Abstrict A mass flow meter for flowing media which works on the Coriolis
Principle includes a straight measuring pipe carrying the flow medium,
an oscillator acting on the measuring pipe and two transducers detecting
Coriolis forces and/or Coriolis oscillations based on Coriolis forces.
The meter also has a carrier pipe holding the measuring pipe, the
oscillator and the transducers and two temperature sensors that
detect the temperature of the measuring pipe and correct the measured
value depending on the temperature of the measuring pipe. The measuring
pipe and the carrier pipe are connected to one another in a way
that excludes relative axial movements, and the axial distance between
the connecting points of the measuring pipe carrier pipe represents
the oscillation length of the measuring pipe. The mass flow meter
is designed so that, in a simple way, the measured value is largely
independent from temperature changes and from forces acting from
the outside and so that a length-change sensor that detects changes
in the oscillation length of the measuring pipe can correct the
measured value depending on the oscillation length of and stress
on the measuring pipe.
Claims I claim:
1. A mass flow meter for flowing media that works on the Coriolis
Principle, with a substantially straight measuring pipe carrying
the flowing medium, at least one oscillator acting on the measuring
pipe, means including at least one transducer detecting either one
or both Coriolis forces and Coriolis oscillations based on Coriolis
forces and producing a measured value of the mass flow, a carrier
pipe that holds said measuring pipe, said oscillator and said transducer
and at least one temperature sensor that detects the temperature
of the measuring pipe to correct the measured value depending on
the temperature of the measuring pipe, wherein the measuring pipe
and the carrier pipe are connected to one another in a way that
excludes relative axial movements, and the axial distance between
the points connecting the measuring pipe and the carrier pipe corresponds
to the oscillation length of the measuring pipe, characterized by
the fact that a length-change sensor that detects changes in the
oscillation length of the measuring pipe is provided for correcting
the measured value depending on said oscillation length of and the
stress on the measuring pipe.
2. The mass flow meter according to claim 1 wherein said temperature
sensor produces temperature signals and said length-change sensor
produces length-change signals, and further including first and
second correction circuits for receiving said temperature and length-change
signals, respectively, and producing first and second correction
signals and an evaluation circuit responsive to the first and second
correction signals for correcting said measured value.
3. The mass flow meter according to claim 1 wherein said temperature
sensor produces temperature signals and said length-change sensor
produces length-change signals, and further including a correction
circuit responsive to said temperature signals and said length-change
signals for producing correction signals and an evaluation circuit
responsive to the correction signals for correcting said measured
value.
4. The mass flow meter according to claim 1 wherein said temperature
sensor produces temperature sensor signals and said length-change
sensor produces length-change sensor signals, and further including
a correction and evaluation circuit responsive to the temperature
signals and the length-change signals for correcting said measured
value.
5. The mass flow meter according to any one of claims 1 to 4 wherein
the length-change sensor comprises at least one strain gage.
6. The mass flow meter according to claim 5 wherein the strain
gage is placed on the measuring pipe.
7. The mass flow meter according to claim 5 wherein the strain
gage is placed on the carrier pipe.
8. The mass flow meter according to any one of claims 1 to 4 wherein
the length-change sensor comprises at least one length-change detector
rod.
9. The mass flow meter according to claim 8 wherein said at least
one length-change detector rod is of a material with a low thermal
expansion coefficient.
10. The mass flow meter according to claim 8 wherein the length-change
sensor is located inside the carrier pipe.
11. The mass flow meter according to claim 9 wherein the length-change
sensor is located inside the carrier pipe.
12. The mass flow meter according to claim 8 wherein the length-change
sensor is located outside the carrier pipe.
13. The mass flow meter according to claim 9 wherein the length-change
sensor is located outside the carrier pipe.
14. The mass flow meter according to claim 8 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
15. The mass flow meter according to claim 9 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
16. The mass flow meter according to claim 10 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
17. The mass flow meter according to claim 11 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
18. The mass flow meter according to claim 8 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
19. The mass flow meter according to claim 9 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
20. The mass flow meter according to claim 10 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
21. The mass flow meter according to claim 11 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
22. The mass flow meter according to any one of claims 1 to 4 wherein
there are two temperature sensors located near the opposite ends
of the measuring pipe.
23. The mass flow meter according claim 22 wherein the length-change
sensor comprises at least one strain gage.
24. The mass flow meter according to claim 23 wherein said at least
one strain gage is placed on the measuring pipe.
25. The mass flow meter according to claim 23 wherein said at least
one strain gage is placed on the carrier pipe.
26. The mass flow meter according to claim 22 wherein each length-change
sensor comprises a length-change detector rod.
27. The mass flow meter according to claim 26 wherein said at least
one length-change detector rod is of a material with a low thermal
expansion coefficient.
28. The mass flow meter according to claim 26 wherein the length-change
sensor is located inside the carrier pipe.
29. The mass flow meter according to claim 27 wherein the length-change
sensor is located inside the carrier pipe.
30. The mass flow meter according to claim 26 wherein the length-change
sensor is located outside the carrier pipe.
31. The mass flow meter according to claim 27 wherein the length-change
sensor is located outside the carrier pipe.
32. The mass flow meter according to claim 26 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair or spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
33. The mass flow meter according to claim 27 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
34. The mass flow meter according to claim 28 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
35. The mass flow meter according to claim 29 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
36. The mass flow meter according to claim 30 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
37. The mass flow meter according to claim 31 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rod and facing one another..
38. The mass flow meter according to claim 26 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
39. The mass flow meter according to claim 27 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
40. The mass flow meter according to claim 28 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
41. The mass flow meter according to claim 29 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
42. The mass flow meter according to claim 30 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
43. The mass flow meter according to claim 31 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
44. A mass flow meter for flowing media that works on the Coriolis
Principle, with a substantially straight measuring pipe carrying
the flowing medium, at least one oscillator acting on the measuring
pipe, at least one transducer detecting either one or both Coriolis
forces and Coriolis oscillations based on Coriolis forces and producing
a measuring value of the mass flow, a carrier pipe that holds said
measuring pipe, said oscillator and said transducer and two temperature
sensors for detecting the temperature of the measuring pipe to correct
the measuring value depending on the temperature of the measuring
pipe wherein the measuring pipe and the carrier pipe are connected
to one another in a way that excludes relative axial movements,
and the axial distance between the points connecting the measuring
pipe and the carrier pipe corresponds to the oscillation length
of the measuring pipe and wherein the two temperature sensors are
electrically connected in series whereby only two external connections
are required from both temperature sensors.
45. The mass flow meter defined in claim 44 and further including
a length-change sensor which detects changes in the oscillation
length of the measuring pipe for correcting the measuring valve
depending upon the stress on the measuring pipe.
46. The mass flow meter according to claim 45 wherein the length-change
sensor comprises at least one strain gage.
47. The mass flow meter according to claim 46 wherein the strain
gage is placed on the measuring pipe.
48. The mass flow meter according to claim 46 wherein the strain
gage is placed on the carrier pipe.
49. The mass flow meter according to claim 45 wherein the length-change
sensor comprises at least one length-change detector rod.
50. The mass flow meter according to claim 49 wherein said at least
one length-change detector rod is of a material with a low thermal
expansion coefficient.
51. The mass flow meter according to claim 49 wherein the length-change
sensor is located inside the carrier pipe.
52. The mass flow meter according to claim 50 wherein the length-change
sensor is located inside the carrier pipe.
53. The mass flow meter according to claim 49 wherein the length-change
sensor is located outside the carrier pipe.
54. The mass flow meter according to claim 50 wherein the length-change
sensor is located outside the carrier pipe.
55. The mass flow meter according to claim 49 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
56. The mass flow meter according to claims 50 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
57. The mass flow meter according to claim 51 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
58. The mass flow meter according to claim 52 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
59. The mass flow meter according to claim 53 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rods and facing one another.
60. The mass flow meter according to claim 54 wherein the length-change
sensor comprises axially aligned length-change detector rods and
a pair of spaced-apart capacitor plates mounted to the adjacent
ends of said rod and facing one another.
61. The mass flow meter according to claim 49 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
62. The mass flow meter according to claim 50 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
63. The mass flow meter according to claim 51 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
64. The mass flow meter according to claim 52 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
65. A mass flow meter according to claim 53 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
66. A mass flow meter according to claim 54 wherein the length-change
sensor comprises a pair of axially aligned length-change detector
rods and a strain gage extending between the adjacent ends of said
rods.
67. The mass flow meter defined in claim 44 wherein the temperature
sensors are temperature-dependent resisters.
68. The mass flow meter defined in claim 44 wherein the temperature
sensors are positioned adjacent to the opposite ends of the measuring
pipe.
Description The invention concerns a mass flow meter for flowing media that
works on the Coriolis Principle, with at least one straight measuring
pipe carrying the flowing medium, at least one oscillator acting
on the measuring pipe, at least one transducer detecting Coriolis
forces and/or Coriolis oscillations based on Coriolis forces, one
carrier pipe that holds the measuring pipe, the oscillator and the
transducer, and at least one temperature sensor that detects the
temperature of the measuring pipe to correct the measured value
depending on the temperature of the measuring pipe, wherein the
measuring pipe and the carrier pipe are connected to one another
in a way that excludes relative axial movements, and the axial distance
between the points connecting the measuring pipe and the carrier
pipe represents the oscillation length of the measuring pipe.
BACKGROUND OF THE INVENTION
Mass flow meters for flowing media that work on the Coriolis Principle
are known in various embodiments (see, for example, German Disclosure
Documents 26 29 833 28 22 087 28 33 037 29 38 498 30 07 361
33 29 544 34 43 234 35 03 841 35 05 166 35 26 297 36 32 800
37 07 777 39 16 285 40 16 907 41 24 295 41 24 296 and 41 29
181 European Patent Disclosure Documents 0 083 144 0 109 218
0 119 638 0 196 150 0 210 308 0 212 782 0 235 274 0 239 679
0 243 468 0 244 692 0 261 435 0 271 605 0 275 367 and 0 282
552 as well as U.S. Pat. Nos. 4491009 4628744 and 4660421)
and are increasingly being used in practice.
Mass flow meters for flowing media that work on the Coriolis Principle
are basically divided into those whose measuring pipes are designed
to be at least basically straight, and those whose measuring pipes
are designed to be loop-shaped. The mass flow meters in question
are also divided into those with only one measuring pipe and those
with two; in designs with two measuring pipes, the pipes may be
fluidically in series or in parallel.
Embodiments of mass flow meters in which the measuring pipe or
pipes are designed to be straight are simple in mechanical design
and consequently can be produced at relatively low cost. Moreover,
the inner surfaces of the pipe are easy to work on, for example,
to polish and they have low pressure losses.
The disadvantage of mass flow meters that work on the Coriolis
Principle and in which the measuring pipe or pipes is designed to
be straight is that both thermally caused changes in length and
thermally caused stresses and also forces and torques working from
those outside can lead to measurement errors and to mechanical damage,
namely to stress cracks.
Experts have already dealt with the measurement errors that occur
due to temperature changes in mass flow meters that work on the
Coriolis Principle.
First of all, it has already been recognized that the temperature
dependence of the modulus of elasticity influences the oscillation
frequency and the elasticity of the measuring pipe and thus the
measured value; the result is that a temperature sensor is often
provided that detects the temperature of the measuring pipe to correct
the measured value depending on the temperature of the measuring
pipe; see, for example, in the German publication "Messen Prufen
Automatisieren", 1987 Vol. 23 No. 5 Pages 301 through 305
the essay Direkte Massendurchflussmessung, insbesondere mit Coriolisverfahren"
by von W. Steffen und Dr. W. Stumm.
Incidentally, with a mass flow meter of the type described at the
outset, the extensive temperature dependence of the measured value
was taken into account in that a temperature sensor is provided
to detect the temperature of the carrier pipe to correct the measured
value, depending on the temperature of the carrier pipe (see German
Disclosure Document 36 32 800 and the corresponding European Disclosure
Document 0 261 435). Here, temperature sensor signals produced by
the two temperature sensors (one for the measuring pipe and one
for the carrier pipe) are put into a correction circuit that should
eliminate the influence of temperature on the measured value. Specifically,
provision is made for the correction circuit to multiply the measured
value by a correction factor K=K.sub.0 +K.sub.1 T.sub.1 +K.sub.2
T.sub.2 +K.sub.3 T.sub.1.sup.2 +K.sub.4 T.sub.2.sup.2 +K.sub.5 T.sub.1
T.sub.2 wherein T.sub.1 is the temperature of the measuring pipe,
T.sub.2 is the temperature of the carrier pipe and K.sub.0 K.sub.1
K.sub.2 K.sub.3 K.sub.4 and K.sub.5 are constant coefficients that
are specific for a certain embodiment of the mass flow meter.
In practice, it was shown that the higher order terms of the above
expression can be ignored, so that temperature compensation is attained
with sufficient precision if the uncorrected measured value is multiplied
by the correction factor K=K.sub.0 +K.sub.1 T.sub.1 +K.sub.2 T.sub.2.
In the known mass flow meter described above, the temperatures of
the measuring pipe and the carrier pipe--more or less as the external
cause of a temperature-dependent measurement error--are considered
correcting; the internal causes resulting from these external causes
have not yet been addressed, however.
Finally, a mass flow meter working on the Coriolis Principle is
known that, like the mass flow meter from which the invention comes,
has a straight measuring pipe carrying the flowing medium, an oscillator
acting on the measuring pipe, two measurement transducers that detect
Coriolis oscillations based on Coriolis forces and a carrier pipe
for the measuring pipe, the oscillator and the transducers, but
in which there is no temperature sensor to detect the temperature
of the measuring pipe, but where, in another way, care is taken
that the measured value is largely non-temperature-dependent, and
temperature changes thus do not lead to measurement errors to a
considerable extent (see German Disclosure Document 41 24 295 and
corresponding U.S. application Ser. No. 07/917577 infra). In this
mass flow meter, the carrier pipe is designed as a so-called compensation
cylinder, through which or in connection with which temperature
changes--as well as forces and torques acting from the outside--are
compensated or at least their effects are largely eliminated. The
structural unit of the measuring pipe and the carrier pipe designed
as a compensation cylinder is more or less "immune" to
temperature changes, and to forces and torques acting from the outside.
Thus, additional measures to "immunize" the cylinder
from temperature changes and forces and torques acting from the
outside are taken. An initial additional measure of this kind consists
of the fact that the measuring pipe is arranged inside the carrier
pipe with tensile prestress. As the temperature increases, the tensile
prestress decreases. A second extra measure to "immunize"
the cylinder consists of using materials for the measuring pipe
and the carrier pipe with the same or almost the same heat expansion
coefficients, especially materials with relatively low heat expansion
coefficients. For further details on this known mass flow meter,
please refer expressly to the contents of German Disclosure Document
41 24 295 and corresponding U.S. application Ser. No. 07/917577
filed Jul. 21 1992 the contents of which are hereby incorporated
by reference herein.
SUMMARY OF THE INVENTION
Proceeding from the state of the art explained above in detail,
the invention is now based on the task of designing and developing
the mass flow meter described at the beginning so that the measured
value is largely independent of temperature changes and forces acting
from the outside in a simple way.
The mass flow meter of the invention, in which the task derived
and presented previously is solved, is now characterized first of
all basically by the fact that it has a length-change sensor that
detects changes in the oscillation length of the measuring pipe
to correct the measured value depending on the oscillation length
of and the stresses on the measuring pipe.
In the mass flow meter in the invention, on one hand, temperature
changes in the measuring pipe, and on the other hand, length changes
in the measuring pipe, and even changes in the oscillation length
of the measuring pipe that influence the measured value are used
to correct the measured value. Thus, both measurement errors based
on temperature changes in the measuring pipe and the carrier pipe
and also such measurement errors that result from forces acting
from the outside are eliminated. This is based on the following:
in mass flow meters of the type in question, the measured value--obtained
from the transducer signals by an evaluation circuit--is primarily
dependent on the oscillation frequency of the measuring pipe. The
oscillation frequency of the measuring pipe is, in turn, dependent
on--i.e., temperature-dependent--the modulus of elasticity of the
measuring pipe, the effective oscillating length, the so-called
oscillation length of the measuring pipe, and the axial stress on
the measuring pipe. The oscillation frequency of the measuring pipe
is thus only indirectly dependent on the temperature of the carrier
pipe and the forces working on the carrier pipe from the outside,
namely only due to the fact that the oscillation length and/or the
axial stress condition of the measuring pipe is changed thereby.
Due to the fact that the mass flow meter of the invention, first
of all, like the state of the art as well, has a temperature sensor
that detects the temperature of the measuring pipe to correct the
measured value, depending on the temperature of the measuring pipe,
the influence of the temperature-dependent change in the modulus
of elasticity on the oscillation frequency of the measuring pipe
can be considered and a measurement error resulting therefrom can
be practically completely eliminated.
Due to the fact that, now, according to the invention, in addition
to the temperature sensor that detects the temperature of the measuring
pipe, there is a length-change sensor that detects changes in the
oscillation length of the measuring pipe, the changes in oscillation
length influencing the oscillation frequency of the measuring pipe
and the axial stress on the measuring pipe can be considered, and
measurement errors resulting therefrom can be eliminated--and this
is especially important--including both those errors that are based
on the temperature changes in the measuring pipe and/or the carrier
pipe and also those that are based on forces that act on the measuring
pipe and/or the carrier pipe from the outside. Thus, length-change
sensor signals given off by the length-change sensor are, naturally,
a direct measurement of changes in the oscillation length of the
measuring pipe, regardless of what these changes are based on, and
an indirect measurement of changes in the axial stress on the measuring
pipe, in turn, regardless of what these changes are based on.
The fact that a change in the axial stress on the measuring pipe
can be determined from a change in the oscillation length of the
measuring pipe results from the fact that a change in the axial
stress on the measuring pipe is linearly dependent on a change in
the oscillation length of the measuring pipe. The length-change
sensor provided according to the invention, which detects changes
in the oscillation length of the measuring pipe, thus provides the
possibility of practically completely eliminating changes in the
oscillation length of the measuring pipe and changes in the axial
stress on the measuring pipe--and thus measurement errors based
on them--in determining the measurement value. What the mass flow
meter in the invention therefore does is make sure that all influences
changing the oscillating frequency of the measuring pipe are considered
so that all measurement errors resulting therefrom can be practically
completely eliminated.
Now, individually, there are a large number of possibilities of
designing and developing the mass flow meter according to the invention,
which applies especially to the design of the length-change sensor.
For this, please refer to the patent claims subordinated to claim
1 on one hand, and, on the other hand, to the following description
of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and objects of the invention,
reference should be had to the following detailed description taken
in connection with the accompanying drawings, in which:
FIG. 1 is a longitudinal sectional view of one embodiment of a
mass flow meter according to the invention;
FIG. 2 is a similar view of a second embodiment of a mass flow
meter according to the invention;
FIG. 3 is a similar view of a third embodiment of a mass flow meter
according to the invention;
FIG. 4 is a fragmentary sectional view on a larger scale of a part
of the FIG. 3 flow meter;
FIG. 5 is a longitudinal sectional view of a fourth embodiment
of a mass flow meter according to the invention, and
FIG. 6 is a view similar to FIG. 4 of a part of the FIG. 5 flow
meter.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The mass flow meter for flowing media according to the invention
is one that works on the Coriolis Principle. Consequently, it has
a straight Coriolis pipeline 1 carrying the flowing medium, an oscillator
2 acting on the Coriolis pipeline 1 and two transducers 3 for detecting
Coriolis forces and/or Coriolis oscillations based on Coriolis forces.
In the embodiment shown specifically in FIG. 1 the oscillator 2
works with a pendulum arm 4 provided on the Coriolis pipeline 1
as described in U.S. application Ser. No. 07/736400 filed Jul.
26 1991 the contents of which are hereby incorporated by reference
herein.
In addition, FIGS. 1 2 and 4 show that the measuring pipe 1 is
provided with solid bodies 5 through whose mass and arrangement
along the pipe the inherent frequency of the measuring pipe 1 can
be influenced within certain limits.
For the mass flow meter from which the invention proceeds and the
mass flow meter of the invention, and for all disclosed embodiments
of the mass flow meter of the invention, it is also true that there
is a carrier pipe 6 that holds the measuring pipe 1 the oscillator
2 the transducers 3 and the solid bodies 5 if there are any, and
at least one temperature sensor 7 that detects the temperature of
the measuring pipe 1 wherein the measuring pipe 1 and the carrier
pipe 6 are connected to one another in a way that excludes relative
axial movement and the axial distance between the connection points
of the measuring pipe 1 and carrier pipe 6 represents the oscillation
length of the measuring pipe 1 and wherein the temperature sensor
7 that detects the temperature of the measuring pipe 1 is used to
correct the measured value, depending on the temperature of the
measuring pipe.
In all embodiments of the mass flow meter of the invention shown,
the measuring pipe 1 is connected to the carrier pipe 6 via two
connecting rings 8 mounted to the ends of the carrier pipe 6. There
is also an outer cylinder 9 which holds the unit consisting of the
measuring pipe 1 the oscillator 2 the transducers 3 and pendulum
arm 4 the mass bodies 5 if there are any, the carrier pipe 6 and
the connecting rings 8. And the cylinder 9 has two connecting rings
10 mounted to its ends, to which connecting flanges 11 projecting
to the outside are connected. Pipes 12 connected to the measuring
pipe 1 project through the connecting rings 10 into the connecting
flanges 11. For this purpose, the measuring pipe 1 and the connecting
pipe 12 are preferably made in one piece; in this way, there is
a fully continuous pipe. To protect the connecting pipe 12 it is
encased in a reinforcing cylinder 13.
For more information concerning the measuring pipe 1 the carrier
pipe 6 the connecting rings 8 and outer cylinder 9 the connecting
rings 10 the connecting flange 11 the connecting pipe 12 and the
reinforcing cylinder 13 and concerning the connection of these components
to one another, refer to the above-mentioned U.S. application Ser.
No. 07/917577.
According to the invention, a length-change sensor 14 that detects
changes in the oscillation length of the measuring pipe 1 is provided
for correcting the measured value depending on the oscillation length
of and the stress on the measuring pipe. The fact that, and the
extent to which, all the oscillation influences changing the oscillation
frequency of the measuring pipe are considered and thus all measurement
errors resulting therefrom can be practically completely eliminated
has already been described in detail.
The drawing figures do not show how the temperature sensor signals
from the temperature sensor 7 and the length-change sensor signals
from the length-change sensor 14 are produced. There are many ways
of doing this. One way is to put the temperature sensor signals
produced by the temperature sensor 7 into a first correction circuit
and length-change sensor signals produced by length-change sensor
14 into a second correction circuit and apply the correction signals
from both correction circuits to an evaluation circuit to correct
the measured value obtained from the transducer 3 signals. It is
also possible to apply the temperature sensor signals produced by
the temperature sensor 7 and the length-change sensor signals produced
by the length-change sensor 14 to a single correction circuit and
have the correction signal from the correction circuit, in an evaluation
circuit, correct the measured value obtained from the transducer
3 signals. Still another possibility is to apply the temperature
sensor signals from the temperature sensor 7 and the length-change
sensor signals from the length-change sensor 14 to a correction
and evaluation circuit and there correct the measured value obtained
from the measured value sensor signals.
Suitable correction and evaluation circuits for deriving from the
temperature sensor signals suitable correction signals for evaluation
and correction of the measured value obtained from the transducer
signals are described in German disclosure document 36 32 800 and
the corresponding European disclosure document 0 261 435 identified
as prior art at the outset and hereby incorporated by reference
herein. See also U.S. Pat. No. 4491009.
As already stated, there are various possibilities, especially
regarding the construction of the length-change sensor 14; some
of those individual possibilities are realized in the mass flow
meter embodiments shown in the drawing figures.
In the embodiments shown in FIGS. 1 and 2 each length-change sensor
14 is designed as a strain gage. In the embodiment in FIG. 1 a
pair of length-change sensors 14 is placed on the measuring pipe
1. Since the measuring pipe 1 and the carrier pipe 6 are connected
to one another in a way that excludes relative axial movements,
the length-change sensor 14 designed as a strain gage, can also
be placed on the carrier pipe 6 as shown in FIG. 2.
In the embodiments of the mass flow meter that are shown in FIGS.
3 (with FIG. 4) and 5 (with FIG. 6), the length-change sensor 14
comprises two length-change detector rods 15 that are made of a
material such as invar with an especially low thermal expansion
coefficient. The low expansion coefficient leads to the fact that
temperature-dependent length changes in the length-change detector
rods 15 have practically no influence on the measurement of changes
in the oscillation length of the measuring pipe 1. Also, in this
embodiment of the length-change sensor, the sensor can be produced
inside the carrier pipe 6 as shown in FIG. 3 or outside the carrier
pipe 6 as shown in FIG. 5.
As shown in detail in FIG. 4 in the meter embodiment in FIG. 3
the ends of the axially aligned length-change detector rods 15 face
one another and carry spaced apart capacitor plates 16. In this
example, a length change is thus measured via a change in capacitance
of that capacitor. On the other hand, FIG. 6 shows for the FIG.
5 meter embodiment that connected between the ends of the length-change
detector rods 15 which face one another, there is a strain gage
17. In this embodiment, then, a length change is reflected by a
change in resistance of the strain gage 17.
Any known means may be provided for measuring the change in capacitance
or resistance of the sensors 14 depicted in FIGS. 3 and 5.
In conclusion, it should be pointed out that in all embodiments
of the mass flow meter according to the invention shown, two temperature
sensors 7 designed as temperature-dependent resistors, preferably
Pt-1000 are provided, and they are on the connecting pipes 12 on
both sides of the measuring pipe 1. When the temperature at the
input end of the measuring pipe 1 is different from the temperature
at the output end of the measuring pipe 1 this gives average temperature
of the measuring pipe 1. Both temperature sensors 7 are connected
in series so that only two external connections are needed to process
the temperature sensor signals produced by the two temperature sensors.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently attained
and, since certain changes may be made in the above constructions
without departing from the scope of the invention, it is intended
that all matter contained in the above description or shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
It is also understood that the following claims are intended to
cover all of the generic and specific features of the invention
described herein.
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