured flows from the pump 40 through the upstream valve 41 into the mass flowmeter 42 to the outside through the downstream valve 43. The measurements were made in first and third order modes, respectively, under the following conditions: Condition I: The pump 40 is stopped (i.e., external vibration is almost null); Condition II: The pump 40 is in operation and the upstream valve 41 is closed (i.e., the vibration of the pump 40 is transmitted to the mass flowmeter 42); Condition III: The pump 40 is in operation and the downstream valve 43 is closed (the vibration of the pump 40 is transmitted through the piping and the fluid to be measured). The apparatus employed is substantially the same as that described in Embodiment I above and uses speed sensors (electromagnetic sensors) as the vibration sensors. The natural frequency of the vibration in a first order mode is 1,000 Hz, which is lower than that of the supporting mechanism, i.e., in an elastic control region. Figs. 13A to 13F illustrate the fluctuation of a zero point recorded on an automatic recorder under the above-described conditions. Figs. 13A to 13C relate to third order mode vibrations and Figs. 13D to 13F relate to first order mode vibrations. Figs. 13A and 13D relate to Condition I, Figs. 13B and 13E relate to Condition II, and figs. 13C and 13F relate to Condition III. The fluctuation shown in Figs. 13A to 13F were read peak to peak and the results were expressed in percentage taking Upper Range Limit (URL) as 100. Table 1 shows the results. Fluctuation of Zero Point (% URL) Condition I Condition II Condition III First order mode 0.072 0.083 0.183 Third order mode 0.014 0.026 0.028 The present invention has been described in detail with respect to an embodiment, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect.

Claims: A Coriolis mass flowmeter (1) comprising: a housing (8); at least one measuring pipe (2) contained in said housing, said at least one measuring pipe enabling measurement of flow rate of a fluid which flows in said pipe under application of a vibration which generates a Coriolis force in said flowing fluid; a supporting mechanism (3a, 3b, 4) connected to both ends of said measuring pipe; a vibration generator (5) which is arranged to apply said vibration to said measuring pipe; at least two sensors (6a; 6b) for detecting Coriolis vibration generated in said measuring pipe; an inlet conduit (7a) arranged to introduce said fluid to be measured into said measuring pipe; and an outlet conduit (7b) arranged to discharge said fluid out of said measuring pipe; characterised in that said Coriolis vibration has a frequency which is at least 2 times the principal natural frequency of said supporting mechanism. The Coriolis mass flowmeter as claimed in claim 1, characterised in that said supporting mechanism (3a, 3b, 4) further comprises a mass (10). The Coriolis mass flowmeter as claimed in claim 2, characterised in that said supporting mechanism (3a, 3b, 4) and said mass (10) are integral to each other. The Coriolis mass flowmeter as claimed in any one of claims 1 to 3, characterised in that said Coriolis vibration is of a higher order mode than a principal natural vibration of said measuring pipe. The Coriolis mass flowmeter as claimed in claim 4 characterised in that said at least two vibration sensors each comprise a speed sensor. The Coriolis mass flowmeter as claimed in claim 4 characterised in that said at least two vibration sensors each comprise an acceleration sensor. The Coriolis mass flowmeter as claimed in any one of claims 4 to 6, characterised in that said mass flowmeter further comprises a signal processor for measuring the flow rate of said fluid flowing through said measuring pipe based on a phase difference between signals from said at least two vibration sensors. The Coriolis mass flowmeter as claimed in claim 7, characterised in that said measuring pipe comprises a straight pipe and wherein said signal processor is arranged to improve the accuracy of the flow rate value based on any two natural frequencies of said measuring pipe. The Coriolis mass flowmeter as claimed in 8, characterised in that said two natural frequencies used for the improvement in accuracy of the flow rate comprise natural frequencies of said Coriolis vibration in first and third order modes. The Coriolis mass flowmeter as claimed in 8, characterised in that said two natural frequencies used for the improvement accuracy of the flow rate comprise natural frequencies of said Coriolis vibration in first and fifth order modes. The Coriolis mass flowmeter as claimed in claim 9 or 10, characterised in that said signal processor is arranged to make a signal amplitude of the frequency of said first order mode vibration detected by said vibration sensors smaller than a signal amplitude of the frequency of said Coriolis vibration detected by said vibration sensors. The Coriolis mass flowmeter as claimed in claim 9 or 10, characterised in that said signal processor is arranged to make a signal amplitude of the frequency of said first order mode vibration detected by said vibration sensors no higher than -40 dB.

Inventors: KISHIRO MASAMI, YAO HIRONOBU, KISHIRO, MASAMI, YAO, HIRONOBU
Applicants: FLOWTEC AG, ENDRESS + HAUSER FLOWTEC AG
Application year: 1996
Main classification: G01F1
IPC codes: G01F1