Abstrict Ultrasonic flow meter having a W-shaped ultrasonic path in the
measurement tube (1) and having a stepped elevation/depression of
the inner wall of the tube at the site of reflection of the parasitic
V-shaped path.
Claims What is claimed is:
1. An ultrasonic flow meter having a measurement tube through which
a fluid flows and having ultrasonic transducers operable as transmission
transducers and reception transducers, which, for a "W"-shaped
ultrasonic path with multiple reflections provided within the measurement
tube, are positioned at a given spacing from one another on a common
side wall of the measurement tube, comprising:
in order to at least reduce a disturbing signal of a parasitic
"V"-shaped ultrasonic path which occurs within the measurement
tube, means for providing at least one of an elevation and a depression
of an inner wall of the measurement tube that is located in the
interior of the tube at a tube wall which is opposite the side wall
provided with the transducers and at which a reflection of a V-shaped
path occurs, said means extending over a first half component of
a reflection region of the V-shaped path, said means having a single
step height, the single step height being dimensioned so that for
ultrasonic radiation of the "V"-shaped path extinguishing
interference occurs between radiation reflected at said first half
component of said means and radiation reflected at a remaining second
half component of the reflection region.
2. The ultrasonic flow meter according to claim 1 wherein at least
one of the first and second half components of the reflection region
comprises individual surface components.
Description BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic flow meter having
a measurement tube through which gas/liquid flows and having ultrasonic
emission/reception transducers, which, for a "W" shaped
ultrasonic path with multiple reflections provided within the measurement
tube, are located at a given spacing from one another on a common
side wall of the measurement tube.
Flow meters operated by ultrasound are known from the prior art
and are proposed in PCT application PCT/EP90/02178 filed 13 Dec.
1990 and in the parallel patent applications GR 89 P 2014 DE and
GR 89 P 2015 DE. The content of the specifications of these applications
is an additional component part of the present invention specification.
The flow meters consist essentially of a tube through which the
pertinent medium flows and within the interior of which an acoustic
path extends. The influencing of the ultrasound by the flowing medium
is the parameter from which, for example, the volume of gas or liquid
flowing through the tube cross section per unit time is to be ascertained.
Such flow meters are suitable, for example, as gas flow meters such
as, for example, as domestic gas meters.
The aforementioned acoustic measurement path is formed between
an acoustic emission transducer and an acoustic reception transducer.
The emission transducer and reception transducer are fitted in a
manner known per se to the side wall of the tube through which the
medium flows, and specifically in such a manner that an acoustic
beam is passed from the emission transducer into the interior of
the tube, with a component oriented parallel to the direction of
flow. Oblique injection of the ultrasound is the method in widespread
use; in this case, the ultrasound is reflected at the opposite wall
of the tube and executes, for example with three successive reflections
at the inner wall of the tube, an in total W-shaped path between
the emission transducer and the reception transducer.
The evaluation of the influencing of the ultrasound by the flow
takes place, as a rule, in such a manner that the measurement path
is traversed by the ultrasound alternately in a downstream and upstream
direction, and the difference value resulting from the influencing
of the flow, for the two ultrasound measurements, is the evaluation
signal. Thus, the emission transducer and reception transducer are
used interchangeably in terms of operation.
The aforementioned W-shaped path is preferred as against an only
V-shaped path, because advantages can be achieved with a predetermined
spacing, axial when related to the tube axis, between the emission
transducer and the reception transducer. It is also possible to
provide an ultrasound path with, instead of three reflections, even
more reflections, especially an odd number thereof. In this case,
it is essential that the ultrasonic transducers do not directly
"see" one another, i.e. that ultrasound does not pass
without reflection from the respective emission transducer to the
pertinent reception transducer.
If use is made of the W-shaped ultrasound path 21 in an arrangement
for example as shown in FIG. 1 in which the lower and upper tube
wall in the Figure are designated by 4 and 5 and the transducers
are designated by 11 and 12 then a signal transmission unavoidably
also takes place via the V-shaped path 22 between the emission transducer
and the reception transducer. This is based on the fact that the
radiation lobes of the emission transducer and the reception lobe
of the reception transducer cannot be directed with an arbitrarily
selectable degree of sharpness.
In the two aforementioned older patent applications, proposals
have been made for the most extensive possible suppression of a
parasitic signal of the V-shaped path as compared with the useful
signal of the W-shaped path. On the one hand, this involves the
use of a markedly rectangular tube cross section, with a ratio of
height H to width B exceeding 2:1 to 15:1 preferably 5:1 to 6:1.
In addition, such a rectangular cross section has the advantage
of acting in a homogenizing manner on the ultrasonic transirradiation
of the respective flow cross section. In the other application,
it is proposed, in the case of a measurement tube having a not necessarily
rectangular cross section, to arrange the transducers 11 and 12
of FIG. 1 in an appropriately modified manner in a "misdirected
position". In FIG. 1 the transducers are disposed with respect
to their axial spacing from one another and with respect to the
angular orientation of their radiatively emitting surfaces 111 and
112 so that the end sections of the W-shaped path do not coincide
with the respective normal to these surfaces 111 112. The misorientation
consists in dimensioning the axial spacing to be greater, and/or
arranging the transducers 11 12 with their surfaces 111 112 "misoriented"
at an angle, i.e. tilted, in comparison with the angular orientation
of the transducers of FIG. 1. It is also possible to provide these
two measures in combination.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a further measure
for the suppression of the signal of the parasitic V-shaped path
as compared with the useful signal of the W-shaped path. In particular,
the object is to achieve the most effective possible, up to virtually
complete, suppression of the parasitic signal.
This object is achieved by the following described ultrasonic flow
meter. In order to reduce/eliminate the disturbing signal of a parasitic
"V"-shaped ultrasonic path which, as is known, occurs
within the measurement tube, in the interior of the tube at a tube
wall, which is opposite a tube wall provided with the transducers
and at which a reflection of a V-shaped path occurs, in the region
of the reflection of the V-shaped path an elevation/depression is
provided that extends over one half component of this reflection
region. The elevation/depression is on the inner wall of the tube
and has a step height. The step height is dimensioned so that for
ultrasonic radiation of the "V"-shaped path extinguishing
interference occurs between the radiation reflected at the half
component on the one hand and the radiation reflected at the remaining
half component of this reflection region on the other hand.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention which are believed to be
novel, are set forth with particularity in the appended claims.
The invention, together with further objects and advantages, may
best be understood by reference to the following description taken
in conjunction with the accompanying drawings, and in which:
FIG. 1 and 1a are cross-sectional views of an ultrasonic flow meter
according to the present invention;
FIG. 2a is a plan view of one embodiment of an inner wall of the
FIG. 1 flow meter at the site of reflection of the V-shaped path;
FIG. 2b depicts the splitting of the reflection at the FIG. 2a
site;
FIG. 3a is a plan view of another embodiment of an inner wall of
the FIG. 1 flow meter at the site of reflection of the V-shaped
path;
FIG. 3b depicts the splitting of the reflection at the FIG. 3a
site;
FIG. 4a is a plan view of another embodiment of an inner wall of
the FIG. 1 flow meter at the site of reflection of the V-shaped
path;
FIG. 4b depicts the splitting of the reflection at the FIG. 4a
site;
FIG. 5a is a plan view of another embodiment of an inner wall of
the FIG. 1 flow meter at the site of reflection of the V-shaped
path;
FIG. 5b depicts the splitting of the reflection at the FIG. 5a
site.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As is evident from FIG. 1 the axial position of the reflection
locations of the W-shaped path, and specifically of the reflections
at the tube wall opposite to the transducers, is different from
the corresponding reflection location of the V-shaped path.
An attempt has already been made to provide at the location of
the reflection of the V-shaped path at the inner wall of the tube
sound-absorbing components such as, for example, a felt covering.
However, such a measure has proved to be disadvantageous to the
flow and also lacked the required service life. Furthermore, the
attainable signal attenuation was only moderate. Structuring cf
the inner wall of the tube, specifically to avoid directional reflection,
also does not lead to any result useful in practice.
The invention is based on the idea of achieving, by means of interference,
a to a large extent total extinction of the acoustic signal of the
V-shaped path. According to the invention, an insertion is undertaken
in the interior of the tube at the tube wall at the pertinent site
of the reflection of the V-shaped path, which insertion splits up
that surface of the inner wall of the tube which enters into consideration
for the purposes of reflection into at least two, but preferably
only two components or halves. Preferably, the original inner wall
of the tube forms one of the halves, and the other half is the surface,
raised by the amount D (see FIG. 1a), of a packing of the inner
wall of the tube. It is also possible to provide a corresponding
depression which, however, is more onerous in preparation. FIGS.
2a, 2b, 3a, 3b, 4a, 4b and 5a, 5b show examples of such a splitting-up,
and specifically FIGS. 2a to 5a in plan view onto the inner wall
at the site of the reflection of the V-shaped path and FIGS. 2b
to 5b the associated sections a and a'. These representations reveal
the significance of the dimension D, without this requiring any
further explanations. In FIGS. 2a, 3a, 4a and 5a a part of the V-shaped
path 22 has been shown. For the surface components X and Y there
are formed, after reflection, two parallel ultrasound paths 22a
and 22b. The thickness is dimensioned as a function of the angle
.beta. so that these two paths 22a and 22b are in extinguishing
interference with one another. Preferably, the first-order interference
is used.
The component X and/or Y can, per se, be split up in terms of area
once more, as is shown, for example, by FIGS. 3b and 5b. In this
manner, it is possible to achieve an even greater measure of reliable
extinction. In any event, it is necessary to apply the rule that
the intensity reflected at the surface component X must be equal
to the intensity reflected at the surface component Y; in this case,
it has to be borne in mind that a non-uniform intensity distribution
prevails within the total reflecting surface.
At acoustic wavelengths of lambda approximately equal to 2 mm,
corresponding to a frequency of 170 kHz in air, the step height
Dk is smaller than/equal to 1 mm with customary dimensions for the
angle beta. As regards the flow, an insertion of such dimensions
in the interior of the tube is insignificant and leads to at most
a slight disruption of the flow. The explanation "interference
of acoustic waves" given hereinbelow is to be taken as a basis
for the dimensioning.
Interference of acoustic waves
The following is applicable:
where c is the velocity of sound in the respective medium, f is
the transducer frequency and .beta. is the wavelength in the medium.
Two acoustic waves of the same direction of propagation, frequency
and amplitude extinguish one another if they possess a path difference
of ##EQU1## In the case of normal incidence, this path difference
.delta. is achieved for the spacing ##EQU2## of the two reflection
planes, i.e. at k=0 for D=.lambda./4.
Having regard to the angle of incidence .beta. of the signal of
the V-shaped path applicable in the measurement tube, the height
D required for extinction is computed from: ##EQU3## or, having
regard to the various media, may be represented as: ##EQU4##
This interference phenomenon is used to suppress the signal of
the V-shaped path. To this end, the direction of propagation can
be regarded as being almost constant for all participating acoustic
waves. For the sake of simplicity, only the center frequency of
the transducer resonance has been taken into consideration in the
present treatment.
On account of the large jump in impedance from air to solid materials
where sound is concerned, a partial reflection at the upper and
lower surface of a thin layer is not possible (such as, for example,
in optics at thin layers). Accordingly, the wave front is not reflected
for example at a uniformly coated surface, but the reflecting surface
is subdivided into two regions of equal surface content, of which
one surface is displaced by the offset D in relation to the other
(FIG. 2a and 2b) and thus a path difference is generated. The phase
jump of .lambda./2 upon reflection at the denser medium occurs in
this case with both components, and accordingly does not need to
be taken into consideration. The path difference leads, in the direction
towards the reception transducer, to the mutual extinction of the
two components of the signal of the V-shaped path which are reflected
at the two partial surfaces, and thus to its effective suppression.
Discussion of application in the measurement tube
Typical values for a measurement tube to be used here are L.sub.m
=174 mm, r=7 mm and h=30 mm, as well as .alpha.=35.degree., and
this gives .beta.=21.4.degree.. At room temperature, the velocity
of sound in air is c=340 m/s and in methane c=440 m/s. For transducers
with r=7 mm, a radial resonant frequency f.apprxeq.170 kHz is in
practice specified. According to (1), this gives for extinction
of the signal of the V-shaped path the possible elevations D.sub.k
in air D.sub.0 =0.182 mm, D.sub.1 =0.546 mm, D.sub.2 =0.912 mm,
D.sub.3 =1.277 mm and in methane D.sub.0 =0.236 mm, D.sub.1 =0.708
mm, D.sub.2 =1.180 mm, D.sub.3 =1.652 mm. For a good suppression
in both media, and for all velocities of sound in the intermediate
range, it is possible to select, for example D.about.0.2 mm or D.about.0.6
mm. Depending upon the particular requirements, it is also possible
to match D precisely to a specified medium. The aforementioned values
of D signify an only slight insertion into the flow. For ultrasonic
transducers with r=10.5 mm and f.about.130 kHz, the result is .beta.=23.9.degree..
According to (1), this gives in air D.sub.0 =0.265 mm, D.sub.1 =0.796
mm, D.sub.2 =1.326 mm, and in methane D.sub.0 =0.343 mm, D.sub.1
=1.03 mm, D.sub.2 =1.716 mm, so that for a good suppression in both
media and in the intermediate range, for example, D.about.0.3 mm
or D.about.0.9 mm should be selected.
Example of measurement: At .beta.=21.4.degree. and r=7 mm, i.e.
for transducers with f.about.170 kHz, the length of the surface
of incidence emerges as 39 mm. With a step height of D=0.6 mm at
a length of 15-20 mm, excellent results were achieved. The signal
of the V-shaped path may be attenuated by approximately 18 dB by
using the 3.lambda./4 plate (D=0.6 mm).
Possible embodiments of the raised surface
In addition to the embodiment shown in FIGS. 2a and 2b, further
variations of the apportionment of the surface are also feasible.
FIG. 2b shows once again the form shown in FIG. 2a, in which the
sequence not raised/raised/not raised occurs in the direction of
flow. FIG. 3b shows an arrangement complementary thereto, having
a sequence raised/not raised/raised. Also feasible would be arrangements
having a sequence transversely to the direction of flow. FIG. 4b
shows, in this sense, an arrangement raised/not raised. FIG. 5b
shows an arrangement raised/not raised/raised. A sequence of the
various surfaces transversely to the direction of flow would signify
less insertion into the flow.
The invention is not limited to the particular details of the apparatus
depicted and other modifications and applications are contemplated.
Certain other changes may be made in the above described apparatus
without departing from the true spirit and scope of the invention
herein involved. It is intended, therefore, that the subject matter
in the above depiction shall be interpreted as illustrative and
not in a limiting sense. |