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.
 This is a Continuation of International Application PCT/DK2003/000534
with an international filing date of Aug. 12 2003 which was published
under PCT Article 21(2) in German, and the disclosure of which is
incorporated into this application by reference.
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 measuring tube.
 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 calculated.
 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
OBJECTS OF THE INVENTION
 One object of the invention is to avoid crosstalk between
the flow meters.
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
 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 plate 6.
 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.
 FIG. 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 be used.
 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