Construction of a micro-thermocouple sensor designed to be incorporated
in a mass flow meter for the circulation of gaseous fluids, includes
of the following steps: depositing an insulating layer of several
microns by electron gun on the sensor tube, then depositing the
components of the thermocouple, by the electron gun, through the
nickel masks, at a residual pressure lower than 10.sup.-6 torr,
at a thickness of several thousand Angstroms, annealing of the capillary
tube for one hour, then the mounting of a heating element on the
capillary tube treated in this manner.
1. A micro-thermocouple sensor for use in a mass flow meter for
a gas circuit, the sensor comprising: a metallic capillary tube
having an insulating layer extending therearound and thereover,
said tube having a central area, a first side area on one side of
said central area and a second side area on an opposite side of
said central area; a first active element deposited on said first
side area; a second active element deposited on said second side
area, said second active element being of an identical material
as said first active element; a third active element deposited on
said central area, said third active element having a first edge
at one end and a second edge at an opposite end, said third active
element being of a different material than said first and second
active elements, said first active element overlapping said first
edge of said third active element, said second active element overlapping
said second edge of said third active element, said first active
element and said second active element and said third active element
and said capillary tube being annealed together; and a heating element
extending over said third active element.
2. The sensor of claim 1 said first and second active elements
being of a bismuth material, said third active element being of
an antimony material.
3. The sensor of claim 1 said heating element being a wire wound
around said third active element in said central area, said wire
being of an alloy of 75% by weight of nickel and 25% by weight of
4. The sensor of claim 1 said heating element being a copper sleeve
onto which resistors are mounted.
5. The sensor of claim 1 said heating element being of a material
identical to a material on said capillary tube.
6. The sensor of claim 1 said heating element being a deposit
of a nickel-chromium alloy on said central area, said heating element
having ends contacting respectively said first and second active
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
REFERENCE TO MICROFICHE APPENDIX
FIELD OF THE INVENTION
The invention presented here is in the domain of devices for the
measurement of the flow rate of liquids in a channel.
The thermocouple according to the invention is designed to be incorporated
in a mass flow meter which is placed in a system for the management
and control of the circulation of high purity gas, for example.
These mass flow meters usually consist of a capillary tube for
the circulation of fluids, on which the representative measurements
of the flow are carried out, and which is arranged in parallel with
the main circuit of fluid circulation.
BACKGROUND OF THE INVENTION
Numerous types of mass flow meters are already known to the professional.
They are most often based on a local heating of the passing fluid
in the capillary tube, and a measure of the variation in the resistance
of the resistive components as a function of the temperature, the
aforementioned measurement being representative of the flow of the
gas in the tube, and thus of the flow rate. The measurement resistances
are most often simply wound around the insulated capillary tube
(U.S. Pat. No. 3938384).
In the patent entitled DIRECTION SENSITIVE FLOW-RATE INDICATOR
(EP 0313 120), Bronkhorst presents a flow indicator device sensitive
to the flow of gas in two perpendicular directions by the use of
two thermocouples placed in orthogonal directions on a substrate.
In another application, MASS FLOW METERS WITH TEMPERATURE SENSORS
(EP 0395 126 B I), Bronkhorst proposes a geometry of the tube having
a very elongated U, and equipped with a series of thermocouples
placed symmetrically and a central heating resistance in two parts,
possibly with Peltier cooling components, for the stage of problems
of errors in measurement associated with a circulation of air to
the outside of the sensor or internal convection to the capillary
There are many other documents, patents or articles in scientific
journals involving thermocouples designed to be integrated into
mass flow meters.
BRIEF SUMMARY OF THE INVENTION
The invention presented here proposes a new process for constructing
According to a second goal of the invention, the process for constructing
the device makes it possible to manufacture thermocouples having
known characteristics in a precise and reproducible manner.
The device which is the object of the invention is thus a process
for constructing the micro-thermocouple sensor designed to be incorporated
into a mass flow meter for the circulation of gaseous fluids, comprising
the following steps: deposit of an insulating layer on the sensor
tube, then the deposit of the components of the thermocouple, then
annealing of the sensor, then the mounting of a heating element
on the tube treated in this manner.
Preferentially: the deposit of the insulating layer on the sensor
tube is done to several microns by electron gun, the deposit of
the thermocouple components is also carried out by electron gun,
obliquely through the nickel masks, at a residual pressure lower
than 10' Torr, at a thickness of several thousand Angstroms; and
annealing of the capillary tube is done for one hour.
These steps make it possible to construct a thermocouple set upon
a capillary tube, with a high degree of manufacturing precision
and the final characteristics of the thermocouple, and an excellent
According to one particular embodiment, the heating component for
the thermocouple for the mass flow meter takes the form of a winding
of a filament in an alloy of nickel (75%) and chromium (25%), having
a diameter of several tens of microns.
According to another embodiment, the heating element takes the
form of a c sleeve on which the CMS resistors are mounted.
According to yet another embodiment, the heating element is deposited
on the tube by electron gun, the material of this element is a nickel
chromium alloy, and the resistor, mounted in the central region
of the thermocouple, includes a central region and two contacts
at its ends, and two successive stages of deposition are carried
out, with the nickel masks adapted to the different geometries of
the resistor and the contacts.
According to yet another embodiment, the heating element includes
one thermocouple materials and the heating effect is obtained by
the Joule effect in an alternative scheme.
These devices correspond to the embodiment variations and make
it possible to obtain either devices which are more economical to
manufacture, or devices which are very precise, according to the
The description which follows, made with regard to the attached
drawings in the goal of explaining and in no way limiting, makes
it possible to understand the advantages, goals, and characteristics
of the invention.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a schematic view showing the functional design of a flow
meter from traditional technology.
FIG. 2A is a plan view of a traditional flow sensor, and FIG. 2B
is a graph illustration showing the functional principle of the
traditional flow sensor.
FIG. 3 is a sectional view showing the design of the structure
of the thermocouple according to the invention.
FIG. 4 is a sectional view showing the design of a structural variation
of the thermocouple according to the invention.
FIG. 5 is a sectional view showing the design of another variation
of the thermocouple according to the invention.
FIG. 6 is another sectional view showing the design of another
variation of the thermocouple according invention.
DETAILED DESCRIPTION OF THE INVENTION
As is depicted according to a traditional arrangement in FIG. 1
a flow meter 1 is inserted into a gas circulation line, and includes
a gas input 2 and a gas output 3 (the direction of the circulation
of the gas is symbolized by the arrows). The circulation of the
gas has a laminar restriction 4 in the flow meter 1 in the main
section of the gas passage, having a branch passage (bypass) 5 in
parallel which circulates a part of the gas flow into a capillary
tube 6 in the sensor 7.
The flow meter 1 also includes a valve 8 for controlling the gaseous
flow 2 which regulates the flow, and an electronic circuit 9 which
is of the comparator type (P.I.D., i.e. Proportional Integrator
Differentiating circuit) between an externally transmitted control
variable and the measurement made by the sensor 7. A control loop
10 of the type known to the professional performs the automatic
functional control of the flow rate 1.
In so far as its principle is concerned, the sensor 7 receives
and heats up a small part of the laminar flow (at full scale 10
cm.sup.3 /min), which is proportional to the total flow. The mass
flow rate is estimated based on the thermal transfer which it generates:
the profile of the temperature without circulation of the gas 11
along the capillary tube 6 of the sensor 7 heated on one part of
its length is changed into an asymmetrical profile 12 when the gas
circulates in the capillary tube 6 and this temperature difference
between the upstream 13 and the downstream 14 of the capillary tube
6 is a measurement of the mass flow.
The flow sensor, in the traditional device not according to the
invention, comprises, as viewed in FIGS. 2A and 2B, two coils 15
16 of resistive wire, which ensure two simultaneous functions: heating
and temperature measurement.
This temperature measurement is obtained by measuring the variation
of the two resistances, mounted in a traditional manner in a Wheatstone
bridge. The application of a constant current between the resistor
connecting terminals which are selected at the equivalent value
R (at the same temperature) induces heating of the capillary tube
at the two adjacent locations.
In the absence of the circulation of gas in the capillary, the
temperature distribution is represented by the curve 11 in FIG.
2 (the curve shows the value of the temperature in the ordinate
(y) axis, and the distance along the tube in the abscissas (x) axis)
and is, of course, symmetrical with respect to the center 17 of
the two coils 15 16. On the contrary, in the case of the circulation
of gas in the tube (curve 12), the temperature distribution is asymmetrical,
and it is seen that between the two points 18 19 equidistant from
the center 17 of the coils 15 16 a temperature difference .DELTA.T,
which results in the different measurements of the resistance for
the two resistive components 15 16 R-.delta.R and R+.delta.R.
In a thermocouple sensor, the mounting with the two resistances
in the Wheatstone bridge is functionally replaced by a thermocouple
which, as is known, includes two different materials such that a
difference of the temperature observed between these materials induces
the appearance of a directly measurable electric current which results
in a mounting which is more simple than the traditional design having
two measurement resistances.
In a preferred embodiment of the invention, the couple Bi(n)--Sb(p)
(bismuth-antimony) is used based on thin film technology. The thermoelectric
power of this couple, on the order of 120 microV/.degree. C., is
achieved by optimizing the parameters of the deposit temperature
of the substrate, the speed of the deposit, temperature (0-100.degree.
C.) and the duration of the annealing.
Various embodiment modes of the thermocouples have been conceived.
In one preferred embodiment detailed in FIG. 3 the structure is
made up of a heating element 20 a wire made of an alloy of nickel
(75%) and chromium (25%) which has the characteristic feature of
having a high resistivity (1.33 Ohm.mm2.m-1) and a low coefficient
of variation of resistance as a function of temperature (10 ppm/.degree.
C.) (its diameter is 28 microns and it is powered by a current of
12.5 mA), and of the thermocouple Bi(n)-Sb(p) comprising of two
side areas 2122 of bismuth surrounding a central area 23 made of
Before the deposit of the active elements, an insulating layer
of zirconia (ZrO.sub.2), not shown, is deposited on the capillary
tube 6 of a metallic nature. The zirconia (ZrO.sub.2), selected
for its good stability and its good dielectric characteristics,
is deposited by electron gun on the capillary tube 6 of the sensor,
at a thickness of 2.5 microns. The substrate, made up of a capillary
tube 6 made of stainless steel 316L, is supported on a heating support
not shown but of a type known to the professional. It is preferable
that the substrate support be in rotation during depositing.
The deposit of the insulating layer is done by the addition of
oxygen into the enclosed space of the deposit. The parameters for
the deposit by electron gun of the zirconia on the capillary tubes
6 are thus the following: target: ZrO.sub.2 thickness deposited:
2 to 3 microns, residual pressure: lower than 10.sup.-6 Torr, O2
pressure: 8 10.sup.-5 Torr, substrate temperature: 300.degree. C.,
speed of deposition: 20 to 40 Angstroms per second.
The depositing of the components of bismuth 21 22 and antimony
23 of the thermocouple are carried out in a manner known to the
professional, with typically a depositing by electron gun, through
the nickel masks, at a residual pressure lower than 10.sup.-6 Torr,
and at a thickness of several microns. As can be seen in FIG. 3
the bismuth deposits 21 22 overlap the edges 24 25 of the central
deposit 23 in antimony (configuration NPN).
The resistive wire making up the heating component 20 is wound
around the zone having the antimony deposit 23.
The functioning mode of the device is identical to the traditional
function the context of a mass flow meter 1 on a thermocouple for
a fluid circuit.
In a variation not shown, the couple Bi2Te3(n)-Bi2Te3(p) (bismuth
telluride), in thin film technology, is used to replace the couple
The sensitivity obtained is on the order of 400 microV/.degree.
C. in the temp (range of 0.degree. C.-150.degree. C.
In another variation presented in FIG. 4 the heating element 20
is a copper sleeve on which the resistors 27 28 of the CMS type
are mounted. The copper sleeve performs the function of making the
temperature on one section of the capillary tube 6 uniform.
In yet another structural variation, presented in FIG. 5 the heating
element 20 is deposited on the tube 6 by a method known to the professional,
for example, by electron gun or by magnetron pulverization. The
material of this element is a nickel-chromium alloy. The resistor,
deposited on the antimony area 23 of the thermocouple, includes
a central zone 29 and two contacts 30 31 at its ends. In the case
of the deposit by electron gun, two stages of successive depositing
are imagined with the nickel masks adapted to the different geometries
of the central region 29 of the resistor and the contacts 30 31.
In a third embodiment of the structure presented in FIG. 6 the
heating component 20 is made of one of the thermocouple materials.
It is then heated by the Joule effect in an alternative scheme.
During the functioning of the mass flow measurement device, when
the gas is circulating, the temperature difference between the two
junctions of the thermocouple is determined by a continuous micro-voltmeter
32 from the impedance value of the elevated input.
The range of the invention presented here is not limited to the
embodiment methods presented but, on the contrary, extends to improvements
and modifications which are conceivable to the professional.