A Coriolis effect fluid flow meter comprises a tube curved about
a first axis and mounted for oscillatory motion about a second axis,
means to drive the tube in a controlled oscillatory motion about
said second axis to generate in fluid flowing in the tube a measurable
bi-direction force, a pair of flexible couplings through which fluid
flow passes arranged to minimise forces on the tube due to the inherent
stiffness of the tube, and means arrange to measure the bi-directional
force in the direction of said first axis and to derive the fluid
What is claimed is:
1. A fluid flow meter comprising a tube curved about a first axis
and mounted for oscillatory motion about a second axis, said tube
having a first end and a second end, means operatively connected
to said tube for driving the tube in a controlled oscillatory motion
about said second axis to generate in fluid flowing in the tube
a measurable bi-directional force, measurement means operatively
connected to said tube and arranged to measure the bidirectional
force, at a specific point on the curved tube, in the direction
of said first axis and to derive the fluid flow therefrom, and a
pair of flexible couplings towards either end of the tube through
which the fluid flow passes into and from the tube and by which
said tube is mounted, the flexible couplings being positioned to
facilitate movement of the tube about a third axis and minimize
forces on the tube due to the stiffness of the tube so that the
signal measured by the measurement means is independent of the mechanical
stiffness of the tube.
2. A fluid flow meter according to claim 1 wherein said first axis
is a vertical axis.
3. A fluid flow meter according to claim 1 wherein said first axis
is a horizontal axis and fluid density is obtained from the gravitational
torque of the fluid in the tube acting about a horizontal axis.
4. A fluid flow meter according to claim 1 comprising two curved
tubes, each tube including a pair of flexible couplings through
which the fluid flow passes arranged to minimise forces on the tube
due to inherent stiffness of the tube, and means to drive the tubes
in a controlled oscillatory motion in which the motion of one tube
is 180.degree. out of phase with the motion of the other tube and
means to measure the difference in oscillatory force between the
two tubes and derive the fluid flow therefrom.
5. A fluid flow meter according to any one of claims 1 to 4 wherein
said curved tube or tubes is or are a U-shaped tube or tubes.
6. A fluid flow meter according to claim 1 wherein the tube is
in the form of a loop.
7. A fluid flow meter according to any one of claim 1 to 4 or 6
wherein the tube or each of the tubes of the flow meter includes
a second pair of flexible couplings through which the fluid flow
passes mounted to facilitate the oscillatory motion of the tube
8. A fluid flow meter according to claim 5 wherein the tube or
each of the tubes of the flow meter includes a second pair of flexible
couplings through which the fluid flow passes mounted to facilitate
the oscillatory motion of the tube or tubes.
FIELD OF INVENTION
This invention relates to apparatus for measuring the flow of a
In this specification the term "fluid" is intended to
refer to any flowing substance whether it may be liquid or a gaseous
or a polyphase substance such as a liquid/solid or a liquid/gas.
BACKGROUND OF INVENTION
Various methods are known for the measurement of the flow of a
fluid mass and in particular it is known to use flow meters based
on the Coriolis effect because they are capable of measuring the
mass flow directly and are relatively insensitive to the properties,
such as density and viscosity, of the fluid being measured. However,
one of the main disadvantages of flow meters based on the Coriolis
effect measurement is because the Coriolis forces generated are
generally very small, the measurements are subject to interference
from effects which are difficult to suppress.
Boom R J in "A User's Perspective for Coriolis Flow Meters"
44th Annual Symposium on Instrumentation for the Processes Industry
1989 referred on pages 7-10 to some patents granted in the 1950's
for "gyroscopic mass flow meters". All of these early
meters involved curved members, a means of oscillation/vibration,
and some sort of flexible coupling or slip joints and all attempted
to measure the torque caused by the gyroscopic (Coriolis) effect.
Such early gyroscopic meters were failures and all exhibited a common
problem that because the Coriolis forces were small compared to
other forces, then it was not possible to obtain accurate readings.
U.S. Pat. No. 3927565 describes an apparatus for measuring the
mass flow of fluid and utilises a rectilinear pipe which can be
rotated about an axis which is not parallel with the longitudinal
axis of the pipe. The apparatus also includes a means for making
the fluid flow through the pipe and means for measuring the force
which the fluid flowing through the pipe exerts on a segment of
U.S. Pat. No. 4109524 describes another method of measuring the
mass flow rate using the Coriolis effect and describes an apparatus
which includes a conduit having sections. A section of one end of
the conduit is reciprocated and the torque generated by the Coriolis
force is measured.
U.S. Pat. No. 4252028 describes an apparatus for measuring mass
flow and utilises a plurality of channels which are provided in
one or more conduits which rotate or oscillate as a common unit.
Additive streams flow to the channels in one direction and subtractive
streams are flowed through the channels in the opposite direction
and the Coriolis force thereby imposed on the conduit is measured.
SUMMARY OF INVENTION
The invention provides an improved or at least alternative form
of Coriolis flow meter.
In broad terms in one aspect the invention comprises a flow meter
based on the Coriolis effect comprising a substantially U shaped
tube driven in a controlled oscillatory motion about an axis to
generate in the fluid within the tube a measurable bidirectional
force, said U tube including flexible couplings intermediate of
the length of the tube and adapted to allow the tube to be oscillated
at a frequency which is not necessarily the resonant frequency of
In broad terms in another aspect the comprises a fluid flow meter
based on the Coriolis effect comprising two U shaped tubes each
including flexible couplings intermediate of their length and each
being adapted to be excited by an oscillatory motion in which the
motion in one tube is 180.degree. out of phase with the other tube
and wherein the fluid flow through the tubes is in the same direction,
means being provided to measure the difference in oscillatory force
between the two tubes.
In broad terms in a further aspect the invention comprises a fluid
flow meter based on the Coriolis effect comprising two U shaped
tubes each including flexible couplings intermediate of their length
and each being adapted to be excited by an oscillatory motion in
which the motion in one tube is in phase with the other tube and
wherein the fluid flow through the tubes is in the opposite direction,
means being provided to measure the difference in oscillatory force
between the two tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described with reference to the accompanying drawings
by way of example and without intending to be limiting, in which:
FIG. 1 is a diagrammatic view of the basic principle of a Coriolis
mass flow meter;
FIG. 2 is a diagrammatic view of a known Coriolis flow meter which
utilises the tubular arrangement as shown in FIG. 1; and
FIG. 3 is a diagrammatic view of a Coriolis flow meter of the present
In the basic type of Coriolis flow meter as shown in FIG. 1 fluid
flows through the tube about the y axis and at the same time the
tube is rotated about the x axis. The two motions will result in
a torque about the z axis. The torque about the z axis is proportional
to the mass flow rate through the tube.
FIG. 2 illustrates a Coriolis mass flow meter utilising this basic
principle. A single U tube has an inlet 1 and an outlet 2 so that
fluid can flow through the tube in the direction indicated by the
arrows. The tube is fixed at AA and the tube is excited into an
oscillatory angular motion about the x axis, at a resonant frequency
determined by the stiffness of the tube, the mass of the tube, and
the fluid with the tube. Various means of providing the oscillatory
angular motion can be utilised such as a magnetic forcer indicated
at 10. The oscillatory angular motion results in Coriolis forces
being generated by the fluid as it is constrained to follow this
tube motion. These forces will result in an oscillatory twisting
of the tube about the z axis by an amount proportional to the mass
flow rate and inversely proportional to the tube stiffness. The
degree of twist and consequently the mass flow rate is obtained
by computing the difference in the transit times of the points 11
and 12 through a reference x,z plane.
This basic design of Coriolis mass flow meter is still relatively
unchanged although a variety of different tube parameters, including
bent tubes, straight tubes, single tubes and split tubes, have been
tried to improve performance and overcome the sensitivity of earlier
designs to various field and environmental effects such as temperature,
pressure, vibration and mechanical stress.
The principal problem with this basic design is that it is barely
viable. It relies on the elastic properties of the flow tube and
hence the design is always a compromise between maximising the tube
diameter for low pressure drop and minimising the tube stiffness
for high flow sensitivity. A larger diameter tube is stiffer and
twists less. Consequently, the wall of the flow tube must be thin
to minimise flexural stiffness which in combination with high flow
velocities can lead to stress and tube erosion problems.
As a result, both the pressure drop across current Coriolis flow
meters and the drop in velocity of the fluid flow is large compared
with other types of flow meter. Another problem is that the signals
to be measured are extremely small and so can lead to inaccuracies.
Typical twists in the tube are less than 10 .mu.m and transit time
differences between the points 11 and 12 are of order 10 .mu.s.
Current Coriolis flow meters are therefore sensitive to external
infuences as mentioned above, and suffer from zero instability which
limits their available range. They also depend on the stability
of the elastic properties of the flow tube which limits their potential
One reason for the design remaining relatively unchanged is that
the density of the flowing fluid can be obtained from the resonant
frequency of the tube or tubes. However, this density measurement
is also dependent on the elastic properties of the flow tube(s)
and is non-linearly related to the measure resonant frequency.
FIG. 3 shows a Coriolis flow meter of the invention which is similar
to the flow meter shown in FIG. 2 except that the flow meter can
be driven in an accurately controlled oscillatory motion about the
axis AA (see FIG. 2) at a frequency that is not necessarily a mechanical
resonance of the flow tubing. The flexible couplings 21 and 22 facilitate
this motion. The frequency and the amplitude of this torsional oscillation
are chosen to enhance the performance of the meter and both these
parameters are actively controlled during flow measurements.
The flow rate is obtained by measuring the oscillatory bi-directional
force in the Y direction at point 23 this force being proportional
to the precessional torque induced about the X axis 24 25 due to
the Coriolis effect. The force may be measured by a sensitive and
accurate sensor such as an electromagnetic force balance device
which also prevents the flow tube from rotating about the axis 24
The flexible couplings 26 27 are utilised to ensure there are
no significant forces at the point 23 resulting from tube stiffness.
Phase sensitive measurement of the force at 23 can be utilised
to improve the signal to noise ratio and hence the precision of
the flow measurement. The tube diameter can be relatively large
to keep both the fluid flow velocity and the pressure drop across
the flow meter small without degrading the performance of the meter.
The fluid density can be derived from the oscillatory torque required
to rotate the flow tube and the fluid within the tube about the
axis AA which will be directly proportion to fluid density. This
torsional drive could be electromagnetic in which case the fluid
density will be proportional to the drive current.
In an alternate form of the invention, if the plane of the flow
meter is horizontal instead of vertical as indicated in FIG. 3
the fluid density can be obtained from the gravitational torque
of the fluid in the tube acting about the axis 24 25. In this form
the fluid density will be proportional to the average force at 23
in the y direction. The use of relatively large tube diameter will
improve the accuracy of the density measurement by increasing either
the mass of fluid accelerated into torsional oscillation or the
mass of fluid exerting a force at the point 23 due to gravity.
The essential feature of the invention is the provision of the
flexible couplings which enables the sensing and oscillatory drive
components to be changed. For example a variation of the modification
shown in FIG. 3 may have the same basic configuration as the mass
flow meter shown in FIG. 2 and in this variation the oscillatory
motion is applied at the point 23 about the axis 24 25 and by utilising
bi-directional force sensors to measure the fluid mass flow at points
24 and 25.
Variations of the invention can be applied by utilising a double
loop version of the mass flow meter indicated in FIG. 3 to which
two tubes are located adjacent to each other and with the fluid
flow through the tubes being in the same direction while the oscillatory
motions of the two tubes are separated in phase by 180.degree..
In this case the sensor at 23 will measure the difference in force
between the two loops.
In a yet further variation of the double loop version the two tubes
can be oscillated in unison about the axis AA and the flows can
be in the tubes in opposite directions.
The foregoing describes the invention and modifications as will
be obvious to those skilled in the art are intended to be included
within the scope of the invention, as defined in the following claims.