Abstrict A mass flow meter operating by the Coriolis principle comprises
a mechanical oscillating system having two straight measuring tubes
which are clamped at both ends. The mechanical oscillating system
is arranged axially in a support tube. In the center of the measuring
tubes an oscillation exciter is disposed which sets the two measuring
tubes into oppositely phased flexural oscillations. Oscillation
sensors sensing the mechanical oscillations at equal distances on
both sides of the oscillation exciter generate electrical oscillation
sensor signals which are characteristic of the frequency and phase
position of the sensed oscillations. An evaluation circuit receives
the oscillation sensor signals and generates from their phase difference
a measuring signal indicating the measured value of the mass flow.
A first temperature sensor is arranged such that it measures the
temperature of the support tube and generates a first temperature
sensor signal indicating said temperature. A second temperature
sensor is so arranged that it measures the temperature of the mechanical
oscillating system and generates a second temperature sensor signal
indicating said temperature. A correction circuit receives the two
temperature sensor signals and imparts to the measuring signal on
the basis of the temperatures measured a correction for eliminating
the temperature influence on the measurement result.
Claims We claim:
1. Mass flow meter operating by the Coriolis principle and comprising
a support tube, a mechanical oscillating system disposed axially
in the support tube and comprising at least one straight measuring
tube clamped at both ends, an oscillation exciter which sets the
or each measuring tube in the the centre in flexural oscillations,
oscillation sensors which sense the mechanical oscillations at equal
distance on both sides of the oscillation exciter and generate electrical
oscillation sensor signals which are characteristic of the frequency
and phase position of the sensed oscillations, and an evaluation
circuit which receives the oscillation sensor signals and from the
phase difference thereof generates a measuring signal indicating
the measured value of the mass flow, characterized in that a first
temperature sensor is so arranged that it measures the temperature
of the support tube and generates a first temperature sensor signal
indicating said temperature, that a second temperature sensor is
so arranged that it measures the temperature of the mechanical oscillating
system and generates a second temperature sensor signal indicating
said temperature, and that a correction circuit receives the two
temperature sensor signals and imparts to the measuring signal on
the basis of the measured temperatures a correction for eliminating
the temperature influence on the measurement result.
2. Mass flow meter according to claim 1 characterized in that
the correction circuit multiplies the measured value of the mass
flow by a correction factor
wherein T.sub.l is the temperature of the oscillating system, T.sub.2
the temperature of the support tube and k.sub.0 k.sub.l, k.sub.2
are constant coefficients specific to the mass flow meter.
3. Mass flow meter according to claim 1 characterized in that
the evaluation circuit and the correction circuit are formed by
a correspondingly programmed micro- computer.
Description The invention relates to a mass flow meter operating by the Coriolis
principle and comprising a support tube, a mechanical oscillating
system disposed axially in the support tube and comprising at least
one straight measuring tube clamped at both ends, an oscillation
exciter which sets the or each measuring tube in the centre in flexural
oscillations, oscillation sensors which sense the mechanical oscillations
at equal distance on both sides of the oscillation exciter and generate
electrical oscillation sensor signals which are characteristic of
the frequency and phase position of the sensed oscillations, and
an evaluation circuit which receives the oscillation sensor signals
and from the phase difference thereof generates a measuring signal
indicating the measured value of the mass flow.
In mass flow measuring devices of this type operating by the Coriolis
principle the mass flow measurement is of course based on the fact
that the measured medium flowing through the oscillating straight
measuring tubes generates Coriolis forces which result in a mutual
phase displacement of the mechanical oscillations at the two ends
of each measuring tube. The magnitude of this phase displacement
is a measure of the mass flow. The phase displacement is measured
with the aid of the two oscillation sensors which convert the oscillations
sensed by them to electrical sensor signals which are characteristic
of the phase position of the oscillations. Proceeding from the phase
difference between the oscillation sensor and signals the evaluation
circuit can generate a measuring signal which indicates the measured
value of the mass flow.
In such mass flow meters the flow value indicated by the measuring
signal may contain temperature-induced errors. The causes of such
temperature-induced errors are in particular a temperature gradient
between support tube and oscillating system, dynamic temperature
changes and different thermal coefficients of expansion of the materials
of which the support tube and oscillating system consist.
The problem underlying the invention is the provision of a mass
flow meter of the type mentioned at the beginning in which temperature-induced
errors in the measuring signal are largely compensated.
According to the invention this problem is solved in that a first
temperature sensor is so arranged that it measures the temperature
of the support tube and generates a first temperature sensor signal
indicating said temperature, that a second temperature sensor is
so arranged that it measures the temperature of the mechanical oscillating
system and generates a second temperature sensor signal indicating
said temperature, and that a correction circuit receives the two
temperature sensor signals and imparts to the measuring signal on
the basis of said temperature sensor signals a correction for eliminating
the temperature influence on the measurement result.
By the separate detection of two temperatures, that is the temperature
of the support tube and the temperature of the oscillating system,
in a mass flow meter with measuring tubes clamped at the ends in
a support tube it is possible in simple manner to obtain a correction
of the measuring signal by which temperature-induced errors are
very extensively compensated. The mass flow meter constructed according
to the invention therefore permits an exact measurement of the mass
flow in a wide range of the ambient temperature and of measured
media of very different and even fluctuating temperatures.
Advantageous further developments and embodiments of the invention
are characterized in the subsidiary claims.
Further features and advantages of the invention will be apparent
from the following description of examples of embodiment which are
illustrated in the drawings, wherein:
FIG. 1 is a partially sectioned side view of a mass flow meter
according to the invention operating by the Coriolis principle,
FIG. 2 is a cross-sectional view of the mass flow meter of FIG.
1 along the section line A-B and
FIG. 3 is a schematic illustration of the mass flow meter with
the block circuit diagram of the connected electronic circuits.
The mass flow meter 10 illustrated in FIG. 1 in a partially sectioned
side view and in FIG. 2 in cross-section comprises a strong support
tube 11 in the interior of which a mechanical oscillating system
12 is disposed. The support tube 11 consists of a tube portion 13
which is welded at each end to an end bush 14 and l5 respectively.
Screwed to each end bush is a connecting piece 16 and 17 respectively
carrying a flange 18 and 19 respectively. By means of the flanges
18 and 19 the mass flow meter can be inserted into a conduit through
which the medium whose mass flow is to be measured flows. The support
tube 11 is surrounded by a sheet metal housing 20.
The mechanical oscillating system 12 consists of two parallel measuring
tubes 21 and 22 which at the two ends are connected by distributor
members 23 and 24 together in such a manner that they are connected
in parallel from the fluid mechanics point of view. The measuring
tubes 21 and 22 extend over the entire length of the tube portion
13 and the distributing members lie in the interior of the end bushes
14 and 15. The two distributing members 23 and 24 illustrated schematically
in FIG. 3 are made and arranged completely identically but in the
partial sectional view of FIG. 1 only the distributing member 23
lying in the interior of the end bush 14 can be seen. However, the
following description of the distributing member 23 applies identically
also to the distributing member 24 arranged at the other end of
the oscillating system.
The distributing member 23 contains in the interior flow passages
25 which divide the flow coming through the connecting piece 16
uniformly amongst the two measuring tubes 21 and 22. In corresponding
manner the distributing member 24 arranged at the other end combines
the flow from the two measuring tubes so that the combined flow
flows off through the connecting piece 17. Of course, the flow direction
may also be the converse.
The distributing member 23 is connected to the inner edge of an
annular conically shaped diaphragm 26. The outer edge of the diaphragm
26 is connected to a holder ring 28 which is clamped in the end
bush 14 and bears axially against the somewhat inwardly projecting
end face of the connecting piece 16. Preferably, the diaphragm 26
and holder ring 28 are made in one piece with the distributing member
23. In the same manner on the distributing member 24 (FIG. 3) an
annular conical diaphragm 27 is integrally formed which merges into
a holder ring 29. Thus, the oscillating system 12 is suspended by
means of the diaphragms 26 and 27 axially in the support tube 11
the sole contact between the oscillating system 12 and the support
tube 11 being via the diaphragms 26 and 27 disposed at the two ends.
Arranged in the centre of the support tube 11 is an oscillation
exciter 30 (FIG. 2) which can set the two measuring tubes 21 22
into oppositely directed flexural oscillations, the oscillation
plane of which lies in the common plane of the two measuring tubes,
i.e. perpendicularly to the plane of the drawing in FIG. 1. The
oscillation exciter 30 consists of an electromagnet 31 which is
secured in the wall of the tube portion 13 and opposite which an
armature 32 lies which is secured to the measuring tube 21. When
an alternating current is sent through the coil of the electromagnet
31 by the alternating attraction forces between the electromagnet
31 and the armature 32 the measuring tube 21 is set in flexural
oscillations which via the distributing members 23 and 24 are coupled
over to the measuring tube 22 so that finally the two measuring
tubes 21 and 22 perform opposite phase flexural oscillations. The
excitation alternating current comes from an electronic excitation
circuit 33 which is accommodated in a circuit housing 34 secured
to the sheet metal housing 20 as indicated in FIGS. 1 and 2 by
the printed circuit board 35.
The measurement of the mass flow is based in such a mass flow meter
on the fact that the measured medium flowing through the oscillating
measuring tubes 21 and 22 generates Coriolis forces which result
in a mutual phase displacement of the mechanical oscillations at
the two ends of each measuring tube. The magnitude of said phase
displacement is a measure of the mass flow. For measuring the phase
displacement on both sides of the oscillation exciter 30 at equal
distances from the latter two oscillation sensors 36 and 37 are
disposed. The oscillation sensors 36 and 37 sense the mechanical
oscillations of the measuring tubes 21 and 22 and convert them to
electrical sensor signals which are characteristic of the phase
position of the sensed oscillations. The oscillation sensor signals
are supplied to an electronic evaluation circuit 38 (FIG. 3) which
is also accommodated in the circuit housing 34 as indicated in FIGS.
1 and 2 by the printed circuit board 39.
The output signal of the oscillation sensor 37 is also supplied
to the excitation circuit 33 (FIG. 3) which furnishes at its output
to the electromagnet 31 of the oscillation exciter 30 an alternating
current which has the same frequency as the output signal of the
oscillation sensor 37 and a phase position such that the measuring
tubes 21 and 22 are stimulated to execute flexural oscillations
with their natural resonance frequency.
The evaluation circuit 38 (FIG. 3) determines the phase difference
between the two oscillation sensor signals and emits at its output
a measuring signal which represents the measured value Q of the
mass flow expressed by said phase difference. Said measured value
Q may however contain errors due to temperature-induced influences
on the oscillation behaviour of the mechanical oscillating system
12.
Such temperature-induced influences may have different causes which
can occur on their own or in conjunction with each other. Even if
the support tube 11 and the oscillating system 12 have the same
temperature temperature-induced mechanical stresses can occur if
the support tube and oscillating system consist of different materials
with different coefficients of thermal expansion. Temperature influences
have an even greater effect on the measurement result if the temperature
of the measuring tubes is different from the temperature of the
support tube. This is the case in particular when the mass flow
is to be measured of a medium whose temperature is different from
the ambient temperature. With very hot or very cold measured media
a very great temperature gradient can exist between the support
tube and the measuring tubes. Finally, dynamic temperature changes
must also be taken into account when the temperature of the measured
medium and/or the ambient temperature are not constant.
The mass flow meter described is equipped with additional means
permitting compensation of such temperature-induced influences on
the measurement result. These means include a first temperature
sensor 40 which is arranged so that it measures the temperature
of the support tube 11 and furnishes a first electrical temperature
sensor signal indicating said temperature. The temperature sensor
40 may be arranged for this purpose at the outside of the tube portion
13 spaced from the two end bushes 14 and 15 of the support tube
11.
A second temperature sensor 41 is so arranged that it measures
the temperature of the mechanical oscillating system 12 and furnishes
a second electrical temperature signal indicating said temperature.
The second temperature sensor could be arranged for this purpose
on a part of the mechanical oscillating system 12 within the support
tube but would then be permanently subjected to the mechanical oscillations,
resulting in problems as regards the fatigue strength. Therefore,
in the example illustrated the second temperature sensor 41 is also
disposed on the support tube 11 but at a point which is substantially
at the temperature of the measured medium, which is also the temperature
of the oscillating system 12. For this purpose in the end bush 14
a recess 42 is formed which extends from the outer surface of the
end bush 14 inclined up to the immediate vicinity of the holder
ring 28. The temperature sensor 41 is disposed at the end of the
recess 42 as close as possible to the holder ring 28 and its connection
leads are led outwardly through the recess 42. Since the holder
ring 28 is at the temperature of the measured medium the temperature
sensor 41 substantially measures the temperature of the mechanical
oscillating system 12. Depending on the form of the temperature
sensor 41 the recess 42 can be an inclined bore or a milled-in inclined
slit.
The temperature sensor 40 and 41 may be of any desired known type.
Preferably, temperature-dependent resistors of metal or semiconductor
material are used.
The temperature sensor signals furnished by the two temperature
sensors 40 and 41 are supplied to a correction circuit 43 (FIG.
3) which is also accommodated in the circuit housing 34 for example
together with the evaluation circuit 38 on the printed circuit board
39. The correction circuit 43 also receives the output signal of
the evaluation circuit 38 representing the uncorrected measured
value Q of the mass flow and furnishes at the output a measuring
signal representing the corrected measured value Q' of the mass
flow. For this purpose the correction circuit multiplies the uncorrected
measured value by a correction factor K which depends on the two
temperatures which are measured by means of the temperature sensors
40 and 41:
For the flow meter described with an oscillating system firmly
clamped at the ends and consisting of straight measuring tubes the
following correction factor applies
Wherein:
T.sub.1 : measuring tube temperature
T.sub.2 : support tube temperature
k.sub.0 k.sub.1 : constant coefficients which are specific to
a given embodiment of the mass flow meter.
It has been found in practice that the terms of higher order are
negligible. A temperature compensation of adequate accuracy is obtained
if the uncorrected measured value Q is multiplied by the correction
factor
The coefficients k.sub.0 k.sub.l and k.sub.2 are determined empirically
for a specific embodiment of the mass flow meter. The expert will
then have no difficulty in designing a correction circuit which
on the basis of the two temperature sensor signals modifies the
measuring signal in such a manner that the uncorrected measured
value Q is multiplied by the above correction factor K. If for example
the output signal of the evaluation circuit 38 is an analog signal
proportional to the measured value Q the correction circuit 43 may
contain an amplifier whose gain is controlled proportionally to
the correction factor K.
In most cases however the evaluation circuit 38 is a microcomputer
which is so programmed that it determines the measured value Q from
the phase displacement of the oscillation sensor signals. In this
case the correction of the measured value Q is preferably made by
an additional correction program in the same microcomputer. |