A device having a first flow meter (2) and a second flow meter
(3). The first and the second flow meters work according to a Coriolis
principle. The first flow meter includes a first measuring tube
(7). The second flow meter includes a second measuring tube (10).
The first flow meter and the second flow meter are disposed in a
common housing (456). The first flow meter and the second flow
meters have different eigenfrequencies because of a first vibration-influencing
device (18) attached to the first flow meter and a second vibration-influencing
device (19) attached to the second flow meter.
What is claimed is:
1. A device comprising: a first flow meter, working according to
Coriolis principle, said first flow meter including a first measuring
tube; a second flow meter, working according to Coriolis principle,
said second flow meter including a second measuring tube; said first
flow meter and said second flow meter being disposed in a common
housing and said first flow meter and said second flow meter having
different eigenfrequencies because of a first vibration-influencing
device attached to the first flow meter and a second vibration-influencing
device attached to the second flow meter.
2. The device of claim 1 wherein the first vibration-influencing
device is located at a relative position different from a relative
position of the second vibration-influencing device.
3. The device of claim 2 wherein at least one of the first and
the second vibration-influencing devices is configured as a passive
4. The device of claim 2 wherein the first and the second flow
meters have similar structures except for the first and the second
5. The device of claim 2 wherein the first and the second measuring
tube each has at least two loops, the two loops in the first measuring
tube being interconnected by at least a first coupling element that
forms the first vibration-influencing device, the two loops in the
second measuring tube being interconnected by at least a second
coupling element that forms the second vibration-influencing device.
6. The device of claim 5 wherein a third and a fourth coupling
elements are provided for the first and the second measuring tubes,
7. The device of claim 6 wherein the coupling elements on each
measuring tube are arranged equidistantly from the ends of the respective
8. The device of claim 5 wherein the coupling elements are plates
and the corresponding measuring tubes are aligned perpendicularly
to the plate.
9. The device of claim 8 wherein the plates are fixed relative
to a first chassis corresponding to the first flow meter and a second
chassis corresponding to the second flow meter, wherein the plates
corresponding to the first flow meter are fixed at a first locating
position on the first chassis and the plates corresponding to the
second flow meter are fixed at a second locating position on the
10. The device of claim 9 wherein the first and the second locating
positions are defined by structures on the first and the second
11. The device of claim 10 wherein the plates are configured to
be inserted into the corresponding structures.
12. The device of claim 10 wherein the first and the second locating
positions are defined by at least three projecting parts each located
in the first and the second chassis, said projecting parts protruding
toward the first and the second measuring tubes respectively.
13. The device of claim 9 wherein the first and the second locating
positions are spaced apart from each other by a distance on the
order of one centimeter.
14. The device of claim 5 wherein a respective strain gauge is
disposed on each of the first and the second coupling elements.
15. The device of claim 1 wherein the first flow meter has a first
electronic control unit and the second flow meter has a second electronic
control unit, wherein the first and the second electronic control
units monitor each other.
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a flow meter device having a first flow
meter, which works according to the Coriolis principles and is provided
with a first measuring tube, and a second flow meter, which works
according to the Coriolis principle and is provided with a second
A flow meter device of this type is known from W. Kiehl, "Difference
measurement using Coriolis mass flowmeters," Flow Meas. Instrum.,
Vol. 2 April 1991 pp. 135 to 138. The use of two Coriolis flow
meters is advantageous, for example, whenever the difference between
two mass flows is to be determined. Determining this difference
is useful if one wants to get information about a leak, for example.
To measure the difference, the first flow meter is used to measure
a first mass flow and a second flow meter to measure the second
mass flow, and the difference between the two mass flows is then
To ensure that the conditions created for the two measurements
are as similar as possible, the two separate flow meters are usually
accommodated in the same housing. To simplify production, the designs
of the two flow meters are practically identical. This also facilitates
the subsequent analysis of the individual signals.
However, when two identical flow meters are used in the same housing,
problems may arise by the two flow meters influencing each other.
In flow meters that work according to the Coriolis principle vibrations
are produced. These vibrations also act on the measuring tubes.
The phase difference between the vibrations along different sections
of the measuring tube is a measure of the mass flow. Under unfavorable
circumstances, however, these vibrations are also transmitted through
the common housing from one flow meter to the other. If both flow
meters vibrate at the same frequency, this transmission causes significant
interference and can distort the flow measurement results.
OBJECTS OF THE INVENTION
One object of the invention is to avoid crosstalk between the flow
SUMMARY OF THE INVENTION
This and other objects are attained by flow meter devices, such
as the ones disclosed, in which the two flow meters are disposed
in a common housing and have different eigenfrequencies.
Because of the different eigenfrequencies, the effects of crosstalk
are reduced. The different eigenfrequencies make it possible to
keep the mutual influence of the two flow meters small enough such
that there is no, or only a tolerable level of, interference with
the measurement result.
In an embodiment, vibration-influencing device is mounted in a
different location on the first measuring tube than on the second
measuring tube. Using the vibration-influencing devices on the two
measuring tubes, the eigenfrequencies of the flow meters are adjusted
differently in relation to each other. The difference between the
eigenfrequencies need not be large. It has been shown that an eigenfrequency
difference of approximately 10 Hz is sufficient for the effects
of crosstalk, i.e., the mutual influence, to become small enough
to be negligible.
The use of a vibration-influencing device on each measuring tube
is a relatively simple measure. The design of the two flow meters
can be practically identical, that is, there is no need to select
a different thickness of material for the measuring tube in one
flow meter than for the measuring tube in the other flow meter.
Nor is it necessary to make any fundamental modifications in the
flow meter. Both flow meters can be configured identically with
respect to the positioning of the sensors and the exciter. If the
two flow meters are identical, or at least practically identical,
they also produce readily comparable results.
Preferably, the vibration-influencing device is designed as a passive
device. In other words, no additional energy is required to generate
different eigenfrequencies in the two flow meters. The structural
complexity is also reduced. Passive devices are much simpler to
produce than active devices. The active devices require an electromagnet,
for example, or some other excitation means.
The two flow meters are preferably identical, with the exception
of the vibration-influencing device. This facilitates not only their
production, as described above. Only a single type of flow meter
needs to be produced. It also facilitates the analysis of the measurement
signals because the measurement signals are in principle based on
the same conditions.
Preferably, the first and the second measuring tube each have at
least two loops that are interconnected by at least one coupling
element, such that the coupling element is the vibration-influencing
device. The configuration of a flow meter with a single measuring
tube having at least two loops is known from WO 92/19940 A1. Between
the loops coupling elements are provided whose function is to prevent
the two loops from vibrating away from each other during operation.
In principle, the coupling elements act as a fixed vibration point
or a vibration node. The vibration of the measuring tube is limited
to one side of the coupling elements while the other side is largely
free from vibrations. This has the advantage that the measuring
tube can be connected to a line without the vibrations being transmitted
to that line. If the coupling elements can simultaneously be used
to generate different eigenfrequencies in the two flow meters, the
resulting structure is relatively simple.
Preferably, two coupling elements are provided for each measuring
tube. This results in a largely symmetrical configuration relative
to the flow through the measuring tube.
The coupling elements on each measuring tube are preferably arranged
equidistantly from the ends of the respective measuring tube. This
has the particular advantage that the coupling elements on the one
measuring tube can be arranged farther away from the ends than those
on the other measuring tube. This changes the eigenfrequency in
a simple manner. However, the measurement result in the measuring
tube itself remains largely unaffected by the coupling elements.
The coupling elements are preferably plates, and the measuring
tubes are aligned perpendicularly to the plates in the area of the
coupling elements. The rigidity of the plates perpendicular to the
direction of movement of the measuring tubes is sufficient to effectively
absorb the vibrations. At the same time, due to the relatively high
rigidity of the plates, the eigenfrequencies of the measuring tubes
are effectively influenced.
The plates of a flow meter are preferably fixed relative to a chassis
of this flow meter. The two chassis each have at least two locating
positions. The plates of the first flow meter are fixed at a first
locating position and the plates of the second flow meter at a second
locating position. The same chassis or housing parts can in turn
be used. Basically, the difference between the eigenfrequencies
of the two flow meters results simply from the fact that the plates
are fixed at different positions, i.e., the so-called locating positions
in each chassis. This is a relatively simple measure, which does
not require any major structural changes in the chassis. This facilitates
production since only a single type of chassis needs to be produced.
The locating positions are preferably created by means of structures
on the chassis. These structures primarily determine the site of
the locating positions. At the same time, structures can also be
used to mechanically fix the individual plates or auxiliary elements.
The structures preferably enable an insertion of the plates. This
facilitates the assembly. The plates merely need to be inserted
into the structure, which defines the respectively desired locating
A locating position is preferably defined by at least three projecting
parts of the chassis, which protrude toward the measuring tube.
The three projecting parts are arranged in a triangle. The plates
can then be inserted into the chassis, such that two projecting
parts are arranged on one side of the plate and one or more projecting
parts on the other side. This configuration adequately secures the
plates within the chassis.
The locating positions are preferably spaced apart by a distance
on the order of one centimeter. Hence, the distance between the
locating positions, that is, the distance between the individual
coupling elements can be relatively small. It has been found that
even small differences are enough to sufficiently change the eigenfrequencies.
Strain gauges are preferably arranged on each of the coupling elements.
These strain gauges are inexpensive. They can register the relative
longitudinal change of the coupling elements. This change can be
used as a measure of the mass flow. In addition, the measurement
by means of strain gauges includes a differential measurement of
the curvature of the measuring tubes, such that the dependence on
the flow direction can be clearly reduced.
Each flow meter preferably has an electronic control unit, such
that the electronic control unit of the one flow meter monitors
the electronic control unit of the other flow meter. The two control
units can very well be structurally combined, e.g., on a common
printed circuit board. However, the printed circuit board is divided
into two sections, which are functionally completely separate, such
that each section controls one flow meter. Furthermore, the two
sections monitor each other by means of a monitoring circuit. If
one section fails, the other section will take over the control
and measurement. This has the advantage of ensuring a reliable operation
even in differential flow measurements, which have to meet high
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in greater detail, by way of
example, with reference to a preferred embodiment depicted in the
drawing, in which:
FIG. 1 shows a flow meter device with two flow meters,
FIG. 2 shows a single flow meter,
FIG. 3 depicts a housing section of a flow meter,
FIG. 4 illustrates a flow meter during assembly,
FIG. 5 shows a coupling element,
FIG. 6 depicts a mounting plate containing a coupling element,
FIG. 7 shows the rest of the mounting plate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A flow meter device 1 depicted in FIG. 1 has two individual flow
meters 2 3 the chassis 4 5 of which are mounted to a common front
The flow meter 2 has a measuring tube 7 which is run in two loops.
Sensors 8 and a driver 9 are disposed between the two loops. The
flow meter 3 likewise has a measuring tube 10 which is run in two
loops. Two sensors 11 and a driver 12 are located between the loops
of the measuring tube 10.
The measuring tubes 7 10 can be formed by parallel loops, that
is to say, by two separate measuring tubes through which the fluid
flows. It is also possible, however, to design the measuring tubes
7 10 as continuous tubes as disclosed in WO 92/19940 A1.
FIG. 2 shows an individual flow meter 2 in detail. The two measuring
tube loops 7a, 7b are mounted on the chassis 4. The measuring tube
7a has an inlet 13a and an outlet 14a. The measuring tube 7b has
an inlet 13b and an outlet 14b. The inlets and outlets 13a, 13b
and 14a, 14b are guided through a base part 15 of the chassis 4.
Straight sections 16 extend from the inlets 13a, 13b and the outlets
14a, 14b and are supported by an anchor 17 behind the base 15. Along
the further course of the measuring tubes 7a, 7b a coupling element
18 is provided for the inlet section 13a, 13b and a coupling element
19 for the outlet section 14a, 14b. The coupling elements 18 19
interconnect the two measuring tubes 7a, 7b on the outside of the
looped section. In the embodiment shown, the fluid can flow parallel
through the two measuring tubes 7a, 7b. It is also possible, however,
to connect the two measuring tubes 7a, 7b in series. For this purpose,
the outlet 14a of the measuring tube 7a is connected to the inlet
13b of the measuring tube 7b, for example. This connection is preferably
effected within the chassis 4.
The position of the coupling elements 18 determines the eigenfrequency
of the flow meter 2. If the coupling elements 18 which are designed
as plates, are shifted along the measuring tubes 7a, 7b, the eigenfrequency
changes because the distance between a vibration node and the driver
9 is lengthened or shortened. The distance relative to the anchor
17 is likewise shortened or lengthened. The anchor 17 together with
the chassis 4 forms a base plate whose mass may be assumed to be
approximately infinite. When the position of the node is changed
by means of the coupling element 18 19 the eigenfrequency of the
measuring tube 7a, 7b changes accordingly. The differences in the
eigenfrequencies of the flow meters 2 3 are adjusted simply by
arranging the coupling elements 18 19 in the one flow meter 2 at
the positions indicated in FIG. 2 and those same coupling elements
18 19 in the other flow meter 3 somewhat closer to the anchor 17.
Otherwise the same identical flow meters can be used, that is, the
measuring tubes 7a, 7b and the chassis 4 5 can be practically identical.
The measuring tubes 7a, 7b are provided with the mounts 8', 8''
for the sensors 8 and the mount 9' for the driver 9.
FIG. 3 shows the chassis 4 with the base 15. The base 15 has four
holes 20 through which the ends 13a, 13b, 14a, 14b of the measuring
tubes 7a, 7b are threaded. Two sides 21 22 extend practically symmetrically
from the base 15 such that they are substantially aligned in a
U-shape relative to the base 15. Each side 21 22 in turn, has
a U-shaped cross section, i.e., it has outwardly angled projecting
parts 23 24 to give the chassis 4 additional stability. The sections
23 24 further make it possible to mount the otherwise identical
chassis 4 5 to the mounting plates 25 26. The mounting plates
25 26 are in turn fixed to the common housing, here the front plate
6. The anchor 17 is thus fixed to the chassis 4. The chassis 4 is
fixed to the mounting plate 26 which is in turn fixed to the front
plate 6 that forms part of the common housing.
The sides 21 22 are provided with recesses 27 in which the anchor
17 can be mounted.
In their end regions, the two sides 21 22 have two groups 28
29 of projecting parts 30. These projecting parts are arranged to
form the corner points of a trapezoid as seen in the top view. The
plates 31 can therefore be inserted between the front and the rear
projecting parts of a group (FIG. 4). Thus, the position of the
plate 31 within the chassis 4 is determined by the selection of
a group 28 29 of projecting parts 30. If, as shown in FIG. 4 the
projecting parts of the group 28 are selected for the positioning
of the plate 31 then the flow meter has a frequency f. If, on the
other hand, the projecting parts 30 of the group 29 are selected
for the plate 31 then the eigenfrequency is f'. The distance between
the two possible positions of the plate 31 is approximately 1 cm,
which results in an eigenfrequency difference of 10 Hz. The frequency
f is preferably 130 Hz and the frequency f' 140 Hz. The essential
factor determining the eigenfrequency is the distance between the
position of the plate 31 and the tip of the loops of the tubes.
FIGS. 5 to 7 show the auxiliary means used to mount the coupling
elements 18 19 at the correspondingly selected positions on the
measuring tubes 7a, 7b. FIG. 5 shows a coupling element 18 which
prior to assembly forms part of the plate 31 shown in FIG. 4. The
plate 31 has a central element 32 and two lateral elements 33 34
which are connected with the two coupling elements 18 19 via rated
break points 35.
The plates 31 are pushed over the measuring tubes 7a, 7b. They
can then be inserted between the projecting parts 30 of the one
group 28 or the other group 29. This determines the position of
the coupling elements 18 19 on the measuring tubes 7a, 7b. The
coupling elements can then be connected, e.g., soldered or glued,
to the measuring tubes 7a, 7b. The rest of the plate 31 i.e., the
central section 32 and the lateral sections 33 34 can then be removed.
The other flow meter 3 is basically constructed in the exact same
way. The only difference is that the plate 31 is inserted between
the projecting parts 30 of the other group 29. The eigenfrequency
differences thus generated are sufficient so that the two chassis
4 5 can be interconnected. The direct connection of the two flow
meters 2 3 provides an excellent mechanical coupling between the
two chassis 4 5 but is unproblematic when the above-described
solution is used. The eigenfrequencies of the flow meters 2 3 differ
sufficiently so that crosstalk interference with the measurement
results can be avoided.
Once the front plate 6 has been mounted to the mounting plate 26
and the base 15 of the two chassis 4 the differential flow meter
depicted in FIG. 1 is nearly completed. It only needs to be inserted
into a housing, e.g., into an aluminum housing.
It is of course also possible to use other markings in the chassis
4 instead of the projecting parts 30. In some cases it is even sufficient
to simply mark or in some other way indicate the position where
the coupling elements 18 19 are to be mounted in the one flow meter
2 or the other flow meter 3. However, the use of structures makes
the assembly easier.
As an alternative to the above-described device, in which a vibration-influencing
device is disposed at different positions on the two measuring tubes,
a point mass can be used as a vibration-influencing device, which
is provided at the same location on both measuring tubes 7 10.
The two point masses may differ with respect to their mass, however.
Instead of point masses other additional masses may of course also
Strain gauges may be fixed to the coupling elements 18 19. The
strain gauges register the relative change in length of the coupling
elements 18 19. This change can be used as the measure for the
mass flow. In this case, the sensors 8' and 8'' can be eliminated.
An additional advantage is that this type of measurement using strain
gauges includes a differential measurement of the curvatures of
the measuring tubes 7 10. This measurement is therefore insensitive
to the flow direction.
The flow meter device consisting of two individual Coriolis flow
meters is controlled by a joint main control unit (not depicted).
This main control unit is arranged within the common housing and
has two electronic control units, one for each flow meter. The control
units can be arranged on the same printed circuit board. In this
case, however, the printed circuit board is divided into two completely
separate sections, such that each section controls one Coriolis
flow meter. The control units can of course also be arranged on
different printed circuit boards.
In addition, the two control units monitor each other by means
of a monitoring device or circuit. If the one control unit fails,
the other control unit takes over the control and measurement of
the flow meter. This ensures a reliable operation even in differential
flow measurements, which have to meet particularly high standards.
The above description of the preferred embodiments has been given
by way of example. From the disclosure given, those skilled in the
art will not only understand the present invention and its attendant
advantages, but will also find apparent various changes and modifications
to the structures disclosed. It is sought, therefore, to cover all
such changes and modifications as fall within the spirit and scope
of the invention, as defined by the appended claims, and equivalents