Mass flow meter working on the coriolis principle

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.

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 109,218), 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 extent laterally. 
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 connection. 
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 generated. 
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.

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 signals. 
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 tubes.