JPH11148945A - Flow velocity sensor and flow velocity-measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the measurement of flow velocities of a wide range by setting a first, a second temperature detection resistance bodies in the vicinity of a first, a second heat generation resistance bodies and one or two surrounding temperature detection resistance bodies on an upper face of a base. SOLUTION: When receiving a voltage Va generated by a first heat generation circuit 400 and a voltage Vb generated by a second heat generation circuit 500, an output circuit 600 calculates Va-Vb and outputs a voltage corresponding to a flow velocity of a gas. This output method enables highly accurate measurement of the flow velocity of the gas without being influenced by a temperature characteristic of a zero point and vibrations. In a heat generation method whereby the first heat generation circuit 400 makes a heat generation resistance element 4 generate heat and the second heat generation circuit 500 makes a heat generation resistance element 6 generate heat, a thermal energy robbed by a fluid is supplemented and the method is substantially equal to a measuring principle of a heat wire type flow velocity meter. Therefore, flow velocities of a wide range from a low velocity to a high velocity can be measured. Since a flow velocity-measuring apparatus 200 is provided with a flow velocity sensor 100 of highly accurate, high-speed responsivity, the apparatus can measure flow velocities of a wide range from a low velocity to a high velocity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow rate sensor used for measuring a flow rate of a fluid, and a flow rate measuring apparatus for measuring a flow rate using the flow rate sensor. The present invention relates to a flow rate sensor that measures the flow rate and a flow rate measurement device that measures the flow rate using the flow rate sensor.

[0002]

2. Description of the Related Art The applicant of the present invention has disclosed in Japanese Patent Publication No. 6-25684 a flow rate sensor having a semiconductor micro-machining configuration realizing high accuracy and high speed response, and a flow rate measuring device using the same.

FIG. 8 is an external view of this flow sensor, and FIG.
Next, a circuit configuration of this flow velocity measuring device is illustrated. here,
FIG. 8A is a perspective view of the flow rate sensor, and FIG. 8B is a cross-sectional view of the flow rate sensor.

As shown in FIG. 8, this flow rate sensor comprises a semiconductor base 1 made of single crystal silicon or the like, and a void 2 is formed in the center of the semiconductor base 1 by anisotropic etching. As a result, a thin film member 3 that is spatially isolated from the semiconductor base 1 is formed above the gap 2.

On the surface of the thin-film member 3, two thin-film temperature-measuring resistance elements A and B which generate heat by electric current are provided.
And a slit D that thermally insulates the two from each other, and furthermore, on the surface of the semiconductor base 1 where no gap 2 is formed, an ambient temperature resistance Element C is formed. E is
In order to achieve thermal insulation, the slits F to K formed on the surface of the thin film member 3 other than between the temperature measuring resistance elements A and B are:
This pad is provided for connection to an external circuit.

The flow rate measuring device shown in FIG. 9 uses this flow rate sensor to measure the flow rate of gas moving on the semiconductor base 1. The temperature difference detecting circuit 20, the constant current circuit 30, And a switching circuit 40.

The temperature difference detecting circuit 20 comprises temperature-measuring resistance elements A and B and resistors 21 and
22; amplifiers 23 and 24 for amplifying the output voltage of the bridge circuit; and a differential amplifier 2 for outputting the difference between the output voltages of the amplifiers 23 and 24.
This bridge circuit is supplied with a constant current from a constant current circuit 30 composed of an ambient temperature resistance measuring element C, transistors 31, 32 and a resistor 33, and has a transistor 41 and a resistor 42. Are intermittently driven by the switching circuit 40 composed of

The ambient temperature measuring resistance element C provided in the constant current circuit 30 is provided to compensate for a change in the ambient temperature. In the flow velocity measuring apparatus disclosed in Japanese Patent Publication No. 6-25684 by the present applicant, the temperature measuring resistance elements A and B generate heat by the constant current supplied from the constant current circuit 30. Here, the resistors 21 and 22
Has a much larger resistance value than the resistance temperature elements A and B, it can be considered that the resistance temperature elements A and B are driven by a constant current.

When the gas flows on the surface of the flow rate sensor, the resistance temperature element AorB located on the upstream side is cooled more strongly than the resistance element AorB located on the downstream side. That is, the downstream-side resistance temperature element AorB receives heat transfer from the upstream-side resistance temperature element AorB, and is hardly affected by the gas.

From now on, two temperature measuring resistance elements Aor
A temperature difference appears between B, and this temperature difference becomes a resistance change,
The bridge circuit loses its balance, and the difference amplifier 25 outputs a voltage corresponding to the temperature difference.

As described above, the applicant of the present invention has disclosed
The flow velocity measurement device disclosed in Japanese Patent No. 84 realizes a function of detecting the flow velocity of gas flowing on the surface of a flow velocity sensor.

[0012]

Certainly, the flow velocity measuring device disclosed by the present applicant in Japanese Patent Publication No. Hei 6-25684 is provided on the thin film member 3 of the semiconductor base 1 in which the sensor element of the flow velocity sensor is thermally insulated. Since it is formed, it has a feature that the gas flow velocity can be detected with an extremely fast response speed and with high accuracy.

However, in this flow velocity measuring device, when the flow velocity of the gas increases, the thermal energy taken away from the temperature measuring resistance element A or B on the downstream side also increases and the temperature decreases. There remains a problem that the flow rate cannot be measured because the difference between the resistance changes of the temperature measurement resistance elements A and B is reduced.

That is, as shown in FIG. 10, when the flow velocity of the gas increases, the difference between the resistance changes of the upstream and downstream temperature-measuring resistance elements A and B decreases, so that the output voltage of the difference amplifier 25 increases. However, there has been a problem that the flow rate cannot be measured because of the decrease.

The present invention has been made in view of the above circumstances, and provides a new flow velocity sensor capable of measuring a wide range of flow velocity, and a new flow velocity measurement device which measures the flow velocity using the flow velocity sensor. For the purpose of providing.

[0016]

In order to achieve this object, a flow rate sensor according to the present invention has a thin film member having an air gap with the base formed on an upper surface of the base, and a thin film member formed on the thin film member.
First and second heat-generating resistors arranged side by side at a predetermined distance and generating heat by an electric current, and arranged near the first and second heat-generating resistors to change the resistance value by the heat generation. A first and a second temperature detecting resistor are provided, and between the first heating resistor and the first temperature detecting resistor, and the second heating resistor and the second temperature detecting resistor. Providing one or more slits to thermally insulate them,
Further, one or two ambient temperature detecting resistors that change the resistance value according to the ambient temperature are provided on the upper surface of the base.

When adopting this configuration, there may be a configuration in which the ambient temperature detecting resistor is not provided. As described above, the flow rate sensor of the present invention can detect the fluid flow rate with an extremely fast response speed and with high accuracy because the sensor element is formed on the thin film member of the base that is thermally insulated. There is.

On the other hand, the flow velocity measuring device of the present invention measures the flow velocity of a fluid using the flow velocity sensor of the present invention having this feature, and comprises a first temperature detecting resistor and an ambient temperature detecting resistor. The first bridge circuit having the components, the second bridge circuit having the second temperature detecting resistor and the ambient temperature detecting resistor as the components, and the first bridge circuit are balanced so that the first bridge circuit is balanced. A first heating circuit for heating the first heating resistor, a second heating circuit for heating the second heating resistor so that the second bridge circuit is balanced,
An output circuit for outputting a difference value between a voltage generated by the first heating circuit and a voltage generated by the second heating circuit is provided.

In this configuration, if the flow velocity sensor adopts a configuration in which no ambient temperature detecting resistor is provided, a fixed resistor is used instead of the ambient temperature detecting resistor.
In the flow velocity measuring device of the present invention configured as described above, the first
The heat generating circuit generates the first heat generating resistor so that the first bridge circuit is balanced, so that the temperature detected by the first temperature detecting resistor is changed to the ambient temperature detected by the ambient temperature detecting resistor. The first heating resistor is heated so that the temperature becomes higher than the specified temperature. And the second heating circuit,
By causing the second heat generating resistor to generate heat so that the second bridge circuit is balanced, the temperature detected by the second temperature detecting resistor is higher by a specified temperature than the ambient temperature detected by the ambient temperature detecting resistor. Then, the second heating resistor is heated.

Here, when adopting a configuration in which a fixed resistor is used instead of the ambient temperature detecting resistor, the first temperature detecting resistor is set so that the temperature becomes higher by a specified temperature than the temperature specified by the fixed resistor. Heated by the first heating resistor,
The second temperature detecting resistor is heated by the second heating resistor so as to be higher than the temperature specified by the fixed resistor by a specified temperature.

The power required for the first and second heating resistors to generate heat increases in accordance with the flow velocity of the fluid flowing on the surface of the flow rate sensor, and the downstream heating resistor has:
The heat is transferred from the heating resistor on the upstream side to the temperature detecting resistor on the downstream side, so that the heat can be generated with small electric power.

From this, the output circuit outputs a difference value between the voltage generated by the first heating circuit and the voltage generated by the second heating circuit, so that a voltage corresponding to the flow velocity of the fluid can be output. become.

The heat generation method in which the first heat generation circuit causes the first heat generation resistor to generate heat and the heat generation method in which the second heat generation circuit generates heat to the second heat generation resistor compensates for heat energy taken away from the fluid. This heat generation method is basically the same as the measurement principle of the hot-wire anemometer. This hot-wire anemometer has the characteristic that it can measure the flow velocity in a wide range from low speed to high speed.

As can be seen from the above, according to the flow velocity measuring apparatus of the present invention, the characteristic of the flow velocity sensor of the present invention, which realizes high accuracy and high speed response, is utilized, and a wide range from a low speed to a high speed is realized. Measurement of flow velocity can be realized.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail according to embodiments. FIG. 1 shows a flow rate sensor 100 according to the present invention.
1 is illustrated. Here, FIG. 1A is a perspective view of the flow sensor 100, and FIG. 1B is a cross-sectional view of the flow sensor 100.

As shown in FIG. 1, a flow rate sensor 100 according to the present invention comprises a semiconductor base 1 made of single crystal silicon or the like, and a void is formed at the center of the semiconductor base 1 by anisotropic etching. 2 are formed, and as a result, a thin film member 3 is formed above the gap 2 so as to be spatially isolated from the semiconductor base 1.

The thin film member 3 formed on the gap 2 has a pair of slits 10 communicating with each other through the gap 2.
a and b are provided at a predetermined interval from each other. Further, a slit 9 is provided between these slits 10a and b at a predetermined interval from each of the slits 10a and b and extending in a direction orthogonal to a straight line connecting the slits 10a and b. The slits 9 form two arranging portions α and β between the slits 10a and 10b along the gas flow direction.

These disposition portions α and β are thermally insulated from each other by a slit 9, and are further disposed on the disposition portion α in such a manner that a thin-film heat-generating element 4 which generates heat by electric current and the thin-film heat-generating element 4 are surrounded by the thin-film heating resistance element 4. In addition, the temperature measuring resistance element 5 whose resistance value is changed by the heat generation is formed by the thin film forming technique, and the disposition portion β is provided with the heat generation resistance element 6 of the thin film which generates heat by the electric current and the surroundings. The temperature measuring resistance element 7 which changes its resistance value by the heat generated is formed by a thin film forming technique.

On the surface of the semiconductor base 1 where the voids 2 are not formed, an ambient temperature measuring resistance element 8 for changing the resistance value according to the ambient temperature is formed by a thin film forming technique.

Reference numerals 11 to 20 denote pads prepared for connection to an external circuit. FIG. 2 shows an embodiment of the flow velocity measuring device 200 of the present invention for measuring the flow velocity of the gas moving on the semiconductor base 1 using the flow velocity sensor 100 of the present invention.

As shown in the figure, the flow velocity measuring device 200 of the present invention comprises a bridge circuit 300 and a first heat generating circuit 4.
00, a second heat generating circuit 500, and an output circuit 600.

The bridge circuit 300 includes a series connection of the temperature measuring resistance element 5 and the resistor R10, a series connection of the ambient temperature temperature measuring resistance element 8 and the resistance R11, and a series connection of the temperature measurement resistance element 7 and the resistance R12. It is configured by connecting in parallel.

The first heating circuit 400 receives the potential at the connection point between the temperature measuring resistance element 5 and the resistor R10 and the potential at the connection point between the ambient temperature temperature measuring resistance element 8 and the resistor R11, and calculates a difference value between them. Is integrated by a resistor R 402 and a capacitor C 403 to generate a voltage Va, and the amplifier Va is applied to the heating resistor element 4.

The second heating circuit 500 receives the potential at the connection point between the resistance temperature measuring element 7 and the resistor R12 and the potential at the connection point between the resistance temperature element 8 and the resistance R11, and calculates the difference between the potential values. Is integrated by a resistor R502 / capacitor C503 to generate a voltage Vb, and the amplifier 501 is applied to the heating resistor element 6.

The output circuit 600 includes a first heating circuit 400
Is calculated and output from the voltage Va generated by the second heat generating circuit 500. In the flow velocity measuring apparatus 200 of the present invention configured as described above, when the temperature measuring resistance element 5 and the ambient temperature temperature measuring resistance element 8 are resistors having a positive temperature coefficient of resistance such as platinum, the voltage Va is reduced. When zero, R5 / R10 <R8 / R11 R5: resistance value of resistance element 5 for temperature measurement R8: resistance value of resistance element 8 for ambient temperature measurement resistance R10: resistance value of resistance R10 R11: resistance value of resistance R11 When the resistors R10 and R11 are selected as described above and the voltage Va is applied by the amplifier 401 of the first heating circuit 400, a current flows through the heating resistor element 4 to generate heat. As a result, the heating resistor element 4 is adjacent to the heating resistor element 4. As the temperature of the resistance temperature element 5 rises, the resistance value R5 of the resistance element 5 increases, and the balance is established where R5 / R10 = R8 / R11. I do.

As described above, the first heat generating circuit 400 adjusts the temperature measurement resistance so that the temperature detected by the temperature measurement resistance element 5 becomes higher than the ambient temperature detected by the ambient temperature temperature measurement resistance element 8 by a specified temperature. An operation is performed to cause the heating resistance element 4 located near the element 5 to generate heat.

On the other hand, when the resistance temperature element 7 and the resistance temperature element 8 are resistors having a positive temperature coefficient of resistance, such as platinum, when the voltage Vb is zero, R7 / R12 <R8 / R11 R7: Resistance value of resistance element 7 for temperature measurement R8: Resistance value of resistance element 8 for resistance temperature of ambient temperature R11: Resistance value of resistance R11 R12: Resistance value of resistance R12 The resistances R11 and R12 are selected. When the voltage Vb is applied by the amplifier 501 of the second heating circuit 500, a current flows through the heating resistor element 6 to generate heat, and as a result, the temperature of the temperature measuring resistor element 7 adjacent to the heating resistor element 6 rises. As a result, the resistance value R7 of the resistance temperature element 7 increases, and the balance is established where R7 / R12 = R8 / R11.

As described above, the second heat generating circuit 500 determines whether the temperature detected by the resistance element 7 is higher than the ambient temperature detected by the resistance element 8 by a specified temperature. The heating resistor element 6 located near the element 7 operates so as to generate heat.

The power required to generate heat from the heating resistance elements 4 and 6 increases in accordance with the flow velocity of the gas flowing on the surface of the flow velocity sensor 100, and the downstream heating resistance element 6 generates heat from the upstream side. The heat is transferred from the resistance element 4 to the temperature measurement resistance element 7 on the downstream side, and the heat can be generated with a small amount of electric power.

The voltage Va generated by the first heating circuit 400 and the voltage V generated by the second heating circuit 500 will now be described.
b increases in accordance with the flow velocity of the gas flowing on the surface of the flow velocity sensor 100 in the form as shown in FIG.

The output circuit 600 receives the voltages Va and Vb, calculates and outputs “Va−Vb”, and outputs a voltage corresponding to the flow velocity of the gas. With this output method, the gas flow velocity can be measured with high accuracy without being affected by the temperature characteristics and vibration at the zero point.

The heating method in which the first heating circuit 400 causes the heating resistor element 4 to generate heat and the heating method in which the second heating circuit 500 causes the heating resistor element 6 to generate heat compensate for the heat energy taken away by the fluid. Method
Basically, the measurement principle is the same as that of the hot-wire anemometer. This hot-wire anemometer has the characteristic that it can measure the flow velocity in a wide range from low speed to high speed.

As can be seen from the above, according to the flow velocity measuring device 200 of the present invention, the characteristics of the flow velocity sensor 100 of the present invention realizing high accuracy and high speed response are utilized, and a wide range from low speed to high speed is realized. Measurement of the flow velocity can be realized.

FIG. 4 shows another embodiment of the flow rate sensor 100 of the present invention. The difference between this embodiment and the embodiment shown in FIG. 1 is that this embodiment employs a configuration having two ambient temperature measuring resistance elements 8a and 8b.

FIG. 5 shows an embodiment of the flow velocity measuring device 200 according to the present invention when the flow velocity sensor 100 is used.
That is, when the flow velocity sensor 100 shown in FIG. 4 is used, as shown in FIG. 5, the flow velocity measurement device 200 is configured such that the bridge circuit 300 includes the series connection of the resistance temperature element 5 and the resistance R10, 8a and resistance R
11a, the temperature measuring resistance element 7 and the resistance R1.
2 and a series connection of the ambient temperature measuring resistor element 8b and the resistor R11b are connected in parallel.

Further, the amplifier 40 of the first heating circuit 400
1 is the electric potential at the connection point between the temperature measuring resistance element 5 and the resistance R10, the ambient temperature temperature measurement resistance element 8a and the resistance R11a.
And the amplifier 501 of the second heating circuit 500 outputs the potential of the connection point between the resistance element 7 and the resistance R12 and the connection point between the resistance element 8b and the resistance R11b. And a potential.

The flow velocity measuring device 200 constructed as described above
Is basically the same as that shown in FIG.
FIG. 6 shows another embodiment of the flow rate sensor 100 of the present invention.

The difference between this embodiment and the embodiment shown in FIG. 1 is that this embodiment employs a configuration in which the ambient temperature measuring resistance element 8 is not provided. FIG. 7 shows a flow rate measuring device 20 according to the present invention when this flow rate sensor 100 is used.
0 illustrates an embodiment.

That is, when the flow velocity sensor 100 shown in FIG. 6 is used, the flow velocity measuring device 200 adopts a configuration using a fixed resistance R13 instead of the ambient temperature resistance element 8.

With this configuration, the first heating circuit 400
Indicates that the temperature detected by the temperature measuring resistance element 5 is the resistance R13
When the temperature becomes higher than the temperature specified by the following equation, the second heating circuit 500 operates to generate heat in the heating resistance element 4 located in the vicinity of the temperature measuring resistance element 5.
Indicates that the temperature detected by the temperature measuring resistance element 7 is the resistance R13
However, there is a difference that if the temperature is set to be higher than the temperature specified by the equation (1), the heating element 6 located near the temperature measuring resistance element 7 operates to generate heat. Va generated by
The difference between the voltage and the voltage Vb generated by the second heating circuit 500 depends on the gas flow rate as shown in FIG. 3, and therefore, as in the embodiment of FIG. Measurement of flow velocity in a wide range can be realized.

Here, when the flow velocity sensor 100 shown in FIG. 6 is used, the ambient temperature measurement resistance elements 8a and 8b of the flow velocity measuring device 200 shown in FIG. 5 are changed to fixed resistances. The flow velocity measuring device 200 may be used.

[0052]

As described above, according to the present invention,
Forming a thin film member having an air gap with the base on the upper surface of the base; and first and second heat generating resistors arranged side by side at a predetermined distance on the thin film member and generating heat by current. , And first and second temperature detecting resistors disposed near the first and second heat generating resistors, the resistance values of which are changed by the heat generated by the first and second heat generating resistors. One or more slits are provided between the temperature detecting resistor and the second heat generating resistor and the second temperature detecting resistor to thermally insulate them, and further, on the upper surface of the base, Using a flow rate sensor configured to provide one or two ambient temperature sensing resistors that change the resistance value with ambient temperature;
The measurement of the flow velocity in a wide range from a low speed to a high speed can be realized while taking advantage of the characteristic of the flow velocity sensor that realizes high accuracy and high speed response.

And, according to the present invention, on the upper surface of the base,
Forming a thin film member having an air gap with the base, and first and second heat generating resistors arranged side by side at a predetermined distance on the thin film member and generating heat by an electric current;
And first and second temperature detecting resistors that are arranged near the heat generating resistor and change the resistance value due to the heat generated by the first heat generating resistor and the first temperature detecting resistor. A high-precision and high-speed response using a flow sensor configured to provide one or a plurality of slits between the second heating resistor and the second temperature detecting resistor to thermally insulate them. The measurement of the flow velocity in a wide range from a low speed to a high speed can be realized while utilizing the characteristic of the flow velocity sensor that realizes the above.

[Brief description of the drawings]

FIG. 1 is an embodiment of a flow rate sensor according to the present invention.

FIG. 2 is an embodiment of a flow velocity measuring device according to the present invention.

FIG. 3 is an explanatory diagram of the present invention.

FIG. 4 is another embodiment of the flow rate sensor of the present invention.

FIG. 5 is another embodiment of the flow velocity measuring device of the present invention.

FIG. 6 is another embodiment of the flow rate sensor of the present invention.

FIG. 7 shows another embodiment of the flow velocity measuring device of the present invention.

FIG. 8 is an explanatory diagram of a conventional technique.

FIG. 9 is an explanatory diagram of a conventional technique.

FIG. 10 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor base 2 Air gap part 3 Thin film member 4 Heating resistance element 5 Temperature measuring resistance element 6 Heating resistance element 7 Temperature measuring resistance element 8 Ambient temperature temperature measuring resistance element 9 Slit 10a Slit 10b Slit

Claims (4)

[Claims] A first thin film member having an air gap with the base formed on an upper surface of the base, and a first and a second thin film member which are arranged on the thin film member at a predetermined distance and generate heat by an electric current; And a first and a second temperature detecting resistor which are disposed near the first and second heating resistors and change the resistance value by the heat generation thereof. One or more slits are provided between the heating resistor and the first temperature detection resistor and between the second heating resistor and the second temperature detection resistor to thermally insulate them, and A flow rate sensor, comprising, on an upper surface of the base, one or two ambient temperature detecting resistors that change a resistance value according to an ambient temperature. 2. A first and second thin film member having an air gap with the base is formed on the upper surface of the base, and are arranged side by side at a predetermined distance on the thin film member and generate heat by an electric current. And a first and a second temperature detecting resistor which are disposed near the first and second heating resistors and change the resistance value by the heat generation thereof. One or more slits are provided between the heating resistor and the first temperature detection resistor and between the second heating resistor and the second temperature detection resistor to thermally insulate them, and A flow rate sensor provided on the upper surface of the base with one or two ambient temperature detecting resistors for changing a resistance value according to an ambient temperature; and the first temperature detecting resistor and the ambient temperature detecting resistor as constituent elements. A first bridge circuit, a second temperature detection resistor, and an ambient temperature. A second bridge circuit having a detecting resistor as a component, a first heat generating circuit for heating the first heat generating resistor so as to balance the first bridge circuit, A second heating circuit for heating the second heating resistor so that the bridge circuit is balanced; and a voltage generated by the first heating circuit and a voltage generated by the second heating circuit. An output circuit that outputs a difference value. 3. A first and a second thin film member having a gap with the base formed on an upper surface of the base and arranged side by side at a predetermined distance on the thin film member and generating heat by an electric current. And a first and a second temperature detecting resistor which are disposed near the first and second heating resistors and change the resistance value by the heat generation thereof. Providing one or more slits between the heating resistor and the first temperature detection resistor and between the second heating resistor and the second temperature detection resistor to thermally insulate them. Characteristic flow velocity sensor. 4. A first and second thin film member having an air gap with the base is formed on the upper surface of the base, and the thin film member is arranged on the thin film member at a predetermined distance and generates heat by electric current. And a first and a second temperature detecting resistor which are disposed near the first and second heating resistors and change the resistance value by the heat generation thereof. A flow sensor provided with one or more slits for thermally insulating them between the heating resistor and the first temperature detection resistor, and between the second heating resistor and the second temperature detection resistor; A first bridge circuit having the first temperature detecting resistor and the fixed resistor as components, a second bridge circuit having the second temperature detecting resistor and a fixed resistor as components, The first heating resistor is activated to balance the first bridge circuit. A first heating circuit for heating, a second heating circuit for heating the second heating resistor so that the second bridge circuit is balanced, and a voltage generated by the first heating circuit. An output circuit for outputting a difference value from a voltage generated by the second heat generating circuit.