Meter for measuring the volume of a flowing fluid

A meter for measuring the volume of a flowing fluid includes a viscosensitive unit through which the fluid flows under laminar flow conditions, a first fluidic oscillator in series with the viscosensitive unit, and a second fluidic oscillator shunting at least the viscosensitive unit. The viscosensitive unit and the fluidic oscillators are adapted to determine the viscosity of the fluid so as to apply to the fluid volume measured by the fluidic oscillators a correction dependent on the Reynolds number.

FIELD OF THE INVENTION
 The present invention concerns a meter for measuring the volume of a
 flowing fluid.
 The invention finds a particularly advantageous application in the field of
 accurately measuring the volume of fluid delivered by a dispenser, in
 particular a fuel dispenser.
 BACKGROUND OF THE INVENTION
 Fuel dispensers are equipped with a unit for measuring the volume of fuel
 dispensed. This unit, also known as a measurer, is generally a mechanical
 positive displacement meter.
 The function of a mechanical measurer is to convert the flow of the fuel
 into a rotary motion in which one complete revolution corresponds to a
 given volume of fuel passing through the measurer. In a known piston type
 measurer for example, the liquid from the pump of the dispenser is
 injected into two or four cylinders which are filled and emptied in
 succession by means of a dispensing slide valve device. The pistons drive
 a crankshaft whose angle of rotation is proportional to the volume of
 liquid that has passed through the cylinders. An optical or magnetic
 encoding system coupled to the rotary motion supplies an electrical signal
 made up of a series of pulses each of which corresponds to a volume
 measurement increment, for example 1 centiliter (cl).
 Piston and cylinder volume meter technology is well established and proven
 but nevertheless has a number of drawbacks, namely:
 many mechanical parts,
 close machining tolerances,
 large overall size,
 moving parts subject to wear by friction that must be compensated by
 periodic calibration,
 problems concerning the ability of the materials used to withstand the
 chemical constituents of fuels,
 mechanical noise,
 high internal volume so that it is not possible to meter a plurality of
 different products sequentially in the same meter.
 OBJECTS AND SUMMARY OF THE INVENTION
 An object of the present invention is to provide a meter for measuring the
 volume of a flowing fluid that remedies the drawbacks of prior art
 mechanical meters, in particular by limiting the number of moving parts.
 This and other objects are attained in accordance with one aspect of the
 present invention which is directed to a meter for measuring the volume of
 a flowing fluid. The meter includes a viscosensitive unit through which
 said fluid flows under laminar flow conditions, a first fluidic oscillator
 in series with the viscosensitive unit, and a second fluidic oscillator
 shunting at least the viscosensitive unit. The viscosensitive unit and the
 fluidic oscillators are adapted to determine the viscosity of the fluid so
 as to apply to the fluid volume measured by the fluidic oscillators a
 correction that depends on the Reynolds number.
 Accordingly, the volume meter of the invention has no moving parts, which
 eliminates all wear and noise problems. It is also of simple mechanical
 design, compact, economical, and insensitive to pressure variations and to
 vibrations. Finally, its small internal volume allows sequential metering
 of more than one fuel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 A fluidic oscillator is a solid state meter in which the flowing fluid
 whose volume is to be measured passes through a cavity designed to impart
 periodic oscillatory motion to the flow of fluid. To a first approximation
 the frequency of the oscillations of the fluid is proportional to the
 flowrate of the fluid through the oscillator. The accuracy obtained is on
 the order of .+-.2%. In many cases this accuracy is sufficient. However,
 in the case of fuels, current legislation requires the much higher
 accuracy of .+-.0.3%. It is therefore necessary to use a corrective term
 depending on the Reynolds number of the fluid in the oscillator. To
 determine the Reynolds number it is necessary to determine the viscosity
 of the fluid, which is done by the volume meter of the invention in
 accordance with the principle described in U.S. Pat. No. 6,073,483.
 The well known general principle of a fluidic oscillator is to cause a
 fluid to flow into a cavity so designed that the fluid passing through it
 is subject to periodic oscillatory motion. There is a reliable
 relationship between the frequency of the oscillations and the flowrate
 through the cavity. An electronic device converts the periodic signal
 supplied by the fluidic oscillator into pulses which correspond to the
 volume measurement increment. Volume is measured directly by counting the
 pulses, with the fluidic oscillator constituting a volume meter.
 FIG. 1 shows a known fluidic oscillator 20 which comprises a first cavity
 201 serving as a flow conditioner. The incoming fluid may be subject to
 internal speed distributions dependent on the installation and that may
 disrupt the measurement. The flow of fluid impinges on an obstacle 203
 which orients the flow in a controlled fashion toward a calibrated slot
 204 independently of the upstream pipework.
 The slot 204 injects the fluid at high speed into a second cavity 202
 containing a second obstacle 205 which diverts the flow alternately to the
 right (continuous arrow) or to the left (dashed line arrow). Fluid flows
 are created dispenser inside the cavity 202 that react on the high-speed
 jet, forcing it to change direction. The phenomenon is reversed
 periodically depending on the fluid flowrate Q. Detection of the change of
 direction of the jet by sensors 206, 207 enables the frequency F of the
 oscillations to be measured and thus the flowrate Q to be determined. For
 a given flowrate Q, the frequency F is perfectly stable, reproducible, and
 reliable. It is therefore possible to determine the volume of fluid
 dispensed corresponding to each individual oscillation. The sensors 206,
 207 are preheated temperature sensors, for example, and variations in
 their resistance induced by variations in the speed of the fluid in
 contact with the sensor are detected. The sensors 206, 207 are supplied
 with a regulated current which heats them by the Joule effect to a
 temperature higher than that of the fluid. In this case the fluid jet
 evacuates the heat generated by the sensors.
 To a first approximation the frequency F of a fluidic oscillator is
 proportional to the flowrate Q of the fluid:
EQU F=KQ (1)
 K being a coefficient characteristic of the fluidic oscillator.
 However, as indicated in FIG. 2, for a given flowrate Q the coefficient K
 from equation (1) depends on the Reynolds number Re of the oscillating
 cavity 202:
EQU Re=Q/h.nu.
 where h is the height of the cavity and .nu. is the kinematic viscosity of
 the fluid.
 The coefficient K can vary by up to .+-.2% as a function of Re, which is
 acceptable in a great many situations. However, in the case of measuring
 the volume of fuel dispensed by a dispenser, the authorities impose an
 accuracy of .+-.0.3%. The variations of K with Reynolds number can be
 represented by the equation:
EQU K=Ko(1+.epsilon.(Re))
 where Ko is an average value and Ko.epsilon.(Re) is the deviation relative
 to Ko for the Reynolds number Re. In FIG. 2, the value of Ko is 50
 oscillations/liter.
 Equation (1) can then be written:
EQU F=Ko(1+.epsilon.(Re))Q (2)
 If qo is the average individual volume corresponding to one oscillation at
 frequency F (qo=1/Ko) and if q is the actual individual volume, with
 .epsilon.(Re) small compared to 1, equation (2) can be written:
EQU Q=qo(1-.epsilon.(Re)) (3)
 Allowing for the spread of the coefficient K as a function of the Reynolds
 number Re therefore amounts to correcting the individual volume qo for
 each oscillation by the amount qo.epsilon.(Re).
 For a given oscillating cavity, the Reynolds number depends essentially on
 viscosity, so it is essential to be able to measure the viscosity of the
 fluid in real time.
 To this end, and as shown in FIGS. 3 and 4, there is provided a meter for
 measuring the volume of a flowing fluid comprising:
 a first viscosensitive unit 10 through which said fluid flows under laminar
 flow conditions,
 a first fluid oscillator 21 in series with said viscosensitive unit 10, and
 a second fluidic oscillator 22 shunting at least the viscosensitive unit
 (10),
 the viscosensitive unit 10 and said fluidic oscillators 21, 22 being
 adapted to determine the viscosity of the fluid.
 The manner in which the volume meters from FIGS. 3 and 4 measure the
 viscosity of the fluid is explained in detail in the above-mentioned
 co-pending patent application. Briefly, in FIG. 3, in which the second
 fluidic oscillator 22 shunts only the viscosensitive unit 10, the dynamic
 viscosity .mu. is given by:
EQU .mu.=(a.sub.2 /k)x.sup.2 Qt/(1-x) (4)
 where x is the ratio Q.sub.1 /Q.sub.2 of the flowrates measured by the
 fluidic oscillators 21, 22, Qt is the total flowrate, here Q.sub.1, and
 a.sub.2 and k are parameters characteristic of the second fluidic
 oscillator 22 and of the viscosensitive unit 10, the ratio (a.sub.2 /k)
 being determined by prior calibration. The kinematic viscosity .nu. is
 related to the dynamic viscosity .mu. by the equation .nu.=.mu./.rho.,
 .rho. being the density of the fluid.
 In FIG. 4, in which the second fluidic oscillator 22 shunts the combination
 of the viscosensitive unit 10 plus the first fluidic oscillator 21, the
 dynamic viscosity .mu. of the fluid is given by the equation:
EQU .mu.=(1/k)(a.sub.2 x.sup.2 -a.sub.1)Qt/(x+1) (5)
 where x is the ratio Q.sub.1 /Q.sub.2 of the flowrates measured by the
 fluidic oscillators 21, 22, Q.sub.t is the total flowrate Q.sub.1 +Q.sub.2
 and Q.sub.1 is a parameter characteristic of the first fluidic oscillator
 21, the ratios a.sub.1 /k and a.sub.2 /k being determined by prior
 calibration.
 The viscosensitive unit 10 is adapted to assure laminar flow of the fluid
 throughout the range of flowrates envisaged. One embodiment of a
 viscosensitive unit of the above kind is the flashback arrestor used in
 fuel dispensers.
 FIG. 5 shows one particular embodiment of the meter shown in diagrammatic
 form in FIG. 4. Note that a flowrate reducing diaphragm 30 has been
 disposed in series with the second fluidic oscillator 22 to balance the
 flowrates between the two branches. The effect of the diaphragm 30 is to
 increase the parameter a.sub.2 of the second fluidic oscillator 22.
 FIG. 6 shows the signal s.sub.osc supplied directly by a fluidic
 oscillator. In the case of an oscillator with two sensors 206, 207 as
 shown in FIG. 1, the signal s.sub.osc is the differential signal from the
 two sensors. Regarding s.sub.osc, there is shown a pulsed signal Imp
 deduced from s.sub.osc by an electronic signal shaping circuit known in
 itself. Each pulse of Imp corresponds to the actual individual volume q of
 the fluid and summing all the pulses of the actual individual volumes q
 gives the total volume of the fluid that has flowed.
 The volume meters from FIGS. 3 and 4 operate in the following manner.
 The fluid flowrates Q.sub.1 and Q.sub.2 across the fluidic oscillators 21,
 22 are determined from the approximate equations:
EQU Q.sub.1.ident.q.sub.01 F1
 and
EQU Q.sub.2.ident.q.sub.02 F2
 Knowing Q.sub.1 and Q.sub.2, x and Q.sub.t are known, from which the
 dynamic viscosity .mu., and thus the kinematic viscosity .nu., can be
 deduced using equations (4) and (5) applied either analytically or in the
 form of tables after calibration.
 The corresponding Reynolds number can then be calculated for each fluidic
 oscillator, namely:
EQU Re.sub.1 =Q.sub.1 /h.sub.1.nu.
 and
EQU Re.sub.2 =Q.sub.2 /h.sub.2.nu.
 Then, knowing the corrections .epsilon..sub.1 (Re.sub.1) and
 .epsilon..sub.2 (Re.sub.2) from calibration, it is possible to determine
 the actual individual volumes corresponding to one pulse of the signals
 Imp.sub.1 and Imp.sub.2 :
EQU q.sub.1 =q.sub.01 (1-.epsilon..sub.1 (Re.sub.1)) q.sub.2 =q.sub.02
 (1-68.sub.2 (Re.sub.2))
 The correction is applied by a microprocessor 100 shown in FIG. 7. A meter
 200 sums pulse by pulse the actual individual volumes q.sub.1 and q.sub.2
 to obtain the total volume of fluid that has flowed and each time an
 individual volume q.sub.0, for example 1 cl, is metered the meter 200
 supplies a pulse Imp' to a computer, such as the computer of a fuel
 dispenser, which then has only to count the pulses Imp' to determine the
 volume of fluid that has flowed. As an alternative to this, the meter 200
 could also supply the cumulative total volume, sent periodically in
 encoded form.
 Some authorities require the volume to be corrected for temperature. The
 volume of a fluid, and especially of a hydrocarbon, varies by
 approximately 0.1% per degree centigrade and, for temperature lying in the
 range -20.degree. C. to 35.degree. C., the relative variation in the
 volume of the fluid can be as great as 5%. The temperature sensors 206,
 207 can then be used also to measure the temperature of the liquid, the
 volume being corrected by the microprocessor 100.