INSTRUMENT DE MESURE DU POIDS SPECIFIQUE DE LIQUIDES PAR VOIE ACOUSTIQUE

MEASURING EQUIPMENT FOR ACOUSTIC DETERMINATION OF SPECIFIC GRAVITY IN LIQUIDS

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
The specification relates to measuring equipment for
the determination of the specific gravity of liquids. The
equipment includes a transducer which is fitted in a measuring
probe consisting of a tubular holder which also contains a
reference unit in full contact with the transducer. A reflection
disc is placed at some distance from the side of the reference
unit opposite one face of the transducer. The tubular holder is
slotted between the reference unit and the reflection disc so
that there is at least one opening in the tube wall. The refer-
ence unit has at least one all-round incision the limitation
surface of which, facing the transducer, is parallel to the in-
terface between the transducer and the reference unit.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A measuring probe for the acoustic determination of
the specific gravity of a liquid, comprising, a holder having
a piezoelectric ceramic disc attached at one end thereof and
a parallel arranged reflection disc attached at the other end
thereof, a reference element having parallel end faces and a
known acoustical impedance, said reference element having one
of said end faces in abutting engagement with said piezo-
electric disc and the other of said end faces in spaced
relation to said reflection disc to form a sampling space
therebetween, said reference element having an annularly shaped
groove formed therein intermediate said end faces and having
a planar surface adjacent to and parallel to said ceramic
disc, said holder having wall means bounding and surrounding
said annular groove, and said wall means forming an opening to
provide ingress and egress to and from said sampling space for
a medium externally of said probe.

Description

The present invention relates to a measuring equipment
for determination o~ the specific gravity of liquids and o~ the
kind stated in the introduction of the claim.
The acoustic impedance of a liquia is the p~oduct o~ the
specific gravity of the liquid and the propagation time of an
acoustic wave in the liquid. This connection is utilized in
specific gravity determination in apparatus in which a trans-
ducer, e.g. a piezoelectric ceramic disc, transmits an ultra-
sonic pulse over a given distance through the li~uid.
In measuring apparatus known so far this given distance
is the distance between two walls of a measuring chamber, or
twice that distance if the same transducer is used as both
transmitter and receiver of the ultrasonic pulses. Such
apparatus, e.g. known from the specifications of U.S. patents
3-028-749 and 2-926-522, require a relatively complicated and
stationary measuring equipmentO
According to the present invention, the transducer is
suggested to be fltted in a measuring probe comprising, a
holder having a piezoelectric ceramic disc attached at one
~0 end thereof and a parallel arranged reflection disc attached
; at the other end thereof, a reference element having parallel
end faces and a known acoustical impedance, the reference
element having one of the end faces in abutting engagement
with the piezoelectric disc and the other of the end faces
; in spaced relation to the reflection disc to form a sampling
space therebetween, the reference element having an annularly
shaped groove formed therein intermediate the end faces and
having a planax surface adjacent to and parallel to the
s d /~J~
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ceramic disc, the holder havin~ wall means boun~ing and
surrounding the annular groove, and the wall means forming
an opening to provide ingress and egress to and from the
sampling space for a medium externally of the probe.
Through this, a simple and mechanically stable
measuring equipment is obtained, and it is easily portable
because the measuring probe may be the size of an ordinary
ball-point pen. Through the electronic circuit, the specific
gravity of a liquid, in the space between the reference unit
and the reflection dise, can be determined by measuring the
refleetor amplitude from the transition between the referenee
unit and the liquid and the propagation time of the acoustie
wave in the liquid.
In the following, the invention will be explained in
detail with refarenee to the drawing whieh shows a cross-
seetion of a measuring probe as speeified in the invention,
for acoustie determination of the specific gravity of liquidsO
The probe shown consists of a piezoelectric ceramic
disc (1), a referenee unit (2) and a refleetion dise (3), fitted
in a tubular holder (4). Between the referenee unit (2) and
the reflection dise (3) the tubular holder (4) is slotted so .
that there is at least one opening (5) in the tube wall.
The eeramie dise (1) is conneeted to an eleetronie
eircuit - not shown in the drawing - whieh can apply eleetrie
pulses to the dise. Then, ultrasonic waves will be transmitted
through the adjoining reference unit (2), pass through the
open space between the referenee unit (2) and the refleetion
disc (3), and be refleeted to the ceramic disc. Here, the
acoustie signals will be converted into eleetrie signals whieh
sd/ ~ -2-

'18~;41 3
will be ~pplied to the elec~ronic circuit.
Thus, the ceramic disc (1) is used as both a transmitter
and a receiver of acoustic pulses.
In the reference unit (2) there is an annular groove
having a planar limitation surface (6) which faces the ceramic
disc (lj and is parallel to the interface between the ceramic
disc (1) and the reference unit (2), i.e. at right angles to
the direction of sonic propagation in the reference unit (2).
When a short electric pulse is applied to the ceramic
disc (1), an ultrasonic wave will be transmitted and
propagated through the reference substance. Part of the sound
will be totally reflected at the limitation surface (6~ if,
on the other side of the limitation surface (6), there is a
substance, e.g. air, the acoustic impedance of which is a
great deal lower than the acoustic impedance of the reference
substance which may, e.g., be glass, aluminium or stainless
steel. The amplitude of Ao of the reflected echo will be
proportional to the amplitude of the acoustic wave transmitted.
The remaining acoustic wave transmitted will be propagated
through the reference substance and be reflected to the
interface between the reference unit (2) and the medium which
has permeated through the opening ~5) between the reference
unit (2) and the reflection disc (3). Thus, if the measuring
probe has been immersed in a liquid, the latter will be the
substance adjoining the terminal surface of the reference
unit (2).
B
sd/c~ -3-
: ,. , . . .. ~ . . .:

T r a n s I a t i o n ~ 48
From this terminal surface a sonic wave is reflected, the
amplitude Al of which can be expressed as follows:
Al - Aj ( p c + g~ c ) ( 1 )
in which e~pression A is the arnplitude of the sonic wave trans-
mitted, ~)r and Pv designate the specific gravity of the reference
unit and the liquid, and c and CV the corresponding propagation
times.
From tl-e above expression tl ) the specific gravity ~'v of the
liquid can be calculated, because
( A. - A ~ )
v r (Aj + Al ) x c (2)
in which expression Zr designates the given acoustic impedance
Srcr of the reference unit (2).
The part of the sonic wave which is not reflected by the
interface between the reference unit (2) and the liquid, will be
propagated through the latter and be reflected from the reflection
disc to the ceramic disc (1). The propagation time in the liquid
cv can be expressed as fol lows:
v T ( 3 )
in which expression I is the distance between the reference unit
12) and the reflection disc (3), and T is the propagation time,
determined by~ the time lag between the sonic waves reflected.
Through the amplitude A the sound reflected from the limita-
tion surface (6) the electronic circuit can adjust the transmission
amplitude so that Aj will be constant irrespective of variations of
the temperature-variable data of the piezoelectric ceramic d;sc (1).
On the basis of expressions (2) and (3) the specific gravity of the
liquid can then be determined by measuring amplitude Al 9 or the
voltage El proportional to that, and the propagation time T,
because
rTv (E j -- El )
Pv 2 X I (Ej ~ E~ where E; = const. x A;
4 ..... s
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T _ a n s I a t I o n 5
Electronically, it may be difficult to get precise figures by
analog multiplication and division of voltages, so the above
expression (4) can be transcribed as fol lows:
v 2 x I X ~nor ~ 2 x I X r nor ~7~
in which expression Znor is the acoustic impedance of a normal
solution, for which purpose water at 20 centi~3rade will be
expedient. If the probe is immersed in water, the second term of
the above expression (5) wiil be zero, and the specific gravity is
determined by measuring the time. As the acoustic impedance of
most liquids is of the same size as that of water, the last term of
expression (S) is of minor importancè in the determination of the
specific gravity. Consequently, any uncertainty in calculation of
the last term will be negligible.
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