Abstrict A flow meter comprises an inflow chamber having a first fluid inlet,
an outflow chamber housed in the inflow chamber and having a second
fluid inlet, which is in fluid communication with the inflow chamber,
and a fluid outlet by which fluid can drain from the outflow chamber,
and a sensing device having a elongate sensor which is positioned
at, or adjacent to and downstream of, the second fluid inlet. The
second fluid inlet is of limited dimensions so that, in use, a head
of fluid is formed in the inflow chamber and the flow rate of the
fluid passing through the second fluid inlet is monitored based
on the extent of the elongate sensor which is covered by the fluid.
Claims What is claimed is:
1. A flow meter comprising an inflow chamber having a first fluid
inlet, an outflow chamber housed in the inflow chamber and having
a second fluid inlet, which is in fluid communication with the inflow
chamber, and a fluid outlet by which fluid can drain from the outflow
chamber, and a sensing device having an elongate sensor which is
positioned at, or adjacent to and downstream of, the second fluid
inlet, the second fluid inlet being of limited dimensions so that,
in use, a head of fluid is formed in the inflow chamber and the
flow rate of the fluid passing through the second fluid inlet is
monitored based on the extent of the elongate sensor which is covered
by the fluid.
2. A flow meter as claimed in claim 1 wherein the elongate sensor
is a wire sensor.
3. A flow meter as claimed in claim 1 wherein the elongate sensor
is a strip sensor.
4. A flow meter as claimed in claim 1 wherein the outflow chamber
comprises a plurality of the said second fluid inlets and one said
elongate sensor is positioned at, or adjacent to, each said second
fluid inlet.
5. A flow meter as claimed in claim 1 wherein the or each second
fluid inlet is an elongate slit, which extends in the direction
of the longitudinal extent of the outflow chamber.
6. A flow meter as claimed in claim 1 wherein the outflow chamber
has spaced parallel inner and outer surfaces between which a throughflow
chamber is formed, the second fluid inlet(s) being formed in the
inner surface and a third fluid inlet, through which fluid can enter
the throughflow chamber, being formed adjacent to the bottom edge
of the outer surface.
7. A flow meter as claimed in claim 1 wherein the outflow chamber
includes a foam overflow inlet in its top surface which bypasses
the or each elongate sensor.
8. A flow meter as claimed in claim 1 wherein the inflow chamber
is frusto-conical, tapering in a downwards direction.
9. A flow meter as claimed in claim 1 wherein the outflow chamber
is frusto-conical, tapering in an upwards direction to, in use,
encourage bubbles in the said fluid in the inflow chamber to move
upwards to the surface of the fluid and not to pass through the
second inlet.
10. A flow meter as claimed in claim 1 wherein the first fluid
inlet of the inflow chamber is positioned tangentially or substantially
tangentially relative to the inner surface of the inflow chamber
so that, in use, fluid flow follows a centrifugal spiral or substantially
spiral path on entering the inflow chamber.
11. A flow meter as claimed in claim 1 wherein a further elongate
sensor is provided as a reference sensor which, in use, is entirely
and continuously immersed in the fluid.
12. A flow meter as claimed in claim 1 wherein the sensing device
includes energising means for driving the or each elongate sensor,
and electronic circuitry for monitoring the electrical characteristic
of the or each elongate sensor.
13. A flow meter as claimed in claim 12 wherein the electronic
circuitry monitors the capacitive impedance, capacitance and/or
conductance of the fluid based on the electrical response of the
or each elongate sensor.
14. A flow meter as claimed in claim 11 wherein each elongate
sensor is driven sequentially to prevent or limit interference therebetween.
15. A flow meter as claimed in claim 12 wherein the energising
means outputs an alternating current, and the impedance of the sensing
device is matched to that of the fluid being monitored.
Description INTRODUCTION
This invention relates to a flow meter and, in particular but not
exclusively, to a flow meter for milk.
Flow meters for determining the flow rate of fluids are known.
However, these are typically mechanical arrangements or incorporate
mechanical mechanisms which can lead to inaccuracies and failure
over time.
Further problems occur when trying to determine the flow rate of
milk, since the specific gravity and viscosity of milk varies throughout
milking, from cow to cow, and depends on the time of year due to,
for example, entrained air, butterfat content, and mastitis.
The entrained air and surface tension of milk can vary considerably.
Foaming can occur due to vacuum creation in the lines transporting
the milk, and with the formation of slugs. It is desirable to separate
or reduce the air from the milk so that volumetric measurements
can be made under calm or relatively calm flow conditions.
Furthermore, the conductive and dielectric properties of milk may
not only vary from cow to cow, but also may vary during the milking
cycle. The constituents of milk exhibit varying impedances when
subjected to alternating and direct currents (AC and DC). Milk cannot
therefore be measured reliably by measuring bulk conductivity and
may, under turbulent flow conditions, appear to become inductive.
The present invention seeks to provide a solution to these problems.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a flow meter
comprising an inflow chamber having a first fluid inlet, an outflow
chamber housed in the inflow chamber and having a second fluid inlet,
which is in fluid communication with the inflow chamber, and a fluid
outlet by which fluid can drain from the outflow chamber, and a
sensing device having an elongate sensor which is positioned at,
or adjacent to and downstream of, the second fluid inlet, the second
fluid inlet being of limited dimensions so that, in use, a head
of fluid is formed in the inflow chamber and the flow rate of the
fluid passing through the second fluid inlet is monitored based
on the extent of the elongate sensor which is covered by the fluid.
BRIEF DESCRIPTION OF THE DRAWING
The present invention will now be described, by way of example,
with reference to the accompanying FIGURE which is a diagrammatic
cross-sectional view of one embodiment of a flow meter, in accordance
with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the FIGURE, a flow meter 10 is shown therein. The
flow meter 10 has an inflow chamber 12 an outflow chamber 14 and
a sensing device 16. The inflow chamber 12 has a first fluid inlet
18 at, or adjacent to, its top surface 20 and is of frusto-conical
shape, tapering in a downwards direction. The first fluid inlet
18 is positioned tangentially or substantially tangentially relative
to the inner surface 22 of the inflow chamber 12.
The outflow chamber 14 is concentrically housed within the inflow
chamber 12 and comprises inner and outer walls 24 and 26 which extend
in parallel with each other and which are spaced apart, thus forming
a throughflow chamber 28 therebetween. A plurality of spaced second
fluid inlets 30 is formed in the inner wall 24 and a fluid outlet
32 is formed in the base of the outflow chamber 14. The second fluid
inlets 30 are elongate slits which extend in the direction of the
longitudinal extent of the outflow chamber 14 and which are in
fluid-communication with the inflow chamber 12. The outflow chamber
14 is central to the housing.
The shapes of the inflow and outflow chambers 12 and 14 are such
as to maximise flow capacity in a minimum volume.
The outer wall 26 is joined to the inner wall 24 at the top of
the outflow chamber 14 to form a rim 34 which defines an overflow
opening 36 into the outflow chamber 14. The outer wall 26 extends
to a position at which its lower free-edge 38 is adjacent to, but
spaced from, the bottom edge 40 of the inner wall 24 and this opening
defines a third fluid inlet 42. The position of the third fluid
inlet 42 is designed to inhibit the through-flow of bubbles.
The sensing device 16 has elongate, typically wire, measuring sensors
44 which are positioned symmetrically in the outflow chamber 14
directly adjacent to and in parallel with each elongate slit 30
and an elongate, typically wire, reference sensor 46 which in use
is coiled, typically horizontally, around the bottom edge 40 of
the inner wall 24 of the outflow chamber 14. The reference sensor
46 is of a length which is proportional to the sum of the lengths
of the measuring sensors 44. Each sensor 44 and 46 may be sheathed
in an electrically insulating, typically plastics, dielectric covering.
The sensing device 16 also includes an Earth ring member 48 which
is located in the base of the throughflow chamber 28. In addition
device 16 acts as an Earth.
Wire sensors are beneficial over plate sensors due to their reduced
surface area and the fact that fluid is less likely to adhere, or
will adhere for shorter periods, to the outer surface of a wire
sensor than a plate sensor. This results in improved accuracy of
measurements.
As an alternative to wire sensors, elongate strip sensors could
be utilised.
The sensing device 16 also includes energising means (not shown)
which supplies an alternating current at high frequency, typically
from 50 kHz to several MHz but below the frequency range at which
molecular resonance may impact the accuracy of the readings, for
driving each sensor 44 and 46 and microcomputer-based electronic
circuitry (not shown), which includes a display, for monitoring
the capacitive impedance, capacitance and/or conductance of the
fluid based on the electrical response of the wire sensors 44 and
46 computing results, and displaying a corresponding value. The
electronic circuitry is set up so that the energising means drives
each sensor 44 and 46 sequentially, thus preventing or minimising
any interference therebetween.
The electronic circuitry filters, to reduce noise and sampling
errors, and digitises the instantaneous response values from the
measuring sensors 44 and reference sensor 46. Further digital filtering,
scaling and linearising may then take place in software. To expedite
on-line calibration of the flow meter 10 the flow meter 10 may
be statically and dynamically calibrated and the resultant values
stored in memory of the electronic circuitry. One calibration method
typically takes the form of calibrating the fluid flows at known
flow rates, and recording the flow measurements whilst taking concurrent
load readings of the outflowing fluid. In this way, linearisation
and scaling factors may be calculated and concurrently applied to
the measurement data such that flow rates and mass flow may be displayed
in real time.
To further ensure the highest possible resolution of the sensing
device 16 and therefore the most accurate measurements, the overall
impendence range of the sensors 44 and 46 are typically matched
to the impedance of the fluid being monitored, even though the provision
of the reference sensor 46 fully compensates for small changes in
fluid impedance during normal operation.
The flow meter 10 normally operates with a partial vacuum in the
inflow, outflow and throughflow chambers 12 14 and 28 and is adapted
so that in use a zero or substantially zero pressure differential
can exist between the first fluid inlet 18 of the inflow chamber
12 and the fluid outlet 32 of the outflow chamber 14.
In use, the afore-mentioned partial vacuum is first set up between
the first fluid inlet 18 of the inflow chamber 12 and the fluid
outlet 32 of the outflow chamber 14 to which the vacuum line is
connected. Under force from this vacuum, milk 50 is drawn tangentially
into the inflow chamber 12 from the first fluid inlet 18. Due to
the tangential orientation of the first fluid inlet 18 the inflowing
milk 50 is centrifugally urged against the inner surface of the
inflow chamber 12 and follows a reducing spiral path downwards,
thereby slowing and enabling flow calming and the separation of
bubbles and foam. The inflowing milk 50 thus accumulates at the
bottom of the inflow chamber 12 and any foam floats to the top.
Since the reference sensor 46 of the sensing device 16 is positioned
around the bottom edge 40 of the inner wall 24 of the outflow chamber
14 this sensor is entirely and continuously immersed only in the
milk 50 and not in any foam. The reference sensor 46 is therefore
able to measure the electrical characteristics of the milk 50 relative
to the Earth ring member 48. Using a differential measurement method,
the measuring sensors 44 can then be calibrated based on the relationship
between the measuring sensors 44 and the immersed reference sensor
46. The calibration values are then stored in the memory of the
electronic circuitry.
The elongate slits 30 act to reduce flow errors and are of limited
dimensions so that a head of milk 50 is formed in the inflow chamber
12 and the throughflow chamber 28. The milk 50 flows around the
bottom edge 38 of the outer wall 26 of the outflow chamber 14 and
into the throughflow chamber 28 before then flowing through the
elongate slits 30 past the wire measuring sensors 44 and out through
the fluid outlet 32.
Any bubbles formed by the milk 50 are discouraged from flowing
into the throughflow chamber 28 and thus through the elongate slits
30 due to the frusto-conical shapes of the inflow and throughflow
chambers 12 and 28 and the position of the third fluid inlet 42.
These bubbles tend to migrate towards the outer surface of the outer
wall 26 of the outflow chamber 14 and then flow upwardly towards
the surface of the milk 50 in the inflow chamber 12 to form foam.
If the amount of foam rises above the rim 34 of the outflow chamber
14 it overflows through the opening 36 and down to the fluid outlet
32 completely bypassing, and thereby not having an effect on, the
measuring sensors 44.
The driven measuring sensors 44 because they are positioned downstream
of the elongate slits 30 and directly in the flow path, only return
data based on the extent they are covered by the fluid flowing through
the elongate slits 30 and not simply the height of the fluid in
the first and throughflow chambers 12 and 28. The electronic circuitry
sequentially scans and measures, typically at several hundred times
per second, the response generated between the measuring sensors
44 and the Earth ring member 48 in contact with the flowing milk
50. Since the extent of the measuring sensors 44 covered by the
milk is proportional to the fluid flow, a corresponding flow value
is generated by the electronic circuitry.
A single measuring sensor could be used if the properties of the
fluid are known and stable. The reference sensor 46 enables on-line
compensation when fluid electrical characteristic variations occur.
More than one reference sensor could be utilised, if the fluid
comprises multiple phases. For example, a further reference sensor
could be suitably positioned to be, in use, covered by the foam.
Furthermore, although the elongate reference sensor is coiled,
providing it is in use entirely and continuously immersed, it could
take any suitable arrangement or position.
The inflow, outflow, and/or throughflow chambers could be cylindrical
or substantially cylindrical, or of any other suitable shape.
Although the flow meter is described with reference to monitoring
milk flow rate, it could be utilised for a single phase fluid, and
a multiple phase fluid, such as a bi-phase fluid, which can be separated
by density and resolved by differences in the electrical characteristics.
Examples of such bi-phase fluids are air/water, air/milk, and oil/water.
Furthermore, although the measurement method described is a differential
method between the reference sensor and the measuring sensors, other
methods could be used. For example, an absolute calibration of the
measuring sensor impedances against a known external reference,
such as a stable electrical network, could be used; differential
measurement between a number of reference sensors and a calibrated
value to return a value proportional to the properties of the fluid
and from which changes in the fluid may be compensated for, may
be used; or a ratiometric measurement between the measuring sensors
and the reference sensor to return a value proportional to the fluid
profile with height, from which the mass flow can be derived, could
be used. This latter method can also be used to calibrate against
simultaneous load measurements, and thus the fluid density can be
calculated.
The flow meter could operate without the outer wall of the outflow
chamber, i.e. the throughflow chamber, if only a single phase fluid
were to be measured, or if the flow readings being influenced by
bubbles and foam passing over the measuring sensors was unimportant.
It is thus possible to provide a flow meter which has no mechanically
moving parts, is not dependent on the specific gravity and viscosity
of the milk, and the readings from which are not impacted by entrained
air, surface tension of the milk, or foaming. Furthermore, because
a high impedance measurement method is being used to measure the
fluid, the power consumption of the flow meter is significantly
reduced.
The embodiment described above is given by way of example only
and further modifications will be apparent to persons skilled in
the art without departing from the scope of the invention as defined
by the appended claims. |