CA2192583A1

CA2192583A1 - Fluidic oscillator and method for measuring the volume of a fluid flowing therethrough

Info

Publication number: CA2192583A1
Application number: CA002192583A
Authority: CA
Inventors: Patrice Ligneul; Philippe Hocquet
Original assignee: Schlumberger SA
Current assignee: Itron Soluciones de Medida Espana SA
Priority date: 1994-06-17
Filing date: 1995-06-08
Publication date: 1995-12-28
Also published as: JPH10501605A; FR2721360B1; WO1995035448A1; AU2742895A; FR2721360A1; PL178702B1; AU703888B2; DE69516316D1; CN1151788A; ES2147292T3; EP0835385B1; DK0835385T3; PL317644A1; ATE191772T1; RU2153603C2; EP0835385A1; DE69516316T2

Abstract

A fluidic oscillator that is symmetrical about a longitudinal plane of symmetry (P) in which a longitudinal fluid flow direction is contained, the oscillator comprising:
means (26b) for generating a two-dimensional jet of fluid that oscillates transversely relative to said longitudinal plane of symmetry (P);
two ultrasound transducers (52, 54);
and means (62-72) firstly for generating an ultrasound signal in the fluid flow travelling from one of said transducers towards the other, and secondly for receiving said ultrasound signal as modulated by the oscillations of the jet of fluid; and means (100) for processing the received signal so as to determine a volume-related quantity concerning the fluid that has flowed through said fluidic oscillator;
characterized in that the ultrasound transducers (52, 54) are substantially in alignment with the longitudinal plane of symmetry (P).

Description

.-~LE, P~i Tl; ~ h...-~J-- 219~!583 ~ T~ RAl~ LATlON
A FLUIDIC OSCILLATOR AND--A METHOD OF MEASURING A VOLUME-REIIATED QUANTITY OF FLUID FLOWING THROUGH SUCH A FLUIDIC
OSCILLATOR
The prèsent invention relates to a fluidic 5 oscillator and to a method of measuring a volume-related guantity of fluid flowing through said fluidic oscillator .
It is known to use fluidic oscillators for measuring for a volume-related guantity of fluid such as flow rate 10 if the frequency of the oscillations is measured, or volume if the number of oscillations is counted. Such a fluidic oscillator is ~cr~hl~, for e~ample, in French patent application No. 92 05301 filed by the Applicant, and based on detecting the freguency of oscillation of a 15 two-~li c~nniql fluid jet in an oscillation chamoer.
The fluid jet is formed as the liguid flow passes through a slot which opens out into the oscillation chamber, and it oscillates transversely relative to the plane of longitudinal ~y Lly of the fluidic oscillator.
20 An obstacle is placed in the oscillation chamber and possesses a cavity in its front portion, which cavity is placed on the path of the fluid ~et so that said jet sweeps the walls of the cavity during oscillation. Flow rate mea ;UL~ ~ is performed, for example, by detecting 25 the ~et sweeping over the bottom of the cavity as it oscillates, with the freguency of oscillation of the jet being proportional to the fluid flow rate.
Also known from patent application GB-A-2 120 384 is a fluidic oscillator operating on a somewhat different 30 principle since it is a Coanda effect oscillator, but its end purpose remains measuring a volume-related guantity of a fluid by detecting the nSr~ t; nn freguency of a jet of fluid. That fluidic oscillator includes three obstacles housed in an oscillation chamber, two of the 35 oscillators being situated on opposite sides of the longitudinal plane of :,y ~ y immediately downstream from the fluid ~flmicclnn opening into said chamber and z 2192583 co-operating with the si~e walls of the oscillation chamber to form two symmetrical ~h~nnPl s, and the third obstacle is disposed facing the fluid .9fl-"1 CR~ nn opening, but downstream from the first two obstacles at the sides.
During its sweeping motion, the fluid ~et meets one of the side obstacles, and attaches thereto, the fluid flow then moving Ul):.i,L~III and taking the channel formed between said obstacle and one of the walls of the oscillation chamber, thereby causing the fluid to 10 circulate again.
When the fluid flow reaches the upstream zone situated close to the fluid ;~rlml cqi nn opening at which the base of the fluid ~et is situated, the flow then causes said ~ et to switch over towards the other side 15 obstacle and the same rhPn~ is reproduced with said other side obstacle. The fluidic oscillator also ~n,~ Pc two ultrasound tr~n~ Prs disposed on either side of a fluid ilow position such that ~ltrasound signals are emitted and received in planes that are 20 substantially transverse to the lons~itudinal direction of fluid flow.
In the meas~rement method described, one of the tr~nq~llnPrs emits towards the other tr~nC~lllrPr which is placed downstream from the first or in the same 25 transverse plane as the first, and the emitted signal is modulated by the oscillations of the fluid jet in the oscillation cham]~er so the other transducer receives said ultrasound signal as modulated in this way.
On the basis of the received signal, it is possible 30 to detect the frequency f of oscillation of the fluid ~et, and to deduce therefrom the flow rate or the volume of the fluid that has flowed through the fluidic oscillator .
That Britis~l patent application then ~Yrl~nc that 35 the received signal is ~ ted and transformed into a pulse signal where each pulse corresponds to a unit volume of fluid swept through by the fluid jet during its oscillation .
That meaYul~ t technique provides the advantage of achieving good measurement repeatability over the usual range of flow rates for a fluidic os~ tor, However, in some cases, it is n~. Pqq~ry to obtain very good accuracy in flow rate or in volume mea,Ul, t, and conseguently it can be advantageous to have a fluidic oscillator whose sensitivity can be easily improved over its usual range of flow rates.
In addition, it is known that a fluidic osclllator cannot measure a volume-related quantity of a fluid when the flow rate of the fluid is so low that it is no longer possible to detect the freguency of fluid ~et oscillations.
In domestic installations, it is also known that, for most of the time, the flow rates of a fluia, e.g.
gas, are very lo~, being typically less -~han 200 liters per hour (l/h). It is therefore particularly important to be able to measure flows at such rates as well as being able to measure the maximum values of flow rate that can happen oo-f~q;nn~lly~ In addition, it is also necessary to be able to detect fluid leaks when they occur and thus it must be pnq%l hl F- to distinguish a leakage rate from a small flow rate.
The present invention seeks to remedy the drawbacks of the prior art by proposing a fluidic oscillator structure that is easily adapted to more accurate meaYul~ t of a volume-related guantity of a fluid over the usual range of flow rates for said fluidic oscillator, should that be n~nPqq;~ry, and which is also easily adaptable to rr~qllr;n~ a small volume-related quantity of a fluid, at which fluid jet oscillations disappear .
The invention also proposes a method of measurement that is adapted to measuring a volume-related guantity of a fluid in each of the above-s~r~f;~ cases.

~ 2192583 Thus, the present invention provides a fluidie . oscillator that is sy-mmetrical ~hout a longitudinal plane of ~y Lly, the oscillator eomprising:
means for generating a two-fl~ ~c1nn:l1 jet of fluid 5 that oseillates transversely relative to said longitudinal plane of ~y LLY;
two ultrasound tr~nc-lllrPrc3;
and means firstly for generating an ultrasound signal in the fluid flow travelling from one of said 10 transdueers towards the other, and secondly for receiving said ultrasound signal as modulated by the oscillations of the ~ et of f luid; and means for proeessing the received signal so as to determine a volume-related quantity ~.r,n.~Prn~ng the fluid 15 that has flowed through said fluidic oseillator, charaeterized in that the ultrasound tr~ncfll~rPrs are substantially in ~ , t with the longitudinal plane of ~Y L1 Y, This novel disposition of the ultrasound transdueers 20 in a fluidic oscillator is highly advantageous firstly beeause by ~ h~Qc~ ns to plaee said tr~neflll~ prs 5pp-~lf~ 1y in the plane of ~y L~Y or slightly offset therefrom, it is possible to promote detection at the oseillation freguency of the ~et or at twiee that 25 frequeney, thereby increasing the sensitivity of said fluidic osclllator, and secondly, while using the same tr~ncfl-lr~.Prs positioned in this way and whether operating at the oscillation frequency or at twice the oscillation frequeney, it is possible to measure small volume-related 30 quantities of a fluid for which the oseillations of the fluid ~et are too small for it to be pQCc~h1P to deteet the frequency thereof.
The fluidie oscillator obtained in this way is referred to as a ", ' ~nPfl" oscillator and it covers a 35 range of flow ~ra~es that is wider than the usual range of flow rates for a conventional fluidic oscillator.

21~2~83 To this end, provision is made for the fluidic oscillator to include:
means for emitting and receiving an ultrasound signal alternately from each of the ultrasound 5 tr~nqfl~ Prq; and means responsive to each received ultrasound signal and tc consecutive pairs of ultrasound signals to determine a value for a magnitude that is characterlstic cf the propagation speed of said ultrasound signal as ~fl~ ~ Pfl by the fluid flow, and to deduce therefrom a volume-related quantity applicable to the fluid that has flowed through said fluidic oscillator.
The ultrasound transducers are disposed in different transverse planes, an "upstream" one of said tr~ncfl~ Prs being ~; qpn5Pfl upstream from the means for generating the two-fl~ -q1nn~1 let of fluid, the other tr~nqdllrPr being a "downstream" trPnqfl11nP~.
Given that the fluid jet oscillatio~ ~h~.n~ `nnn iS
observed over a large range of flow rates covering the higher values cf the flow rate, the range of rates or which fluid jet oscillations are too weak to be capable of being detected is relatively small.
Consequently, in this small range, it is possible to use tr~nqA11rPrs that are highly resonant, and therefore of relatively simple design.
For exampler the magnitude representative of the speed of propagation of the ultrasound signal may be the propagation time of said signal. Alternatively said magnitude may be the phase of said signal.
In an emoodiment of the inven~ion, the fluidic oscillator comprises:
means for generating a two-fll- ,q~nn~l cscillating fluid ~et, which means are formed by a fluid admission opening of transverse dimension or width d and of height h;
an oscillation chamber connected at one oi its ends to said ~luid ;lflm~qq~ nn opening and at its opposite end ~ 2~25g~
.

to a fluid outlet opening, said openings being in Al; ~, ~ L in gaid longitudinal plane o symmetry; and at least one obstacle disposed in sald oscillation chamber between the A~m~ CC~ nn opening and the fluid 5 outlet opening.
According to other characteristics ol~ the fluidic oscillator:
the upstream ~rAnqfl~lr.Pr is disposed U~L1~:dlll from the fluid Aflm~ cc1 nn opening;
the obstacle has a front portion in which a cavity is formed facing the fluid Afl~ cq~ nn opening;
the downstream trAncfl~ .or is secured to the obstacle; and the downstream transducer is disposed in the cavity 15 of the obstacle.
According to other characteristics o~ the inventicn:
the oscillator ~n~1~7flPc, upstream from the obstacle, a passage for the fluid that is defined by two walls that are perppnfll slll Ar to the longitudinal plane o~ :,y Lly 20 and that are spaced apart by a distance _;
the oscillator ~nrlllflPq, upstream from the fluid Aflm~ cqi nn opening, a longitudinally-extending channel iorming at least a portion of the passage for the fluid, said channel being of substantially ccnstant width d that 25 is perpendicular to the distance h;
the channel pcssesses, at one of its ends, a "downstream" opening that corresponds to the fluid A(9Ti qq~ nn opening, and at its opposite end, an "upstream"
opening which, iII a plane parallel to the flow direction 30 of the fluid and perpf~nfl~ l =r to the longitudinal plane Of ~y ~ Lly, is .iullv~l!,~llt in shape, its width tapering progressively do~n to the width d;
it ~ n~-l llflPq two fluid inlets disposed symmetrically abcut the longitudinal plane of ~y LLY and opening out 35 into the passage, upstream from the channel;

it ~ nrl t~ q two flui-d inlets disposed :,y l.Llcally about the longitudinal plane of symmetry and openlng out into the passage, upstream from the channel;
two side passages e~tending in a direction that is generally parallel to the longitudinal plane of symmetrical and in particular to the longitudinal direction of the channel each constitutes a f luid inlet, each of said E)assages being connected f irstly via one end to a common first chamber perpl~nrlirlll~r to said plane and secondly, via an opposite end, to a common second chamber parallel to said first chamber, said first chamber being provided with a fluid feed;
an empty space forming another portion of the passage for the fluid is provided upstream of the channel and the two fluid inlets open out into said empty space;
the upstream transducer is disposed upstream f rom the empty space;
in that the ultrasound tr~nqflllrF~rs ~re situated on the same side in a direction perpf~nril r~ r to the longitudinal direction of the fluid flow, and contained in the longitudinal plane of ~_y [,Ly;
in that bot~l ultrasound tr~nC~ rprs are secured to the same one of the walls flef~nins the passage for the f luid;
the ultrasound tr~nq~lllrGrs are offset in a direction perp~n~llr--l~r to the longitudinal direction of fluid flow and contained in the longitudinal plane of symmetry; and in that each ultrasound transducer is secured to a respective one of the walls ~ f;n~n~ the passage for the fluid.
The invention also provides a method of measuring a volume-related s~uantity of a fluid ilowing through a fluidic oscillator in which a jet of fluid oscillates transversely about a longitudinal plane of symmetry, said method consisting successively in:
emitting an ultrasound signal into the fluid flow from an ultrasound tr;~nc~lr-pr;

~ ~ 219,~,583 .

receiving said ultrasound signal as modulated by the oscillations of the ~et of fluid by uslng another ultrasound transducer; and procPcc~ng the received signal so as to determine said volume-related quantity of the fluid that has flowed through the oscillator;
the method being characterized in that it consists in emitting an ultrasound signal in a direction that is substantially contained in the longitudinal plane of ~y Lly.
Advantageously, by ~lign1nJ the ultrasound tr~ncA~IrPrs accurately in the longitudinal plane of ~>y l,Ly of the fluidic oscillator and by emitting an ultrasound signal in said plane, and in the fluid flow direction, said ultrasound signal which is modulated by the oscillations of the ~et of fluid and as picked up in said longitudinal plane of sylrmetry is affected mostly by the frequency 2f where f represents the ~Erequency of oscillation ol~ said ~et of fluid. By detecting the frequency 2f, it is therefore possible to double the sensitivity of the fluidic oscillator over its usual range of flow rates, i.e. at flow rates for which the oscillations of the ~et o~ fluid are detectable. This frequency of 2f cannot be detected when applying the tprhn~r~l tP~rh~nfJ of patent application GB-A-2 120 384.
In contrast, when it is desired to use the combination fluidic oscillator to cover the widest possible range oi flow rates without seeking to improve the sensitivity of said oscillator, it is not nPrPQC1~ry to position the ultrasound tr~ncfll~rprc accurately in the longitudinal plane of symmetry. The ultrasound transducers are then substantially aligned with the longitudinal ~lane of symmetry in such a manner as to present an inclination of 1 to 2 relative to the longitudinal direction of said plane of symmetry.
Given triqnsfl- rPrs disposed in this way for measuring a volume-related quantity of flui~ at a high flow rate, 2~58~
.
g i.e. when the oscillations of the jet of fluid are strong - enough to enable the frequency thereof to be detected, it is possible to emit an ultrasound signal in the f low direction of the fluid through the fluidic oscillator.
5 The ultrasound signal as modulated by the oscillations of the jet of fluid and as picked up is then affected mostly by the freguency of oscillation f of said jet of fluid.
With the transducers in this disposition, an ultrasound signal i5 preferably emitted in the direction opposite to lO the fluid flow direction through the fluidic oscillator for the purpose of improving detection of the oscillation frequency f over the situation where the ultrasound signal is L,L~a~c l lng in the same direction as the fluid flow direction.
By having the tr~nqA~ rs in this advantageous disposition, it is also possible to measure a volume-related ~uantity of a fluid at a low flow rate, i.e. when the oscillations of the jet of fluid arP' too small to be detectable and the method of the invention then consists successively in:
emitting an ultrasound signal from one of the transducers towards the other in a direction that is substantially contained in the longitudinal plane of ~y I,Ly;
receiving said ultrasound signal whose speed of propagation has been modified by the flow of the fluid;
de~Prm1n;n~ a first value of a magnitude characteristic oi` said speed of ~,L~,~a~ ion of the received ultrasound signal;
repeating the above steps after interchanging the emitter and receiver functions of the ultrasound tr~ncfl~P~s and de~Prm~n~n~ a second value for said magnitude characteristic of the speed of propagation for another ultrasound signal; and APfl~ n~ therefrom the measurement of a small volume-related quantity of the fluid.

2~S83 .

The invention i5 particularly advantageous in the field of gas metering.
Other characteristics and advantages appear from the following description given hy way of non-limiting 5 illustrative example and made with re~erence to the ~c~ ylng drawings, in which:
Figure 1 is a dia~L Llc longitudinal section view on the longitudinal plane of ':jy Lly P of an embodiment of the fluidic oscillator of the invention Figure 2 is a dia5~1 Llc view of the Figure 1 fluidic oscillator on plane Pl;
Figure 3 is a fr~ Lclly diayl Llc vlew in section on A through the embodiment of the fluidic oscillator as shown in Figure l;
Figure 4 i5 a dia~l Llc view on the plane Pl showing a variant embodiment of the fluidic oscillator shown in Figure 2;
Figure 5 shows a first variant embo~Liment of the disposition of tlle ultrasound transducers shown in Figure l;
Figure 6 shows a second variant embodiment of the disposition oi the ultrasound tr~ncu1~ c shown in Figure l;
Figure 7 shows a third varlant f~nho~l 1 nt of the disposition of the ultrasound transducers shown in Figure 1;
Figure 8 shows a fourth variant embodiment of the disposition of the ultrasound transducers shown in Figure l;
Figure 9 is a diagrammatic view on the plane Pl of Figure 1 through a second embodiment of a f luidic oscillator o the invention;
Figure 10 is a block diagram showing a portion of the electronic circuit used for measuring the volume of gas flowing through the fluidic oscillator;
Figure 11 is a detailed view on a larger scale of the electronics block 100 in Figure 10;
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ .

` 21~258~
.

Figure 12 shows the~modulated ultrasound signal as l;f;~fl by ~mrl;f;Pr 102 in Figure 11;
Figure 13 shows the ultrasound signal of Figure 12 after it has been rectified by circuit 104 in Figure 11;
Figure 14 shows the ultrasound signal of Figure 13 after it has been filtered by electronics block 106 of ~igure 11;
Figure 15 shows the ult~asound signal of Figure 14 after passing through electronic ~mr1;f;r~tion block 108 and through electronic peak detection block 112;
Figure 16 shows the operation performed on a peak by the peak detection electronics block 112; and Figure 17 is a calibration curve for the fluldic oscillator of the invention.
As shown in Figures 1 and 2, and as given overall ref erence 1, the f luidic oscillator of the inventlon is aprl; t~.;`hl F', for example, to domestic gas metering, and it rmq5~qs~oq a longitudinal plane of symme~y P which is disposed vertically and which corresponds to the plane of 20 ~igure 1.
It should be observed that the fluidic oscillator may also operate in a position such that its plane P is horizontal or even in some other position without that disturbing the measurement of the volume-related quantity 25 of a fluid (e.g. flow rate or volume proper). The fluid flowing through said fluidic oscillator is a gas, but it could equally well be a liquid, e. g . water.
The f luidic oscillator shown in Figure 1 has a vertical gas ~eed 10 that is centered relative to the 30 longitudinal plane of -;y - Lly P and opens out into a "top" first horizontal chamber 12 which is of large size and disposed symme~rically about said plane P. The flow section of said top chamber 12 is rectangular in shape and parallel to the longitudinal plane of symmetry P, and 35 it sub~ects the flow of gas that enters via the feed 10 to a sudden increase in section, e . g . equal to a f actor .. _ .... _ .. . . . _ _ _ _ _ _ . . . . _ . . ... .. . ., _ _ 21g2~8~

of 4, in order to destro~ the turbulent :,~ u~,LuLt: of the flow by reducing its speed.
The top chamber 12 possesses two opposite "end"
openings 12a and 12b each opening out into a respective 5 vertical side passage 14 or 16 ( as shown in Figures 2 and 3 ) of rectangular flow section identical to the flow section of said first chamber 1. The two vertical side passages 14 and 16 are symmetrical to each other about the longitudinal plane of :,y ~Ly P.
Each side passage 14 (16) ~ r~tes firstly at a "top" one of its ende 14a (16a) that ro;nr~q with the corr~qp~-nl~n~ end 12a (12b) of the top first chamber 12, and secondly at its opposite or "bottom" end 14b (16b) with a "bottom" second chamber 18 that is identical to the first chamber 12, as shown in Figure 1. The top and bottom chambers 12 and 18 are symmetrical to each other about the plane Pl shown in Figure 1 and they are parallel to each other, but it would als~ be possible or the volume of the bottom chamber 18 to be smaller.
Each of the two vertical side passages 14 and 16 constitutes a gas inlet and has a middle side opening 14c, 16c of ~ow section parallel to the longitudinal plane of symmetry P (Figure 2). The gas inlets 4 and 16 open out via their side openings 14c and 16c into an empty space 20 situated halfway between the bottom and top chambers 18 and 12. The empty space 20 which forms an in~ te chamber of smaller size than the chamber 12, possesses a "transverse" long ~; q;rn which is perpPn~irul~ to the longitudinal plane of symmetry P, and it is defined along said ~ q;~ln firstly by an "upstream" end wall 22 and secondly by a "downstream" end wall 24. The walls 22 and 24 are spaced apart by a distance which corresponds to the longitudinal ~1; nc;rn of the side openings 14c and 16c. A channel 26 in ~ on the longitudinal plane of symmetry P is provided through the downstream wall 24. This channel 26 is ref erred to as the " main " channel and has a transverse . , ... ,, . , . , .. , . , . .... .,, . . , . . _ , _ _ ,, ,,, , _ _ _ 21~2~83 nn or "width" d that is substantially constant along the entire longitudinal ~ n or "length" of the channel . The length of the channel is pref erably greater than lOd in order to obtain good accuracy in 5 measuring the volume-related ~uantity of a gas at low flow rates, i.e. when the oscillation8 of the ~et of gas are too weak for their frequency to be detectable. As shown in Figure 2, the upstream opening 26a of the mzin channel 26 has a shape that Cullv~ 8 in the plane Pl.
10 Each portion of the upstream opening 26a which is on one side or the othe~ of the plane P has a convex profile, e. g . a substantially rounded profile, thereby contributing to progressively reducing the width of said opening down to the width d of the main channel 26.
In a variant of the invention shown in Figure 4, the gas inlets or vertical side passages 14 and 16 (not shown in this figure) open out via side openings 14c and 16c into respective "gas inlet" horizontal ~ nnF~l c 15 and 17. The flow section of each horizontal channel tapers 20 progressively. The two gas inlet r~h~nn-~l c 15 and 17 are symmetrical about the plane P and they meet in a zone 19 situated in said plane P immediately upstream from the upstream opening 26a of the main channel. The horizontal ~hFInnPl C are defined firgtly by a common "upstream" end 25 wall 21 that pro ~ects in a downstream direction, and secondly by a common " downstream" end wall 23 . As described above, a main channel 26 in ~1 i!j -nt on the longitudinal plane of gymmetry P is provided in the downstream wall. The gas inlet ~.h~nni~l s 15 and 16 and 30 the main channel 16 thus form a horizontal passage for the gas which is defined above and below by a top wall and by a bottom ~all ( not shown in the plane of Figure 4 ) that are spaced apart by a height h.
The fluidic oscillator shown in Figures 1 and 2 35 ;n~ means for generating a two-~ nc;r~n~l jet of gas that oscilla~es transvergely relative to the longitudinal plane of gymmetry P . These means are f ormed 21~2583 by a gas ~11m~ ec1 on openi~g into an "oscillation" chamber 32, which opening nn1nc1tlpe with the downstream opening 26b of the main channel and is rectangular in shape. The oscillation chamber 32 has one of its ends connected to 5 the downstream opening 26b of the main channel 26 and has its opposite end connected to a gas outlet opening 24 of width greater than d. The gas ~m1sc1nn and outlet openings 26b and 34 are in ~ L on the plane P.
The fluidic oscillator also includes an obstacle 36 10 of height h disposed in the center of the oscillation chamber 32 between the gas A~9m1 c.51 nr~ opening 26b and the gas outlet opening 34. A horizontal passage for the gas situated upstreanl from the obstacle 36 is formed in part by the empty space 20 and the channel 26 ana is defined 15 above and below respectively by a "top" wall 28 and by a "bottom" wall 30 ( Figure 1 ) . These two walls 28 and 30 are separated from each other by a height =. Such an obstacle 36 has already been described i-~ French patent application No. 92 05301. The obstacle 36 has a front 20 portion 36a in which a cavity 37 is formed, referred to as the "central`' cavity, which cavity faces the ~lm1 es~ on opening 26b of the oscillation chamber 32.
Two secondal~y cavities 38 and 39 are also provlded in the front portion 36a of the obstacle 36 symmetrically 25 about the plane P. The oscillation chamber 32 possesses side walls 40 and g2 of a shape that substantially matches the outside shape of the obstacle 3 6, thereby co-operating ~ith said obstacle to provide two ~y Ll lcal secondary r.h;~nn~l e Cl and C2 situated on either side of 30 the longitudinal plane of :,y L~ y P.
The width of the secondary nh~nnPl c C1 and C2 is substantially ~ull~ L,--t in order to avoid disturbing the ~low of gas. The secondary rh;snnPl e Cl and C2 pass round the obstacle 36 and meet again downstream therefrom in a 35 zone 44 situated immediately upstream from the outlet opening 36 of the oscillation chamber 32. This outlet opening 34 opens out into a vertical passage 36, at a ........ . .... . ... . . .. . . _ ... , , _,, , _ 2~258~

point halfway up it, as shown in Figure 1. The vertical passage 46 is, for example, symmetrical about the longitudinal plane of symmetry P and at a "top" one of its ends 46a it has a vertical gas outlet 48 centered 5 relative to said plane P. The configuration described with reference to Figures 1 to 3 has the advantage of conferring satisfactory compactness to the fluidic oscillator.
The v~ 1~ o~ the flow of gas in the fluidic 10 oscillator are now described. A vertical flow of gas is fed to the fluidic ncn7llRtnr via the vertical feed 10 and penetrates into the top chamber 12 of said fluidic oscillator, where it splits into two portions. These two portions of the "main" flow travel hori70ntally through 15 the top cha7nber 12 of the fluidic oscillator in opposite directions perppnfl~ 7l ;7r to the longitudinal plane of symmetry P. As shown in Figure 3, each portion of the flow passes through an end opening 12a (~2b) of the top chamber 12 of the fluidic oscillator and penetrates into 20 one of the vertical side passages 14 ( 16 ), performing rotary motion prior to being engulfed in the empty space 20 via one of the side openings 14c (16c).
This configuration is flPsl ~nPfl to enable the gas to get rid of any polluting particles with which it might be 25 charged ( dust . . ) as it passes through the vertical side passages 14 and 16 where, under the e~fect of gravity and of the rotary motion of the flow, such particles are sent towards the bottom chamber 18 of the fluidic oscillator.
h7hen the two portions of the flow of gas penetrate 30 symmetrically about the plane P into the empty space 20, they meet on sai~ plane P and are engulfed in the main channel 26 via its upstream opening 26a. The flow of gas then travels along the main channel 26 and is trnn~f~ --7 into an oscillating two-flir-nc~nn~l jet at the downstream 35 opening 26b thereof. Within the oscillation chamber 32, the f low of gas alternates between the channel C1 and the channel C2 prior to reaching the outlet opening 34 and then flowing up the vertical passage 46 towards the vertical gas outlet 48.
Given that the vertical passage 46 extends vertically to a level below that of the oscillation 5 chamber 32, it too can serve to rid the gas of certain polluting particles if they have not already been eliminated. As ~entioned above, the fluidic oscillator may be placed in some otXer position in which there is no need to provide a bottom chamber 18 for the purpose of 10 removing dust from the gas.
In accordance with the invention, the fluidic oscillator has two ultrasound tr~nq~llrprq 52 and 5g that are substantially in ~ L with its longitudinal plane of ~y LLY. The advantage of removing the ma~or 15 fraction of polluting particles from the gas is to avoid contaminating the tr~nq~llrprs~ thereby increaslng their lifetime .
In the embodiment of the invention ~hown in Figures 1 to 3, the ultrasound tr~nqd~lrprs 52 and 54 are offset 20 angularly by about 1. 5 from the longitudinal plane of ~,y LLY P in order to respond mainly to the frequency of oscillation f of the jet of gas in the ultrasound signal as modulated by the oscillations of said ~ et of gas .
This angular offset serves to distinguish the freguency f 25 from the freguency 2f in the modulated ultrasound signal.
If the angular offset exceeds 2, then there is a risk of the ultrasound signals being multiply reflected in the main channel 26, thereby degrading the guality of the signal, and in particular reducing its signal/noise 30 ratio. In the event that it is desired to improve the sensitivity of the fluidic oscillator over its usual range of flow ra~es ( e. g. 100 l/h to 6000 lJh ), then, in order to enhance detection of the freguency 2f, it is necessary to place the ultrasound transducers very 35 accurately in the longitudinal plane of ~y ~ LLY P and it is also preferable to emit the ultrasound signal from the upstream end towards the downstream end.
, . . , ,, . . . . .. , .. ,, ... , .. , ,, _ _ _ _ _ _ _ _ _ _ _ .

~ 2192583 .

As shown in Figures 1 and 2, the ultrasound transducers 52 and 54 are disposed facing each other in different transverse planes. The term "transverse plane"
is used herein to designate a plane perrPnfl~ rlll Al- to the longitudinal plane o symmetry P and to the gas f low dlrection. If the ultrasound tr~ncfl~lrP~s æe placed in the same transverse plane, as they are in the prior art, then they are not suitable for measuring a volume-related (luantity of gas at low flow rates since the emitted ultrasound signals cannot pic~ up in~ormation rnnrP~n;n~
the speed of f 10W of the gas .
The upstream trAnc~lrpr 52 is situated upstream from the Aflm~ c~ nn opening 26b, and more precisely upstream from the empty space 20. As shown in Figures 1 and 2, the upstream trAncfl~lcp~ 52 is reoeived in the upstream end wall 22 and is thus protected from the gas flow. The downstream trAnc;lllrPr 54 is secured to the obstacle 36 and, more precisely, it is disposed in t~e main cavity 37 of said obstacle.
As shown in the variant embodiment of Figure 4, the upstream trr~sducer 52 is disposed in the middle portion of the upstream end wall 21, i.e. in its portion closest to the main channel 26, whereas the downstream tr~ncfl~
54 is secured to the obstacle 36 as described above.
In the embodiment described with reference to Figures 1 and 2, the upstream and downstream tr~ncfll~rP~s 52 and 54 are situated at the same height relative to the height _ of the channel 26 of the fluidic oscillator. In a variant of the invention, the upstream and downstream ultrasound tr;lncfll-rPrc may also be situated at di~:Eerent heights relati~e to the height h of the channel 26 of the fluidic oscilIator, but they must always face each other.
For e~ample, as shown in Figure 5, the difference in height between t~le upstream and do~Tnstrer m ultrasound tr~ncfl~lrP~s may be substantially e(aual to h.
In another ~rariant of the invention, as shown in Figure 6, the upstream and downstream ultrasound 2i92583 .

trAn~ rPrs 52 and 54 are likewise situated at different heights, but the upstream tr~nC~ ~ 52 is mounted at the bottom of a recess 53 formed in the bottom wall 30 of the fluidic oscillator, beneath the empty space 20. The 5 downstream tr~nQ~ P~ 54 is mounted in the top of a recess 54 mounted in the top wall 28 of the fluidic oscillator substantially over the obstacle 36. The upstream and downstream tr~nC:~1llr~rs are disposed facing each other.
In yet anotller variant as shown in Figure 7, the upstream and downstream ultrasound transducers 52 ana 54 are situated at the same height and they no lon~er face each other. The tr~nc~ rpr~s are mounted in respective recesses 53 and 55 both of which are formed in the top wall 28 of the fluidic oscillator The downstream tr~nQ~9~ rf~ 54 is situated substantially over the obstacle 36 and the recess 55 in which it is installed does not extend as far as the channel 26 in order:~ to avoid disturbing the formation of the ~et of gas. In addition, the downstream transducer 54 must be based downstream from the channel 26 so that the ultrasound signals can be modulated sufficiently by the oscillations of the ~et of gas. The path followed by the ultrasound signals in the longitudinal plane of ::;y tLy P i5 thus a V-shaped path.
The variant shown in Figure 8 also serves to obtain a V-shaped path for the ultrasound signals in the longitudinal plane of :,y Lly P, but with the ultrasound tr~nQ~ s being disposed at different heights. The upstream transducer 52 is mounted in a recess formed in the end wall 22 so as to faoe the obstade 36. The downstream t~ilnqr~ ~ is still installed in the same manner as that described with reference to Figure 7.
It should be observed that by placing the downstream tr~nQ~ ~ 54 above or below the obstacle 36, the oscillation of the ~et of gas is disturbed less than when the transducer is placed in the central cavity of said ~1925~3 .

obstacle, thereby improving the 5~uality of the ultrasound signal modulated by the oscillations of said ~ et of gas .
It would also be possible to tilt the upstream tr~nq~ -Pr 52 to~ards the bottom wall 30 of the fluidic 5 oseillator.
A seeond ~ml~s~ L of the invention is shown in part in Figure 9, and referenees to the varigus Pl Ls of this figure are preceded by the digit 2. The fluidic oscillator 201 is said to be "in line" since it has a gas feed 210 and a gas outlet 212 which are both in aliç~nment in the longitudinal plane of symmetry P, unlike the embodiment shown in Figures 1 to 7 where the --,v~ t of the flow of gas is round a loop. The gas feed is connected to a passage 214 which has its downstream end opening out into a first chamber 216 that is in ~ t with said passage on the plane P. The first chamber is of a shape that f lares in a downstream direction until it comes level with a transverse plane P2 ~hat is perp~n~ r to the plane P, after whieh it tapers so as to communicate with an upstream end 218a of a main ehannel 218 having the same eharacteristies as the main ehannel 2 6 shown in Figures 1 to 7 . The f irst ehamber also ~n~ lcll~q a strP;~ml ;n~ element 220 situated substanti~Llly in the middle the~eof and in ~ t on the plane P. This element has a reeess 222 that faees downstream and that reeeives an "upstream" ultrasound tr~nc~l17-~r 224, thereby proteeting it from the flow. The strP~ml; n~rl element 220 may also serve to ealm the gas flow.
The main channel 218 is aligned on the longitudinal plane of :~y ~ L~ y P and opens out into a second chamber 26 constituting an oscillatlon ehamber having the same characteristics as the chamber 32 ~i~ql-r~ h~d above with reference to Figures 1 to 8. This oscillation chamber inel~ q an obstacle 228 identical to the obstacle 36 shown in Figures 1 to 8. The obstacle 228 has a central cavity 230 situated facing the downstream open end 218b .. _ _ . ,, ... , , . _ _ . . .

~ 2ig2s83 .

of the main channel 218 and it also has two secondary eavities 231 ana 232 located on either side of said eentral eavity 230. A seeond ultrasound tr~nC~ rPr 234 is reeeived in the eentral eavity 230 so that the two ultrasound tr~nq~ rPrs are su-hstantially in ~ on the longitudinal plane of ~y tL y P .
There follo~s a description with referenee to Figures 10 to 16 of the metbod of measuring a volume-related quantity of a gas such as the volume of gas that 10 flows ~hrough the fluidic oscillator as described above with reference to Figures 1 to 3.
By way oi' example, the range of gas flow rates to be measured extends from 5 l/h to 6000 l/h (a domestic gas meter ) .
An electronics unit 60 is shown diagrammatically in Figure 10 and it serves firstly to feed the various functional blocks ~PcrrthP~ below with electricity and secondly to control the method of measur-ing gas volume.
The electronics ~mit 60 comprises a microcontroller 62 connected to an electricity power supply 64, e. g . a battery, and to a crystal clock 66 whose freguency is 10 MHz, for example, and which is also powered ~rom the power supply 64. The microcontroller 62 is also connected to an F~m; C51 rn block 68 and to a reception block 70, each of which i5 powered by the power supply.
Each of these blocks comprises, for e~ample, an operational iqmrlifiPr and a .,ullv~:LLer, spPrif;r;~lly a digital-to-analog ~:u--v~r~er for the ~miCsirn block 68 and an analog-to-digital ~_:UIIVt:L l,~' for the reception block 70. The eleetronies unit 60 also ;nrll~9Pc a switehing cireuit 72 that is powered by the power supply 64 and that is eonnected firstly to the pm1cc;rn and reeeption blocks 68 and 70, and seeondly to the two ultrasound tr~nc~ r~rs 52 and 54.
When the oseillations of the jet of gas in the oseillation chamber 32 are too weak for it to be possible to deteet the fregueney thereof, i.e. when the flow rate ~ 2192583 of the gas is below a transitioII value which i5 egual to 100 l/h, for example, then the ultrasound tr~nC(ll~r~rs 52 and 54 are used to measure the flow rate and thus the volume of the gas in the following manner ( low flow rate conditions ):
the upstream transducer 52 emits an ultrasound signal towards the downstream tr;~nc~tlrrr 54;
the downstream transducer 54 receives said ultrasound signal whose speed of propagation c is modified by the speed of the flow of gas vg (c+vg);
a first value is det~rm~ nP~ for a magnitude that is characteristic of the propagation speed of the received ultrasound signal, e. g . its propagation time;
the emitter and receiver functions of the ultrasound tr~nc~-r.Prc 52 and 54 are interchanged;
the downstream tr~nc~ r~r 54 now emits an ultrasound signal towards the upstream transducer 52;
the upstream tr~nc~llr~r 52 receives-; this ultrasound signal that propagates at a speed ( c-vg );
. a second value is detPrm~ nP~ for the propagation time of the ultrasound signal; and a mea:,uL. t of the ~as flow rate is deduced therefrom which, by integration, serves to provide a mea~ ul~ t of the total volume of gas that has passed through the fluidic oscillator.
With reference to ~igure 10, a measurement is triggered as follows: the seguencer ( not shown ) of the microcontroller activates the ~m~ c5~ nn block 68 to send an electrical signal to the upstream transducer 52, and also activates t~le power supply 64 to set the switching circuit 72 so that the ~m~ c5~ nn block 68 is connected to the upstream transducer 52 and so that the reception block 70 is connected to the downstream transducer 54.
The electrical signal excites the upstream transducer 52 which emits an ultrasound signal into the gas in the flow direction of the gas at a specific instant which is determined by the clock 66. The signal travels through . _ . _ . . . .. . .

21g2583 the gas at speed c while the gas itself travels at a speed vg. After a time lapse tl measured by the clock 66, the downstream tr;tnC~lt~rpr 54 receives the ultrasound signal which appears to have been propagating at the speed c+vg.
To measure the propayation time tl of the ultrasound signal, rPfPrPnre may be made to the method described in European patent application No. 0 426 309. That method consists successively: in gen-erating and transmitting an ultrasound signa~ made up of a plurality of cycles or pulses, ;nr~l(lin~ a phase change within the signal; in receiving the ultrasound signal; and in detecting the phase change wit~lin the received signal, such that the instant that corresponds to said phase change enables the propagation time tl to be detPrm; nP~ . Everything reguired for implementing that method o~ measuring propagation time is described in European patent application No. 0 426 309, and is theref~tre not described again herein.
Thereafter, the se~uencer o~ the microcontroller 62 causes the switc~ling circuit 72 ~o swap connections so that the pm; c5; nn block 68 is now cu~ e~ d to the downstream tr~tnct~lllrpr 54 while the reception block 70 is connected to the upstream trrtnctrltlrpr 52. A second ultrasound sisnal is emitted in li~e manner by the downstream transducer 54 towards the upstream transducer 52 so a to travel in the opposite direction to the gas flow direction, and the clock 66 determines the time t2 reguired for said ultrasound signal to propagate, in the manner described above and in European patent application No. 0 426 309 Given that the propagation times tl and t2 can be expressed by the following relat;nncth;r~:
tl = I./ ( c-vg ) t2 = ~/ ( c+vg ) the arithmetic and logic unit (not shown) of the microcontroller 62 calculates the velocity of the gas vg by applying the following relationship:
.. .. . . .. ... .. . _ ...

~192~3 L
vg [ ]
2 t2 tl from which a gas flow rate measurement Qm is deduced 5 where:
L
Qm = S - t-- - --]
2 t2 tl 5 being the internal section o~ the channel 26.
The microcontroller 62 compares each measured flow rate value with the predet~m1n~o~ transition flow rate value as stored in its memory in order to determine whether the next mea- uL~ t of flow rate should be performed using the above method or by detecting the 15 frequency of oscillation of the ~et of gas in the oscillation chamber 32 of the fluidic oscillation ( high ilow rate conditions ) . If the measured flow rate value is below the transition flow rate, then the flow rate of the gas is measured again after a prede~rml nrrl time 20 interval using the above method.
It should be observed that the fluidic oscillator of the present invention, referred to as a "combination"
fluidic oscillator, makes it possible to use propagation time meaYuL Ls on an ultrasound signal in the flow of --25 gas at flow rate values that are small enough to avoidintroducing errors into the flow rate measurements due to instabilities in the flow of gas, which instabilities are generated by the transition from laminar flow to turbulent flow. The low flow rate mea~u" 1_-. thus have 30 the advantage of being accurate and repeatable. In addition, given that this technique is used to cover a relatively narrow range of flow rates, it is pr,Rq1h1~ to make do with narrow band ultrasound tr~nc~ rp~R that typically have a resonance frequency of 40 kHz, rather 35 than using transducers that are more sophisticated, more expensive, and that are resonant at lO0 kHz.
If the measured flow rate has a value that is greater than the transition flow rate value, then the 21g258~
.

oscillations of the jet of gas are strong enough for the freguency thereof to be detected (high flow rate conditions). Under such Cil~; ~culCeS, the gequencer of the microcontroller 62 controls the switching circuit 72 5 so that the pm~ qc1 rn block 68 is connected to the downstream tr~ncd~r~r 54 and the reception block 70 is connected to the u~ l L~am tr;~,nRdl r-F~r 52. The sequencer also causes a switch 74 to operate so that the slgnal coming from the upstream tr~ncfl~lrPr 52 is now treated by 10 the electronics unit 100 that can be seen on the right of Figure 10. This unit is described below in greater detail with reference to Figure ll.
Under high flow rate conditions, the microcontroller 62 causes the pm; qc~ nn block 68 to generate a permanent 15 electrical signal for exciting the downstream ultrasound tr~ncfl 7r~r 54, e.g. a squarewave signal, at a frequency ~u so that the downstream tr~ncfl ~rPr continuously emits an ultrasound signal of frequency fu tow~rds the upstream transducer 54 in a direction that is ~ nrl ~ n~ at about 20 1. 5 to the longitudinal plane o~ :,y ~Ly P. The ultrasound signa~ received by the upstream tr~nC~llrpr is a signal of irequency fu modulated by the frequencies f and 2f that are characteristic of the oscillation rhPnl -'nn O~ the ~et of gas. By way of example, the 25 frequency fu may be equal to 40 kHz and the amplitude of the electrical excitation signal may be 20 mV.
The Applicant has been able to observe that by emitting the ultrasound signal against the flow of gas, it is pr~Rs~hlP to reduce rl~nR~flPrably the influence of 30 the llydLudy,lamic pregsure of the jet, thereby reducing the energy of the signal that is received in respect of the frequency 2f. By way of example, a difference of lO dB has been observed on the amplitude of the signal received at the frequency 2f, and that suffices to make 35 it possible to distinguish the frequency f from the frequency 2f in the modulated signal while using electronic equipment that is simple, cheap, and consumes little energy. The Appl~cant has also observed that by emitting the ultrasound signal ayainst the flow of gas, the modulated ul trasound signal presents periodicity in time, thereby facilitating detection of the frequency f.
Thus, when the upstream LLC11:~dUC~CL 52 receives an ultrasound signal modulated by the oscillations of the ~et of gas, this signal is initially amplified by a low noise analog ~mrl; f ; ~r 102 . The analog -~rl; f ; l~r 102 is a non-inverting :qmrl;f;l~r l~C~nP-l for coupling to the 10 electronic circuit that performs mea:iuL~ ~i under low flow rate conditions, and it is constituted by an operational ~mrl; f; er A1 whose non-inverting input is connected firstly to the modulated signal as received by the upstream tr~n~ rc~r 52 and secondly to ground via a 15 resistor Rl. The $nverting input of this operational amplifier A1 is connected firstly to ground via a resistor RZ and secondly to the output Bo of the amplifier via another resistor R3. The modulated ~nd amplified ultrasound signal then has the appearance shown in Figure 20 12. ::
A conventional halfwave rectlfier circuit 104 is shown in Figure ll and comprises a resistor R~ connected between the output Bo of the amplifier A1 and the inverting input B1 of an operational ~m?1~f;~r Az whose 25 non-inverting input is connected to ground. The inverting input of the ~mrl~f;~r A2 is connected to the output B2 of said ;~mrl; f; ~r via two parallel-connected branches: a first branch is constituted by a resistor R5 in series with a diode D1 that is reverse-connectedi and 30 a second branch which is constituted by a diode D2. In ~,c -lv~lLlonal manner, when the difference VB1_VB2 is greater than the threshold voltage of the diode Dl, then it conducts giving VB3 = ( R~/R~ )VB2 . Conversely, when the value VB1_VBZ drops below the threshold voltage of the 35 diode D1, then the diode D2 becomes conductive and VB3 = , the rectified signal having the appearance given in Figure 13.
. , . . . .. . . . . . .. _ , . .. . _ _ . . _ _ . . _ . .. . .

~1~2583 In order to retain only the oscillation frequency f of the ~et of gas, the rectified signal is then filtered by electronics block 106 which acts as a (second order) lowpass filter. As shown in Figure 11, the block 106 has 5 two resistors R6 and R8, and a capacitor C1 f orming a T -filter which is subjected to negative feedback by a resistor R~ and a capacitor C2 together with an operational ~mrl; f i Pr A3 whose non-inverting input is connected to ground. The filtered signal obtained at Bs 10 has the appearance shown in Figure 14.
This signal is then injected into an ~mrl;f;cation electronics block 108 that comprises two stages: a first stage 109 that acts as a bandpass ~mrl if ~Pr having a gain of 50, for e~ample, and having a cutoff requency lying in the range 0 . 5 Hz to 50 Hz; and a seaond stage 110 that acts as a lowpass ;~mrlif;Pr having a gain equal to 5, for example, and a cutoff freguency e~ual to 50 Hz.
The first stage comprises a resistc~r R9 and a capacitor C3 connected in series between the output Bs of block 106 and the inverting input of an operational ~rl~l;f;Pr A~. The non-inverting input of the ~mrl;f;Pr A~
ls connected to ground, and its inverting input is connected to its outputs B6 via a capacitor C6 and a resistor RlC connected in parallel.
The second stage 110, downstream from B6, comprise~3 a resistor R1l connected to the inverting input of an operational amplifier As~ which input is also connected to the output B~ of the ~mrl i f i Pr via a resistor Rl2 and a capacitor Cs ccnnected in parallel. The non-inverting input of the ;:~mrlif;Pr As is connected to ground.
The electrcnics block 108 serves to shift the signal from the filter 106 so as to place it on either side of zero, and to amplify said signal. The signal as amplified in this way that appeared at B~ is in~ected into the following block 112 which transforms it into a pulse signal as shown in Figure 15.
.... . .... ....... . . .. . ..... .. . . . _ . .

2~2~83 The electronics block 112 comprises an operational amplifier A6 whose non-inverting lnput is connected to the output B~, and whose inverting input is connected firstly to the output B8 of the ~mrl ~ fiP~ A6 via a resistor Rl~, and secondly to the output of a ~ v~-tlonal follower circuit that comprises an operational ~mrl~f1pr A~. Because of the negative feedback from the follower circuit, the amp~ifier A6 makes it poc~lhlP to amplify small amplitude signals more-than large amplitude signals.
This block also includes a resistor Rls connected to the output B3 of the amplifier A6 and to a point B9, and it also includes two diodes D3 and D~ mounted head to tail between the point Bg and a point B1o. The point Blo is connected firstly to ground via a capadtor C6 and secondly to the non-inverting input of the follower circuit A~ and to the inverting input of a further operational Amrl; f;P~ A3. The output of ~operational amplifier A8 (point Bl2) is looped back to its non-inverting input via a resistor R1,. The non-inverting input of this ~mrl~ f~P~ is also connected to the diodes D3 and D~ via a resistor R16. When the amplitude of the voltage VB9_V310 increases to exceed the threshold of diode D~, then the diode conducts and the value of the voltage signal at point B9 minus the voltage drop across diode D;
is stored in capacitor C6. The differential amplifier A8 then compares the value of the voltage at point B11 as given by:
V R + V R

Rl7 + Rl6 with the value of the voltage on capacitor C6, and it produces a high value signal when the voltage at point B9 is greater than the voltage on the capacitor C6.
Once a peak has been reached and the amplitude of the signal (lPrP~qPq~ the difference between the value of the signal at point B9 and the value of the signal stored - ~ ' 21g2~83 by the capacitor C6 drops below the threshold of the diode D~ so the diode D4 becomes non-conducting. The value of the sigl~al stored on capacitor C6 then remains unchanged. When the amplitude of the signal at point B5 drops below the value of the signal stored on the capacitor C6, then the amplifier A8 provides a low level signal showing that a peak has occurred. When the amplitude of the signal drops below the value of the signal stored on the capacitor C6 by an amount that corresponds to the threshold of tl~e diode D3 plus the voltage at B1o, then the diode D3 starts to conduct and the value of the signal as stored on the capacitor C6 drops to the vallle of the signal at point Bg minus the voltage drop across the diode D3. When a negative peak is reached and passed, the diode D3 will again become non-conducting, and the ~mrl;fiP~ A3 will indicate a change in state once the signal at point Bll has increased to above the value of the signal stored ~n the capacitor C6 ~
In Figure 16, curve 150 shows how the voltage of the first signal at point Bg varies, and curve 151 shows how the voltage across capacitor C6 varies. Initially, the capacitor voltage 151 is e~Iual to the signal 150 minus the value Vd that corresponds to the voltage drop across the diode D" so the ~mrl if i~ A8 provides a high level signal. When a peak is reached at time T0, and while the voltage of the signal 150 has dropped under the threshold of the diode D~, the voltage on the capacitor 151 remains unchanging. At time Tl the volta~e of the signal 150 drops below the ~oltage stored on the capacitor 150 so the output oi ~the ;~ 7l i f; c.~ A8 provides a low level signal. At time T2, the difference between the voltage of the first signal 150 and the voltage stored on the capacitor 151 becomes greater than the threshold voltage for the diode D~ so the capacitor voltage again tracks the voltage of t~le iirst signal. The electronic circuit corresponding to block 112 is thus a peak detector. The :
2~2583 amplifier A8 of l igure lr is a ~ CLuL with hysteresis that compares the values ûf the two voltages 150 and 151 shown in Figure 16. Thus, when the value of the voltage at point B.., i.e. V31l, the voltage on the non-inverting input of the amplifier A8, is greater than the value of the voltage at point Blo~ i.e. VD10~ the voltage on the inverting input of the amplifier A8, then the amplifier provides a constant output voltaye equal to +Vcc where Vcc is the power supply voltage of said amplifier, and the voltage at B11 becomes:
VccR16 + VDgR17 Rl6 + Rl7 Conversely, and as shown at instant Tl in Figure 16, when VD11 is less than VD10~ then the output voltage of the cmplifier A~ i5 egual to -Vcc so the voltage at B~, becomes:
-VccR16 + VDgR17 VD11 =
R1~ + R17 As a result the output from the block 112 is in the form o E a pulse signal in which each pulse represents a unlt volume of gas being swept by the j et of gas during a single oscillation ( Figure 15 ) . --An electronic counter (Figure 10), e.g. a 16-bit counter, then serves to count the total number of pulses, thereby Pni~hl ~n~ the mi~ilu~;ullLLuller to deliver the volume of gas that has flowed through the fluidic oscillator .
It should be observed that if the transducers 52 and 54 are accurately aligned on the longitudinal plane of -y ~ tly P in order to enhance received signal energy at the frequency 2f, then the above-described electronic circuit 100 needs to be adapted sp~r~ f ~ ~~P l l y to detect 3 5 that f requency .
When the oscillations of the ~ et of gas become too weak for the frequency thereof to be detectable, i.e.
when the flow rate of the gas becomes less than the 2192~83 above-mentioned transition flow rate, provision is made to use the ultrasound trAncrl~ rPrs 52 and 54 to measure the ~lu~aya~lon time of ultrasound signals in the flow of gas as r9PC~r~ hPfl above ( low f low rate conditions ) . In 5 order to decide ~hen to use low flow rate conditions or high flow rate conditions, it is possible, for example, to measure the time interval between two successive pulses and to compare the time interval measured in this way with a prede~Prm; nP~ value that corresponds to the 10 transition flow rate. If the measured time interval exceeds the predetPrm~ nPCl value, then the ultrasound tr~nR-lllrPrs are used alternately as emitter and receiver.
It is also pOcR~ hl P to provide for an overlap range, e. g. extending from 100 l/h to 150 l/h, over which both 15 operating conditions of the combination fluidic oscillator may be used. Thus, if the fluidic oscillator is operating under low flow rate conditions, it may continue to measure flow rate in that way until the high value of the overlap range i5 reached, at which point 20 measurement switches over to high flow rate conditions.
Similarly, when the fluidic oscil 1 ator is operating under high flow rate conditions, then it is nPrPgc~ry for the flow rate to drop below the lower value of the overlap range be~ore the fluidic f~R~ r switches over to 25 measuring under low flow rate conditions.
The advantage of having an overlap range is to make it unlikely that it will be necessary to switch mea:iuL t conditions back again to the preceding conditions immediately after a switchover has taken 30 place.
The combination fluidic oscillator o the present invention can be adapted to various flow rate ranges, and is capable, in particular, of covering flow rates at greater than 6000 l/h. Figure 17 is a calibration curve 35 for the combination fluidic oscillator operating over flow rates extending within a range of about lO l/h tû
about 7000 l/h. The curve shows the relative error ~ 2192583 applicable to measul~ ~ throughout the above range. It can thus be seen that the combination fluidic oscillator is entirely suitable for mea:,u~ ~ t purposes throughout this large range of flow rates.
It will be observed that the invention has the a~v~cy~: of also being applicable to other types of 1uidic oscillators, e. g . that described in patent application GB-A-2 120 384, which is based on the Coanda effect. It is quite possible to envisage using the combination fluidic oscillator o~ the invention both to cover a range of 1uid 10w rates over which oscillations o the jet of 1uid are strong enough for the frequency thereo to be detected, and also to use the low flow rate operating conditions as P~rl~;n~d above solely for det~rm;n;n~ a leakage flow rate. For example, a 1uidic oscillator of the invention could be used as a commercial gas meter ~flow rate range 0.25 m3~h to 40 m3/h) or as an industrial gas meter (flow range 1 m3/h to 160 m3/h) that is capable o measuring a leakage rate.
It is also possible to increase meter sensitivity by positioning the ultrasound transducers accurately in the longitudinal plane o symmetry of the fluidic oscillator.

Claims

1/ A fluidic oscillator that is symmetrical about a longitudinal plane of symmetry (P) in which a longitudinal fluid flow direction is contained, the oscillator comprising:
means (26b) for generating a two-dimensional jet of fluid that oscillates transversely relative to said longitudinal plane of symmetry (P);
two ultrasound transducers (52, 54);
and means (62-72) firstly for generating an ultrasound signal in the fluid flow travelling from one of said transducers towards the other, and secondly for receiving said ultrasound signal as modulated by the oscillations of the jet of fluid; and means (100) for processing the received signal so as to determine a volume-related quantity concerning the fluid that has flowed through said fluidic oscillator;
characterized in that the ultrasound transducers (52, 54) are substantially in alignment with the longitudinal plane of symmetry (P).

2/ A fluidic oscillator according to claim 1, characterized in that for small volume-related quantities of fluid flowing through said fluidic oscillator, the ultrasound transducers (52, 54) are suitable for measuring said small volume-related quantities of fluid.

3/ A fluidic oscillator according to claim 2, characterized in that it includes:
means (62-74) for emitting and receiving an ultrasound signal alternately from each of the ultrasound transducers (52, 54); and means responsive to each received ultrasound signal and to consecutive pairs of ultrasound signals to determine a value for a magnitude that is characteristic of the propagation speed of said ultrasound signal as modified by the fluid flow, and to deduce therefrom a volume-related quantity applicable to the fluid that has flowed through said fluidic oscillator.

4/ A fluidic oscillator according to any one of claims 1 to 3, characterized in that the ultrasound transducers are disposed in different transverse planes, an "upstream" one of said transducers (52) being disposed upstream from the means (26b) for generating the two-dimensional jet of fluid, the other transducer (54) being a "downstream" transducer.

5/ A fluidic oscillator according to claim 3, characterized in that the magnitude characteristic of the propagation speed of the ultrasound signal is the propagation time of said signal.

6/ A fluidic oscillator according to claim 3, characterized in that the magnitude characteristic of the propagation speed of the ultrasound signal is the phase of said signal.

7/ A fluidic oscillator according to any preceding claim, characterized in that the means for generating an oscillating two-dimensional jet of fluid are formed by a fluid admission opening (26b) of transverse size or width d and of height h, and in that it comprises:
an oscillation chamber (32) connected at one of its ends to said fluid admission opening (26b) and at its opposite end to a fluid outlet opening (34), said openings being in alignment in said longitudinal plane of symmetry (P); and at least one obstacle (36) disposed in said oscillation chamber (32) between the admission opening (26b) and the fluid outlet opening (34).

8/ A fluidic oscillator according to claims 4 and 7, characterized in that the upstream transducer (52) is disposed upstream from the fluid admission opening (26b).

9/ A fluidic oscillator according to claim 7, characterized in that the obstacle (36) has a front portion (36a) in which a cavity (37) is formed facing the fluid admission opening (26b).

10/ A fluidic oscillator according to claims 4 and 7, characterized in that the downstream transducer (54) is secured to the obstacle (36).

11/ A fluidic oscillator according to claims 9 and 10, characterized in that the downstream transducer (54) is disposed in the cavity (37) of the obstacle (36).

12/ A fluidic oscillator according to any one of claims 7 to 11, characterized in that it includes, upstream from the obstacle (36), a passage for the fluid that is defined by two walls (28, 30) that are perpendicular to the longitudinal plane of symmetry (P) and that are spaced apart by a distance h.

13/ A fluidic oscillator according to claim 12, characterized in that it includes, upstream from the fluid admission opening (26b), a longitudinally-extending channel (26) forming at least a portion of the passage for the fluid, said channel being of substantially constant width d that is perpendicular to the distance h.

14/ A fluidic oscillator according to claim 13, characterized in that the channel (26) possesses, at one of its ends, a "downstream" opening (26b) that corresponds to the fluid admission opening, and at its opposite end, an "upstream" opening (26a) which, in a plane parallel to the flow direction of the fluid and perpendicular to the longitudinal plane of symmetry (P), is convergent in shape, its width tapering progressively down to the width d.

15/ A fluidic oscillator according to claims 4 and 13, characterized in that upstream transducer (52) is disposed upstream from the channel (26).

16/ A fluidic oscillator according to any one of claims 13 to 15, characterized in that it includes two fluid inlets (14, 16) disposed symmetrically about the longitudinal plane of symmetry (P) and opening out into the passage, upstream from the channel (26).

17/ A fluidic oscillator according to claim 16, characterized in that an empty space (20) forming another portion of the passage for the fluid is provided upstream from the channel (26) and in that the two fluid inlets (14, 16) open out into said empty space (20).

18/ A fluidic oscillator according to claims 4 and 17, characterized in that the upstream transducer (52) is disposed upstream from the empty space (20).

19/ A fluidic oscillator according to any one of claims 1 to 18, characterized in that the ultrasound transducers (52, 54) are situated on the same side in a direction perpendicular to the longitudinal direction of the fluid flow, and contained in the longitudinal plane of symmetry.

20/ A fluidic oscillator according to claims 12 and 19, characterized in that both ultrasound transducers are secured to the same one of the walls (28, 30) defining the passage for the fluid.

21/ A fluidic oscillator-according to any one of claims 1 to 18, characterized in that the ultrasound transducers (52, 54) are offset in a direction perpendicular to the longitudinal direction of fluid flow and contained in the longitudinal plane of symmetry.

22/ A fluidic oscillator according to claims 12 and 21, characterized in that each ultrasound transducer (52, 54) is secured to a respective one of the walls (28, 30) defining the passage for the fluid.

23/ A method of measuring a volume-related quantity of a fluid flowing through a fluidic oscillator in which a jet of fluid oscillates transversely about a longitudinal plane of symmetry (P), said method consisting successively in:
emitting an ultrasound signal into the fluid flow from an ultrasound transducer;
receiving said ultrasound signal as modulated by the oscillations of the jet of fluid by using another ultrasound transducer; and processing the received signal so as to determine said volume-related quantity of the fluid that has flowed through the oscillator;
the method being characterized in that it consists in emitting an ultrasound signal in a direction that is substantially contained in the longitudinal plane of symmetry (P).

24/ A method according to claim 23, characterized in that said method consists in emitting the ultrasound signal in the flow direction of the fluid flowing through the fluidic oscillator.

25/ A method according to claim 24, characterized in that after receiving the ultrasound signal and on the basis of said ultrasound signal, the method consists in detecting a frequency of oscillation that is equal to twice the frequency of oscillation of the jet of fluid, thereby making it possible to improve measurement sensitivity.

26/ A method according to claim 23, characterized in that said method consists in emitting an ultrasound signal in the opposite direction to the flow direction of the fluid through the fluidic oscillator.

27/ A method according to any one of claims 23 to 26, characterized in that for small volume-related quantities of fluid flowing through the fluidic oscillator, said method consists successively in:
emitting an ultrasound signal from one of the transducers towards the other in a direction that is substantially contained in the longitudinal plane of symmetry (P);
receiving said ultrasound signal whose speed of propagation has been modified by the flow of the fluid;
determining a first value of a magnitude characteristic of said speed of propagation of the received ultrasound signal;
repeating the above steps after interchanging the emitter and receiver functions of the ultrasound transducers and determining a second value for said magnitude characteristic of the speed of propagation for another ultrasound signal; and deducing therefrom the measurement of a small volume-related quantity of the fluid.

28/ A method according to claim 27, characterized in that the magnitude characteristic of the propagation speed of the ultrasound signal is the propagation time of said signal.

29/ A method according to claim 27, characterized in that the magnitude characteristic of the propagation speed of the ultrasound signal is the phase of said signal.

30/ The use of a fluidic oscillator according to any one of claims 1 to 22 and a measurement method according to any one of claims 23 to 29 for measuring a volume-related quantity of a gas.