Note: Descriptions are shown in the official language in which they were submitted.
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Eile Ref.: PCT/EP 90/02177 GR ~9 P 2017
Siemens Aktienge~ellschaft
SOUND-INSULATED HOLDER FOR AN ULTRASOUND TRANSDUCER
The present invention relates to a holder for an
ultrasound transducer for transmitting and receiving
ultrasound radiation into and from a gaseous medium.
Emission of ultrasound from a vibrating solid
ob~ect surface of a transducer into the medium of gas
leads to only a low power transmission ratio from the
solid ob~ect into the gaseous medium and vice versa, as
a consequence of the very large difference in the acou~-
tic characteristic lmpedance of the material of the solid
ob~ect ~urface and that of the medium, this difference
being in the order of magnitude of the 4th power.
DE-A-3,832,947, which has not previously been
published but iæ also described in the following text,
describes how, by constructing the transducer body, which
is to be excited into oscillations, with a matching
layer, approximate characteristic impedanco matching can
be achieved. In this case, a rubber ring is provided to
hold the transducer body, which rubber ring rests on the
one hand in an annular groove in the oscillation node of
the transducer and, on the other hand, in an annular
groove of the outer holder. The transducer is adequately
held by corresponding ~wo-surface push-~it contacts in
the grooves. ~he suxface pressure on the two ~ides
(transducer and holder3 has, however, been found not to
be optimum, namely, in addition to the Lmprovement in the
30und emission achieved (3ic) with the matching layer,
also to achieve a large attenuation of body sound
transmis~ion from the transducer body ~sic].
US-A-3,964,308 likewise describes a holder for
attenuating body sound transmi~sion, which provides three
individual 0-ring~, each having two surface contacts.
However, it is not the transducer body which is held in
this case, but a tubular pro~ecting piece of the same. In
~his ca~e, there iæ a gap between the pro~ecting piece
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and the holder which is not sealed against the ingress of
dust from the surrounding atmosphere.
Ultrasound transducers are used in ultrasound
measurement appliances, especially for flowing gases, for
example for determining the gas volume flowing through a
tube. An application of such an appliance is a~ a ga~
meter, for example as a domestic gas meter.
In such appliance~, the ultrasound transducers
which determine the measurement path within a tube are
arranged with respect to the tube axis a~ a dis~ance of,
for example, 240 mm, which i~ still relatively small. In
general, these transducers are in~erted into the tube
wall and emit the ultrasound in an oblique direction with
respect to the tube axis through the flowing medium of
gas. In this case, one transducer is the transmitting
transducer, the other transducer being the receiving
transducer. In general, the measurement is carried out
alternately upstream and downstream and by subtracting
the resulting signals. Such ultrasound flowmeters are
described in detail in the PCT Patent Applications of
equal priority, of W091~09282, W091/09280, W091/09281, of
which the present invention is an advantageous
dovelopment.
In the case of the short transducer distance as
specified, there are no exacerbating problems as such, as
a consequence of flow noise occurring. However, the
evaluation of the received signal can be critically
adversely affected by sound intensity which is
transmitted by the tube wall originating from, for
30 example~ sound sources which are even at a relatively
long range. In order to minimize thi~ body sound
transmission, the tran~ducer~ used, i.e. e~pecially the
receiving transducer, mu6t be mounted in th~ appliance or
~n the tube wall ~uch that the body of the transducer and
the tube wall are largely insulated from one another with
respect to acoustic sound transmis~ion. The at least
partial use of material having a sound~attenuating
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characteristic on the transducer can achieve a further
contribution.
The ob~ect of the present invention is to specify
a sound-insulated holder for a solid body transducer in
5 an appliance as appropriate, suitable especially for mass
production.
Thi~ ob~ect is achieved by means of the features
of Patent Claim 1.
Figure 1 shows an illustration of the principle
of an appropriste ultra~ound measurement appliance 1 in
tubular form, on whoqe tube 5 carrying the flowing medium
there are fit~ed two ultrasound transducers 11, 12. As
can be seen, the~e ultrasound transducers are located in
obliquely pointing connecting pieces 51 which are
inserted into the tube wall 5. Especially in the case of
mea3urement appliance~ which are provided for measuring
those gases which must not escape to the exterior,
suitable seal~ must be provided at those points at which
the appliance can be opened, for example for insertion or
replacement of the transducers 11, 12. An acoustic
radiation path between the transducers 11 and 12 i~
dosignated by 21.
Fig. 2 shows a detailed reprQsentation of a
lateral tubular pro~ecting connecting piece 51 in lon-
gitudinal ~ection, of the type shown in Fig. 1. This
connecting piece 51 i8 the actual holder for the transdu-
cer 11. The tubular connecting piece 51, which is cut off
obliguely, ha~ a step 52 as shown. The oute tubular 0nd
of the connecting piece 51 i8 clo~Pd by mean~ of a cover
58, a sealing ring 57 being provided in a groove 56. The
cover 58 has a clamping ring 55, a~ can be ~een from the
f~gure, with an edge as a pro~ection. Thi~ clamping ring
55 i8 dimensioned such that it iB matched to the diameter
of the annular step 52 of the connecting piece 51. An
0-ring 54 of rubber or of a comparably permanently
elastic material i~ provided, which i8 positioned as can
bo seen in Fig. 2. Thi~ 0-ring 54 i8 not only used a~ a
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further seal as known but also, on its own, holds the
transducer 11 positioned in the connecting piece 51. The
step 52, together with the clamping ring 55, provide~
precise positioning of the 0-ring 54 in the connecting
piece 51, and the annular groove 59 in the outer wall 53
of the transducer 11 en~ures its exact positioning with
respect to the 0-ring 54.
For the sake of clarity, the electrical connec-
tions of the transducer 11 are omitted. Leads can be
passed through the cover 58 or ducts can al~o be provided
on the sides in the wall of the connecting piece 51.
These connections mu~t be constructed such that, for
their part, they cannot produce any noticeable body ~ound
tran~mission, as is ensured, for example, by tha use of
connecting braided cables.
The holder according to the invention of the
transducer 11 i8 of such a type that advantages are
achieved over a holder for an ultrasound transducer which
i~ described in an older Patent Application of the not
20 prev10u21y published DE-A-3,832,947. The invention
achieves an optimum of a sound-insulated holder with a
"linear" compres~ion of the 0-ring 54, the compression on
the cylindrical clamping ring 55 provided being reduced.
In the case of the invention, thi~ ~linear~ (double) edge
contact res~ with only a minimal ~urface contact in the
parasitic sound path. The actual gas-tight closure is
formed by the additional ~ealing ring 57 which i~ pres6ed
into the groove 56 over a large area, as shown, when the
cover 58 i8 screwed on and thus ensures a permanently
reliable seal.
By me~ns of the ~easures according to the
invention, a body sound level of less than -140 dB can be
achieved, which i lost in ~he noi~e of the received
signal. By means of the invention, a gas-tight
con~truction i~ achieved which nevertheles~ has good
acoustic decoupling and yat axactly defined transducer
positioning.
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A development of the invention is to provide a
second O-ring 154 of permanently elastic material, namely
in a region close to the sound-emitting front surfaces of
the transducer 3. This ring 154 is used to close the gap
60 which necessarily exists between the outer wall of the
transducer ll and the annular inner surface of the
tubular connecting piece 51. Specifically, it has been
found that preferably dust, small particles and the like
are deposited in this gap which are carried along in the
gas flowing through the measuring tube 4. This is preven-
ted by the ring 154 which is inserted in the groove 156
o~ th~ transducer 11 and into the groove 15~ of the inner
~urface of the connecting piece 51.
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