Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2001-12-27
2004-02-03
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06684694
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to a flow sensor for detecting the flow amount and flow rate of a fluid, a method of manufacturing the same and a fuel cell system using the flow sensor.
FIGS. 1 and 2
are conceptual diagrams of a flow sensor
1
having a conventional structure.
FIG. 2
is a sectional view taken along the line X
1
—X
1
in FIG.
1
.
FIG. 1
shows a heater and a temperature measuring element in an exposed state, and
FIG. 2
shows the same members covered with a protective film
10
or the like. In the flow sensor
1
, a gap section
3
in the form of a recess is formed on a top surface of a silicon substrate
2
; an insulating thin film
4
is provided on the top surface of the silicon substrate
2
such that it covers the gap section
3
; and a part of the insulating thin film
4
forms a bridge section
5
in the form of a thin film over the gap section
3
. The bridge section
5
is thermally insulated from the silicon substrate
2
by the space (air) in the gap section
3
. A heater
6
is provided in the middle of a surface of the bridge section
5
, and temperature measuring elements
7
and
8
are provided in respective positions which are symmetrical about the heater
6
. An ambient temperature measuring resistive element
9
is provided on a surface of the insulating thin film
4
located outside the bridge section
5
. Further, the silicon substrate
2
is coated with a protective film
10
such that the heater
6
, temperature measuring elements
7
and
8
, and ambient temperature measuring resistive element
9
are covered with the same.
Various elements are used as the temperature measuring elements
7
and
8
. For example, Japanese unexamined patent publication No. S60-142268 disclosed the use of thin film resistors made of an iron-nickel alloy. In an article titled, “Low power consumption thermal gas-flow sensor based on thermopiles of highly effective thermoelectric materials”, is disclosed the use of BiSb—Sb thermopiles as temperature measuring elements. Further, transistors or the like may be used as temperature measuring elements. The following description is based on an assumption that thermopiles formed from BiSb—Sb thermocouples are used as the temperature measuring elements
7
and
8
.
When thermopiles formed from BiSb—Sb thermocouples are used as the temperature measuring elements
7
and
8
, thin wires made of BiSb and thin wires made of Sb are alternately provided across edges of the bridge section to form a group of hot contacts
11
at points where the BiSb thin wires and Sb thin wires are connected in the bridge section
5
, and to form a group of cool contacts
12
at points where the BiSb thin wires and Sb thin wires are connected outside the bridge section
5
.
A voltage V
1
output by the temperature measuring element
7
(a voltage across the same) and a voltage V
2
output by the temperature measuring element
8
(a voltage across the same) are respectively expressed by Equations 1 and 2 as follows where n represents the quantities of the hot contacts
11
and cool contacts
12
of the temperature measuring elements (thermopiles)
7
and
8
; Tc represents the temperature of the cool contacts
12
(which is equivalent to the ambient temperature at the time of measurement); Th
1
represents the temperature of the hot contacts of the temperature measuring element
7
; and Th
2
represents the temperature of the hot contacts
11
of the temperature measuring element
8
.
V
1
=
n·a
(
Th
1
−
Tc
) Equation 1
V
2
=
n·a
(
Th
2
−
Tc
) Equation 2
“a” represents a Seebeck coefficient.
The flow sensor
1
is placed in a channel
13
through which a fluid flows as shown in
FIG. 3
, and the outputs of the temperature measuring elements
7
and
8
are monitored with the heater
6
heated by a current applied thereto. When there is no wind, or when no gas flows, since the temperature distribution on the surface of the insulating thin film
4
is symmetric about the heater
6
, as indicated by the solid line in
FIG. 5
, the temperature Th
1
of the hot contacts of the temperature measuring element
7
and the temperature Th
2
of the hot contacts of the temperature measuring element
8
are equal to each other because of the symmetry of their positions, and the voltage V
1
output by the temperature measuring element
7
and the voltage V
2
output by the temperature measuring element
8
are therefore equal to each other.
On the contrary, when a fluid flows from the temperature measuring element
7
toward the temperature measuring element
8
as indicated by the arrow in
FIG. 4
, the temperature distribution on the surface of the insulating thin film
4
is asymmetric, as indicated by the broken line in FIG.
5
. Specifically, the temperature Th
1
of the hot contacts of the temperature measuring element
7
located upstream decreases because the element is cooled by the flow of the fluid, and the output voltage V
1
=n·a(Th
1
−Tc) decreases. Meanwhile, the heat of the heater
6
is transported by the fluid downstream to increase the temperature Th
2
of the hot contacts of the temperature measuring element
8
located downstream, which results in an increase in the output voltage V
2
=n·a(Th
2
−Tc). The flow amount of the fluid can be measured from a resultant change &Dgr;V=V
2
−V
1
in the output voltage. When the flow amount of the fluid is small, since the difference &Dgr;T=Th
2
−Th
1
between the temperatures of the temperature measuring elements
7
and
8
is proportionate to the mass flow of the fluid, the temperature difference can be obtained from Equation 3 shown below by measuring the output voltages V
1
and V
2
of the temperature measuring elements
7
and
8
, and the mass flow of the fluid can be calculated by performing further calculation processes that are required.
Δ
T
=
Δ
V
/
(
n
·
a
)
=
(
V2
-
V1
)
/
(
n
·
a
)
Equation
3
The ambient temperature measuring resistive element
9
measures the ambient temperature of the flow sensor
1
. The ambient temperature is measured with the ambient temperature measuring resistive element
9
to maintain the heating temperature of the heater
6
at a temperature which is higher than the ambient temperature by a constant value at any flow rate (this effect is hereinafter referred to as “constant temperature rise of the heater
6
”) and to correct temperature characteristics of the flow sensor
1
.
In the flow sensor
1
, when the heating temperature of the heater
6
increases, the output voltages of the temperature measuring elements
7
and
8
increase in proportionate to the same, which improves the resolution of a temperature measured by the temperature measuring elements
7
and
8
. The higher the heating temperature of the heater
6
, the greater the power consumption of the heater
6
. Therefore, the heating temperature of the heater
6
is set by a user at an arbitrary constant temperature taking both factors into consideration.
However, the heating temperature of the heater
6
changes depending on the flow rate of a fluid. In an environment in which the flow sensor
1
is used, the ambient temperature normally changes. For those reasons, a change in the difference between the ambient temperature and the heating temperature of the heater
6
results in a change in a temperature gradient around the heater
6
and a change in the relationship between the output voltages of the temperature measuring elements
7
and
8
and the quantity or rate of the flow of a fluid, which deteriorates the accuracy of measurement.
Under such circumstances, a heater control circuit
14
as shown in
FIG. 6
is used in the conventional flow sensor to automatically adjust the heating temperature of the heater
6
to a temperature which is higher than the ambient temperature detected by the ambient temperature measuring resistive element
9
by a constant value (a constant temperature
Fujiwara Takeshi
Nakamura Ken'ichi
Neda Tokudai
Sasaki Syo
Lefkowitz Edward
Omron Corporation
Rosenthal & Osha L.L.P.
Thompson Jewel V.
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