The invention relates to a mass flow meter which operates on the
coriolis principle. The meter has two straight and parallel measuring
tubes with two axially aligned compensating supply and discharge
tubes arranged in parallel to the measuring tubes. The compensating
tubes have approximately the same coefficient of expansion as said
measuring tubes. A mounting member having supply and discharge passage
has the juxtaposed ends of the compensating tubes connected thereto.
The opposite ends of the compensating tubes connect to the respective
juxtaposed ends of the measuring tube. An oscillator drives the
measuring tubes in opposite directions and sensors are provided
for detecting movements of the measuring tubes from which mass flow
can be determined.
1. A mass flow meter operable on the Coriolis principle, comprising,
two straight parallel arranged measuring tubes of the same length
and having their respective ends in juxtaposition, two straight
axially aligned compensating tubes disposed in parallel to said
measuring tubes, said compensating tubes having inner ends in juxtaposition
with each other and outer ends in juxtaposition with the corresponding
ends of said measuring tubes, a mounting member having supply and
discharge passages with external ends connectable to external supply
and discharge means and internal ends in juxtaposition, said compensating
tubes having their inner ends connected to either said supply or
discharge passages and their outer ends connected to the corresponding
outer ends of said measuring tubes, oscillator means for driving
said measuring tubes in opposite directions, and sensor means for
detecting movements of said measuring tubes from which mass flow
can be determined.
2. A mass flow meter according to claim 1 wherein said compensating
tubes have approximately the same coefficient of expansion as said
3. A mass flow meter according to claim 1 wherein said juxtaposition
ends of said compensating tubes are in fixed relation to each other
in said mounting member.
4. A mass flow meter according to claim 1 wherein said compensating
tubes are in the same plane as said measuring tubes.
5. A mass flow meter according to claim 1 wherein said compensating
tubes are disposed between said measuring tubes.
6. A mass flow meter according to claim 1 including a housing for
enclosing said measuring and compensating tubes, said housing being
connected only to said mounting member.
7. A mass flow meter according to claim 1 wherein said oscillator
means is mounted midway between the ends of said measuring tubes,
said sensor means being spaced near the ends of said measuring tubes
and closer thereto than to said oscillator means.
The invention relates to a mass flow meter working on the Coriolis
principle, in which two juxtaposed measuring tubes are mechanically
interconnected at their ends and connected in parallel mechanically
with the aid of two tube connectors which, in turn, are each connected
to a connection by a supply and a discharge passage, wherein an
oscillator driving the measuring tubes in opposite senses is provided
and wherein the measuring tubes are associated at a spacing from
the oscillator with sensors for receiving measuring signals from
which the mass flow can be determined.
In a known meter of this kind (EP-OS 109218), a cylindrical container
provided at its ends with connections for the supply and discharge
of the medium to be measured and with dividing walls in the middle
carries two U-shaped bent tubes which communicate with the interior
of the container at both sides of the dividing walls. The container
therefore forms the tube connectors and the supply and discharge
passages. The adjacent limbs of the U-shaped tubes are mechanically
interconnected near the container by lugs which define the ends
of the actual measuring tubes which can be oscillated in opposite
senses by the oscillator. The oscillator engages the middle of the
web of the U. The sensors are disposed at the transition of the
web with the rectilinear tube limbs. The particular flow of mass
can be determined at both ends of the web of the U from the difference
of the phases of the oscillatory motion. Since the oscillating measuring
tubes must have a certain length but project laterally from the
container, the resulting meter is bulky in the lateral direction.
The invention is based on the problem of providing a mass flow
meter of the aforementioned kind which projects to only a small
This problem is solved according to the invention in that the measuring
tubes are straight and parallel, that compensating tubes respectively
extend from a tube connector to a central zone approximately at
the centre of the tube and have substantially the same coefficient
of thermal expansion as the measuring tubes, and that the supply
and discharge passages extend from this central zone to the associated
In this construction, rectilinear measuring tubes are used instead
of the bent measuring tubes. Consequently, the lateral extent is
small. The measuring tubes can extend parallel to the pipe in which
the meter is changed over. However, since the tube connectors have
a large axial spacing from each other, changes in length occur as
a result of temperature fluctuations. If the tube connectors form
a fixed constructional unit together with the connections as is
usual, the position of the unit being spacially fixed by being applied
to the pipe, the change in length leads to axial stresses in the
measuring tubes, through which the oscillatory behaviour is altered
and thus the measurement is falsified. Axial stresses can also occur
because of wrong clamping of the appliance or for other reasons.
Consequently, the invention provides for the compensating tubes.
Upon fluctuations in temperature, these undergo the same change
in length as the measuring tubes so that no axial stresses are exerted
on the measuring tubes themselves. The result of the measurement
is therefore independent of temperature.
Preferably, the compensating tubes are fixed to each other in the
central zone. In this way, the measuring tubes, the compensating
tubes and the tube connectors form a structural unit of high strength.
In a preferred example, a common carrier for holding the measuring
and compensating tubes is fixed to the compensating tubes in the
central zone. By means of this carrier, the entire measuring appliance
can be mounted on a support. Practically no noise is transmitted
through this mechanical bridge despite the oscillations that are
Desirably, the compensating tubes extend in the plane of the measuring
tubes. This gives a very compact construction.
With particularr advantage, the compensating tubes are disposed
between the measuring tubes. This gives a symmetrical construction
which facilitates even more accurate measurements. In addition,
because of the symmetrical construction, a defined node is produced
in the central zone for all the six possible linear and rotary movements
so that, when fixed at this point, it is possible to obtain perfect
insulation of the resonance frequency from the surroundings.
In a preferred embodiment, the measuring and compensating tubes
are enclosed by a housing and connected thereto only by way of the
supply and discharge passages at the places where they go through
the wall of the housing. Such a housing can be closed with a hermetic
seal and possibly evacuated so that no condensation is formed on
the tubes that might influence the accuracy of measurement. Since
the connection is only by way of the supply and discharge passages,
the measuring tubes remain uninfluenced by stresses that could arise
at the housing as a result of the securing.
Preferably, the throughgoing positions are adjacent to the central
zone. Since no temperature-dependent elongations occur at that position,
there is no tendency at all for the housing to transmit interfering
forces to the tube system.
In a meter in which the sensors are disposed in front of and behind
the oscillator, which is arranged in the middle of the measuring
tubes and detect the measuring tube positions relatively to each
other, it is advisable for the spacing of the sensors from the ends
of the measuring tubes to be less than that from the centres of
the measuring tubes. With this spacing of the sensors, one can detect
the largest phase difference. The sensors should, however, also
have a small spacing from the ends of the measuring tubes so that
an adequately large measuring signal can be detected. The optimum
position can be readily found by trial and error.
Examples of the invention will now be described in more detail
with reference to the drawing, wherein
FIG. 1 is a longitudinal section on the line A--A in FIG. 2;
FIG. 2 is a horizontal section on the line B--B in FIG. 1;
FIG. 3 is a section on the line C--C in FIG. 1;
FIG. 4 is a spacial diagram of the measuring and compensating tubes;
FIG. 5 is a diagrammatic representation of the circuit for operating
the oscillator and sensors; and
FIG. 6 is a longitudinal section through a modified embodiment.
In the mass flow meter accordinig to FIG. 1 two straight and parallel
measuring tubes 1 and 2 extending in the same plane are connected
at their ends E to tube connectors 3 and 4 respectively.
Two compensating tubes 5 and 6 each having about half the length
of one measuring tube extend in the same plane as the measuring
tubes and between same from the tube connector 3 or 4 up to a central
zone 7 at the centres of the measuring tubes.
The confronting sides of the compensating tubes 5 and 6 are interconnected
by being connected to a common carrier 8. In the present example,
the connection is effected by inserting bent tubular spigots 9 or
10 in the bores defined by the ends of supply and discharge passages
11 or 12 and by soldering to an upstanding wall 13 of the carrier.
The two ends of the carrier 8 form connections 14 and 15 to which
tube sections 16 or 17 of a conventional flow pipe can be connected
by their flanges 18 or 19 with the aid of screws 20.
In the central zone 7 there is an oscillation generator 21 which
is adapted to set the measuring tubes 1 and 2 into oppositely directed
oscillations in their plane. Oscillation takes place over the free
length of the measuring tubes 1 and 2 that is to say between their
ends E at which they are mechanically fixed to the tube connectors
3 or 4. Sensors 22 and 23 which detect the particular spacing of
the measuring tubes 1 and 2 from each other in the central zone
are so placed that they have a smaller spacing from the ends E than
from the centres of the tubes. Their construction is shown by way
of example in FIG. 4.
The measuring tubes 1 2 the tube connectors 3 4 and the compensating
tubes 5 6 are disposed in a housing 24 which in practice consists
of an upper portion and a lower portion and has a through passage
25 for the supply and discharge passages 11 12 hermetically sealed
at the central zone 7. The housing 24 is connected to the tube system
only at this through passage. The interior 26 is evacuated so that
the formation of condensation on the measuring and compensating
tubes is not possible.
The material of the measuring tubes 1 2 and the compensating tubes
5 6 should have substantially the same coefficient of thermal expansion.
Preferably, the material is the same, it only being necessary for
the cross-section of the compensating tubes to be somewhat larger
than that of the measuring tubes. Consequently, upon a change in
temperature, the sum of the changes in length of the compensating
tubes 5 6 is equal to the change in length of the measuring tubes
1 2. The measuring tubes therefore undergo no axial stresses caused
by temperature that might falsify the measuring result.
It will be assumed that a medium, particularly a liquid, flows
through the meter in the direction of the arrows. The two measuring
tubes 1 and 2 will then form a parallel circuit. If, now, the measuring
tubes 1 2 are set into oscillation in opposite senses in their
planes by means of the oscillator 21 at resonance frequency if
at all possible, then Coriolis forces exerted by the mass of the
flowing medium bring about a phase dispacement in the oscillation
of the measuring tubes along their length. By reason of the oscillations
in opposite senses, this phase displacement can be very readily
determined by sensors 22 23 which detect the positions of the measuring
tubes 1 2 relatively to each other. Since the sensors are disposed
near the ends E, the phase displacement is comparatively large.
Since a certain spacing remains from the ends E, the measuring signal
is still sufficiently large in comparison with all interference
In FIG. 4 the measuring and compensating tubes are illustrated
diagrammatically. One extreme oscillatory position of the measuring
tubes 1 2 is shown in broken lines. It will be seen that, because
of the opposed movement, the oscillatory forces at the tube connectors
3 4 balance each other out and therefore oscillations are not diverted
to the outside in this plane by way of the central carrier, so that
the associated noise is likewise not transmitted.
Because of the oscillation convexity, there is periodic upsetting
of the compensating tubes 5 6. Since the upsetting forces are equal
and opposite, they balance each other out in the region of the carrier
8. They are therefore likewise not transmitted to the surroundings.
Because of the symmetric construction, the same applies to all other
translatory and rotary movements that could occur as a result of
the oscillations. The junction of the two compensating tubes 5
6 therefore forms a node K, so that no or hardly any oscillating
noises are transmitted to the outside through the carrier.
The supply and discharge passages 11 12 extend within the carrier
8 which is sufficiently strong to carry the entire arrangement.
Since the connections 14 15 of the carrier are likewise still in
the central zone 7 there is no fear of disruptive thermal elongation.
The same applies with respect to the connecting point between the
housing 24 and carrier 8 in the region of the throughway 25. Any
changes in the dimensions of the housing and carrier caused by the
temperature are likewise negligibly small at this position.
The most varied kind of measuring signal detectors are suitable
for determining the phase difference in the oscillations between
the two sensors 22 and 23. In particular, the sensors should work
without contact. This can be done optically, magnetically, capacitatively
or otherwise. Determination of the phase position can for example
take place by measuring the acceleration, the velocity or the amplitude.
The measuring signal need not be an oscillation instead, one can
measure the period during which the spacing of the measuring tubes
exceeds or falls below certain limiting values.
In the FIG. 5 embodiment, in which corresponding integers are referenced
with numerals increased by 100 in relation to FIGS. 1 to 4 there
are electromagnetic sensors 122 and 123 each comprising an induction
coil 126 or 127 secured to the one measuring tube 102 and a permanent
magnet 128 or 129 secured to the other measuring tube 101. Because
of the relative oscillatory movement between the two parts of the
sensors, an A.C. voltage is induced in the induction coil, that
is applied by conductors 130 and 131 to a detector 132 which is
provided with indicating means 133 for the through-flow.
The oscillator 121 is defined by a drive coil 134 connected to
measuring tube 102 and a permanent magnet 135 connected to measuring
tube 101. Drive coil 134 is fed by a driving circuit 136 with an
A.C. voltage that determines the oscillations of the measuring tubes
101 102. It should be as close as possible to the resonance frequency
of these tubes so that the least possible power will produce the
transverse motion of the measuring tubes necessary for the measurement.
By feeding the measuring signal back along the conductor 130 the
resonance condition is particularly easy to achieve.
The two compensating tubes are in this embodiment in a different
plane from the measuring tubes 101 and 102. For this purpose, the
tube connectors 103 104 have upwardly projecting spigots 137 138
by which the compensating tubes are connected in a plane above that
of the drawing.
In the FIG. 6 embodiment, parts corresponding to those in FIGS.
1 to 4 have reference numerals increased by 200. The compensating
tubes 205 and 206 are again in a plane between the parallel measuring
tubes (not shown). In the central zone 207 the compensating tubes
are interconnected by way of a securing point 213. The supply and
discharge passages 211 and 212 are substantially parallel to the
measuring and compensating tubes. The places 225 and 225a where
they pass through the housing 224 are at opposite end walls of the
said housing. This is also the place of connection to the housing.
If the housing 224 and the supply and discharge passages 211 and
212 have different coefficients of thermal expansion, this will
not influence the measurement because any axial stresses in the
passages will balance each other out and not affect the measuring