Abstrict The flow rate of a fluid can be measured using an ultrasonic flow
meter according to the present invention with a higher measuring
accuracy and a lower cost. The feature of the invention consists
in an ultrasonic flow meter comprising a measurement tube for flowing
a fluid to be subjected to measurement, a first oscillator fitted
to the outer circumference of the measurement tube, a second oscillator
fitted to the outer circumference of the measurement tube at a predetermined
interval along the flow of the fluid from the first oscillator and
an acoustic filter fitted to the measurement tube for cutting high
frequency range of the oscillating wave propagating through the
oscillating tube, in which the flow rate of a fluid is measured
based on a time difference between a time required when the ultrasonic
wave from the upstream oscillator reaches the downstream oscillator
and another time required when the ultrasonic wave from the downstream
oscillator reaches the upstream oscillator.
Claims What is claimed is:
1. An ultrasonic flow meter comprising a measurement tube for flowing
a fluid to be subjected to measurement, said tube being constructed
of a metallic material capable of propagating ultrasonic wave and
being free from obstacles hindering the flow of the fluid in the
tube, a first oscillator fitted to the outer circumference of the
measurement tube, a second oscillator fitted to the outer circumference
of the measurement tube at a predetermined interval along the flow
of the fluid from the first oscillator and a plurality of acoustic
filters fitted to the measurement tube between the first and second
oscillators for cutting high frequency range of the oscillating
wave propagating through the oscillating tube, in which the flow
rate of a fluid is measured based on a time difference between a
time required when the ultrasonic wave from the upstream oscillator
reaches the downstream oscillator and another time required when
the ultrasonic wave from the downstream oscillator reaches the upstream
oscillator, wherein the acoustic filters are separated by an inner
distance and comprise flanges having a thickness and the product
of the inner distance of the acoustic filters and the thickness
of the flanges is inversely proportional to said high frequency
range of the oscillating wave.
2. An ultrasonic flow meter comprising a measurement tube for flowing
a fluid to be subjected to measurement, said tube being constructed
of a resin and being free from obstacles hindering the flow of the
fluid in the tube, a first oscillator fitted to the outer circumference
of the measurement tube, a second oscillator fitted to the outer
circumference of the measurement tube at a predetermined interval
along the flow of the fluid from the first oscillator, and a plurality
of acoustic filters fitted to the measurement tube between the first
and second oscillators for cutting high frequency range of the oscillating
wave propagating through the oscillating tube, in which the flow
rate of the fluid is measured based on a time difference between
a time required when the ultrasonic wave from the upstream oscillator
reaches the downstream oscillator and another time required when
the ultrasonic wave from the downstream oscillator reaches the upstream
oscillator, wherein the acoustic filters are separated by an inner
distance and comprise flanges having a thickness and the product
of the inner distance of the acoustic filters and the thickness
of the flanges is inversely proportional to said high frequency
range of the oscillating wave.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ultrasonic flow meter suitable for
use in tube or pipe line.
2. Description of the Prior Art
As prior art techniques for measuring a flow rate of a fluid such
as gases or liquids flowing in a tube or pipe line, various apparatus
such as orifices and nozzles are known and of late, ultrasonic flow
meters utilizing ultrasonic wave have widely been spread.
FIG. 7 shows a figure for illustrating the principle of an ultrasonic
flow meters, in which when a first oscillator 102 and second oscillator
103 are fitted to a measurement pipe 101 through a constant distance
L, and an ultrasonic wave is transmitted from one of the first oscillator
102 and second oscillator 103 and received by the other of the second
oscillator 103 and first oscillator 102 there occurs a difference
between a time t1 required when the ultrasonic wave from the first
oscillator 102 reaches the second oscillator 103 and another time
t2 required when the ultrasonic wave from the second oscillator
103 reaches the second oscillator 102.
When the sonic velocity in a fluid is c and a velocity of the fluid
is v, the following calculation formula is concluded:
That is, a formula 3 is derived by subtracting the recipropcal
number of a formula 2 from the recipropcal number of a formula 1
and this formula 3 is rearranged based on v to obtain a formula
4. According to this formula 4 the flow velocity v of the fluid
can be given if the distance L, time t1 and t2 are determined. When
the thus obtained flow velocity v is multiplied by the inner cross-sectional
area of the measurement pipe 101 the flow rate can be obtained.
As an invention based on the above described principle, for example,
there is an "ultrasonic flow meter" disclosed in JP-A-10-122923
which is constructed of such a simple structure that a measurement
tube 1 is provided with ring-shaped ultrasonic oscillators 2A and
2B, and a gap with the tube 1 is filled with grease 3 3 to acoustically
combine the measurement tube 1 with the ultrasonic oscillators 2A
and 2B, as shown in FIG. 3 of the same publication. These marks
are same as described on this publication. The inner part of the
measurement tube 1 so smooth that cleaning of contamination is not
required and the apertures of the ultrasonic oscillators 2A and
2B can be rendered smaller to comply with the aperture of the measurement
tube 1 thus reducing the aperture of the measurement tube 1.
FIG. 8 shows a figure for illustrating the problems of an ultrasonic
flow meters of the prior art, in which when ultrasonic oscillators
2A and 2B are fitted to the measurement pipe 1 the ultrasonic oscillator
2A being used as a sending side and the ultrasonic oscillator 2B
being used as a receiving side, an oscillating wave fundamentally
propagates along an arrow 1 in a fluid (medium) flowing in the measurement
tube 1. During the same time, the measurement tube 1 itself functions
as a propagation material and an oscillating wave shown by an arrow
2 propagates to the ultrasonic oscillators 2B.
Thus, the oscillating wave of the arrow 2 sometimes becomes a noise
to affect unfavourably the flow measurement. When the propagation
speed of the arrow 2 is close to that of the arrow 1 in particular,
influences thereof are larger.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an ultrasonic
flow meter capable of moderating influences of the oscillating wave
propagating through the measurement tube itself, whereby the above
described problems of the prior art can be solved.
The above described object can be attained by an ultrasonic flow
meter comprising a measurement tube for flowing a fluid to be subjected
to measurement, said tube being constructed of a metallic or metal-like
material capable of propagating ultrasonic wave and being free from
obstacles hindering the flow of the fluid in the tube, a first oscillator
fitted to the outer circumference of the measurement tube, a second
oscillator fitted to the outer circumference of the measurement
tube at a predetermined interval along the flow of the fluid from
the first oscillator and an acoustic filter fitted to the measurement
tube for cutting off high frequency range of the oscillating wave
propagating through the oscillating tube, in which the flow rate
of a fluid is measured based on a time difference between a time
required when the ultrasonic wave from the upstream oscillator reaches
the downstream oscillator and another time required when the ultrasonic
wave from the downstream oscillator reaches the upstream oscillator.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross-sectional view of an ultrasonic flow meter for
illustrating the principle (First Example) according to the present
invention.
FIG. 2 is a cross-sectional view for an acoustic model of an acoustic
filter according to the present invention.
FIG. 3 is a cross-sectional view of an ultrasonic flow meter for
illustrating the principle (Second Example) according to the present
invention.
FIG. 4 is a cross-sectional view of an ultrasonic flow meter for
illustrating the principle (Third Example) according to the present
invention.
FIG. 5 are graphs in Comparative Experiments, showing the effects
of acoustic filters.
FIG. 6 are graphs showing time differences measured by the ultrasonic
flow meter according to the present invention.
FIG. 7 is a drawing for illustrating the principle of an ultrasonic
flow meter.
FIG. 8 is a drawing for illustrating the problem of an ultrasonic
flow meter according to the prior art.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have tried to add a function for attenuating an oscillating
wave to the measurement tube in order to moderate the influences
of the oscillating wave propagating through the measurement tube
itself and accordingly, have succeeded in developing the following
first method and second method. The first method comprises, when
the measurement tube is constructed of a metallic material or the
like capable of well propagating an oscillating wave, fitting an
acoustic filter to the measurement tube and thereby cutting off
or reducing the oscillating wave. The second method comprises constructing
the measurement tube itself of a material capable of attenuating
an oscillating wave.
Specifically, the first invention provides an ultrasonic flow meter
comprising a measurement tube for flowing a fluid to be subjected
to measurement, said tube being constructed of a metallic or metal-like
material capable of propagating ultrasonic wave and being free from
obstacles hindering the flow of the fluid in the tube, a first oscillator
fitted to the outer circumference of the measurement tube, a second
oscillator fitted to the outer circumference of the measurement
tube at a predetermined interval along the flow of the fluid from
the first oscillator and an acoustic filter fitted to the measurement
tube for cutting off high frequency range of the oscillating wave
propagating through the oscillating tube, the oscillator being preferably
of a material for piezoelectricity.
Fundamentally, the flow rate of a fluid is measured based on a
time difference between a time required when the ultrasonic wave
from the upstream oscillator reaches the downstream oscillator and
another time required when the ultrasonic wave from the downstream
oscillator reaches the upstream oscillator. During the same time,
the oscillating wave propagating through the measurement tube is
effectively cut off by the acoustic filter, so that influences of
this oscillating wave upon measurement of the flow rate of the fluid
may be rendered very small. Therefore, precision of the measurement
of the flow rate can largely be increased according to the first
invention.
In the second invention, the acoustic filter is a flange of preferably
W, Cu or stainless steel. Since the acoustic filter is composed
of a simple flange, increase of the cost for the ultrasonic flow
meter can be suppressed.
The third invention provides an ultrasonic flow meter comprising
a measurement tube for flowing a fluid to be subjected to measurement,
said tube being constructed of a resin being free from obstacles
hindering the flow of the fluid in the tube, a first oscillator
fitted to the outer circumference of the measurement tube, a second
oscillator fitted to the outer circumference of the measurement
tube at a predetermined interval along the flow of the fluid from
the first oscillator. Fundamentally, the flow rate of a fluid is
measured based on a time difference between a time required when
the ultrasonic wave from the upstream oscillator reaches the downstream
oscillator and another time required when the ultrasonic wave from
the downstream oscillator reaches the upstream oscillator in the
similar manner to the first invention. Since the oscillating wave
propagating through the measurement tube is attenuated by the measurement
tube itself during the same time, this oscillating wave hardly affects
measurement of the fluid.
According to the third invention, therefore, the measuring accuracy
of the flow rate can largely be increased.
Embodiments of the present invention will now be illustrated based
on the accompanying drawings.
FIG. 1 is a cross-sectional view of an ultrasonic flow meter (First
Example) to illustrate the principle of the present invention, in
which an ultrasonic flow meter 10 comprises a measurement tube 11
a first oscillator 12 and second oscillator 13 being arranged at
a constant distance L on the the measurement tube 11 a first oscillator
12 and a second oscillator 13 being arranged at a constant distance
L on the measurement tube 11 a power source 17 and an amplifier
18 being connected to the first oscillator 12 through a first switch
14 and to the second oscillator 13 through a second switch 15 and
flanges 21 and 22 as an acoustic filter, fitted to the measurement
tube 11. In FIG. 1 a white arrow shows a flowing direction and
so on.
The first oscillator 12 and the second oscillator 13 are closely
fitted to the measurement tube 11 through acoustic binders 19 19.
The acoustic binder 19 is combined to such an extent that the oscillators
12 13 are prevented from shifting to the axial direction of the
measurement tube 11 for example, through receiving an oscillation,
etc., and consists of a material capable of well transmitting oscillations,
preferably, epoxy resins or greases.
The first switch 14 and the second switch 15 are changed into an
A side as shown in FIG. 1 to convert the first oscillator 12 into
a generator and to convert the second oscillator 13 into a receiver
and thus, the propagation time of the normal flow can be measured,
while the first switch 14 and the second switch 15 are then changed
into a B side reversely to FIG. 1 and the propagation time of the
reverse flow can be measured using the first oscillator 12 as a
receiver and the second oscillator 13 as a generator.
During the same time, the flanges 21 and 22 as acoustic filters
exhibit a function to cut off an oscillating wave propagating through
the measurement tube 11 itself, in particular, a high frequency
range wave. The principle of this function will be illustrated below.
FIGS. 2(a) to (c) show acoustic models of an acoustic filter according
to the present invention. (a) is a rough figure of FIG. 1 in which
the thickness of the flanges 21 and 22 as the acoustic filter,
is defined as L1 the inner distance of the acoustic filters 21
and 22 is defined as L2 and the inner diameter and outer diameter
of the measurement tube 11 are respectively defined as d and D.
(b) is a cross-sectional view of the measurement tube 11 having
the inner diameter of d and the outer diameter of D and accordingly,
an effective cross-sectional are a2 in the measurement tube 11 is
(.pi./4).times.D.sup.2 -(.pi./4).times.d.sup.2.
(c) is a cross-sectional view of square flanges 21 and 22 of W.times.W,
having a hole made with a diameter of D and accordingly, an effective
cross-sectional area a1 is W.sup.2 -{(.pi./4).times.D.sup.2 }. Considering
the above described model as a propagating path, this will be a
low frequency passing type filter, i.e. high frequency cutting off
filter, during which a cuttoff frequency .omega.2 is as follows:
##EQU1##
That is, .omega.2 is represented by a formula 5 consisting of a
function of the stiffness s and mass m, while the stiffness s is
represented by a formula 6 and the mass m is represented by a formula
7. Thus, when these formulas are inserted in the formula 5 and rearranged,
a formula 8 is obtained and a frequency f2 can be given by a formula
9. Consequently, f2 can be concluded from c, L1 L2 a1 and a2.
FIG. 3 is a cross-sectional view of an ultrasonic flow meter (Second
Example) to illustrate the principle of the present invention, in
which a damper 24 as an acoustic filter, is provided to the measurement
tube 11 so that, for example, an oscillating wave propagating through
the measurement tube 11 itself of an ultrasonic wave generated in
the first oscillator 12 is absorbed by the damper 24 and a considerable
part of the oscillating wave can thus be cut off. Accordingly, the
acoustic filter of the present invention can be any of the flanges
21 and 22 in FIG. 1 the damper 24 in FIG. 3 and the like capable
of exhibiting equal functions, without limiting the structures thereof.
FIG. 4 is a cross-sectional view of an ultrasonic flow meter (Third
Example) to illustrate the principle of the present invention, in
which the first oscillator 12 is fitted to the center of the measuring
tube 11 the second oscillatiors 13A and 13B are respectively fitted
to the downstream side and upstream side from the first oscillator
12 the power source 17 is connected with the above described first
oscillator 12 the amplifiers 18A and 18B are respectively fitted
to the second oscillators 13A and 13B, and the flanges 21 and 22
as the acoustic filter, are fitted to the measurement tube 11 between
the first oscillator 12 and the second oscillators 13A and 13B.
Since, for the ultrasonic wave generated from the first oscillator
12 the time required for reaching the second oscillator 13A of
the downstream side is shorter and the time required for reaching
the second oscillator 13B of the upstream side is longer, the flow
rate of the fluid can be determined by the time difference thereof.
Total of the three members of the oscillators 12 13A and 13B are
required, but no switch is required. Furthermore, the flanges 21
and 22 as the acoustic filters, attenuate the oscillating wave
propagating through the measurement tube 11 itself, so the measurement
accuracy is consequently increased.
When the measurement tube 11 is a stainless tube, carbon steel
tube or glass tube, each having a higher ultrasonic wave propagating
property, the above described acoustic filter is required. When
the measurement tube 11 is a resin tube such as tetrafluoroethylene
having a much larger attenutation property of the oscillating wave,
as compared with metals, however, attenuation of the oscillating
wave is rendered possible by the resin itself. Consequently, in
this case, provision of attachments such as flanges is not necessary,
resulting in simplification of the structure of the ultrasonic flow
meter as well as improvement of the appearance.
EXAMPLES
The present invention will now be illustrated by the following
experimental examples without limiting the same.
The experimental model is fundamentally as shown in FIG. 2 with
the following dimensions: d=2.8 cm D=3.2 cm L1=1.5 cm L2=1.5 cm
W=7 cm
From these numerals, it is apparent that the cross-sectional area
a1 is 41 cm.sup.2 and the cross-sectional area a2 is 1.9 cm.sup.2.
The material of the measurement tube is stainless steel and the
propagation speed (sonic speed) c is about 5.3.times.10.sup.5 (cm/s).
The fluid (medium) to be subjected to measurement is water and the
propagation speed c of water at 1 atm and 0.degree. C. is about
1.4.times.10.sup.5 (cm/s),
a1=41 (cm.sup.2), a2=1.9 (cm.sup.2), L1=1.5 (cm), L2=1.5 (cm) and
c=5.3.times.10.sup.5 (cm/s) are substituted in the foregoing formula
8 and based on the resulting value, a frequency f2 is calculated
by the foregoing formula 9 to obtain a value of about 17 kHz. It
can be expected that a higher frequency ultrasonic wave than this
frequency is cut off.
FIGS. 5(a) to (c) are graphs in Comparative Experiments, showing
the effects of acoustic filters, in which (b) shows a receiving
wave form when the first oscillator of an acoustic filter-free measurement
tube is subjected to oscillation under such a dry state that a measurement
objective (medium) is not flowed and (c) shows a receiving wave
form when the first oscillator of an acoustic filter-fitted measurement
tube is subjected to oscillation. (a) is a figure of a transmission
wave form, as a prerequisite thereof, in which the propagating frequency
is adjusted to about 10 times, i.e. 170 kHz considering that the
foregoing frequency f2 is about 17 kHz. It can be understood from
(b) that the amplitude is very large, while, on the other hand,
it is apparent from (c) that the amplitude is very small. (c) teaches
that the amplitude can largely be reduced by providing the measurement
tube with an acoustic filter.
FIGS. 6(a) and (b) are graphs showing a time difference measured
by the ultrasonic flow meter according to the present invention.
(a) is the same figure of a transmission wave form as that of FIG.
5 with a carrier frequency of 170 kHz, while (b) is a figure of
a receiving wave form when water is caused to pass through the acoustic
filter-fitted measurement tube and the first oscillator is subjected
to oscillation, and t1 is a measurement time obtained. In the figure
of (b), the resulting wave form is almost considered due to ultrasonive
wave propagating through the water, since (b) corresponds to passage
of water through the measurement tube having given the results of
the foregoing FIG. 5(c). Accordingly, reliability of the measurement
time t1 is very high.
If water is passed through the measurement tube of the foregoing
FIG. 5(b) and the acoustic filter-free measurement tube, on the
other hand, a receiving wave will surely be given through a superimposed
wave of FIG. 5(b) with that of FIG. 6(b) or through a synthesized
wave thereof. If so, the receiving wave is a synthesized wave of
a wave propagating through the measurement tube itself and a wave
propagating through the water and therefore, the receiving wave
cannot be said to precisely measure the speed of water. As apparent
from this illustration, the acoustic filter of the present invention
is capable of exhibiting marked advantages.
Advantages of Present Invention
The present invention can exhibit the following advantages according
to the above described construction.
In the first invention, the first oscillator and the second oscillator
are arranged outside the measurement tube and no obstacle is placed
in the measurement tube. If there is an obstacle in the measurement
tube, foreign matters tend to be accumulated on the obstacle and
cleaning thereof is troublesome. The first invention is excellent
in this respect, since there is no obstacle in the measurement tube,
foreign matters are hardly accumulated in the measurement tube and
even if cleaning is carried out, its operation for cleaning can
simply be finished. In addition, the acoustic filter is fitted to
the measurement tube to reduce an oscillating wave propagating through
the measurement tube and consequently, the oscillating wave according
to the fluid as the measurement objective can predominantly be received,
thus resulting in increase of the measurement precision.
The second invention is characterized in that the acoustic filter
is a flange. Since the acoustic filter is composed of a simple flange,
increase of the cost of the ultrasonic flow meter can be suppressed.
In the third invention, the first oscillator and the second oscillator
are arranged outside the measurement tube and no obstacle is placed
in the measurement tube. If there is an obstacle in the measurement
tube, foreign matters tend to be accumulated on the obstacle and
cleaning thereof is troublesome. The third invention is excellent
in this respect, since there is no obstacle in the measurement tube,
foreign matters are hardly accumulated in the measurement tube and
even if cleaning is carried out, its operation for cleaning can
simply be finished. In the third invention, it is not required to
provide the measurement tube with the acoustic filter, but this
can be dispensed with only examination of the material. Accordingly,
the ultrasonic flow meter can be prevented from being complicated.
Furthermore, the oscillating wave propagating through the measurement
tube can be attenuated and the oscillating wave according to the
fluid as the measurement objective can predominantly be received,
thus resulting in increase of the measurement precision. |