Abstrict A non-contaminating flow meter having an isolation member is disclosed.
The flow meter includes two pressure sensor transducers, located
on opposite sides of a restriction in a conduit. Each of pressure
transducers is isolated by an isolation member from exposure to
fluids flowing through the conduit. The flow meter may be positioned
in-line within a fluid flow circuit carrying corrosive materials,
wherein the square root of the difference in pressure between the
two pressure sensors is calculated to thereby determine the rate
of flow within the fluid flow conduit. The flow meter compensates
for changes in temperature within the fluid flow circuit and further
activates an alarm if the pressure or flow rate within the fluid
flow conduit exceeds or falls below a threshold level. The flow
meter of the present invention also avoids the introduction of particulate,
unwanted ions, or vapors into the flow circuit.
Claims What is claimed is:
1. A chemically inert fluid flow meter adapted to be connected
in-line with a fluid flow circuit, comprising:
(a) a chemically inert housing having a longitudinal bore of a
first predetermined cross-sectional area extending through said
housing forming a fluid flow conduit, wherein an inlet end and an
outlet end of the conduit are adapted for connection, in-line, to
the fluid flow circuit, said housing further having first and second
spaced apart cavities each extending transversely from an external
surface of said housing into the longitudinal bore of said housing,
the longitudinal bore having a constriction disposed between said
first and second cavities, the constriction having a second bore
being of a second cross-sectional area that is less than the first
predetermined cross-sectional area;
(b) first means for sensing a first pressure within the flow circuit,
said first means for sensing being contained within said first cavity;
(c) second means for sensing a second pressure within the flow
circuit, said second means for sensing being contained within said
second cavity;
(d) means for constraining the first and second means for sensing
in a fixed position within the cavities of the housing; and
(e) an electronic circuit contained within the housing and coupled
to the first and second means for sensing, whereby the electronic
circuit receives signals proportional to the sensed first and second
pressures within the bore.
2. The fluid flow meter as recited in claim 1 wherein the electronic
circuit further produces a signal proportional to a rate of fluid
flow within the bore.
3. The fluid flow meter as recited in claim 2 wherein the electronic
circuit further includes a means for adjusting the control signal
to compensate for a pressure differential present when the rate
of flow is zero.
4. The fluid flow meter as recited in claim 1 wherein the constriction
of the longitudinal bore comprises a removable insert.
5. The fluid flow meter as recited in claim 4 wherein said removable
insert is manufactured from sapphire.
6. The fluid flow meter as recited in claim 1 further comprising
a chemically inert flexible membrane contained within each of said
first and second cavities, proximate the longitudinal bore of said
housing, each membrane having first and second opposed major surfaces,
said first major surface being exposed to fluid flowing in the fluid
flow conduit.
7. The fluid flow meter as recited in claim 1 wherein a drain
extends transversely from the external surface of said housing into
at least one of said first and second spaced apart cavities.
8. The fluid flow meter as recited in claim 7 wherein said drain
further comprises a sensor positioned within said drain for sensing
the presence of fluids within said drain.
9. The fluid flow meter as recited in claim 1 wherein the first
and second means for sensing each comprise a sapphire sensor.
10. The fluid flow meter as recited in claim 1 wherein the electronic
circuit includes a means for adjusting the control signal to compensate
for fluctuations in temperature within the flow circuit.
11. A chemically inert fluid flow meter adapted to be connected
in-line with a fluid flow circuit, comprising:
(a) a chemically inert housing having a longitudinal bore of a
first predetermined cross-sectional area extending through said
housing forming a fluid flow conduit, wherein an inlet end and an
outlet end of the conduit are adapted for connection, in-line, to
the fluid flow circuit, said housing further having first and second
spaced apart cavities each extending transversely from an external
surface of said housing into the longitudinal bore of said housing,
the longitudinal bore having a constriction disposed between said
first and second cavities, the constriction having a second bore
being of a second cross-sectional area that is less than the first
predetermined cross-sectional area;
(b) first means for sensing a first pressure within the flow circuit,
said first means for sensing being contained within said first cavity;
(c) second means for sensing a second pressure within the flow
circuit, said second means for sensing being contained within said
second cavity;
(d) means for constraining the first and second means for sensing
in a fixed position within the cavities of the housing; and
(e) an electronic circuit contained within the housing and coupled
to the first and second means for sensing, whereby the electronic
circuit receives signals proportional to the sensed first and second
pressures within the bore and thereafter produces an electrical
signal proportional to a rate of fluid flow within the bore determined
from the sensed first and second pressures.
12. The fluid flow meter as recited in claim 11 wherein the constriction
of the longitudinal bore comprises a removable insert.
13. The fluid flow meter as recited in claim 12 wherein said removable
insert is manufactured from sapphire.
14. The fluid flow meter as recited in claim 11 wherein the housing
comprises first and second housing halves each including a longitudinal
bore and a counter bore for receiving a restriction member therein
when the first and second housing halves are juxtaposed with the
longitudinal bore of each housing half being aligned.
15. The fluid flow meter as recited in claim 11 wherein said means
for constraining comprises two spacers and two hold down rings,
wherein each hold down ring has threads formed on an external surface
which mate with threads formed on an internal surface of each cavity,
said spacers being positioned between said first and second means
for sensing and said hold down rings, whereby when each hold down
ring is screwed relative to the internal surface of each cavity,
each hold down ring presses the associated spacer against the respective
means for sensing.
16. The fluid flow meter as recited in claim 11 and further including
a cover adapted to be attached to the housing in covering relation
to the cavities, the cover including a chemically inert electrical
connector electrically coupled to the electronic circuit.
17. The fluid flow meter as recited in claim 16 wherein a gasket
is positioned in sealing relation between said cover and said housing.
18. The fluid flow meter as recited in claim 11 wherein the first
and second means for sensing each comprise an alumina ceramic diaphragm
sensor.
19. The fluid flow meter as recited in claim 11 wherein the first
and second means for sensing each comprise a sapphire sensor.
20. The fluid flow meter as recited in claim 11 wherein the electronic
circuit includes a means for adjusting the control signal to compensate
for fluctuations in temperature within the flow circuit.
21. The fluid flow meter as recited in claim 11 wherein the electronic
circuit further includes a means for adjusting the control signal
to compensate for a pressure differential present when the rate
of flow is zero.
22. The fluid flow meter as recited in claim 11 further comprising
a chemically inert flexible membrane contained within each of said
first and second cavities, proximate the longitudinal bore of said
housing, each membrane having first and second opposed major surfaces,
said first major surface being exposed to fluid flowing in the fluid
flow conduit.
23. The fluid flow meter as recited in claim 11 wherein a conduit
extends transversely from the external surface of said housing into
at least one of said first and second spaced apart cavities.
24. The fluid flow meter as recited in claim 23 wherein said conduit
further comprises a conductive sensor positioned within said conduit
for sensing the presence of fluids within said conduit.
25. A chemically inert fluid flow meter adapted to be connected
in-line with a fluid flow circuit, comprising:
(a) a chemically inert housing having a longitudinal bore of a
first predetermined cross-sectional area extending through said
housing forming a fluid flow conduit, wherein an inlet end and an
outlet end of the conduit are adapted for connection to the fluid
flow circuit, said housing further having first and second spaced
apart cavities each extending transversely from an external surface
of said housing into the longitudinal bore of said housing, the
longitudinal bore having a constriction disposed between said first
and second cavities, the constriction having a second bore being
of a second cross-sectional area that is less than the first predetermined
cross-sectional area;
(b) a chemically inert flexible membrane contained within each
of said first and second cavities, proximate the longitudinal bore
of said housing, thereby isolating an interior of said first and
second cavities from fluid flowing in the fluid flow conduit, each
membrane having first and second opposed major surfaces, said first
major surface being exposed to fluid flowing in the fluid flow conduit;
(c) first means for sensing a first pressure within the flow circuit,
said first means for sensing being contained within said first cavity;
(d) second means for sensing a second pressure within the flow
circuit, said second means for sensing being contained within said
second cavity;
(e) means for constraining the first and second means for sensing
in a fixed position within the cavities of the housing; and
(f) an electronic circuit contained within the housing and coupled
to the first and second means for sensing, whereby the electronic
circuit receives signals proportional to the sensed first and second
pressures within the bore and thereafter produces an electrical
signal proportional to a rate of fluid flow within the bore determined
from the sensed first and second pressures.
26. The flow meter as recited in claim 25 wherein the first cavity
further comprises a first seal member positioned to sealably engage
said flexible membrane to said housing and a second seal member
aligned to sealably engage said first means for sensing to said
housing, wherein said housing further has a drain channel extending
from the first cavity of said housing to the outer surface of said
housing between said first and second seal members.
27. The flow meter as recited in claim 25 wherein the second cavity
further comprises a first seal member positioned to sealably engage
said flexible membrane to said housing and a second seal member
aligned to sealably engage said second means for sensing to said
housing, wherein said housing further has a drain channel extending
from the second cavity of said housing to the outer surface of said
housing between said first and second seal members.
28. The fluid flow meter as recited in claim 25 wherein said means
for constraining comprises two spacers and two hold down rings,
wherein each hold down ring has threads formed on an external surface
which mate with threads formed on an internal surface of each first
and second cavity, said spacers being positioned between said first
and second means for sensing and said hold down rings, whereby when
each hold down ring is screwed relative to the internal surface
of each cavity, each hold down ring presses the associated spacer
against the respective means for sensing.
29. The pressure transducer module as recited in claim 25 wherein
said housing further has a vent extending from the outer surface
of said housing into each cavity of said housing.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to flow meters, and more particularly,
to a flow meter which may be connected in-line within a chemically
corrosive fluid flow circuit in either a liquid or gaseous state,
wherein the flow meter includes two chemically inert pressure sensors
that are contained within a non-contaminating body on opposite sides
of a constriction and may be isolated from the fluid flow circuit.
2. Discussion of the Related Art
Frequently, caustic fluids are used in the processing of sensitive
materials. During the production of these sensitive materials, the
susceptibility to contamination is a significant problem faced by
manufacturers. The manufacturers may, for example, process semiconductor
wafers using caustic fluids. Various manufacturing systems have
been designed to reduce the contamination of the sensitive materials
by foreign particles and generated vapors.
The processing equipment used in the manufacturing systems include
designs that attempt to eliminate all sources of damaging contaminants.
The processing of the sensitive materials often involves the direct
contact of the sensitive materials with the caustic fluids. Hence,
it is critical that the caustic fluids are delivered to the processing
site in an uncontaminated state. Various components of the processing
equipment are commonly designed to reduce the amount of particulate
generated and to isolate the processing chemicals from contaminating
influences.
Liquid transporting systems carry the caustic chemicals from supply
tanks through pumping and regulating stations and through the processing
equipment itself. The liquid chemical transport systems, which includes
pipes, tubing, monitoring devices, sensing devices, valves, fittings
and related devices, are frequently made of plastics resistant to
the deteriorating effects of the toxic chemicals. Metals, which
are conventionally used in such monitoring devices, cannot reliably
stand up to the corrosive environment for long periods of time.
Hence, the monitoring and sensing devices must incorporate substitute
materials or remain isolated from the caustic fluids.
The processing equipment commonly used in semiconductor manufacturing
has monitoring and sensing devices, such as pressure sensors and
flow meters. These monitoring and sensing devices are connected
in a closed loop feedback relationship and are used in monitoring
and controlling the equipment. These monitoring and sensing devices
must also be designed to eliminate any contamination which might
be introduced. For example, a turbine flow meter known in the art
has moving parts that wear out and become corrupted when exposed
to the caustic fluids. Further, these turbine flow meters tend to
trap fluid that contaminate subsequent process fluids. In avoiding
the use of turbine flow meters, the monitoring and sensing devices
may incorporate sensors. These devices must also be designed to
avoid the introduction of particulate, unwanted ions, or vapors
into the processing steps.
The general use of pressure sensors in flow meters is known in
the art. For example a venturi, nozzle, orifice, or weir meter may
be used for measuring or controlling the rate of flow. A differential
sensor or pivot tube with sensors may be used, however, both these
arrangements require small ports or capillaries that can easily
plug or trap contaminants. The prior art does not disclose a flow
meter which may be positioned in-line within a fluid flow circuit
carrying corrosive materials, and which does not contaminate the
processing fluids. Hence, a need exists for a flow meter having
non-contaminating pressure sensors which may be positioned in-line
within a fluid flow circuit carrying corrosive material. Also, a
need exists for a flow meter, wherein the accuracy of the flow meter
is not affected by thermal changes within the fluid flow circuit.
Mechanical processing equipment handling fluids are often subject
to potential leakage and such leakage can create extremely hazardous
conditions, both to the processing of the sensitive materials or
other products and also to personnel who may have to tend and maintain
the processing equipment. Hence, the chemical transport system must
be designed such that leakage is avoided.
An in-line mechanical fluid pressure responsive gauge separated
from the fluid flow by a protective membrane is known in the art.
The gauge is contained within a housing having a cavity filled with
a sensor fluid. The cavity is formed adjacent the fluid flow and
separated by the protective but flexible membrane. The sensor fluid
contained within the cavity is typically a silicone oil. A change
in pressure within the fluid affects the silicone oil pressure within
the cavity. The oil pressure is detected by the mechanical pressure
responsive gauge.
The fluid within the cavity typically has large coefficient of
thermal expansion, which may cause a significant deflection in the
membrane. The large deflection changes in the protective membrane
increases the likelihood that the fluid within the cavity will leak
into the fluid flow, contaminating the flow circuit. Also, the accuracy
of the pressure gauge is negatively affected by the large thermal
expansions of the sensor fluid. Hence, a need exists for an in-line
pressure gauge that does not leak contaminating fluids into the
fluid flow circuit. Also, a need exists for a pressure gauge, wherein
the accuracy is not affected by thermal changes within the fluid
flow circuit.
Collins et al., in U.S. Pat. No. 5316035 (the '035 patent) describes
the use of a capacitance proximity monitoring device in corrosive
atmosphere environments. In one embodiment of the '035 patent, the
capacitance proximity device is described as being incorporated
into a functional apparatus, such as a piping system including valves
and couplings. The capacitance proximity device serves as a functional
portion of the apparatus and creates a sensing region within a predetermined
area. It is then used to determine the change of electrical characteristics
within the predetermined area as various fluids flow past the predetermined
area. Monitoring changes in the current related to the sensing field
when the liquid target media is present, versus air or gas in the
piping when the liquid target media is absent, thereby produces
an indication of the presence or absence of the target media. The
complex valving is used to control fluid flow and the possibility
exists that the fluid will leak and contaminate the processing fluid
flow.
None of the above identified prior art discloses or even considers
a device capable of determining the fluid flow rate within the caustic
chemical transport system of chemical processing equipment. Further,
none of the above identified prior art discloses a device that determines
either or both the fluid flow rate and the pressure within the fluid
flow. Monitoring the fluid flow within the chemical transport system
is useful for several reasons. First, a change in flow within the
system may be indicative of leakage within the system. Second, the
flow within the transport system is regulated to avoid exceeding
predetermined safety limits. Third, a change in fluid flow may indicate
an obstruction or infiltration of contaminants into the fluid flow
circuit.
Therefore, a need exists for a non-contaminating fluid flow meter
which may be positioned in-line within a fluid flow circuit carrying
corrosive materials, wherein the flow meter determines the rate
of flow based upon a pressure differential measurement taken in
the fluid flow circuit, wherein the determination of the rate of
flow is not adversely affected by thermal changes within the fluid
flow circuit. A need also exists for a flow meter that avoids the
introduction of particulate, unwanted ions, or vapors into the flow
circuit. The present invention addresses these needs.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide a flow meter
that may be coupled in-line to a flow circuit transporting corrosive
fluids, where the rate of flow may be determined from a differential
pressure measurement taken within the flow circuit. The flow meter
includes two pressure sensors contained within a non-contaminating
body, wherein the pressure sensors are separated in the flow circuit
by a constricting member.
The flow meter compensates for changes of temperature within fluid
flow circuit and provides a zeroing feature which compensates for
differences in pressure between the two sensors when the fluid is
at rest. In the preferred embodiment, the components of the flow
meter include a housing, a cover, an electrical connector, pressure
fittings, isolation membranes, sealing rings, two pressure sensors,
a circuit board and electronic circuitry, spacer rings and hold
down rings.
The flow meter's housing has a bore extending therethrough, which
forms a passage or conduit through which fluids flow, when the housing
is connected in-line in a fluid flow circuit. Aligned and sealably
connected to the opposed open ends of the bore are pressure fittings.
The pressure fittings are constructed from a chemically inert material
and are readily available and known to those skilled in the art.
The housing also has two pressure transducer receiving cavities
extending from an external surface thereof, wherein each such cavity
communicates independently with the bore. In the preferred embodiment,
the bore tapers to a constricting region located between the two
cavities. The restricted region results in a pressure drop within
the bore across points adjacent the two cavities. This change in
pressure may be detected by pressure sensor transducers placed within
each of the two cavities. The rate of flow is determined from the
change in pressure. The determination of the rate of flow using
the two pressure sensors is discussed below.
An isolation membrane, pressure sensor, sealing members, spacer
ring and hold down ring are contained within each cavity of the
housing. These components and variations thereof are discussed in
co-pending application Ser. No. 08/538478 filed Oct. 3 1995
and assigned to the same assigns as the present application, the
entire disclosure of which is incorporated herein by reference.
A hybrid or fully integrated electronic circuit disposed in the
housing is operatively coupled to both pressure sensor transducers
and to an electrical connector contained in the cover. The electronic
circuit develops a signal which is a measure of the rate of flow
within the flow circuit from information sensed by both pressure
sensors. Further, the electronic circuit may develop a signal corresponding
to one or the other of the downstream or upstream static pressures
within the fluid flow circuit, such that the orientation of the
flow meter within the flow circuit is interchangeable and the direction
of flow may be indicated by comparing the sensed pressure from each
pressure sensor. When sensing the static pressures of gases flowing
through the flow circuit, a correction may be made to the sensed
pressures to correct for non-linearities as a result of gas density
differences.
This electronic circuit may also be used in combination with temperature
sensitive components to adjust the pressure measurement associated
with each cavity based upon temperature changes within the flow
circuit. Further, a switch may be incorporated into the electronic
circuit that allows zeroing of the flow meter by the user.
The electronic circuit is coupled by electrical leads to the electrical
connector and power may be transmitted to the electronic circuit
through the electrical leads mating at the connector with an external
power supply. Further, an analog output such as a standard 4-20
milliamps signal proportional to the calculated rate of flow may
be transmitted through additional electrical leads.
In an alternate embodiment, the housing comprises two symmetric
housing halves. Each housing half includes a longitudinal bore and
counter bore for receiving a restriction member therein. In another
embodiment, a portion of the longitudinal bore disposed between
the two cavities has a third cavity which receives a removable insert.
The insert has a channel extending therethrough, thereby forming
a passage from one bore section to the other. In yet another alternate
embodiment, a bushing is friction fit within the bore between the
two cavities to thereby create the constriction. In a further alternate
embodiment, inert sapphire pressure transducers are positioned within
respective cavities and in direct contact with the fluid flow, thereby
eliminating the isolation membrane.
OBJECTS
It is accordingly a principal object of the present invention to
provide a non-contaminating flow meter adapted to be connected in-line
in a fluid flow circuit.
Another object of the present invention is to provide a flow meter,
wherein inert pressure sensor components of the flow meter may be
situated in direct contact with the fluid flow.
Yet another object of the present invention is to provide a flow
meter having isolation members that are in direct contact with the
associated pressure sensors, the isolation members acting to isolate
the sensors and associated electronic circuitry from potentially
corrosive processing chemicals and precluding introduction of contaminating
substances into the processing fluids being transported.
Still another object of the present invention is to provide a flow
meter, wherein a pressure of the flow circuit is measured non-intrusively
at two independent points within the flow circuit to thereby determine
the rate of flow within the flow circuit.
A further object of the present invention is to provide a non-contaminating
flow meter that compensates for initial pressure differences between
the two pressure sensor transducers when the flow rate is zero.
Yet another object of the present invention is to provide a non-contaminating,
chemically inert flow meter that determines either the rate of flow
or pressure within the fluid flow circuit.
These and other objects, as well as these and other features and
advantages of the present invention will become readily apparent
to those skilled in the art from a review of the following detailed
description of the preferred embodiment in conjunction with the
accompanying drawings and claims and in which like numerals in the
several views refer to corresponding parts.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of the flow meter made in accordance
with the present invention;
FIG. 2 is a top plan view of the flow meter of the type shown in
FIG. 1;
FIG. 3 is a partial sectional side elevational view of the flow
meter of the type shown in FIG. 1;
FIG. 4 is an enlarged, partial sectional side elevational view
of an alternate embodiment of the flow meter with the cover and
electric circuit removed;
FIG. 5 is an enlarged, partial sectional end elevational view of
an alternate embodiment of the flow meter;
FIG. 6 is an exploded view of the flow meter shown in FIG. 5;
FIG. 7 is a side elevational view of an alternate embodiment of
a flow meter in accordance with the present invention;
FIG. 8 is a partial sectional side elevational view the flow meter
of the type shown in FIG. 7 with the pressure transducer, electronic
circuit, isolation membrane hold down ring, spacer ring and seals
removed for clarity;
FIG. 9 is a top plan view of the flow meter of the type shown in
FIG. 8 with the cover and electric circuit removed;
FIG. 10 is an enlarged sectional view of a restriction member used
in the embodiment of FIG. 8;
FIG. 11 is a partial sectional side elevational view of another
embodiment of a flow meter housing in accordance with the present
invention; and
FIGS. 12 and 13 together is a schematic diagram of the electric
circuit used in a flow meter in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIGS. 1 and 2 the flow meter is generally identified
by numeral 10. The flow meter 10 generally includes a housing or
body 12 mounting fastener slots 14 pressure inlet/outlet fittings
16 and 18 and a cover 20. An electrical connector 22 of known
construction, may be removably attached in the cover 20. The housing
12 and cover 20 are preferably manufactured from a chemically-inert,
non-contaminating polymer such as polytetrafluoroethylene (PTFE).
The cover 20 has bores 24 extending through it for mounting the
cover 20 to the housing 12 with appropriate screws (not shown).
A suitable gasket (not shown) is preferably positioned between the
cover and housing to allow the cover 20 to be sealed to the housing
12. Without any limitation intended, a gasket or seal manufactured
from a multi-layer fabric, sold under the GOR-TEX.RTM. trademark
by W. L. Gore & Assoc., Inc., allows venting of an internal
area of the housing 12 for true atmospheric pressure reference,
while restricting the flow of liquids into the internal area of
the housing 12.
Referring to FIGS. 3-6 the internal construction of the flow meter
10 is shown. A longitudinal bore 26 extends through the housing
12 forming a conduit. Thus, when the flow meter 10 is connected
in-line with a fluid flow circuit (not shown), via pressure fittings
16 and 18 the bore 26 serves as the fluid flow passage within the
fluid flow circuit. The orientation of the flow meter 10 within
the fluid flow circuit, may be reversed without affecting its effectiveness.
First and second transversely extending cavities 28 and 30 extend
all the way from an outer surface 32 of the housing 12 to the bore
26. Those skilled in the art will appreciate that cavities 28 and
30 may each extend into the housing from a different sidewall of
the housing. The two cavities 28 and 30 are separated a predetermined
distance by dividing wall 34. The bore 26 also includes a constriction
or restriction 35 located between the two cavities 28 and 30. Near
the region within the housing where each cavities 28 and 30 and
bore 26 intersect, an annular lip 36 is formed. Each lip 36 surrounds
and further defines the opening to each cavity 28 and 30 from the
bore 26.
A thin flexible polymer disk or isolation membrane 38 is positioned
on the lip 36 of each cavity 28 and 30. Without limitation, the
membrane is preferably constructed to have a thickness in a range
between 0.001 and 0.040 inches. The upper surface of membrane 38
is abraded so as to create a pattern of grooves or channels. Preferably,
the flexible membrane 38 is manufactured from tetrafluoroethylene
fluorocarbon polymers. One such tetrafluoroethylene fluorocarbon
polymer is sold under the TEFLON.RTM. trademark by E. I. dupont
Nemours.
The isolation membranes 38 may have a thin film formed on its surface,
to act as a buffer to any caustic chemicals leaking into the pressure
transducer cavities. The thin film may further act as an electric
shield for capacitance sensors, thereby obviating inaccuracy problems
resulting from changes in dielectric properties as fluids flow through
the flow meter. The thin film may for example be a carbon powder
and epoxy ink painted onto the isolation membrane 38 or a thin film
of carbon may be surface molded into the isolation membrane. The
coated disk membrane 38 is preferably molded, since spraying or
manufacturing by some other process may leave pinhole paths therein.
Alternatively, the isolation membrane may be reinforced with carbon
fibers, thereby increasing the elastic behavior of the isolation
membrane 38 and reducing the cold flow tendencies of the PTFE isolation
membrane.
When the flow meter 10 is fully assembled, the annular surface
contact between each flexible membrane 38 and each cavity's lip
36 is such that a hermetic seal is formed therebetween. Various
features of each lip 36 and isolation membrane 38 without any limitation
intended, may be altered as described in the aforementioned co-pending
application Ser. No. 08/538478.
Each pressure transducer 42 and 44 is held in place within their
respective cavities 28 and 30 by spacer ring 48 and externally threaded
hold down ring 50. The isolation membranes 38 and transducers 42
and 44 are sealed within the housing 12 by chemically inert o-ring
seals 52 and 54. A redundant seal is created by the positioning
of o-rings 52 and 54. The seals 52 and 54 are readily available
and of known construction to those skilled in the art. An additional
spacer ring 56 (see FIG. 4) may be necessary, depending upon the
dimensions of the pressure transducer.
A drain or conduit 40 shown in FIGS. 5 and 6 may be formed extending
through the housing wall 32 into each cavity 28 and 30 between the
redundant seals 52 and 54 thereby draining the area between the
redundant seals. In this manner, the drain acts as a drainage, passageway
or outlet, in the event that fluids leak past seal 52 from the fluid
flow circuit. A sensor 41 is positioned within the drain 40 and
is electrically connected (by leads not shown) to the electric circuit
46. Those skilled in the art will appreciate that a conductive sensor,
capacitive sensor or non-electric fiber optic sensor may equally
be used to sense the presence of fluids in the drain 40. When fluid
leaks past the first seal, the fluid activates the sensor 41 thereby
transmitting a signal to the electric circuit 46 which subsequently
sets off an alarm.
The redundant sealing arrangement helps prevent exposure of the
pressure transducers 42 and 44 and electric circuit 46 from the
damaging affects of the caustic fluids. The redundant seal also
further isolates the fluid flow, thereby reducing the potential
contamination of the fluids. Additionally, a tube (not shown) may
be connected to the drain 40 to thereby carry away the caustic
fluids to a non-contaminating area.
Referring again to FIGS. 3 and 4 the pressure sensors 42 and 44
are positioned on top of the associated flexible isolation membrane
38. Each pressure sensor may be of a capacitance type or piezoelectric
type known to those skilled in the art. The base of each pressure
sensor is in direct contact with the membrane 38 and may be either
in pressure contact with or bonded to the membrane by an adhesive,
thermal welding or by other known means. The base may have a carbon
film formed thereon, to act as an additional shield against potential
leakage and further to act as electric shield.
In one embodiment, an alumina ceramic pressure sensor may be used,
wherein the alumina ceramic pressure sensor comprises a thin, generally
compliant ceramic sheet having an insulating spacer ring sandwiched
between a thicker, non-compliant ceramic sheet. The first thin ceramic
sheet or diaphragm is approximately 0.005 to 0.050 inches in thickness
with a typical thickness of 0.020 inches. The thicker ceramic sheet
has a thickness range between 0.100 to 0.200 inches. The spacer
ring may be constructed of a suitable material such as a glass,
polymer or alternatively the ceramic sheets may be brazed together.
The opposed faces of ceramic disks are metalized by metals such
as gold, nickel or chrome to create plates of a capacitor. A similar
capacitive pressure transducer is described by Bell et al. in U.S.
Pat. No. 4177496 (the '496 patent). Other capacitive pressure
transducers similar to that described in the '496 patent are available
and known in the art.
It is contemplated that the flexible membrane 38 could be eliminated
if the pressure sensor used is of the sapphire capacitive pressure
transducer type. A sapphire transducer is inert, and is resistant
to wear when subjected to caustic fluids. Having a sapphire sensor
in direct communication with the fluid flow, further enhances the
pressure measurements of each transducer.
Referring next to FIGS. 7-9 an alternate embodiment of the flow
meter's housing 12 is shown. The housing 12 is split into two sections
or halves 58 and 60 wherein the downstream section 58 has a cavity
62 and longitudinal bore 66 and upstream section 60 has a cavity
64 and longitudinal bore 68 formed therein. The longitudinal bores
66 and 68 of each section 58 and 60 respectively, have respective
counter bores 70 and 72. The two sections 58 and 60 are aligned
and engaged, such that the counter bores 70 and 72 are aligned,
thereby forming a hollow cavity in which a restriction member 74
is inserted (see FIGS. 8 and 9). The restriction member 74 has a
central opening 76 extending therethrough (see FIG. 10), which is
aligned with the longitudinal bores 66 and 68. The restrictions
central opening 76 is smaller in diameter than either section's
longitudinal bores 66 and 68. The restriction member 74 is sealably
engaged with each housing section's bore 66 and 68 by chemically
inert sealing o-rings known in the art. Without any limitation intended,
the restriction member 74 and associated sealing rings are preferably
constructed of polytetrafluoroethylene.
Referring now to FIG. 11 yet another alternate embodiment of the
housing 12 is shown. In this embodiment, a third cavity 78 extends
from a bottom outer surface of the housing 12 and into communication
with the bore 26. The third cavity 78 is formed midway between the
first and second cavities 28 and 30. A replaceable insert or plug
80 is inserted and sealed within the third cavity 78. The plug 80
has an opening or channel 82 (not shown) extending therethrough,
wherein the channel 82 is aligned with the longitudinal bore 26
thereby forming a passage from one bore section to the other. It
is contemplated that the third cavity 78 may extend from a top outer
surface of the housing 12. In this arrangement, once the cover 20
is sealed to the housing 12 external access to the third cavity
78 would be limited.
The diameter of the channel 82 is less than the diameter of either
bore section 26 thereby creating the constriction or restricted
region. The plug 80 engages with the third cavity 78 by chemically
inert o-rings 84 known in the art. Without any limitation intended,
the plug 74 and sealing rings 84 are constructed of polytetrafluoroethylene,
wherein the sealing rings 84 seal the plug 74 within the third cavity
78. The plug may alternatively be constructed of sapphire, a material
resistant to wear from the fluid flow.
In another alternate embodiment the constriction 35 is formed by
a bushing that is friction fit between the cavities 28 and 30. Of
course, the bushing includes a bore of smaller cross-section than
longitudinal bore 26 which interconnects the remaining bore sections.
Those skilled in the art will appreciate that the constriction 35
may be created by a variety of forms including those discussed above.
The constriction may be constructed of sapphire, thereby extending
the longevity of the flow meter. Those skilled in the art will appreciate
that sapphire is an inert substance, highly resistant to wear from
caustic flowing fluids.
Those skilled in the art will appreciate that the cavities and
housings of each embodiment are dimensioned to universally accept
the same pressure fittings 16 and 18 cover 20 isolation membrane
38 pressure sensors 42 and 44 electronic circuit 46 spacer ring
48 and hold down ring 50. Also, each embodiment has a constricting
area of varying construction between the sensor receiving cavities
to create a pressure drop as the fluid flow traverses the restriction.
Referring again to FIG. 3 the electronic circuit module 46 is
positioned above the ceramic pressure transducers 42 and 44 and
is electrically coupled to the conductive surfaces of the ceramic
pressure transducers 42 and 44. The electronic circuit module 46
is also connected by suitable leads (not shown) to internal contacts
of the connector 22 (FIG. 1). In the preferred embodiment the electrical
connector 22 is made of a chemically inert material and preferably
may be of a type available from Pneumatico, part number po3rsd-00004-24.
Referring to FIGS. 12 and 13 there is shown an electrical circuit
schematic diagram of the electronic circuit module 46. The electronic
circuit is used to convert the pressure readings from the two pressure
transducers 42 and 44 to a 4-20 mA analog representation of flow
or, alternatively, a pressure reading of the downstream pressure
transducer 44. The raw analog signal from the upstream transducer
42 is supplied to input terminal 102 and, likewise, the raw analog
transducer output signal from the downstream transducer 44 is supplied
to the input terminal 104. Terminals 106 and 108 are power input
terminals and terminals 110 and 112 are connected to the ground
bus 114 (see FIG. 12).
Connected between the +5 volt power bus 116 and the ground bus
114 is an analog temperature compensation chip 118 which may preferably
comprise a type LM 45 device available from National Semiconductor,
Inc. The temperature compensation chip 118 has its v.sup.+ terminal
connected by conductor 120 to the +5v bus 116 and its v.sup.- input
connected through a semiconductor diode 122 to the ground bus 114.
The diode 122 provides an offset, so that the signal proportional
to temperature produced on output terminal 124 of the temperature
compensation chip 118 can go below 0.degree. C., i.e., assuming
a negative value.
The raw sensor signals produced on input terminals 102 and 104
together with the temperature compensation signal produced at terminal
124 are individually applied to a four channel sigma delta type
analog to digital (A/D) converter chip 126. The chip 126 may preferably
comprise an AD7714 integrated circuit chip supplied by Analog Devices
Corporation. Those wishing details of the mode of operation of that
integrated circuit are referred to the data sheets available from
Analog Devices Corporation.
The sigma delta A/D converter 126 includes a digital filtering
capability for the analog pressure inputs where the cut off frequency
of the low pass filter is a programmable quantity set by the software
executed in the microprocessor chip 128. Without limitation, the
microprocessor 128 may comprise a type PIC 16C73 integrated circuit
available from Microchip Technology Corporation. The resistive voltage
divider including resistors 130 and 132 which are connected between
the positive bus 116 and the ground bus 114 provide voltage compensation
when the pressure data is being linearized.
The A/D chip 126 provides its serial output data stream on line
134 to a data input terminal 136 on the microprocessor 128. The
serial data from the A/D converter chip 126 is clocked out, under
control of timing signals provided by a crystal controlled clock
circuit indicated generally by numeral 138. This clock circuit 138
also provides timing pulses over line 140 to the clock input terminal
142 of the microprocessor chip 128 for controlling its timing.
The microprocessor 128 is programmed to compute the instantaneous
pressure differences being picked up by the upstream and downstream
transducers 42 and 44 and to perform any necessary zeroing adjustments
and scaling. A switch 143 is connected to the microprocessor 128.
When switch 143 is switched to the on position, the difference in
pressure between the two sensors 42 and 44 is calculated. This value
is then stored. Typically, the user will activate switch 143 to
test mode when there is no fluid flow. Any difference in pressure
during no fluid flow will be stored in the microprocessor. The user
then de-activates the switch 143. During fluid flow, the stored
value is subtracted from the difference in pressures, thereby performing
the zeroing adjustment. A test is also made to determine whether
the thus scaled pressure difference is above or below preestablished
high/low limits. If the pressure difference is above or below the
pre-established high/low limits, an alarm is activated. When the
pressure difference is within the preestablished limits, the rate
of flow is computed.
It is known that, in steady-state flow, the flow rate is the same
at any point. The flow rate (I) may be expressed as I.sub.m =.rho.vA.
Where .rho. represents the density of the fluid, v represents the
velocity of the fluid, and A represents the area through which the
fluid travels. Using the continuity equation A.sub.1 v.sub.1 =A.sub.2
v.sub.2 the rate of flow within the flow meter 10 may be found
to be equal to a constant multiplied by the .sqroot.P.sub.1 -P.sub.2
. The microprocessor 128 thus computes the rate of flow from the
data received from the two pressure sensors. Those skilled in the
art will recognize that with laminar flow, the rate of flow approximates
more closely a constant multiplied by P.sub.1 -P.sub.2. Hence, a
low flow limit could be built into the system, such that if the
"Reynolds number" is below a certain threshold, the flow
meter identify the flow rate as zero.
FIG. 13 illustrates the circuitry used to convert the rate of flow
computed by the microprocessor 128 into an analog signal falling
in the range of from 4 mA to 20 mA for use by existing analog control
systems. That is to say, the digital value of flow computed by the
microprocessor 128 is converted to an analog signal whose current
amplitude is directly proportional to the computed flow value and
is in the range between 4 mA and 20 mA.
In addition to providing an analog current in the 4 mA to 20 mA
range indicative of flow, the system may also be used to provide
a 4 mA-20 mA current signal proportional to the pressure sensed
by the downstream transducer 44 or upstream transducer 42. More
particularly, as seen in FIG. 13 the circuitry is partitioned into
substantially identical upper and lower channels where the 4 mA
to 20 mA current signal proportional to flow becomes available across
the output terminals 144 and 146 and the 4 mA-20 mA current signal
proportional to pressure becomes available across the output terminals
148 and 150.
Referring back to FIG. 12 the microprocessor 128 provides a clocking
signal on line 152 which connects to a corresponding line 152 in
FIG. 13. Likewise, first and second data output lines 154 and 156
respectively, coming from the microprocessor 128 connect to the
corresponding lines 154 and 156 at the left hand side of the schematic
FIG. 13. Signals for determining which of the two channels in FIG.
13 is to be operative is also provided by way of a digital to analog
converter chip select signal emanating from the microprocessor 128
on line 158. This signal is provided to an opto isolator circuit
160 and 162 whose output goes to the "chip select" terminal
on either the digital-to-analog (D/A) converter chip 164 or the
digital-to-analog chip 166. In each case, the D/A converters 164
and 166 may comprise a 12 bit device, such as a type MAX538 D/A
converter chip available from Maxim Corporation.
As can be seen from FIG. 13 the clock signals on line 152 as
well as the data signals on lines 154 and 156 are also optically
isolated via opto couplers 168 170 172 and 174 with the resulting
signals being applied to the respective D/A converters.
The circuitry to the right of the vertical-line 176 functions to
convert the analog signal output, from either the digital to analog
converter 164 or the digital to analog converter 166 to a current
signal in the range of from 4 mA to 20 mA depending upon the amplitude
of the voltage output from the D/A converters 164 and 166. As can
be seen, the output from the D/A converter 164 is coupled through
a resistor 178 to the non-inverting input of an operational amplifier
180. The inverting input thereof is connected to ground 194. The
output of the operational amplifier 180 is connected to the gate
electrode of a FET device 182 as are bias resistors 184 and 186.
A voltage reference for the FET device 182 and for the D/A converter
164 is obtained by means of series connected diodes 188 and 190
which are connected in series across the positive voltage bus 192
and the ground bus 194. In that the voltage to current converter
circuitry associated with the D/A converter 166 illustrated to the
right of the vertical line 176 is substantially identical to what
has already been described in association with the D/A converter
164 it is deemed unnecessary to repeat that description.
The output lines 196 and 198 shown coming from the microprocessor
128 in FIG. 12 are applied via corresponding numbered lines in
FIG. 13 to an opto coupler 200. The output from the opto coupler
200 indicates that power is present on lines 144 and 146 and also
on lines 148 and 150.
Referring back to FIG. 12 the microprocessor chip 128 has associated
with it a RS232 serial port indicated generally by numeral 202.
As such, the flow meter device of the present invention is capable
of communicating with a variety of peripheral devices including
a further central processing unit (not shown). The electronic circuit
46 may also adjust the pressure and flow output as the temperature
within the flow circuit changes by including a thermistor or like
component therein. Each pressure transducer is corrected for temperature
independently. One means of temperature compensation is disclosed
in U.S. Pat. No. 4598381.
In use, the user couples the flow meter 10 into a fluid flow circuit
through pressure fittings 16 and 18. As fluid flows through the
flow circuit, the pressure adjacent each of the two cavities is
detected by the electric circuit 46 whereby the rate of flow is
calculated from the two detected pressures. The gauge pressure or
absolute pressure may equally be used. From the determination of
the flow rate, an alarm is activated if the flow rate or downstream
pressure increases or decreases above or below predetermined limits,
or the processing equipment is turned off.
Those skilled in the art will recognize that the flow rate may
be calibrated so that minimum desired output values are associated
with minimum pressure and maximum desired output pressures are associated
with maximum pressure. For example, a pressure sensor intended to
measure 0 to 100 psig (pounds per square inch gauge) can be calibrated
to read 4 mA (milliamps) at 0 psig and 20 mA at 100 psig.
By providing the inert Teflon isolation membrane in intimate contact
with the pressure sensors, the working fluid does not contact the
surfaces of the sensor which could lead to contamination. The sealing
arrangements disclosed, ensures that the working fluid does not
enter the cavities of the housing 12 and adversely affect the electronic
circuitry 46.
This invention has been described herein in considerable detail
in order to comply with the patent statutes and to provide those
skilled in the art with the information needed to apply the novel
principles and to construct and use such specialized components
as are required. However, it is to be understood that the invention
can be carried out by specifically different devices, and that various
modifications, both as to the equipment details and operating procedures,
can be accomplished without departing from the scope of the invention
itself. |