Abstrict A mass flow meter for flowing media, which operates according to
the Coriolis principle, has a straight Coriolis measuring tube,
an oscillation generator acting on the Coriolis measuring tube,
and at least one sensor detecting Coriolis forces and/or Coriolis
oscillations based on Coriolis forces. The problem which results
from the fact the mass flow meter has only one straight Coriolis
measuring tube is largely eliminated by designing the Coriolis measuring
tube as a flow channel of a thick-walled body, namely a thick-walled
tube, the thick-walled tube having recesses accessible from the
outside and reaching very close to the Coriolis measuring tube.
A vibration transducer as the oscillation generator acts upon the
residual material of the thick-walled tube remaining in the area
of the recesses, and the Coriolis forces or Coriolis oscillations
appearing in the area of the residual material of the thick-walled
tube are detected by each sensor.
Claims What is claimed is:
1. A mass flow meter for flowing media, which operates according
to the Coriolis principle, with one at least essentially straight
Coriolis measuring tube having a thick wall with an outside and
an inside surface defining a flow channel with at least one vibration
converter acting on the Coriolis measuring tube, so that a pressure
wave is propagated through the wall of the tube, said pressure wave
having a wavelength, and with at least one sensor detecting at least
one of Coriolis forces and Coriolis oscillations based on Coriolis
forces, wherein said vibration converter is on the outside of the
Coriolis measuring tube and the vibration frequency generated by
the vibration converter is such that the thickness of said tube
wall is an integral multiple of half the wavelength of the pressure
wave propagating through the tube wall whereby a maximum deflection
of said inner surface takes place.
2. A mass flow meter in accordance with claim 1 wherein the Coriolis
measuring tube consists of metal or of a metal alloy.
3. The mass flow meter in accordance with claim 1 wherein the
Coriolis measuring tube consists of plastic, in particular of polytetrafluorethylene
(PTFE), of perfluoroalkoxy-polymer (PFA), or of polyetheretherketone
(PEEK).
4. The mass flow meter in accordance with any one of the claims
1 to 3 wherein said Coriolis measuring tube has a round, an oval,
an elliptical, or a rectangular, in particular a square, cross-section.
5. The mass flow meter in accordance with any one of the claims
1 to 3 wherein the outside of said thick wall has a round, or oval,
an elliptical, or a rectangular, in particular a square, cross-section.
6. The mass flow meter in accordance with any one of the claims
1 to 3 wherein at least one temperature sensor is provided for
compensation of thermal influences on the measurement accuracy and
the zero point of the mass flow meter.
7. The mass flow meter in accordance with any one of the claims
1 to 3 wherein two oscillation generators are provided and both
oscillation generators are arranged symmetrical to the longitudinal
axis of the Coriolis measuring tube.
8. The mass flow meter in accordance with claim 7 wherein the
oscillation generators are operated in phase.
9. The mass flow meter in accordance with claim 7 wherein the
oscillation generators are operated in phase opposition.
10. The mass flow meter in accordance with any one of the claims
1 to 3 wherein the sound frequency is adapted to the flowing medium--by
control or regulation--so that even-numbered multiples or even-numbered
fractions of the wave length in the flowing medium correspond at
least approximately to the inner diameter of the Coriolis measuring
tube or the distance from one inner wall to the opposite inner wall
of the Coriolis measuring tube.
11. The mass flow meter in accordance with any one of claims 1-3
wherein at least one temperature sensor is provided for compensattion
of thermal influences on the measurement accuracy or the zero point
of the mass flow meter.
12. A mass flow meter for flowing media which operates according
to the Coriolis principle with one at least essentially straight
Coriolis measuring tube having a thick wall with an outside and
an inner surface defining a flow channel, with at least one sensor
detecting at least one of Coriolis forces and Coriolis oscillations
based on Coriolis forces, and with two vibration converters being
provided on the outside of the Coriolis measuring tube opposite
to each other and generating vibrations so that a pressure wave
is propagated through the tube wall, said pressure wave having a
wavelength, the frequencies of said vibrations being such that the
thickness of said tube wall is an integral multiple of half the
wavelength of said pressure wave whereby maximum deflection of said
inner surface takes place.
13. A mass flow meter in accordance with claim 12 wherein the
Coriolis measuring tube consists of metal or of a metal alloy.
14. The mass flow meter in accordance with claim 12 wherein the
Coriolis measuring tube consists of plastic, in particular of polytetrafluorethylene
(PTFE), of perfluoroalkoxy-polymer (PFA), or of polyetherketone
(PEEK).
15. The mass flow meter in accordance with claim 12 wherein said
Coriolis measuring tube has a round, an oval, an elliptical, or
a rectangular, in particular a square, cross-section.
16. The mass flow meter in accordance with claim 12 wherein at
least one temperature sensor is provided for compensation of thermal
influences on the measurement accuracy and the zero point of the
mass flow meter.
17. The mass flow meter in accordance with claim 12 wherein both
vibration generators are arranged symmetrical to the longitudinal
axis of the Coriolis measuring tube.
18. The mass flow meter in accordance with claim 17 wherein the
vibration generators are operated in phase.
19. The mass flow meter in accordance with claim 17 wherein the
vibration generators are operated in phase operation.
20. The mass flow meter in accordance with claim 12 wherein the
sound frequency is adapted to the flowing medium--by control or
regulation--so that even-numbered multiples or even-numbered fractions
of the wavelength in the flowing medium correspond at least approximately
to the inner diameter of the Coriolis measuring tube or the distance
from one inner wall to the opposite inner wall of the Coriolis measuring
tube.
21. The mass flow meter in accordance with claim 12 wherein at
least one temperature sensor is provided for compensation of thermal
influences of the measurement accuracy or the zero point of the
mass flow meter.
Description The invention concerns a mass flow meter for flowing media, which
operates according to the Coriolis principle, with one at least
essentially straight Coriolis measuring tube, with at least one
oscillation generator acting on the Coriolis measuring tube, and
with at least one sensor detecting Coriolis forces and/or Coriolis
oscillations based on Coriolis forces.
BACKGROUND OF THE INVENTION
Mass flow meters for flowing media, which operate according to
the Coriolis principle, are known in different designs (cf. e.g.
German Patent Specification 41 24 295 and German Offenlegungsschrift
41 43 361 and the publications cited there in each case in column
1 lines 20 to 27 German Patent Specification 42 24 397 and the
publications cited there in column 1 lines 23 to 30 as well as
German Offenlegungsschrift 196 01 342) and have been increasingly
used in practice for some time.
In the case of mass flow meters for flowing media, which operate
according to the Coriolis principle, basically, there are, on the
one hand, those whose Coriolis measuring tube is made at least essentially
straight, as a rule exactly straight, and, on the other hand, those
whose Coriolis measuring tube is made loop-shaped. In addition,
in the case of the mass flow meters under discussion, there are,
on the one hand, those which have only one Coriolis measuring tube
and, on the other hand, those which have two Coriolis measuring
tubes. In the case of the embodiments with two Coriolis measuring
tubes, they can be hydraulically in series or parallel to one another.
Mass flow meters of the type in question, in the case of which
the Coriolis measuring tube is, or the Coriolis measuring tubes
are, made straight, with respect to their mechanical construction
are simple and can consequently be produced at relatively low cost.
In this case, it is also possible to finish or polish the inner
surfaces of the Coriolis measuring tube or Coriolis measuring tubes
well; they can be polished easily. In addition, they have a relatively
low pressure loss. In the case of mass flow meters which operate
according to the Coriolis principle, and whose Coriolis measuring
tube is made straight, or the Coriolis measuring tubes are made
straight, it can be disadvantageous that both thermally caused expansions
or stresses, as well as forces and moments acting from outside,
can lead to measurement errors and to mechanical damage, namely
stress cracks.
The experts have already dealt with the above-mentioned problems
in mass flow meters with straight Coriolis measuring tubes (cf.
in particular German Patent Specification 41 24 295 German Offenlegungsschrift
41 43 361 and German Patent Specification 42 24 379). Altogether,
a mass flow meter operating according to the Coriolis principle,
with a straight Coriolis measuring tube, which has a measurement
error of only about 0.1% (cf. the prospectus "Zulassung des
Corimass G-Gerates zum eichpflichtigen Verkehr" of the KROHNE
Me.beta.technik GmbH & Co. KG), was made successfully.
Mass flow meters operating according to the Coriolis principle,
which have only one straight Coriolis measuring tube, have considerable
advantages as compared with those mass flow meters which have either
two straight Coriolis measuring tubes or one loop-shaped Coriolis
measuring tube. The advantage as compared with mass flow meters
with two straight Coriolis measuring tubes in particular is to be
seen in the fact that flow separators or flow combiners, which are
required in the case of mass flow meters with two Coriolis measuring
tubes, are not needed. The advantage as compared with flow meters
with one loop-shaped Coriolis measuring tube, or with two loop-shaped
Coriolis measuring tubes, in particular is to be seen in the fact
that a straight Coriolis measuring tube is easier to produce than
a loop-shaped Coriolis measuring tube, that the pressure drop in
the case of a straight Coriolis measuring tube is less than in the
case of a loop-shaped Coriolis measuring tube, and that a straight
Coriolis measuring tube can be cleaned better than a loop-shaped
Coriolis measuring tube.
However, mass flow meters which operate according to the Coriolis
principle and have one straight Coriolis measuring tube, also have
a physically, or mechanically, predetermined disadvantage (cf. European
Offenlegungsschrift 0 521 439).
Mass flow meters operating according to the Coriolis principle
require that the Coriolis measuring tube be put into oscillation
by means of at least one oscillation generator; the Coriolis forces,
or the Coriolis oscillations, do indeed result from the fact that
the Coriolis measuring tube oscillates and from the flowing of mass
through the Coriolis measuring tube.
In the case of mass flow meters with two straight Coriolis measuring
tubes, or with one loop-shaped Coriolis measuring tube, or with
two loop-shaped Coriolis measuring tubes, the Coriolis measuring
tubes, or the parts of the loop-shaped Coriolis measuring tubes
causing oscillation, are designed identically and located and excited
into oscillation so that they oscillate opposite one another. This
has the positive consequence that the oscillating system as a whole
is not acting as such outwards. The position of the center of mass
remains constant and forces which appear are compensated. Consequently,
no oscillations are introduced into the pipeline system in which
this mass flow meter is installed, and oscillations of the pipeline
system do not influence the measurement result.
In the case of mass flow meters operating according to the Coriolis
principle, which have only one straight Coriolis measuring tube,
the positive consequence of Coriolis measuring tubes oscillating
opposite one another, explained above, naturally does not occur.
The center of mass does not remain constant and forces which appear
are not compensated. The consequence of this is, on the one hand,
that oscillations are transferred into the pipeline system in which
a mass flow meter is installed, and on the other hand, that oscillations
of the pipeline system can also influence the measurement result.
SUMMARY OF THE INVENTION
The object of the invention now is to provide a mass flow meter
operating according to the Coriolis principle, in the case of which
the problem, previously discussed in detail, which results from
the fact that the mass flow meter has only one straight Coriolis
measuring tube, is of less consequence.
The mass flow meter in accordance with the invention, in the case
of which the problem derived and presented previously in detail
is solved, now in the first place and essentially is characterized
by the fact that the Coriolis measuring tube is designed as a flow
channel of a thick-walled body, in particular a thick-walled tube,
that the thick-walled tube has recesses accessible from the outside,
reaching very close to the Coriolis measuring tube, that the oscillation
generator acts upon the residual material of the thick-walled tubes
remaining in the area of the recesses, and that the Coriolis forces
or Coriolis oscillations appearing in the area of the residual material
of the thick-walled tube are detected by the sensor or the sensors.
In the case of the mass flow meter in accordance with the invention,
the mass of the residual material of the thick-walled tube acting
as a Coriolis measuring tube as a whole is relatively small in relation
to the mass of the thick-walled tube. From this it results that
the "center of mass not constant" problem discussed initially
does remain qualitatively, but quantitatively has practically no
effect. This may be seen as a summary of the advantages achieved
by the invention.
In particular, there are now a number of possibilities for designing
and further developing the mass flow meter in accordance with the
invention. We refer, on the one hand, to the patent claims, and,
on the other hand, to the description of the preferred embodiments
in connection with the drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 shows a longitudinal section through a first embodiment
of a mass flow meter in accordance with the invention,
FIG. 2 shows a longitudinal section through a second embodiment
of a mass flow meter in accordance with the invention,
FIG. 3 shows a longitudinal section through a third embodiment
of a mass flow meter in accordance with the invention,
FIG. 4 shows a longitudinal section through a fourth embodiment
of a mass flow meter in accordance with the invention,
FIG. 5 shows a longitudinal section through a fifth embodiment
of a mass flow meter in accordance with the invention,
FIG. 6 shows a longitudinal section through a sixth embodiment
of a mass flow meter in accordance with the invention,
FIG. 7 shows a longitudinal section through a seventh embodiment
of a mass flow meter in accordance with the invention,
FIGS. 8a to 8c show a longitudinal section through an eighth embodiment
of a mass flow meter in accordance with the invention,
FIG 9. shows a possible cross-section of a mass flow meter in accordance
with the invention,
FIG. 10 shows a further possible cross-section of a mass flow meter
in accordance with the invention,
FIG. 11 again shows a possible cross-section of a mass flow meter
in accordance with the invention,
FIG. 12 shows a cross-section through an entirely different embodiment
of a mass flow meter in accordance with the invention, and
FIG. 13 shows a schematic representation for explaining another
entirely different embodiment of a mass flow meter in accordance
with the invention.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
The mass flow meter for flowing media in accordance with the invention
is one which operates according to the Coriolis principle. In the
first place, as a rule, but not necessarily, the mass flow meter
in accordance with the invention has a housing not shown in the
figures. An at least essentially, as a rule and in the embodiments
shown exactly, straight Coriolis measuring tube 1 at least one
oscillation generator 2 acting on the Coriolis measuring tube 1
and at least one sensor 3 as a rule two sensors 3 detecting Coriolis
forces and/or Coriolis oscillations based on Coriolis forces are
functionally necessary to the mass flow meter in accordance with
the invention.
As FIGS. 1 to 5 and 7 to 11 show, the Coriolis measuring tube 1
is made as a flow channel of a thick-walled body, namely a thick-walled
tube 4. The thick-walled tube 4 has recesses 5 accessible from outside
and reaching close to the Coriolis measuring tube 1. In this way,
only residual material 6 remains in the areas of the recesses 5
of the thick-walled tube 4; therefore the otherwise thick-walled
tube 4 has--at least partially--a relatively small wall thickness
in the area of the recesses 5. The oscillation generator 2 acts
on the residual material 6 of the otherwise thick-walled tube 4
remaining in the area of the recesses 5. This leads to the fact
that the relatively thin-walled residual material 6 of the otherwise
thick-walled tube 4 acting as the actual Coriolis measuring tube
1 is made to oscillate, so that Coriolis forces and resulting Coriolis
oscillations appear in connection with the mass of a flowing medium.
The Coriolis forces, or the Coriolis oscillations based on these
Coriolis forces, appearing in the area of the residual material
6 of the otherwise thick-walled tube 4 are detected by each sensor
3 and then evaluated in the usual way.
In the case of the mass flow meter in accordance with the invention,
the thick-walled tube 4 can consist of metal, or of a metal alloy,
or of plastic, in particular of polytetrafluorethylene (PTFE), of
perfluouro-alkoxy-polymer (PFA), or of polyether etherketone (PEEK).
With respect to the geometry of the Coriolis measuring tube 1
and with respect to the geometry of the thick-walled tube 4 the
designer carrying out the theory of the invention has considerable
freedom here in particular. Both the Coriolis measuring tube 1 as
well as the thick-walled tube 4 can have a round, an oval, an elliptical,
or a rectangular, in particular square, cross-section. FIGS. 9
10 and 11 show only as examples different cross-sectional shapes
with respect to the Coriolis measuring tube 1 and the thick-walled
tube 4.
In the case of mass flow meters which operate according to the
Coriolis principle, thus also in the case of the mass flow meter
in accordance with the invention, thermal influences can affect
the measurement sensitivity and/or the zero point--and thus the
measurement accuracy of the flow meter as a whole. Consequently,
there is a proposal to provide at least one temperature sensor T
(FIG. 7) in the flow meter, in order to compensate for thermal influences
on the measurement sensitivity and/or on the zero point of the flow
meter.
Initially, it was stated that mass flow meters of the type under
consideration, therefore also the mass flow meter in accordance
with the invention, include at least one oscillation generator 2
acting on the Coriolis measuring tube 1. However, preferably, as
also is shown partially in the drawing figures, two oscillation
generators 2 are provided and the two oscillation generators 2 are
arranged symmetrical to the longitudinal axis of the Coriolis measuring
tube 1 respectively to the thick-walled tube 4. In this case, the
oscillation generators 2 can be operated in phase or in phase opposition.
There follows the details of the embodiments of mass flow meters
in accordance with the invention shown in FIGS. 1 to 8.
The theory embodied in the mass flow meter in accordance with the
invention, as discussed previously, essentially consists in the
fact that the Coriolis measuring tube 1 is made as a flow channel
of a thick-walled body, in particular a thick-walled tube 4 and
that the thick-walled tube 4 has recesses 5 accessible from outside,
reaching relatively close to the Coriolis measuring tube 1 so that
only relatively thin-walled residual material 6 remains in the area
of the recesses 5. The recesses 5 provided in accordance with the
invention can, as is shown in FIG. 5 be realized on one side on
the initially thick-walled tube 4 as is shown in FIGS. 1 to 4 and
6 to 8. However, there is also the possibility, as is shown in FIG.
5 of making the recesses 5 on both sides in the otherwise thick-walled
tube 4. This is required when, as is also shown in FIG. 5 two oscillation
generators 2 are provided and the oscillation generators 2 are to
be located symmetrical to the longitudinal axis of the Coriolis
measuring tube 1 or of the thick-walled tube 4.
Moreover, in the mass flow meter in accordance with the invention,
the oscillation generator 2 (or the two oscillation generators 2)
and the sensor(s) 3 can be designed completely differently.
In the embodiment shown in FIG. 1 the oscillation generator 2
and the sensors 3 are designed conventionally in the broadest sense.
In the case of the embodiment shown in FIG. 2 the oscillation generator
2 and the sensors 3 in connection with a common carrier 7 are made
out of piezo-material, separate layers for the oscillation generator
2 and the sensors 3 being provided. The embodiment shown in FIG.
3 concerns a magneto-restrictive oscillation generator 2 while
the sensors 3 consist of piezo-material and they project out of
the recesses 5 of the thick-walled tube 4 so that the sensors 3
are largely decoupled from the temperature of the thick-walled tube
4. FIG. 4 shows an embodiment whose oscillation generator 2 is operated
in longitudinal resonance, while the sensors 3 are magneto-dynamic
sensors. The embodiment shown in FIG. 5 as already discussed, has
two oscillation generators 2 which can be operated either in phase
or in phase opposition. It is to be noted here that basically it
is also possible to have embodiments of mass flow meters in accordance
with the invention, in the case of which one and the same component
acts both as an oscillation generator 2 and as a sensor 3. However,
in the embodiment shown in FIG. 5 the sensors 3 are designed as
ultrasonic transmitters and receivers. The ultrasonic waves emitted
by the sensors 3 are introduced into the Coriolis measuring tube
1 and reflected back through the inner wall of the Coriolis measuring
tube 1--through an acoustic impedance step change--and received
by the sensors 3 again. The Coriolis forces or the Coriolis oscillations
resulting therefrom acting on the Coriolis measuring tube 1 can
be evaluated by interference measurement. In the case of this embodiment
the thick-walled tube 4 preferably consists of a material with a
low modulus of elasticity E. In the embodiment shown in FIG. 6
it is made clear that there are two oscillation generators 2 which
are located and mechanically coupled on both sides of the thick-walled
tube 4. The embodiment shown in FIG. 7 provides a magnetic oscillation
generator 2 which excites the Coriolis measuring tube 1 therefore
the residual material 6 of the otherwise thick-walled tube 4 in
the bending mode. Moreover, FIG. 7 shows an embodiment in the case
of which three sensors 3 are provided, it being possible to use
the middle sensor 3 for correction or compensation purposes.
FIGS. 8a, 8b, and 8c, show an embodiment of the mass flow meter
in accordance with the invention, in which it is shown that different
excitation modes can be realized. One excitation mode is indicated
in FIG. 8a, another in FIG. 8b. FIG. 8c shows a concrete embodiment
for realizing the excitation mode shown in FIG. 8b. In the case
of this embodiment, the oscillation generator 2 is designed to be
magnetically inductive. It is immediately clear that this embodiment
and application of the oscillation generator 2 lead to the excitation
mode shown in FIG. 8b.
It has been explained above that, in the case of the mass flow
meter in accordance with the invention, the mass of the residual
material 6 of the thick-walled tube 4 acting as a Coriolis measuring
tube 1 is relatively small in relation to the mass of the thick-walled
tube 4 as a whole, and that from this it results that the "center
of mass not constant" problem explained initially does remain
qualitatively, but quantitatively has practically no effect when
the Coriolis oscillations--in the case of a thick-walled tube 4--are
decidedly small. This is attainable when, according to a further
embodiment of the invention oscillation, generators in the form
of vibration converters 8 are provided on the outside of the thick-walled
tube 4 and the vibration frequency is chosen so that a maximum deflection
of the inner wall of the thick-walled tube 4 takes place. FIGS.
12 and 13 show embodiments of the previously described mass flow
meter in accordance with the invention, in the case of which the
recesses provided in the case of the embodiments of the mass flow
meters in accordance with the invention described above are not
realized. High cross-currents to the direction of flow, which in
connection with the mass of the flowing medium lead to relatively
great Coriolis forces, the reaction of which on the inner wall of
the thick-walled tube 4 can be detected, appear as a result of the
theory of the invention described previously.
Finally, it can be advantageous to adapt the sound frequency to
the flowing medium--by control or regulation--so that even-numbered
multiples or even-numbered fractions of the wave length in the flowing
medium correspond at least approximately to the inner diameter of
the Coriolis measuring tube 1 or the distance from one inner wall
to the opposite inner wall of the Coriolis measuring tube 1.
The oscillation generators in the form of vibration converters
8 i.e., vibration transducers, are preferably provided on the outside
of the thick-walled tube 4 acting on the Coriolis measuring tube
1 opposite to each other. The thick-walled tube 4 is exited by the
vibration converters 8 such that a maximum deflection of the inner
wall of the inner surface of the thick-walled tube 4 takes place.
A typical vibration converter or vibration transducer is comprised
of an electrical coil with an axially movable ferromagnetic core.
When an AC current is applied to the coil, the core performs an
harmonic oscillation relative to the coil.
The effect which is achieved with such an arrangement is the following.
The vibration converters provided on the outside of the thick-walled
tube generate pressure waves which propagate radially through the
wall of the Coriolis measuring tube as variations of density of
the material of the tube. At the interface between the media flowing
through the tube and the media of the Coriolis measuring tube, these
density waves are partly reflected. The waves generated by the vibration
converter and the reflected wave have the same frequency and the
same amplitude. Two waves with the same frequency and the same amplitude
propagating in opposite directions towards each other generate a
standing wave. This standing wave has oscillation nodes with a distance
from each other of half the wavelength, and between these nodes
antinodal points with maximum vibration. On both sides of the nodes,
the particles vibrate in phase such that the media in the nodes
is alternately compressed and dilated, respectively. Nodes of the
velocity of the movement of the particles refer to antinodes of
the pressure or density. The pressure wave and the wave of the deflection
of the particles have a phase different of .pi..
At the position where the vibration converter is fixed, a pressure
node, i.e., a maximum deflection is generated. In order to achieve
a maximum deflection of the inner surface of the thick-walled tube,
a pressure node has to be generated at this surface too. Thus, for
achieving a maximum deflection of the inner surface of the thick-walled
tube, the vibration frequency of the vibration converter has to
be chosen such that the thickness of the thick-walled tube has to
be an integral multiple of half the wavelength of the pressure wave
propagating through the wall of the tube. |