A mass flow meter employs discrete chip-type temperature sensors
to sense a fluid flow rate. The sensor can be a semiconductor chip
such as SiC or silicon, or thin film tungsten on an AlN substrate.
The sensors can be distributed symmetrically with respect to the
conduit through which the fluid flows, and can be connected in a
four-sensor bridge circuit for accurate flow rate monitoring. An
output from the mass flow meter can be used to control the fluid
1. A mass flow meter (MFM) structure, comprising: a conduit for
conducting a fluid flow, and at least four mutually spaced temperature
sensors disposed to sense the temperature of a fluid flowing within
said conduit, said sensors connected in a 4-sensor bridge circuit
to sense the mass flow rate of a fluid flowing through said conduit,
wherein said sensors are mounted to said conduit by respective first
layers on said sensors and conduit of a material selected from the
group comprising TiW and Ni, and respective second layers on said
sensors and conduit of Au, with said sensor and conduit second layers
bonded to each other.
2. The MFM structure of claim 1 wherein said sensors are discrete
and are distributed symmetrically with respect to said conduit.
3. The MFM structure of claim 1 said sensors comprising semiconductor
4. The MFM structure of claim 3 said sensors comprising SiC chips.
5. The MFM structure of claim 4 further comprising a SiC oxide
interfacing between said SiC chips and said conduit.
6. The MFM structure of claim 3 said sensors comprising silicon
7. The MFM structure of claim 6 further comprising a silicon oxide
interfacing between said silicon chips and said conduit.
8. The MFM structure of claim 1 further comprising an electrically
insulative film enclosing said sensors, and a circuit on the exterior
of said film and extending through the film to contact said sensors.
9. The MFM structure of claim 1 wherein said sensors comprise
semiconductor chips, further comprising an oxide of said semiconductor
interfacing between said sensors and said sensor first layers.
10. The MFM structure of claim 1 said sensors comprising thin
film tungsten layers on respective AlN substrates.
11. The MFM structure of claim 1 said sensors comprising a pair
of upstream sensors distributed symmetrically with respect to said
conduit at an upstream location, and a pair of downstream sensors
distributed symmetrically with respect to said conduit at a downstream
12. The MFM structure of claim 11 said bridge circuit including
extended leads between said upstream and downstream sensors long
enough to be substantially non-thermoconductive.
13. The MFM structure of claim 1 wherein said sensors include
respective AlN substrates that are mounted to said conduit.
14. The MFM structure of claim 1 further comprising electronic
circuitry for actuating said sensors and determining the mass flow
rate of a fluid flowing through said conduit from said sensors.
15. The MFM structure of claim 14 wherein said electronic circuitry
operates without amplification of the sensor outputs.
16. The MFM structure of claim 14 further comprising a control
valve governing the fluid flow through said conduit under the control
of said circuitry.
17. The MFM structure of claim 1 wherein said sensors are mounted
inside said conduit on protective shields and protected from the
environment within the conduit by said shields.
18. A mass flow meter (MFM) comprising: a conduit for conducting
a fluid flow, at least one temperature sensor disposed to sense
the temperature of a fluid flowing through said conduit, each sensor
comprising an AlN substrate bearing a temperature sensing circuit,
and electronic circuitry for actuating said sensors and determining
from said sensors the mass flow rate of a fluid flowing through
said conduit, wherein each said sensor is mounted to said conduit
by respective first layers on said sensor and conduit of a material
selected from the group comprising TiW and Ni, and respective second
layers on said sensor and conduit of Au, with said sensor and conduit
second layers bonded to each other.
19. The MFM of claim 18 said temperature sensing circuits comprising
respective thin film tungsten layers on said AlN substrates.
20. The MFM of claim 18 wherein each AlN substrate is mounted
to the outer surface of said conduit to conduct heat from said conduit
to its respective temperature sensing circuit.
21. The MFM of claim 18 further comprising a control valve governing
the fluid flow through said conduit under the control of said circuitry.
22. A fluid mass flow meter (MFM), comprising: a conduit for conducting
a fluid flow, at least one discrete chip-type temperature sensor
carried by to said conduit to sense the temperature of a fluid within
said conduit, and electronic circuitry for actuating said at least
one sensor and sensing the mass flow rate of a fluid flowing through
said conduit from said at least one sensor, wherein each said sensor
is mounted to said conduit by respective layers on said sensor and
conduit of a material selected from the group comprising TiW and
Ni, and respective second layers on said sensor and said conduit
of Au, with said sensor and conduit second layers bonded to each
23. The MFM of claim 22 each said sensor comprising a semiconductor
24. The MFM of claim 23 each said semiconductor chip comprising
a SiC chip.
25. The MFM of claim 24 further comprising a SiC oxide interfacing
between each SiC chip and said conduit.
26. The MFM of claim 23 each said semiconductor chip comprising
a silicon chip.
27. The MFM of claim 26 further comprising a silicon oxide interfacing
between each silicon chip and said conduit.
28. The MFM of claim 22 each said sensor comprising a thin film
tungsten layer on a respective AlN substrate.
29. The MFM of claim 22 further comprising an electrically insulative
film enclosing each said sensor, and a circuit on the other side
of said film and extending through the film to contact each sensor.
30. The MFM of claim 22 wherein each said sensor comprises a respective
semiconductor chip, further comprising an oxide of said semiconductor
interfacing between said sensor and said first sensor layer.
31. The MFM of claim 22 wherein said electric circuitry senses
the temperature within said conduit as a function of the sensor
32. The MFM of claim 22 wherein each said sensor is mounted to
the outer surface of said conduit in thermal communication with
a fluid flowing through the conduit.
33. The MFM of claim 22 wherein each said sensor is mounted within
a respective opening in a wall of said conduit.
34. The MFM of claim 22 wherein each said sensor is mounted to
an inner surface of said conduit.
35. The MFM of claim 22 wherein each said sensor is mounted inside
said conduit on a protective shield and protected by said shield
from the environment within said conduit.
36. The MFM of claim 22 wherein each said sensor projects into
the interior of said conduit.
37. The MFM of claim 22 said at least one temperature sensor comprising
a plurality of temperature sensors that are symmetrically arranged
with respect to said conduit.
38. The MFM of claim 22 said at least one temperature sensor comprising
a plurality of temperature sensors that are electrically connected
in a bridge circuit.
39. The MFM of claim 38 said bridge circuit incorporating a pair
of upstream temperature sensors and a pair of downstream temperature
40. The MFM of claim 22 further comprising a control valve governing
the fluid flow through said conduit under the control of said circuitry.
41. A method of forming a temperature sensor, comprising: bonding
at least one discrete chip-type temperature sensor to a conduit
by respective first layers on each said sensor and said conduit
of a material selected from the group comprising TiW and N, and
respective second layers on each said sensor and said conduit of
Au, with said sensor and conduit second layers bonded to each other,
and electrically connecting each said sensor to sense the temperature
of a fluid flowing through said conduit.
42. The method of claim 41 wherein each said temperature sensor
is bonded to said conduit through a thermally conductive insulator.
43. The method of claim 41 wherein multiple temperature sensors
are bonded to said conduit at symmetrical locations with respect
to said conduit.
44. The method of claim 43 wherein a pair of upstream and a pair
of downstream temperature sensors are bonded to said conduit, with
each pair symmetrically arranged with respect to said conduit.
45. The method of claim 44 said electrically connecting step comprising
connecting said upstream and downstream temperature sensors in a
4 sensor bridge network.
46. The method of claim 41 further comprising using said sensed
temperature as an indication of the mass flow rate of a fluid flowing
through said conduit, and controlling said fluid flow rate as a
function of the indicated mass flow rate.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to mass flow meters, and more particularly
to chip-type temperature sensors and four-sensor bridge circuits
for mass flow meters.
2. Description of the Related Art
Numerous different methods are employed to measure the flow rate
of gases and liquids. They can generally be divided into two categories:
those that measure volumetric flow, and those that measure mass
An example of a volumetric flow meter is a tapered tube through
which the gas or liquid travels, displacing a float in the tube.
When there is no flow, the float rests at the bottom of the tube,
sealing its narrower end. As the fluid flows through the tube, the
float rises proportionally to the volumetric fluid flow.
A principal problem with volumetric flow meters relates to the
measurement of gas flow rate. Changes in the pressure or temperature
of the gas can cause inaccuracies in the flow measurements.
Mass flow meters (MFMs) are conventionally used to operate a valve
which controls the flow rate of a fluid through a conduit; the combined
MFM and valve is referred to as a mass flow controller (MFC). These
devices are used in many systems requiring precise control of gas
or liquid flow rate, such as in the semiconductor processing industry
to deliver gases whose atoms are used to grow or dope semiconductor
materials, where gas flow rates are crucial yield parameters. MFCs
for the semiconductor industry are discussed in general in "Results
from the workshop on Mass flow measurement and control for the semiconductor
industry", National Institute of Standards and Technology (NIST),
on May 15-16 2000 results published Jul. 20 2000. An advantage
of MFCs over volumetric flow measurement is that mass flow is less
susceptible to accuracy errors due to variations in line pressure
and temperature. Known types of MFCs include immersible thermal
MFCs, thermal MFCs, and differential pressure MFCs.
Thermal MFCs are the most commonly used type of MFC in the semiconductor
processing industry. They can be made from relatively inexpensive
components, and provide a good compromise between price and performance.
With immersible MFCs, one or more sensors are located directly in
the flow stream, while with capillary tube MFCs a capillary tube
parallels the main fluid conduit, and one or more sensors are provided
on the outside of the tube.
In immersible thermal MFCs, an immersed temperature sensor also
acts as a heater, heating up as electric current passes through
it. The temperature sensor remains at some known constant temperature
when the fluid is not flowing. A flowing fluid reduces the sensed
temperature, due to the fluid's carrying heat away from it. The
magnitude of the sensed temperature drop is proportional to the
fluid's mass flow rate. The sensor may be encapsulated for applications
where there is a concern about the sensor material contaminating
the flowing fluid stream, or itself being contaminated by the fluid.
In an alternate submersible thermal MFC, a heater is immersed upstream
and a temperature sensor downstream. The amount by which the fluid
temperature at the sensor location rises due to operation of the
upstream heater can be correlated with the fluid's mass flow rate.
In capillary tube thermal MFCs, a known fraction of the incoming
flow stream is directed through a heated capillary tube, while the
remainder of the flow stream by-passes the capillary tube. The tube
is heated by metal wire that is wound around its outer surface at
an upstream location, with a temperature sensing winding at a downstream
location. Platinum wire is typically used because its resistance
change, as a function of temperature, is well known, allowing it
to act as both a heater and a temperature sensor. Some MFC manufacturers
use thin film platinum resistance temperature devices, consisting
of a thin layer of platinum on a thin film insulator (typically
alumina) that is deposited onto the outer surface of the capillary
tube. The platinum thin film layer changes resistance as a function
The gas diverted through the capillary tube absorbs some of the
heat from the upstream windings. If no gas is flowing, the tube
will be heated uniformly and the up and downstream sensors will
sense equal temperatures. Once the gas begins to flow through the
tube, its heat absorption capacity cools the upstream portion of
the tube while heating the downstream portion; the temperature differential
increases with increasing gas flow. On-board or remotely located
electronics provide an excitation voltage or current for the sensors,
and also monitor the sensor response. For example, if a current
is applied, the voltage across the winding is monitored so that
the winding's resistance is known. Since the resistance of the sensor
varies as a known function of temperature, the temperature at the
sensor can be determined from its current and voltage.
Thermal MFCs can be either constant current or constant temperature
devices. In a constant current device, the temperature sensors are
electrically connected as two of the resistive elements in a bridge
circuit; the other elements are passive resistors. The constant
excitation current is converted to heat by the sensor resistances,
providing a uniform temperature gradient along the capillary tube.
In a constant temperature device, the sensors are again connected
in a two-sensor bridge circuit, but the MFC electronics provide
a constant voltage rather than a constant current to the bridge
circuit. A fluid flowing through the tube causes a reduction in
the temperature of the upstream sensor, which reduces its resistance
(for a positive temperature coefficient sensor), causing more current
to flow through it. The increase in excitation current causes the
sensor to give off more heat, which replaces the heat lost to the
fluid. The additional current is proportional to the fluid's mass
flow rate. Platinum is typically used as the sensing element.
While they are in widespread use, presently available MFCs suffer
from one or more of the following characteristics: relatively high
temperature sensor drift, low sensitivity, long response times,
waste associated with the difficulty of handling ultra-fine platinum
wire during manufacture, additional electronics required to quantify
the electrical responses of low sensitivity temperature sensors,
and errors resulting from the circuitry for low sensitivity temperature
sensors, when used in conjunction with high sensitivity sensors.
SUMMARY OF THE INVENTION
In one aspect of the present invention, at least four mutually
spaced temperature sensors are disposed to sense the temperature
of a fluid flowing through a conduit, and connected in a four-sensor
bridge circuit to provide an indication of the fluid's mass flow
rate. The sensors are discrete and distributed symmetrically with
respect to the conduit, preferably with a pair of sensors on opposite
side of the conduit at each of two locations along the flow path.
In another aspect of the invention, the temperature sensors are
discrete chip-type elements. Options for the sensors include semiconductor
materials such as SiC or silicon, with an oxide interfacing between
the chip and the conduit for electrically conductive conduits, and
a thin film tungsten layer on an AlN substrate. The sensors can
be enclosed by an electrically insulative film, with a circuit on
the other side of the film extending through the film to contact
the sensors. The sensors can be mounted to the conduit by means
of TiW or Ni layers on both the sensor and conduit, each supporting
a layer of Au. Various sensor positions can be used, including on
the conduit's outer surface, on an inner conduit surface, within
openings in the conduit wall, or projecting into the interior of
The chip-type sensor enables multiple sensors to be symmetrically
positioned around the conduit, either at a single location or at
multiple locations along the fluid flow path. The symmetrical placement
enables a more accurate temperature sensing, and is useful for both
four-sensor bridge circuits and other MFM configurations employing
one or more sensor pairs.
The described MFM can be used to govern the fluid flow through
the conduit by applying its output to a flow control valve for the
These and other features and advantages of the invention will be
apparent to those skilled in the art from the following detailed
description, taken together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified sectional view of a capillary tube MFM in
accordance with the invention;
FIG. 2 is a schematic diagram of a four-sensor bridge circuit that
provides a MFM output in response to the sensors of FIG. 1;
FIG. 3 is a graph illustrating the linear relationship of the temperature
between the upstream and downstream sensors to the bridge output;
FIG. 4 is a simplified combined perspective view and schematic
diagram of chip-type temperature sensors arranged in a MFM four-sensor
bridge in accordance with the invention;
FIG. 5 is a sectional view of the structure illustrated in FIG.
4 with the addition of an insulative sleeve around the sensors;
FIG. 6 is a perspective view of an alternate chip-type sensor configuration,
with a thin film tungsten sensor on an AlN substrate;
FIG. 7 is a simplified sectional view of a sensor chip mounted
to the interior wall of a fluid conduit;
FIG. 8 is a simplified sectional view and schematic diagram of
a sensor mounted within an opening in a fluid conduit;
FIG. 9a is a sectional view of a MFM sensor mounted within an environmental
shield in the interior of a conduit;
FIG. 9b is a sectional view taken along the section line 9b--9b
of FIG. 9a;
FIG. 10 is a sectional view of a sensor in accordance with an embodiment
of the invention immersed within a fluid flow tube; and
FIG. 11 is a simplified sectional view of a MFC system incorporating
a MFM of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A capillary tube MFM in accordance with one embodiment of the invention
is illustrated in FIG. 1. A minor portion of a fluid flow, either
gas or liquid, through a main conduit 2 is diverted to a capillary
tube 4; the main conduit and capillary tube structure can be conventional.
The cross-sections of the main conduit and capillary tube are precisely
machined in a conventional manner to assure their fluid flow rates
are equal. However, instead of the platinum wire windings previously
provided around the capillary tube, a pair of chip-type temperature
sensors U1 and U2 are provided at an upstream location along the
tube, and another pair of chip-type sensors D1 and D2 at a downstream
location along the tube. The sensors of each pair are preferably
positioned symmetrically at 180.degree. intervals, on opposite sides
of the tube. This allows for a more accurate sensing of the fluid
temperature within the tube, in case the fluid temperature varies
slightly from one side of the tube to the other. For example, if
the sensors were located along a horizontal portion of the capillary
tube rather than a vertical portion as shown, rising heat would
make the upper sensor detect a slightly higher temperature, and
the lower sensor a slightly lower temperature. The symmetrical sensor
placements tend to cancel out such discrepancies, and also to enable
a more uniform introduction of heat into the conduit. The capillary
tube wall has a high thermal conductivity, so that the fluid temperature
within the tube is accurately transmitted to the sensors; stainless
steel is commonly used for this purpose.
FIG. 2 illustrates a four-sensor bridge circuit used to sense the
mass flow rate of a fluid through the capillary tube of FIG. 1.
The bridge circuit is organized into left and right branches, with
each branch having upper and lower sections. The left branch includes
a pair of the upstream and downstream sensors, with the upstream
sensor U2 in its upper section and the downstream sensor D2 in its
lower section. The right branch also includes a pair of up and downstream
sensors, but their relative positions within the branch are reversed;
the downstream sensor D1 is in the upper section, while the upstream
sensor U1 is in the lower section.
An actuating current from a current source I1 is fed into the top
of the bridge, with current flowing out of the bridge through a
resistor R to a ground reference. The bridge outputs are the voltages
Vo1 and Vo2 at the connections between D1 and U1 in the right branch,
and between U2 and D2 in the left branch, respectively.
Each sensor has the same structure, and accordingly the same temperature
coefficient of resistance. A fluid flowing through the capillary
tube tends to transfer heat from the upstream sensors U1 and U2
to the downstream sensor D1 and D2. Accordingly, for sensors with
a positive temperature coefficient of resistance, this will result
in a higher resistance for D1/D2 than for U1/U2. The total resistance
of the bridge's left branch will remain equal to the total resistance
of its right branch, and equal currents will flow through each branch.
However, because of the differences in individual resistance levels,
the upper section of the right branch will experience a greater
voltage drop across D1 than the voltage drop across the lower section's
U1 while conversely the upper section of the left branch will experience
a lower voltage drop across U2 than the voltage drop across D2 in
its lower section. Thus, Vo1 will be at a higher voltage level than
Vo2 with the voltage differential representing the mass flow rate.
The circuit of FIG. 2 achieves a higher degree of sensitivity than
prior two-sensor bridge circuits, since Vo2 drops in addition to
Vo1 rising in response to fluid flow, thus producing a compound
effect. It has been found that a useful monitoring of mass flow
rate can be achieved with this circuit without amplifying the sensor
FIG. 3 illustrates the modeled output voltage differential as a
function of the temperature difference between upstream and downstream
sensors for a four-sensor bridge circuit. The curve is substantially
linear, so that only two points along the curve need be determined
to know the flow rate over the full linear range, avoiding the need
for a more complicated equation that might have to be embedded in
a microprocessor microchip.
FIG. 4 illustrates an arrangement of chip-type temperature sensors
on the fluid conduit 4 in accordance with the invention. SiC and
silicon chips are preferred; they have similar sensitivities in
the positive regions of their temperature coefficients of resistance.
SiC is capable of a higher operating temperature without diffusion
than is silicon. Other semiconductor materials may also be employed,
but in general they tend to be harder to use, do not form native
oxides, and are no more sensitive than SiC or silicon. Electrical
contact pads 6 are provided as metallization layers on opposite
ends of each chip, and enable the connection of electrical leads
to the chips so that excitation voltages or currents can be applied,
and the response of the chips monitored.
The lead wire arrangement shown in FIG. 4 corresponds to the four-element
bridge circuit of FIG. 2 with one end of upstream chip U1 connected
by lead 8 to the downstream end of the downstream chip D2 the upstream
end of the other upstream chip U2 connected by lead 10 to the downstream
end of the other downstream chip D1 the facing ends of U1 and D1
connected together by lead 12 the facing ends of U2 and D2 connected
together by lead 14 I1 applied to lead 10 resistor R connected
to lead 8 Vo1 taken from lead 12 and Vo2 taken from lead 14. Although
leads 12 and 14 are illustrated as being short, in practice their
lengths would be considerably extended, such as by connecting them
to pin-out electronics. This can increase the thermal path length
between the connected chips to a point at which the leads can be
considered substantially non-thermally conductive for purposes of
mass flow rate monitoring, thus preventing an additional thermal
path between the chips that would interfere with the measurements.
The other leads would be arranged in a similar fashion.
FIG. 5 is a not-to-scale sectional view illustrating how the symmetrically
arranged chips U1 and U2 can be bonded to the conduit 4. Electrically
insulating but thermally conductive layers 16-1 and 16-2 are formed
on the faces of U1 and U2 respectively, to allow the electrically
conductive chips to be in direct contact with the stainless steel
conduit 4 and bonding material, without creating an electrical short
circuit. The electrically insulating layers 16-1 and 16-2 can be
monolithically integrated, deposited or bonded onto the chip sensors.
With SiC or silicon used for the sensors, the electrically insulating
layers are preferably formed by oxidizing the surfaces of the chips
that are to face the conduit.
A thermally conductive bonding material 18 adheres the chip-type
sensors to the conduit. If the bonding material, typically a solder,
will not adhere directly to the chip oxide and conduit material,
a suitable intermediary bonding material is first deposited onto
these surfaces. In FIG. 5 the bonding material 18 is eutectic gold/tin
solder, which will not adhere directly to the electrically insulating
oxide layers 16-1 and 16-2 or to the stainless steel tube 4. To
produce a good bond, layers 20-1 and 20-2 of TiW or Ni, preferably
about 400-1500 Angstroms thick, are deposited on the upper and lower
chip oxide layers 16-1 and 16-2 respectively. A similar TiW or
Ni layer 20-3 is deposited around the outer surface of the tube
4. Gold (Au) layers 22-1 22-2 and 223 preferably about 4000-25000
Angstroms thick, are then deposited on the intermediate TiW or Ni
layers 20-1 20-2 and 20-3 respectively. The eutectic gold/tin
solder 18 which adheres to the Au surfaces, can then be applied
to bond the two chips U1 and U2 to opposite sides of the tube 4.
Gold is a preferred bonding material because it has a very high
thermal conductivity and does not readily oxidize. Numerous solders
are available for gold-to-gold bonding.
For further protection of the overall assembly, and to provide
additional support to hold the sensors in place, the assembly can
be shielded in an insulative sleeve 24 such as the flexible polyimide
film provided under the trademark KAPTON.RTM. by E.I. DuPont de
Nemours and Company. Contact pad metallizations 26 are deposited
or printed on the outer surface of the sleeve in alignment with
the sensor contacts. When the assembly is heated, the contact metal
26 migrates through the sleeve to mate with the sensor contacts,
thus providing a vehicle for external electrical access to the sensors.
Another advantageous sensor chip configuration, illustrated in
FIG. 6 consists of a tungsten thin film 28 deposited as the sensor
element on an insulating AlN substrate 30. The tungsten conductor
preferably traces a serpentine pattern on the substrate 30 for even
heat distribution when a heating current is applied to it, and terminates
at each end in a pair of spaced contact pads 32. Tungsten provides
a high degree of thermal sensitivity, and can tolerate a wide temperature
range when used in conjunction with an AlN substrate because of
their closely matched temperature coefficients of expansion. The
thin film tungsten layer is generally about 10-1000 microns thick.
Such a temperature sensor is the subject of copending patent application
Ser. No. 10/608737 filed on the same date as the present application
in the name of James D. Parsons, one of the present inventors.
FIG. 7 illustrates the positioning of a sensor 34 in accordance
with the invention along an interior wall of the conduit 4. The
sensor is bonded to the wall in a manner similar to an exterior
sensor, with or without the addition of an electrically insulating
layer between the sensor and conduit wall as determined by the type
of material used for the sensor. Sensor lead wires 36 can be brought
out through a remote location of the conduit, or directly through
bushings in the conduit wall. The placement of a sensor inside the
conduit as illustrated allows for very rapid and accurate tracking
of the temperature of the fluid flowing through the conduit, but
requires that neither the sensor nor the bonding material be reactive
with the fluid.
Refer now to FIG. 8 another alternative for mounting a sensor
36 is illustrated, with the sensor bonded within an opening in the
wall of conduit 4 so that its surface facing the conduit interior
is directly heated by the fluid. The sensor contacts 38 are on its
exterior surface and easily accessible. The sensor, and the conduit
opening within which it fits, should be small enough that the sensor
does not protrude too far into the conduit, and a good bond can
be obtained around its periphery to hold the sensor in place and
prevent fluid loss from the conduit.
The sensor can be part of a four-element bridge as described above,
either the upstream or downstream element of a two-sensor MFM, or
operate by itself in a single-sensor MFM. A current source I2 is
shown directing a current through the sensor, with a voltmeter 40
monitoring the sensor's voltage response to the applied current.
A comparison of the applied current and measured voltage yields
the sensor's resistance; this can be compared with either its resistance
at zero flow, or with the resistance of an up or downstream sensor,
to determine the fluid mass flow rate within the conduit.
To prevent the fluid from reacting with the sensor or its bonding
materials, this preserving the sensor and/or preventing contamination
of the fluid, a chip-type sensor 42 depicted in FIGS. 9a and 9b
can be mounted to the interior of a protective shield 44 that forms
a closed compartment within the conduit 4 sealed off from fluid
flowing through the conduit. For a stainless steel conduit, the
shield 44 would preferably also be stainless steel. Lead wires 46
can extend up from the sensor, through bushings (not shown) in the
FIG. 10 illustrates an immersible type thermal MFM in which a semiconductor
or thermistor chip-type sensor 48 is held in the fluid flow stream
within the conduit 4 at the end of a ceramic die substrate 50. The
substrate bears electrical leads 52 formed from thin layers of deposited
metal, which allow an excitation voltage or current to be applied
to the sensor 48 and also allow onboard or remotely located MFM
electronics to monitor the sensor's resistance.
FIG. 11 illustrates an MFC which utilizes the immersible MFM of
FIG. 10 although it can also be used with any of the other MFM
embodiments contemplated by the invention. A flow control valve
54 is located upstream from the immersed sensor 48 with the sensor's
voltage-current characteristics monitored by an electronics package
56 via the lead traces (not shown) on substrate 50. An electrical
interface 58 on the exterior of a housing 60 for the MFC system
provides electrical inputs and outputs to the system. The electronics
56 provide a signal to the valve control actuator 62 via lead wires
64 to control the operation of the valve in response to the detected
fluid mass flow rate, allowing the flow rate to be maintained at
a desired level despite disturbances such as upstream or downstream
line pressure or temperature variations.
While particular embodiments of the invention have been shown and
described, numerous variations and alternate embodiments will occur
to those skilled in the art. Accordingly, it is intended that the
invention be limited only in terms of the appended claims.