Abstrict A mass flow meter operating by the Coriolis principle includes
a mechanical oscillating system having two straight measuring tubes
which are clamped at both ends. In the center of the measuring tubes
an oscillation exciter is disposed which sets the two measuring
tubes into oppositely phased flexural oscillations. The mechanical
oscillations are sensed by oscillation sensors which are arranged
at equal distances on both sides of the oscillation exciter. The
phase displacement of the sensed oscillations is a measure of the
mass flow. The mechanical oscillating system is arranged axially
in a support tube. For decoupling the mechanical oscillations from
the support tube the mechanical oscillating system is freely suspended
in the support tube at each end by an annular diaphragm. The entire
mechanical oscillating system including the diaphragms preferably
consists of titanium.
Claims We claim:
1. Coriolis type mass flow meter comprising a support tube, a mechanical
oscillation system disposed axially in the support tube and comprising
at least one straight measuring tube, an oscillation exciter which
sets the center of said at least one straight measuring tube in
flexural oscillations, and oscillation sensors for sensing the mechanical
oscillations at equal distances from the center on both sides of
the oscillation exciter, and plural continuously arcuate diaphragm
means each having an inner end connected to one end of the straight
measuring tube and an outer end clamped to the support tube for
providing the sole support for the straight measuring tube.
2. Mass flow meter according to claim 1 in which the outer end
of each diaphragm means is connected to a holder ring clamped at
the support tube.
3. Mass flow meter according to claim 1 in which the mechanical
oscillating system comprises at least two parallel measuring tubes
which at each end are connected fluidly in parallel by a distributing
member and that the connection of the inner end of the diaphragm
to the straight measure tube is provided by connection of the inner
end of each diaphragm to the distributing member disposed at the
same end of the measuring tube.
4. Mass flow meter according to claim 3 in which each diaphragm
is made integrally with an associated distributing member.
5. Mass flow meter according to claim 1 in which each diaphragm
has a non-wavy configuration.
6. Mass flow meter according to claim 1 in which each diaphragm
is made conical.
7. Mass flow meter according to claim 1 in which each measuring
tube consists of titanium.
8. Mass flow meter according to claim 7 in which the entire mechanical
oscillating system including the diaphragms and the distributing
members consist of titanium.
9. The flow meter of claim 7 wherein the diaphragms also consist
of titanium.
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 and oscillation sensors for sensing the mechanical
oscillations at equal distances on both sides of the oscillation
exciter.
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. However, the problem
is encountered that the mechanical oscillating system clamped at
both ends transfers oscillation energy to the support tube and to
the connected conduits. Furthermore, temperature-induced mechanical
stresses arise when the measuring tubes have a temperature different
to that of the support tube, as is in particular the case when measuring
the mass flow of measured media whose temperature differs from the
ambient temperature. Such temperature-induced stresses alter the
oscillation behaviour of the mechanical oscillation system so that
compensation or correction is necessary in order to avoid measurement
errors.
The problem underlying the invention is to provide a mass flow
meter of the type mentioned a the beginning which operates by the
Coriolis principle and in which the mechanical oscillating system
is mechanically well decoupled from the support tube and temperature-induced
mechanical stresses are reduced.
According to the invention this problem is solved in that the mechanical
oscillating system is freely suspended at each end by an annular
diaphragm in the support tube.
In the mass flow meter according to the invention the mechanical
oscillating system is connected to the support tube solely by the
annular diaphragms disposed at the two ends. These diaphragms effect
an excellent mechanical decoupling of the oscillating system from
the support tube and the connected conduits. Furthermore, the diaphragms
can compensate differences in the temperature-induced length changes
of the support tube and mechanical oscillating system.
A further substantial improvement of a mass flow meter operating
by the Coriolis principle is obtained according to the invention
in that at least the or each measuring tube of the oscillating system,
preferably however the entire oscillating system including the two
diaphragms and any distributor members present, consist of titanium.
For the following problem exists in all mass flow meters operating
by the Coriolis principle and having straight oscillating measuring
tubes: to enable an adequately large measuring signal to be obtained
at the location of the oscillation sensors a correspondingly large
oscillation amplitude must be present. The oscillation amplitude
is the greater the longer the measuring tubes. On the other hand,
however, the natural resonance frequency of the oscillating system
is the lower the longer the measuring tubes. Since the oscillating
system is stimulated to natural resonance oscillations the natural
resonance frequency of the oscillating system is the operating frequency.
The lower the operating frequency the greater the occurrence of
uncontrollable interference oscillations which superimpose themselves
on the measuring signal. Consequently, as high an operating frequency
as possible of the mass flow meter would be desirable.
If the measuring tubes or preferably the entire oscillating system
consist of titanium measuring signals of adequate magnitude can
be obtained with substantially shorter measuring tubes than with
an oscillating system made from steel. The natural resonance frequency
of such an oscillating system of titanium is thus considerably higher
than the natural resonance frequency of an oscillating system of
steel furnishing measuring signals of the same magnitude. With the
higher operating frequency of the mass flow meter having an oscillating
system made from titanium undesired interference oscillations are
practically eliminated. In addition, of course, a shorter overall
length of the mass flow meter is also very advantageous for constructional
reasons.
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
operating by the Coriolis principle according to a first embodiment
of the invention,
FIG. 2 is a cross-sectional view of the mass flow meter of FIG.
1 along the section line A-B,
FIG. 3 is a partially sectioned side view of a mass flow meter
operating by the Coriolis principle according to a second embodiment
of the invention and
FIG. 4 is a side view of the two end portions of the oscillating
system of the mass flow meter of FIG. 3 one end portion being shown
sectioned.
The mass flow meter 10 illustrated in FIG. 1 in a partially sectioned
side view and in FIG. 2 in cross-section along the line A-B of FIG.
1 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 15 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 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 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 arranged
at the other end of the oscillating system.
The distributing member 23 is a solid circular disc of considerable
thickness which contains in the interior flow passages 24 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 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.
Arranged in the centre of the support tube 11 is an oscillation
exciter 25 (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 25 consists of an electromagnet which is secured
in the wall of the tube portion 13 and opposite which an armature
27 lies which is secured to the measuring tube 21. When an alternating
current is sent through the coil of the electromagnet 26 by the
alternating attraction forces between the electromagnet 26 and the
armature 27 the measuring tube 21 is set in flexural oscillations
which via the distributing members 23 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 which is accommodated
in a circuit housing 28 secured to the sheet metal housing 20 and
which is indicated only by its printed circuit board 29. The excitation
circuit is designed so that the oscillating system 12 is stimulated
to oscillate at its natural resonance frequency.
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 25 at equal
distances from the latter two oscillation sensors 30 and 31 are
disposed. The oscillation sensors 30 and 31 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. Said sensor signals are supplied
to an electronic evaluation circuit which is also accommodated in
the circuit housing 28 and only indicated by its printed circuit
board 32. The evaluation circuit determines the mass flow from the
phase difference of the sensor signals supplied thereto.
The peculiarity of the mass flow meter illustrated resides in the
suspension of the mechanical oscillating system 12 in the support
tube 11. Integrally formed on the distributing member 23 is a short
tubular extension 34 which is connected to the inner edge of an
annular diaphragm 35. The outer edge of the diaphragm 35 is connected
to a holder ring 36 which is clamped in the end bush 14 and bears
axially against the somewhat inwardly projecting end face of the
connecting piece 16. Thus, the oscillating system 12 is suspended
axially in the support tube 11 by means of the diaphragm 35 and
the corresponding diaphragm disposed at the other end. The distributing
member 23 has smaller cross-section dimensions than the cavity in
the interior of the end bush 14 so that round the distributing member
between the peripheral surface thereof and the inner face of the
end bush 14 there is an intermediate space 37. The only contact
between the oscillating system 12 and the support tube 11 is thus
via the diaphragms 35 attached to the two ends.
The tubular extension 34 the diaphragm 35 and the holder ring
36 may be made in one piece with the distributing member 23. The
diaphragm 35 is then simply a thin wall of the material of the distributing
member 23 the thickness of which is so small that the diaphragm
can deform resiliently under load. Of course, the diaphragm could
also be a separately made part which is sealingly connected at the
outer edge in suitable manner to a separate holder ring and at the
inner edge to the connecting piece, e.g. by welding. In every case
this assembly is designed such that on the one hand the coupling
over from the one measuring tube to the other measuring tube through
the solid distributing member is as hard as possible but the coupling
of the oscillating system to the outer tube by the diaphragm is
as soft as possible. The diaphragm suspension of the oscillating
system 12 thus gives an excellent decoupling of the mechanical flexural
oscillations from the support tube 11 and the connecting pieces
16. Furthermore, different thermal expansions of the measuring tubes
21 22 and support tube 11 are compensated by the diaphragm 35 because
the latter can deform resiliently under an axial load. The axial
movement of the connecting piece 23 necessary for this is possible
without constraint because of the intermediate space 37.
In the embodiment illustrated in FIG. 1 the annular diaphragm 35
is planar and arranged perpendicularly to the flow direction of
the measured medium This arrangement gives the optimum diaphragm
effect but unfavourable flow conditions because the diaphragm 35
forms an obstruction projecting perpendicularly into the flow cross-section.
This gives at the entrance side of the mass flow meter a sudden
constriction and at the outlet side a sudden widening of the flow
cross-section.
FIG. 3 shows a modified embodiment of the mass flow meter which
differs from that of FIG. 1 only in another form of the diaphragm.
All the other components are unchanged; they are therefore designated
with the same reference numerals as in FIG. 1 and will not be described
again. Similarly, the cross-sectional view of FIG. 2 also applies
to the mass flow meter of FIG. 3.
The annular diaphragm 40 is not planar in the embodiment of FIG.
3 but conical so that it extends inclined inwardly from the holder
ring 36 to the connecting piece 23. This provides at the entry side
a gradual reduction of the flow cross-section and at the exit side
a corresponding gradual widening of the flow cross-section, thereby
substantially improving the flow conditions compared with the embodiment
of FIG. 1.
Due to the conical form the diaphragm 40 in the embodiment of FIG.
3 is substantially stiffer than the planar diaphragm 35 of FIG.
1 but the relative movements between the oscillating system 12 and
the support tube 11 to be compensated by the diaphragm are so small
that in the case of
FIG. 3 as well excellent decoupling of the oscillation movements
from the support tube and conduit system is obtained.
As in the embodiment of FIG. 1 in the embodiment of FIG. 3 as
well the diaphragm can be formed in one piece with the distributing
member 23 and the holder ring 36 FIG. 4 shows the end portions of
the oscillating system 12 of FIG. 3 with the two distributing members
23 the diaphragms 40 and the holder rings 36 in a side elevation
which is seen at right angles to the sectional view of FIG. 3 so
that the end portions of the two parallel straight measuring tubes
21 22 can be seen. This illustration also clearly shows the identical
configuration of the two end portions of the oscillating system.
The diaphragm suspension described for the oscillating system of
a mass flow meter is not restricted to the case where the oscillating
system comprises two parallel straight measuring tubes. It is also
suitable for mass flow meters whose oscillating system comprises
more than two parallel straight measuring tubes which are connected
from the fluidic point of view in parallel by distributing members,
and also for mass flow meters whose oscillating system comprises
only one straight measuring tube, in which case the sole measuring
tube is connected directly to the diaphragm without interposition
of a distributing member.
A further substantial improvement of such mass flow meters is obtained
if at least each measuring tube, preferably however the entire mechanical
oscillating system including the distributing members and the diaphragms,
is made from titanium. This makes it possible to eliminate a number
of restrictions and disadvantages present in such mass flow meters
when they have oscillating systems made from steel. In particular,
when using an oscillating system of titanium measuring signals of
adequate magnitude can be obtained with substantially shorter measuring
tubes than when using an oscillating system of steel This gives
the advantage of a higher natural resonance frequency of the oscillating
system because the natural resonance frequency is the higher the
shorter the measuring tubes. Since the mechanical oscillating system
is stimulated to oscillate at its natural resonance frequency an
oscillating system of titanium can be operated at a substantially
higher frequency than an oscillating system of steel furnishing
measuring signals of the same magnitude. Whereas a mass flow meter
of the type illustrated in the drawings with measuring tubes of
steel must be operated at a frequency lying in the order of magnitude
of about 100 to 200 Hz a similar mass flow meter with measuring
tubes of titanium supplying measuring signals of the same magnitude
with correspondingly reduced overall length can be operated at a
substantially higher frequency which is of the order of magnitude
of 1000 Hz. With such a high operating frequency undesired mechanical
interference oscillations are negligible. The occurrence of such
interference oscillations is the greater the lower the operating
frequency. They superimpose themselves on the measuring signal and
falsify the measurement result.
These advantageous properties of the oscillating systems of titanium
result from the fact that titanium has the optimum combination of
specific weight and elasticity modulus for this purpose In addition,
titanium has a favourable coefficient of thermal expansion so that
a mass flow meter having an oscillating system of titanium and a
support tube of steel is suitable for measurements in a large temperature
range. Finally, titanium is very resistant to corrosion so that
such a mass flow meter is also suitable for measurements of chemically
aggressive media. |