GB1591974A

GB1591974A - Mass throughflow meter

Info

Publication number: GB1591974A
Application number: GB1840478A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1977-10-19
Filing date: 1978-05-09
Publication date: 1981-07-01
Also published as: DE2746890A1

Description

(54) MASS THROUGHFLOW METER (71) We, ROBERT BOSCH GmbH, of Postfach 50, 7000 Stuttgart 1, Federal Republic of Germany, a limited liability company, organised under the laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particular described in and by the following statement:- The present invention relates to mass throughflow meter. A mass throughflow meter is known (DT-OS 2,013,153 and DT-OS 2,013,154), in which a baffle body is arranged in a flow path and has an oscillation forced on it by a drive device. The drive device is a synchronous motor and is driven by an alternating current source of constant frequency and variable amplitude.The amplitude of this alternating current is influenceable through an electronic circuit by the signal of an angle meter determining the deflection of the baffle body. The driving moment on the baffle body is thus determined by an electromagnetic coupling between alternating current and constant magnetic field of the synchronous motor, wherein the amplitude of the alternating current is proportional to the driving moment. The damping of the baffle body motion in consequence of the flowing medium is cancelled by the driving moment. The amplitude of the alternating current is however a measure of the mass flow only when the absolute values of the predetermined target value for the deflection of the forced oscillation can be kept constant and the regulating magnitude exactly regulated to this value. As further prerequisite the frequency of the oscillation must remain absolutely constant.This requires a very great effort in apparatus.

Also there is the disadvantage in known mass throughflow meters that measurement errors arise through changes in the mechanical system, for example through deposits or erosions at the baffle body and changes of the restoring force as a result of temperature fluctuations and ageing phenomena.

Furthermore, it is a disadvantage that the measuring signal present as an alternating current amplitude must be translated into a direct current signal, characterising the mass flow, of the indication of the mass throughflow.

It is likewise disadvantageous in known throughflow meters that for the forcing of the oscillation, there is required a moment, which is to be exerted electromechanically and which is composed of a damping moment in consequence of the flow and a moment for acceleration of the inertia masses, wherein the latter according to the dynamic demands on the device can amount to a multiple of the damping moment.

A mass throughflow meter is known (DT-OS 2,501,380) with at least one baffle body which is arranged in the flow path and which executes an oscillation in and against the flow direction against a restoring force in an electromagnetic field, and a speed transmitter which scans the oscillation and which influences the degree of an electromagnetic coupling between the baffle body and the electromagnetic field to balance the damping of the oscillation effected by the flow, wherein current conductors, directed perpendicularly to the field lines of the electromagnetic field, are arranged on the baffle body and flowed through by a current, the current strength of which corresponds to the speed of the baffle body scanned by the transmitter and the field strength of the electromagnetic field is variable in dependence on the signal of the speed transmitter so that the baffle body executes a natural oscillation and the field strength represents a direct measure of the mass flow rate.

According to the present invention there is provided a mass throughflow meter for measuring the mass throughflow of a fluid flowing along a flow path, comprising an impeller wheel mounted within the flow path to oscillate about the wheel axis, the meter being provided with electrical conductor means disposed on the wheel to be flowed past by the fluid and with means to provide a magnetic field perpendicular to the conductor means, wherein means responsive to impeller wheel velocity are disposed to detect changes in the velocity caused by the flowing fluid and to change the strength of the magnetic field by an amount which restores the velocity to that corresponding to zero through flow and which is indicative of the mass throughflow of the fluid.

The impeller wheel may comprise vanes each having major surfaces extending parallel to the flow path.

The impeller wheel may be resiliently mounted to a tension band fastened within the flow path.

The impeller wheel may be mounted to at least one pair of leaf spring members which cross each other and which each have one end portion connected to the impeller wheel and the respective opposite end portion to means bounding the flow path.

The axial length of each vane of the impeller wheel may decrease with increasing distance from the oscillatory axis.

The length of each vane may be given by

wherein v, is the speed of flow of the fluid, w the effective angular speed of the impeller wheel, and Ra the largest and R, the smallest radius of each vane.

The mass throughflow meter may comprise guide surfaces each inclined to the intended direction of flow and disposed in the flow path upstream of the impeller wheel.

Each vane of the impeller wheel may be inclined to the intended direction of flow.

Each vane of the impeller wheel may be wedge-shaped, symmetrical about a plane parallel to the intended direction of flow and taper to point against the intended direction of flow.

The mass throughflow meter may comprise a further such impeller wheel, each impeller wheel being co-axial with the other and being connected to a common carrier by respective torsion means to be oscillatable in opposite phase with each other.

Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings in which: Figs. 1 to 3 show a first embodiment of a mass throughflow meter, Figs. 4 to 5 show a second embodiment of a mass throughflow meter, Figs. 6 to 7 show block diagrams of a regulating device of a mass throughflow meter, Figs. 8 to 11 show schematic representations for the mathematical calculation of the construction of a mass throughflow meter, Figs. 12 to 14 show mass throughflow meters with spin components, and Fig. 15 shows a mass throughflow meter with two impeller wheels oscillating in opposite phase.

Referring to the accompanying drawings, in the throughflow meter shown in Figs. 1 to 3, the flow cross-section 1 of the medium flowing in the arrow direction is determined by the clear width of a pipe 2 in which an impeller wheel 4 is resiliently mounted on a tension band 3 serving as a torsion spring. The tension band 3 is fastened parallel to the direction of flow on webs 5 and 6 fast with the walls of the pipe so that the impeller wheel 4 can execute an oscillation about an oscillatory axis parallel to the flow direction. The impeller wheel 4 possesses at least one vane 7 which is directed parallel to the flow direction and fastened on a hub 8. The mass throughflow meter may for example serve to determine the quantity of air sucked in by an internal combustion engine through the suction duct and, by reason of throughflow meter signal, control a fuel injection plant in such a manner that a quantity of fuel proportional to the quantity of air sucked in is metered. The springmass system provided by the impeller wheel 4 and the tension band 3 can execute rotary oscillations qs(t) around the oscillatory axis formed by the tension band 3.

When the spring-mass system is left to itself after a single forced deflection, which can take place arbitrarily in any desired mode and manner, it executes a damped natural oscillation, the frequency of which corresponds approximately to the natural frequency f and is calculated in a known manner from the spring constant c and the system moment of inertia 0: f= The damping of the oscillating system with a technically favourable construction is composed, to only a very small extent, of internal self-damping (air friction, internal friction in spring and spring fastening) but is substantially dependent upon the mass throughflow rri. When energy loss caused by the damping is replaced through an electromagnetic coupling, then the energy replenished in electrical form can serve as a measure of the mass throughflow ni.The peripheral speed at a given time of the impeller wheel can be determined by a speed transmitter 10 of any desired construction which may be fastened in the wall of the pipe 2 and in this embodiment contains a permanent magnet with a winding. The speed transmitter 10 is arranged in the pivotal range of a vane 7, wherein the vane at its end facing the speed transmitter 10 is provided with magnetically highly conducting material.

Electrical current conductors 12 are arranged on the hub 8 of the impeller wheel 4 perpendicularly to the field lines of an electromagnetic field. The electromagnetic field is generated by a coil 13 and the course of the field lines is determined by a magnetically highly conducting, U-shaped core 14, which surrounds the hub 8 in the region of the electrical current conductors 12. Figs. 4 and 5, show at least one pair of leaf springs 15 and 16 which are fastened crosswise to each other and which can serve as the axially rotatable resilient fastening of the impeller wheel 4. The leaf springs 15 and 16 are fastened at their one end to a block 17 within the tubularly constructed hub 8 which is connected with the pipe wall 2 by a carrier member 18 fast with the housing.The other ends of the leaf springs 15 and 16 are fastened to a block 19 which is connected with the hub 8 of the impeller wheel 4. The oscillatory axis of the impeller wheel 4 passes through the apparent intersection, in Fig. 5, of the two leaf springs 15 and 16. The two leaf springs 15 and 16 can be moved so far apart from each other in the axial direction that a core 14, on which a coil 13 is arranged, can be disposed between the springs. Electrical current conductors 12 are arranged in the interior of the hub 8 perpendicularly to the electromagnetic field generated by the coil 13, the course of the field lines of which is determined by the core 14.

The construction of the mass throughflow meter with an impeller wheel 4 oscillating about an oscillatory axis parallel to the direction of flow has the advantage that no baffle effect is required for the measurement so that a negligibly small pressure drop results at the mass throughflow meter. Furthermore, a better averaging of the mass throughflow results over the entire flow cross-section through as great as desired a number of the vanes. The mass throughflow meter is insensitive to backfiring suction pipe ignitions particularly in the use in the suction pipe of an internal combustion engine.

The operation of the mass throughflow meters described in Figs. 1 to 3 and 4 and 5 is as follows: The gaseous or liquid medium flowing through the flow cross-section 1 exerts partial forces, which damp the constant periodic natural motion of the impeller wheel, on the vanes 7 of the impeller wheel 4 which is rotatably journalled by a tension band 3 or by leaf springs 15 and 16. The energy loss which the impeller wheel 4 experiences through the flowing medium for each periodic duration is proportional to the product of the linear speed VE at radius r and the throughflow quantity m of the flowing medium. The mass of the medium flowing through per unit time can be determined from the energy feed to the impeller wheel at the known actual speed. For this, the speed VE may be measured by the speed transmitter 10.The reluctance of the magnetic circuit of the permanent magnet arranged in the speed transmitter is varied when the magnetically highly conducting material at the end of the vane 7 moves in the region of the speed transmitter 10, whereby a voltage UVE proportional to the actual speed VE of the impeller wheel is induced in the winding arranged on the permanent magnet. The voltage UVE, induced in the speed transmitter and proportional to the actual speed, is amplified in an operational amplifier (see Fig. 6) and applied to the current conductors 12.The energy loss of the impeller wheel 4 in consequence of the damping through the flowing medium is balanced by variation of the field strength of the electromagnetic field, which is generated by the coil 13 and in which the current conductors 12, arranged perpendicularly to the field lines of the electromagnetic field, experience forces K, whereby an accelerating moment of the impeller wheel 4 is produced. The force K on a conductor flowed through by current in a perpendicular magnetic field is given by the relationship: K=(ixB)1 Wherein i is the current strength proportional to the actual speed VE, B is the flux density of the electromagnetic field and I is the effective length of the conductor flowed through by current.On the assumption that the current strength i is proportional to the speed VE and the change from zero throughflow condition of the maximum value of the speed VE is equal to zero, the flux density B is exactly proportional to the mass throughflow m of the flowing medium.

Designated by 4 in the block diagram shown in Fig. 6 is an impeller wheel arranged in the flow cross-section 1 and carrying out a constant periodic actual motion with the speed VE detected by the speed transmitter 10 at which a voltage UVE proportional to the speed VE of the impeller wheel is picked off, amplified by an operational amplifier 38 and applied to the current conductors 12. The voltage Uvt, proportion to the speed VE, is compared with a target voltage Uso" in a comparator 39, at the output of which lies a monostable multivibrator 40 which drives a reversible counter 41, the counting state of which is converted in a digitial differential analyser multiplier 42 into a frequency proportional to the mass flow rate of the medium.This frequency is converted back in a digital-analogue converter 43 into a voltage which through an RC-member and through an operational amplifier 44 lies amplified at the coil 13, to which an electromagnet field is generated, the field strength B of which is proportional to the mass throughflow so that an accelerating moment M, which balances the damping moment on the impeller wheel produced by the mass throughflow, is produced on the current conductors 12.

The digital-analogue converter 43 contains a field-effect transistor 45, the one control electrode of which lies directly, and a field-effect transistor 46, the control electrode of which lies through an inverter 47 at the output of the digital differential analyser multiplier 42. In accordance with which of the field-effect transistors 45 and 46 is switched through, the output of the digital-analogue converter 43 is connected to a reference voltage Urge, or to earth.

In the case of the block schematic diagram shown in Fig. 7, the voltage UVE, proportion to the speed VE of the impeller wheel 4, by contrast to Fig. 6, lies at the input of a pulse-shaped stage 49, at the output of which a rectangular voltage of constant amplitude can be taken off and fed to the current conductors 12.

Furthermore, the driving of the reversible counter 41 takes place through an R-S flip-flop 50.

Figs. 8 and 9 serve for the explanation of the damping caused by throughflow at the mass throughflow meter. Shown schematically in Fig. 8 is the flow to a vane of length L. When the impeller wheel 4 carries out a rotary oscillation in the time t: (1) x,b(t)=0sinit with the angular speed w, the speed VE of the vane 7 at the radius r amounts to: (2) Thereby, a resultant onflow speed Vst arises which one obtains according to the amount and direction by vectorial addition of the actual speed VE and the flow speed v, of the medium. Vst is the flow speed which an observer moving along on the vane would perceive:

Only the projection of the vane area extending perpendicularly to the oncoming flow acts at the baffle area, whereby the effective vane length L is shortened to L': :

An annular section dm out of the measurement flow, using the stated approximations, acts on the vane by the partial moment: 2 VE 2 (5) dM=rdK=rdrL'cw#VSt=rdr Lcw#w#VL=cw#VLrdrvEL VL where cw is the co-efficient of resistance and p is the density. 27rpv,rdr corresponds to the partial flow dm through the annular area: c (6) dM-- dinvL= dihUacoswt 2# 2# The work, caused by the flow and to be exerted over a period T=27r/co, is obtained by equation (2):

an work dAft is proportional to the mass throughflow dm.

Integration of equation (5) give a total moment M=JdM proportional to the mass flow only for homogeneous flows dvJdr=0. In the case of inhomogeneous flows of random rotationally symmetrical profile v,(r), the dependence of VE on radius causes a false valuation of the partial zones. This error influence can be compensated by a radius-dependent vane length const L(r)= r (Fig. 10).

A spin is imparted by the oscillating impeller wheel to the medium on entry into the mass throughflow meter. The torque MD to be expended for this is practically independent on the length L of the impeller wheel. In the further course of the throughflow, the medium is however centrifuged radially outwards by reason of the rotation motion. To overcome the inertia forces or d'Alembert forces (here Coriolis forces), an additional torque component AM is required which likewise does not depend on the vane length L insofar as all medium components are centrifuged completely from the inner radius R, up to the outer radius Ra of a vane 7. For flows above a certain limiting speed v,max, the influenceof the centrifugal acceleration and thereby the additional moment AM reduces with given vane length.

An annular medium mass element of length dL=VLdt (Fig. 11) and of inertia moment d#o generates a spin moment thrust: (8) dMDdt=#d#o=2#r dr#VL#dt By the differential throughflow element (9) drii=27rrdrpv, there is obtained: (10) dMD=dmr# According to equation (10), the throughflow components are weighed in dependence on radius in the case of a very short impeller wheel. The total moment

is interlinked with the total flow

by a constant which applies exactly only to a certain flow profile v,(r).For a homogeneous profile (dv,/dr=o), there applies for the spin moment for example: (11) MD=#VL#(Ra4-Ri4)=-m(Ra2+Ri2) 2 2 The radial acceleration of a circular rotating mass element dm is given by (12) r=re2 The differential equation (12) has the solution: (13) r(t)=rch(t) The mass element dm by reason of the Coriolis acceleration arising during its radial motion causes the moment: (14) dMc=dm2ur r=2## r drdL#sh(#t) The parameter time t lets itself also be replaced by the time t=L/v, for the time required for the axially traversed travel path L.The mass element dm attains the vane radius Ra at VL Ra (15) L=-Arch(-) # r The total moment of a flow line thus results in

According to equation (16), the Coriolis component of the moment appearing additionally in the case of a long impeller wheel is also weighed in dependence on radius.While however the spin component weighs the outer throughflow components more highly, it is just the inner, axially near components which experience a higher valuatiom through the Coriolis acceleration so that the entire moment of an annular element dm no longer has undesirable radius-dependent weighting: (17) dMD+dMc=dm#r+dm#(Ra2#(Ra2-r)=dm#ra2 Integrated over the entire height of the vane, the connection between the moment M and throughflow m is obtained independently of the flow profile v,(r): (18) M= Ram wherein the total moment is composed of a spin component MD according to equation (II) as well as an addition Coriolis component:: a) (19) QM=-in(Ra-R2,) 2 Rv ecuation (15) there is obtained from this condition the required vane length L:

in which, in an inhomogeneous flow v,(r), the arising Coriolis forces by reason of their centrifugal effect cause a compensation of the undesired radius valuation and the calibration factor of the throughflow meter becomes independent on the flow speed profile v,(r). For increasing the measuring sensitivity of the mass throughflow meter by combination of a spin pulse effect acting transversely to the flow and a baffle pressure effect parallel to the flow, it can be advantageous to let the medium flow at a certain angle a on the vanes of the impeller wheel.Thus, in Fig. 12, guide surfaces 53, which are inclined at an angle a to the direction of flow and which lend the flowing medium the desired spin angle a, are arranged upstream of the impeller wheel 4 with the vanes 7 directed parallel to the flow. This arrangement has the additional advantage that a falsifying spin speed, possibly present in the medium, is eliminated by the rectifying effect of the guide surfaces 53.

As a further possibility, the vanes 7 of the impeller wheel 4 themselves can, as shown in Fig. 13, be inclined at an angle a relative to the direction of flow. Special guide surfaces can then be dispensed with.

Furthermore, it is possible to construct the vanes 7 of the impeller wheel 4, as shown in Fig. 14, to be wedge-shaped in the longitudinal direction with a wedge angle 2a, wherein the surface of symmetry of the vanes 7 is directed parallel to the flow and the points of the wedge-shaped vanes are directed against the flow. In each direction of motion, a wedge surface is more strongly loaded by the flow, i.e.

damped, than a wedge-free vane. This arrangement provides the advantage of a higher measuring effect.

In the case of the mass throughflow meters shown in Figs. I to 5, the entire energy required for oscillation must be absorbed by the walls of the pipe 2, i.e. it is transmitted through the webs 5 and 6 or the carrier member 18 to the walls of the pipe 2. By contrast hereto, in the case of the mass throughflow meter according to Fig. 15, two impeller wheels 4 and 55 are co-axially arranged in the flow direction in the flow cross-section 1 and one-sidedly mounted through torsion springs 56 and 57 on a carrier body 58 which is connected fast with the pipe 2.As described above, in relation to the other embodiments, an electromagnetically generated force, balancing the damping brought about by the flowing medium, acts on the impeller wheel 4 and is brought about by the force effect between the electromagnetic field 13 and 14 and the electrical current conductors 12, while a force, displaced in phase through 1800 relative to the force engaging at the impeller wheel 4, engages at the same time in a corresponding manner through conductors 12 and the electromagnetic field 13 and 14 at the impeller wheel 55 so that the impeller wheels 4 and 55 oscillate in opposite phase. Hereby the oscillatory energies of the two oppositely oscillating impeller wheels 4 and 55 cancel each other in the carrier body 58 arid are not transmitted to the pipe walls.

The embodiments of the present invention described by way of example above have the advantage of a negligibly small pressure drop at the meter so that it meets the requirements of today on such a meter and in which neither a change of the predetermined target value nor of parameters of the mechanical system, such as for example the natural frequency, falsifies the measurement result. Furthermore, the measuring signal representing the mass flow is delivered directly as a direct current signal. Also the need for electromagnetic application of a moment required for acceleration of the inert masses is avoided. Furthermore accelerating forces engaging at the system in or against the direction of flow have no influence on the measuring result.

WHAT WE CLAIM IS: 1. A mass throughflow meter for measuring the mass throughflow of a fluid flowing along a flow path, comprising an impeller wheel mounted within the flow path to oscillate about the wheel axis, the meter being provided with electrical conductor means disposed on the wheel to be flowed past by the fluid and with means to provide a magnetic field perpendicular to the conductor means, wherein means responsive to impeller wheel velocity are disposed to detect changes in the velocity caused by the flowing fluid and to change the strength of the magnetic field by an amount which restores the velocity to that corresponding to zero throughflow and which is indicative of the mass throughflow of the fluid.

2. A mass throughflow meter as claimed in Claim 1, wherein the impeller wheel comprises vanes each having major surfaces extending parallel to the flow path.

3. A mass throughflow meter as claimed in either Claim 1 or Claim 2, wherein

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

in which, in an inhomogeneous flow v,(r), the arising Coriolis forces by reason of their centrifugal effect cause a compensation of the undesired radius valuation and the calibration factor of the throughflow meter becomes independent on the flow speed profile v,(r). For increasing the measuring sensitivity of the mass throughflow meter by combination of a spin pulse effect acting transversely to the flow and a baffle pressure effect parallel to the flow, it can be advantageous to let the medium flow at a certain angle a on the vanes of the impeller wheel.Thus, in Fig. 12, guide surfaces 53, which are inclined at an angle a to the direction of flow and which lend the flowing medium the desired spin angle a, are arranged upstream of the impeller wheel 4 with the vanes 7 directed parallel to the flow. This arrangement has the additional advantage that a falsifying spin speed, possibly present in the medium, is eliminated by the rectifying effect of the guide surfaces 53.

As a further possibility, the vanes 7 of the impeller wheel 4 themselves can, as shown in Fig. 13, be inclined at an angle a relative to the direction of flow. Special guide surfaces can then be dispensed with.

Furthermore, it is possible to construct the vanes 7 of the impeller wheel 4, as shown in Fig. 14, to be wedge-shaped in the longitudinal direction with a wedge angle 2a, wherein the surface of symmetry of the vanes 7 is directed parallel to the flow and the points of the wedge-shaped vanes are directed against the flow. In each direction of motion, a wedge surface is more strongly loaded by the flow, i.e.

damped, than a wedge-free vane. This arrangement provides the advantage of a higher measuring effect.

In the case of the mass throughflow meters shown in Figs. I to 5, the entire energy required for oscillation must be absorbed by the walls of the pipe 2, i.e. it is transmitted through the webs 5 and 6 or the carrier member 18 to the walls of the pipe 2. By contrast hereto, in the case of the mass throughflow meter according to Fig. 15, two impeller wheels 4 and 55 are co-axially arranged in the flow direction in the flow cross-section 1 and one-sidedly mounted through torsion springs 56 and 57 on a carrier body 58 which is connected fast with the pipe 2.As described above, in relation to the other embodiments, an electromagnetically generated force, balancing the damping brought about by the flowing medium, acts on the impeller wheel 4 and is brought about by the force effect between the electromagnetic field

13 and 14 and the electrical current conductors 12, while a force, displaced in phase through 1800 relative to the force engaging at the impeller wheel 4, engages at the same time in a corresponding manner through conductors 12 and the electromagnetic field 13 and 14 at the impeller wheel 55 so that the impeller wheels 4 and 55 oscillate in opposite phase. Hereby the oscillatory energies of the two oppositely oscillating impeller wheels 4 and 55 cancel each other in the carrier body 58 arid are not transmitted to the pipe walls.

The embodiments of the present invention described by way of example above have the advantage of a negligibly small pressure drop at the meter so that it meets the requirements of today on such a meter and in which neither a change of the predetermined target value nor of parameters of the mechanical system, such as for example the natural frequency, falsifies the measurement result. Furthermore, the measuring signal representing the mass flow is delivered directly as a direct current signal. Also the need for electromagnetic application of a moment required for acceleration of the inert masses is avoided. Furthermore accelerating forces engaging at the system in or against the direction of flow have no influence on the measuring result.

WHAT WE CLAIM IS: 1. A mass throughflow meter for measuring the mass throughflow of a fluid flowing along a flow path, comprising an impeller wheel mounted within the flow path to oscillate about the wheel axis, the meter being provided with electrical conductor means disposed on the wheel to be flowed past by the fluid and with means to provide a magnetic field perpendicular to the conductor means, wherein means responsive to impeller wheel velocity are disposed to detect changes in the velocity caused by the flowing fluid and to change the strength of the magnetic field by an amount which restores the velocity to that corresponding to zero throughflow and which is indicative of the mass throughflow of the fluid.
2. A mass throughflow meter as claimed in Claim 1, wherein the impeller wheel comprises vanes each having major surfaces extending parallel to the flow path.
3. A mass throughflow meter as claimed in either Claim 1 or Claim 2, wherein

the impeller wheel is resiliently mounted to a tension band fastened within the flow path.
4. A mass throughflow meter as claimed in either Claim I or Claim 2, wherein the impeller wheel is mounted to at least one pair of leaf spring members, which cross each other and which each have one end portion connected to the impeller wheel and the respective opposite end portion to means bounding the flow path.
5. A mass throughflow meter as claimed in Claim 2, wherein the axial length of each vane of the impeller wheel decreases with increasing distance from the oscillatory axis.
6. A mass throughflow meter as claimed in any one of Claims 2 to 5, wherein the length of each vane is given by

where v, is the speed of flow of the fluid, a) the effective angular speed of the impeller wheel, and Ra the largest and R the smallest radius of each vane.
7. A mass throughflow meter as claimed in any one of the preceding claims, comprising guide surfaces each inclined to the intended direction of flow and disposed in the flow path upstream of the impeller wheel.
8. A mass throughflow meter as claimed in any one of Claims 2 to 7, wherein each vane of the impeller wheel is inclined to the intended direction of flow.
9. A mass throughflow meter as claimed in any one of Claims 2 to 8, wherein each vane of the impeller wheel is wedge-shaped, symmetrical about a plane parallel to the intended direction of flow and tapers to point against the intended direction of flow.
10. A mass throughflow meter as claimed in any one of the preceding claims comprising a further such impeller wheel, each impeller wheel being co-axial with the other and being connected to a common carrier by respective torsion means to be oscillatable in opposite phase with each other.
Il. A mass throughflow meter substantially as hereinbefore described with reference to Fig. I to Fig. 3 of the accompanying drawings.
12. A mass throughflow meter substantially as hereinbefore described with reference to Fig. 4 to Fig. 5 of the accompanying drawings.
13. A mass throughflow meter as claimed in either Claim 11 or Claim 12 and modified substantially as hereinbefore described with reference to Fig. 6 or Fig. 7 of the accompanying drawings.
14. A mass throughflow meter as claimed in any one of Claims 11 to 13 and modified substantially as hereinbefore described with reference to Fig. 8 to Fig. 11 of the accompanying drawings.
15. A mass throughflow meter as claimed in any one of Claims 11 to 14 and modified substantially as hereinbefore described with reference to any one of Figs.

12 to 14 of the accompanying drawings.
16. A mass throughflow meter as claimed in any one of Claims 11 to 15 and modified substantially as hereinbefore described with reference to Fig. 15 of the accompanying drawings.