Abstrict A flow meter, particularly applicable to the instantaneous measurement
of an automobile vehicle's fuel consumption, includes an enclosure
separated into two chambers by a flexible membrane which operates
in conjunction with compression springs. The fuel enters the bottom
chamber and, in the top chamber, goes through a pipe which provides
a calibrated pressure drop. A pick-up measures the resultant membrane
movements.
Claims I claim:
1. A flow meter connectable to the piping in which circulates the
fluid whose flow is to be measured, said flow meter comprising:
flow restrictor means for introducing a calibrated pressure drop;
a hermetic enclosure connected to said piping; a flexible membrane
imperviously separating said enclosure into a first and a second
chamber; means for positioning said membrane allowing it to move
resiliently in a direction perpendicular to its surface; and a transducer
component converting said movement into a flow-indicating electrical
signal, said flow restrictor means being housed in the second chamber,
and communication piping between the two chambers, fitted in one
wall of the enclosure, making the flow restrictor means communicate
with the first chamber, a fluid intake hole being formed in one
wall of the first chamber, and an exit hole being formed in one
wall of the second chamber.
2. A flow meter according to claim 1 wherein said membrane includes
a plane centre part surrounded by a semi-annular part and two cup-shaped
metal parts respectively integrated with the two faces of the centre
part and each operating in conjunction with the one end of a helicoidal
compression spring centred on the perpendicular line in the centre
of the membrane, the said end engaging the said cup-shaped part,
the other end of the spring being fixed to a wall of the enclosure.
3. A flow meter according to claim 1 wherein said transducer component
comprises two superimposed coils unconnected with each other, generator
means for supplying the respective coils with the same rectangular
electric current signals, the top coil being mounted close to the
membrane and means for measuring the difference in amplitude between
the currents which go through said coils when the membrane is moved.
4. A flow meter, according to claim 1 wherein the second chamber
is superimposed on the first.
5. A flow meter, according to claim 1 wherein said communication
piping between the two chambers has an intake hole positioned substantially
midway up the first chamber, opposite said fluid intake hole.
6. A flow meter, according to claim 1 wherein the portion of said
communication piping between the two chambers which is connected
to the flow restrictor means is formed in a removable part of the
wall of the second chamber.
Description BACKGROUND OF THE INVENTION
The present invention relates to flow meters. Known flow meters
have drawbacks which make them unsuitable for measuring microflows,
particularly under certain conditions, notably those which are encountered
when measuring the pulsed flow of fuel consumed by an automobile
vehicle. Vane flow meters are only suitable for relatively large
flows and are very complicated when they are intended for measuring
instantaneous flows. The electronics of thermometric bridge flow
meters are elaborate, therefore costly and are sensitive to the
thermal parameters of fluids, to ambient temperature, and to inclination.
Float flow meters have the drawback that they do not supply a reading
in the form of an electrical magnitude. Vortex flow meters are sensitive
to dynamic variations and unsuitable for low flows.
OBJECT OF THE INVENTION
The present invention aims at overcoming these drawbacks and its
object is to provide an accurate instantaneous flow meter for measuring
liquid or gas flows, insensitive to shocks and vibrations, to variations
of thermal magnitudes and to lack of horizontality. Further, the
flow meter can be made simply and cheaply, with a view to being
applied to automobiles.
SUMMARY OF THE INVENTION
The flow meter according to the invention includes: a piping unit
connectable to the piping in which the fluid whose flow is to be
measured circulates, said piping unit comprising means for introducing
a calibrated pressure drop; and means for measuring the difference
in fluid pressures upstream and downstream of the pressure drop.
It is characterised in that said measuring means include: an hermetic
enclosure connected to said piping; a flexible membrane imperviously
separating said enclosure into a first and a second chambers; means
for positioning said membrane allowing it to move resiliently in
a direction perpendicular to its surface; and a transducer component
converting said movement into a flow-indicating electric signal.
According to a preferred embodiment, the first chamber comprises
a fluid intake hole, said piping unit connects the first chamber
to the second, and the second chamber is provided with a fluid exit
hole.
Other features and advantages of the invention, will be apparent
from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
In the attached drawing:
FIG. 1 is a vertical section taken on the line I--I in FIG. 2
of a flow meter according to the preferred embodiment of the invention;
FIG. 2 shows the flow-meter in horizontal section taken on the
line II--II in FIG. 1;
FIG. 3 shows the flow-meter in vertical section taken on the line
III--III in FIG. 2 the components inside the body of the instrument
having been removed;
FIG. 4 is a basic diagram of the movement pick-up with which the
flow-meter is fitted.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The instrument is in the form of a flat cylindrical body advantageously
constituted by a two-piece mounting 1 and 2 fitted together and
assembled by screws such as 12 forming a bottom chamber 3 and a
top chamber 4 separated by a membrane 5. The latter, made of a hydrocarbon-resistant
metal or elastomer, is very flexible and has a thin centre disc,
e.g. 3 mm thick, surrounded by a semi-annular part 51 (FIG. 1),
itself surrounded by a flat annular rim 52 embedded between the
two parts of the cylindrical body. The centre part of the membrane
is sandwiched between two metal cheeks 6 and 7 in the shape of cups,
fixed to the membrane by means of a centre rivet 8 and moving with
it.
Two helicoidal compression springs 9 and 10 centred on the perpendicular
line in the centre of the membrane, are respectively fixed at one
end to the horizontal wall of chambers 3 and 4 and, at the other
end, to the two respective cheeks in which they engage. The springs
are thus accurately positioned and the centre part of the membrane
can only move in a direction perpendicular to its surface.
The cheeks also rigidify the centre part of the membrane, while
the outer half-ring unfolds to a greater or lesser degree according
to the difference in pressure between the two chambers and prevents
the rigidity of the membrane from intervening to any notable extent
to exert a spurious return effect on the springs.
These requirements proved to be essential to obtain a reading at
low flows, when the movement of the membrane is very slight, of
the order of 1/100th of a millimeter, for example.
The instrument is positioned between the pump, connected to an
intake hole 11 formed in the side wall of chamber 3 and the vehicle's
engine, connected to an exit hole 13 formed in the side wall of
chamber 4 or between the pump and the tank. The fuel goes into
chamber 3 at 11 and passes into chamber 4 through a vertical channel
comprising two portions 14 and 15 with different sections, to facilitate
centring.
Channel portion 15 opens laterally into a pressure drop unit 16
arranged horizontally in chamber 4 and which comes out close to
exit hole 13.
Channel 15 is formed in a removable part 17 on which rests a part
cut out of sheet metal 18 screwed at 19. This part also serves
to position the jet. Seals 20-22 are provided.
A movement pick-up comprising two superimposed coils 23-24 (FIGS.
1 and 4) unconnected to each other, is housed in chamber 3 the
top coil 24 being situated close to cheek 7 (FIGS. 1 to 3 are, for
example, twice scale size. The unit with the two coils is embedded
in an insulating resin cladding and the electric conductors go through
the bottom of chamber 3.
As FIG. 4 shows, coils 23 and 24 are excited, through resistors
26 and 27 respectively, by a rectangular signal supplied by a multivibrator
25 and having e.g. a frequency of 30 kHz. The field induced in coil
24 gives rise, in cheek 7 to Foucault currents whose intensity
varies in terms of distance. The resulting signal is rectified at
28 at the terminals of coil 24 then transmitted to a peak detector
29 itself connected to one input of a differential amplifier 32
where it is compared with the reference signal obtained, from the
terminals of coil 23 by rectification at 30 and detection of the
peak value at 31.
By way of variant, the coil could be excited by a sinusoidal current
and the phase displacement between the currents obtained in the
two coils measured.
In both cases, the output signal of the instrument is applied to
an instantaneous flow calibrated measuring instrument 33 possibly
to a flow integrator.
The principle on which the flow meter described works is as follows:
Pipe 16 constitutes a calibrated pressure drop unit which introduces
a pressure drop unit which introduces a pressure difference governed
by the flow between the two chambers. This pressure difference moves
the membrane and this movement is measured by the pick-up.
Since the flows to be measured vary, e.g. from 1 to 25 liters/hour,
it is important to obtain a good degree of accuracy at low flows,
and the pressure drop should not be too great at high flows. This
result is obtained by taking, for pipe 16 a length/internal diameter
ratio at which the pressure drop is due to two suitably balanced
effects, viz: a friction effect against the inside wall of an elongated
pipe, which gives a substantially linear response, and a shock effect
against a hole, which gives a quadratic response. Experience has
shown that, in the case of petrol, dimensions close to 45 mm length
and 2 mm diameter gives satisfactory results. The response obtained
corresponds, at flows exceeding a few liters/hour, to a substantially
linear curve and, at low flows, to a curve such that a pressure
difference becomes apparent as soon as a non-nil flow exists. For
example, a pressure variation of 0.2 millibars is obtained at 1
liter/hour, 2.7 millibars at 5 liters/hour, 8.5 millibars at 10
liters/hour and 21 millibars at 17 liters/hour.
It will be noted that a capillary tube would be unusable in practice,
because of the length that it would have to have.
It will also be noted that the pressure drop is insufficient, even
at high flows, to stall the engine. In case of sudden acceleration,
the resultant movement of the membrane has the effect of expelling
a certain volume of petrol from the chamber 4 which moreover helps
to prevent the engine stalling.
Although the relative position of the two chambers can be reversed,
it is preferable for the intake chamber 3 to be at the bottom. It
then serves to decant the petrol, preventing solid particles from
falling on the membrane and changing the instrument's zero.
It will be noted that the communicating hole between chamber 3
and channel 14 is opposite intake 11 and about mid-way up the chamber.
This positioning allows some degree of de-gassing of the petrol,
preventing gas bubbles from reaching the membrane.
The centre portion of the membrane is moveover protected by annular
projections such as 52 53 comprised therein and which operate in
conjunction with the cheeks to provide a seal.
The instrument is insensitive to shocks and vibrations and only
slightly sensitive to temperature. The electronic circuits of the
pick-up can moveover comprise a resistor with a negative temperature
coefficient designed to compensate for the effect thereof.
The movement pick-up, since it has no contact with the membrane,
introduces no distortion into the measurement.
By way of variant, the pressure drop unit could be positioned outside
body 1 2 and connected to a hole formed in the wall of the bottom
chamber opposite the intake hole. A pipe, positioned downstream
of this unit, would then be connected to the top chamber. In this
variant, the fluid exit would be downstream of the pressure drop
and the fluid would not go through the top chamber, only the pressure
downstream of the pressure drop being transmitted thereto. This
variant, satisfactory for a gas, is less satisfactory for a liquid,
since the rise of liquid in the pressure transmission piping can
give rise to error and to a de-gassing problem in the top chamber.
Application of the instrument to the measurement of a pulsed flow
of fuel is not limitative but is of particular industrial utility.
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