Thermo-sensitive flow sensor for measuring flow velocity and flow rate of a gas

Claims

What is claimed as new and is desired to be secured by Letters Patent of the United States is:

1. A thermo-sensitive flow velocity sensor comprising:

a heating element and a heating-element temperature detecting element provided on a beam formed by etching a substrate;

a fluid temperature detecting element provided on said substrate near said beam for measuring a temperature of fluid;

a sensor drive circuit connected to the heating element for receiving, via an amplifier, an output value from a bridge circuit, including said heating-element temperature detecting element, said fluid temperature detecting element and at least a temperature setting resistor, and wherein a flow velocity of the fluid is measured from radiant quantities of said heating element connected to said sensor drive circuit; and

adjusting means provided at an output of said sensor drive circuit for providing an input to the bridge circuit which adjusts an output value of said bridge circuit by, in a balancing mode, forming a closed loop of the sensor drive circuit and adjusting means while short-circuiting the temperature setting resistor and disconnecting the heating element, and then reconnecting the heating element and temperature setting resistor.

2. The thermo-sensitive flow velocity sensor according to claim 1, further comprising a protective resistor connected between said sensor drive circuit and said adjusting means.

3. The thermo-sensitive flow velocity sensor according to claim 1, further comprising a virtual load resistor having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

4. The thermo-sensitive flow velocity sensor according to claim 1, further comprising a common-emitter circuit having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

5. The thermo-sensitive flow velocity sensor according to claim 1, further comprising a common-source circuit having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

6. The thermo-sensitive flow velocity sensor according to claim 1, further comprising a variable resistor connected to said bridge circuit at a side opposite to a side to which said fluid temperature detecting element is connected.

7. A thermo-sensitive flow velocity sensor comprising:

a heating element and a heating-element temperature detecting element provided on a beam formed by etching a substrate;

a fluid temperature detecting element provided on said substrate near said beam for measuring a temperature of fluid;

a sensor drive circuit connected to said heating element for receiving, via an amplifier, an output value from a bridge circuit, including said heating-element temperature detecting element, said fluid temperature detecting element and at least a temperature detecting resistor, and wherein a flow velocity of the fluid is measured from radiant quantities of said heating element connected to said sensor drive circuit; and

adjusting means provided at an output of said sensor drive circuit for providing an input to the bridge circuit which adjusts an offset voltage of said amplifying circuit by, in a balancing mode, forming a closed loop of the sensor drive circuit and adjusting means while short-circuiting the temperature setting resistor and disconnecting the heating element, and then reconnecting the heating element and temperature setting resistor.

8. The thermo-sensitive flow velocity sensor according to claim 7, further comprising a protective resistor connected between said sensor drive circuit and said adjusting means.

9. The thermo-sensitive flow velocity sensor according to claim 7, further comprising a virtual load resistor having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

10. The thermo-sensitive flow velocity sensor according to claim 7, further comprising a common-emitter circuit having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

11. The thermo-sensitive flow velocity sensor according to claim 7, further comprising a common-source circuit having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

12. A thermo-sensitive flow velocity sensor comprising:

a heating element and a heating-element temperature detecting element provided on a beam formed by etching a substrate;

a fluid temperature detecting element provided on said substrate near said beam for measuring a temperature of fluid;

a sensor drive circuit connected to said heating element for receiving, via an amplifier, an output value from a bridge circuit, including said heating-element temperature detecting element, said fluid temperature detecting element and at least a temperature setting resistor, and wherein a flow velocity of the fluid is measured from radiant quantities of said heating element connected to said sensor drive circuit;

first adjusting means provided at an output of said sensor drive circuit for adjusting an output value of said bridge circuit by, in a balancing mode, forming a closed loop of the sensor drive circuit and adjusting means while short-circuiting the temperature setting resistor and disconnecting the heating element, and then reconnecting the heating element and temperature setting resistor; and

second adjusting means provided at the output of said sensor drive circuit for adjusting an offset voltage of said amplifying circuit by, in the balancing mode, forming the closed loop from the sensor drive circuit and adjusting means while short-circuiting the temperature setting resistor and disconnecting the heating element, and then reconnecting the heating element and temperature setting resistor.

13. A thermo-sensitive flow velocity sensor comprising:

a heating element and a heating-element temperature detecting element provided on a beam formed by etching a substrate;

a fluid temperature detecting element provided on said substrate near said beam for measuring a temperature of fluid;

a sensor drive circuit connected to said heating element for receiving, via an amplifier, an output value from a bridge circuit, including said heating-element temperature detecting element, said fluid temperature detecting element and resistors, and wherein a flow velocity of the fluid is measured from radiant quantities of said heating element connected to said sensor drive circuit;

first adjusting means provided at an output of said sensor drive circuit for providing an input to the bridge circuit which adjusts an output value of said bridge circuit;

second adjusting means provided at the output of said sensor drive circuit for providing an input to the bridge circuit which adjusts an offset voltage of said amplifying circuit; and

switching means for switching between an adjustment of the offset voltage of said amplifying circuit, an adjustment of the output value of said bridge circuit and a measurement of the flow velocity.

14. A thermo-sensitive flow velocity sensor comprising:

a heating element and a heating-element temperature detecting element provided on a beam formed by etching a substrate;

a fluid temperature detecting element provided on said substrate near said beam for measuring a temperature of fluid;

a sensor drive circuit connected to said heating element for receiving, via an amplifier, an output value from a bridge circuit, including said heating-element temperature detecting element, said fluid temperature detecting element and resistors, and wherein a flow velocity of the fluid is measured from radiant quantities of said heating element connected to said sensor drive circuit;

first adjusting means provided at an output of said sensor drive circuit for providing an input to the bridge circuit which adjusts an output value of said bridge circuit;

second adjusting means provided at the output of said sensor drive circuit for adjusting an offset voltage of said amplifying circuit; and

switching timing control means for controlling timings of switching between an adjustment of the offset voltage of said amplifying circuit, an adjustment of the output value of said bridge circuit and a measurement of the flow velocity.

15. The thermo-sensitive flow velocity sensor according to claim 12, further comprising a protective resistor connected between said sensor drive circuit and said first and second adjusting means.

16. The thermo-sensitive flow velocity sensor according to claim 12, further comprising a virtual load resistor having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

17. The thermo-sensitive flow velocity sensor according to claim 12, further comprising a common-emitter circuit having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

18. The thermo-sensitive flow velocity sensor according to claim 12, further comprising a common-source circuit having a resistance value approximate to that of said heating element connected at the output of said sensor drive circuit.

19. The thermo-sensitive flow velocity sensor according to claim 12, further comprising a variable resistor connected to said bridge circuit at a side opposite to a side to which said fluid temperature detecting element is connected.

20. A thermo-sensitive flow sensor comprising:

a microbridge formed across a trench on a substrate;

a heating element provided on said microbridge;

an upstream temperature detecting element provided upstream of said heating element in a gas flow direction;

a downstream temperature detecting element provided downstream of said heating element in the gas flow direction;

wherein a flow rate of the gas is measured by detecting a temperature difference between said upstream and downstream temperature detecting elements;

drive stopping means for stopping a power supply to said heating element;

a temperature-difference detecting bridge circuit including said upstream and downstream temperature detecting elements and reference resistors, each of the reference resistors being connected in series to a corresponding one of said upstream and downstream temperature detecting elements; and

bridge adjusting means for providing an input to the temperature-difference detecting bridge circuit for performing an output adjustment of said temperature-difference detecting bridge circuit by, in a balancing mode, forming a closed loop of the sensor drive circuit and adjusting means while short-circuiting the temperature setting resistor and disconnecting the heating element, and then reconnecting the heating element and temperature setting resistor.

21. A thermo-sensitive flow sensor comprising:

a microbridge formed across a trench on a substrate;

a heating element provided on said microbridge;

an upstream temperature detecting element provided upstream of said heating element in a gas flow direction;

a downstream temperature detecting element provided downstream of said heating element in the gas flow direction;

wherein a flow rate of the gas is measured by detecting a temperature difference between said upstream and downstream temperature detecting elements;

drive stopping means for stopping a power supply to said heating element;

a temperature-difference detecting bridge circuit including said upstream and downstream temperature detecting elements and reference resistors, each of the reference resistors being connected in series to a corresponding one of said upstream and downstream temperature detecting elements; and

bridge adjusting means for providing an input to the temperature-difference detecting bridge circuit for performing an output adjustment of said temperature-difference detecting bridge circuit while the power supply to said heating element is stopped by said drive stopping means;

wherein said bridge adjusting means includes a differential amplifier for amplifying a potential difference between output terminals of said temperature-difference detecting bridge circuit, an error detecting amplifier provided at an output side of said differential amplifier, a bridge circuit adjusting circuit provided between said error detecting amplifier and said temperature-difference detecting bridge circuit, switching means for switching connection states between said differential amplifier and said error detecting amplifier and between said error detecting amplifier and said bridge circuit adjusting circuit, and switching timing control means for controlling switching timings of said switching means and said drive stopping means before measurement of the gas flow rate.

22. The thermo-sensitive flow sensor according to claim 21, further comprising short-circuiting means for short-circuiting input terminals of said differential amplifier, and zero-adjusting means for adjusting an output value of said differential amplifier to zero while the power supply to said heating element is stopped by said drive stopping means and the input terminals of said differential amplifier are short-circuited by said short-circuiting means.

23. A thermo-sensitive flow sensor comprising:

a microbridge formed across a trench on a substrate;

a heating element provided on said microbridge;

an upstream temperature detecting element provided upstream of said heating element in a gas flow direction;

a downstream temperature detecting element provided downstream of said heating element in the gas flow direction;

wherein a flow rate of the gas is measured by detecting a temperature difference between said upstream and downstream temperature detecting elements;

drive stopping means for stopping a power supply to said heating element;

a temperature-difference detecting constant current circuit for feeding a constant current to each of said upstream and downstream temperature detecting elements; and

constant current adjusting means for rendering voltages across said upstream temperature detecting element and said downstream temperature detecting element equal to each other while the power supply to said heating element is stopped by said drive stopping means.

24. The thermo-sensitive flow sensor according to claim 23, wherein said constant current adjusting means includes a differential amplifier for amplifying a difference between the voltages across said upstream temperature detecting element and said downstream temperature detecting element of said temperature-difference detecting constant current circuit, an error detecting amplifier provided at an output side of said differential amplifier, a constant current circuit adjusting circuit provided between said error detecting amplifier and said temperature-difference detecting constant current circuit, switching means for switching connection states between said differential amplifier and said error detecting amplifier and between said error detecting amplifier and said constant current circuit adjusting circuit, and switching timing control means for controlling switching timings of said switching means and said drive stopping means before measurement of the gas flow rate.

25. The thermo-sensitive flow sensor according to claim 24, further comprising short-circuiting means for short-circuiting input terminals of said differential amplifier, and zero-adjusting means for adjusting an output value of said differential amplifier to zero while the power supply to said heating element is stopped by said drive stopping means and the input terminals of said differential amplifier are short-circuited by said short-circuiting means.

26. A thermo-sensitive flow sensor comprising:

a fluid temperature detecting element provided on an electrically insulating film of a substrate;

a microbridge formed across a trench on said substrate;

a heating element provided on said microbridge;

a heating-element temperature detecting element provided near said heating element;

an upstream temperature detecting element provided upstream of said heating element in a gas flow direction;

a downstream temperature detecting element provided downstream of said heating element in the gas flow direction;

wherein a flow rate of the gas is measured by detecting a temperature difference between said upstream and downstream temperature detecting elements;

drive control means for controlling a power supply to said heating element so as to hold constant a temperature difference between said fluid temperature detecting element and said heating element temperature detecting element;

drive stopping means for stopping the power supply to said heating element;

a temperature-difference detecting bridge circuit including said upstream and downstream temperature detecting elements and reference resistors, each of the reference resistors being connected in series to a corresponding one of said upstream and downstream temperature detecting elements; and

bridge adjusting means for providing an input to the temperature-difference detecting bridge circuit for performing an output adjustment of said temperature-difference detecting bridge circuit while the power supply to said heating element is stopped by said drive stopping means.

27. The thermo-sensitive flow sensor according to claim 26, wherein said bridge adjusting means includes a differential amplifier for amplifying a potential difference between output terminals of said temperature-difference detecting bridge circuit, an error detecting amplifier provided at an output side of said differential amplifier, a bridge circuit adjusting circuit provided between said error detecting amplifier and said temperature-difference detecting bridge circuit, switching means for switching connection states between said differential amplifier and said error detecting amplifier and between said error detecting amplifier and said bridge circuit adjusting circuit, and switching timing control means for controlling switching timings of said switching means and said drive stopping means before measurement of the gas flow rate.

28. The thermo-sensitive flow sensor according to claim 27, further comprising short-circuiting means for short-circuiting input terminals of said differential amplifier, and zero-adjusting means for adjusting an output value of said differential amplifier to zero while the power supply to said heating element is stopped by said drive stopping means and the input terminals of said differential amplifier are short-circuited by said short-circuiting means.

29. A thermo-sensitive flow sensor comprising:

a fluid temperature detecting element provided on an electrically insulating film of a substrate;

a microbridge formed across a trench on said substrate;

a heating element provided on said microbridge;

a heating-element temperature detecting element provided near said heating element;

an upstream temperature detecting element provided upstream of said heating element in a gas flow direction;

a downstream temperature detecting element provided downstream of said heating element in the gas flow direction;

wherein a flow rate of the gas is measured by detecting a temperature difference between said upstream and downstream temperature detecting elements;

drive control means for controlling a power supply to said heating element so as to hold constant a temperature difference between said fluid temperature detecting element and said heating element temperature detecting element;

drive stopping means for stopping the power supply to said heating element;

a temperature-difference detecting constant current circuit for feeding a constant current to each of said upstream and downstream temperature detecting elements; and

constant current adjusting means for rendering voltages across said upstream temperature detecting element and said downstream temperature detecting element equal to each other while the power supply to said heating element is stopped by said drive stopping means.

30. The thermo-sensitive flow sensor according to claim 29, wherein said constant current adjusting means includes a differential amplifier for amplifying a difference between the voltages across said upstream temperature detecting element and said downstream temperature detecting element of said temperature-difference detecting constant current circuit, an error detecting amplifier provided at an output side of said differential amplifier, a constant current circuit adjusting circuit provided between said error detecting amplifier and said temperature-difference detecting constant current circuit, switching means for switching connection states between said differential amplifier and said error detecting amplifier and between said error detecting amplifier and said constant current circuit adjusting circuit, and switching timing control means for controlling switching timings of said switching means and said drive stopping means before measurement of the gas flow rate.

31. The thermo-sensitive flow sensor according to claim 30, further comprising short-circuiting means for short-circuiting input terminals of said differential amplifier, and zero-adjusting means for adjusting an output value of said differential amplifier to zero while the power supply to said heating element is stopped by said drive stopping means and the input terminals of said differential amplifier are short-circuited by said short-circuiting means.

32. A method of adjusting a thermo-sensitive flow sensor comprising a heating element, a heating element detecting element, a bridge circuit including a temperature detecting element and a temperature setting resistor, an operational amplifier to receive an output of the bridge circuit, and a first adjusting amplifier to receive an output of the operational amplifier and a reference voltage, and to input a first feedback signal to the bridge circuit, comprising the steps of:

disconnecting the heating element from the operational amplifier;

short-circuiting the temperature setting resistor and the operational amplifier;

first adjusting the bridge circuit until an output of the operational amplifier is equal to zero.

33. The method according to claim 32, wherein the flow sensor further comprises a second adjusting amplifier to receive an output of the operational amplifier and the reference voltage and to input a second feedback signal to the bridge circuit, further comprising steps of:

reconnecting the operational amplifier to the bridge circuit;

disconnecting the first adjusting amplifier from the bridge circuit; and

second adjusting the bridge circuit until an output of the operational amplifier is equal to the reference voltage.

34. A method of adjusting a thermo-resistive flow sensor including a heating element, a heating element driving circuit connected to a power source, a heating element detecting element, a bridge circuit including an upstream and a downstream temperature detecting element, a differential amplifier to receive an output of the bridge circuit, and a first adjusting amplifier connected to receive an output from the differential amplifier and a reference voltage, and to input a first feedback signal to the bridge circuit, comprising the steps of:

stopping a supply of power to the heating element driving circuit;

short-circuiting the differential amplifier;

first adjusting the bridge circuit until an output of the differential amplifier becomes equals to zero.

35. The method according to claim 34, wherein the flow sensor further comprises a second adjusting amplifier to receive an output of the differential amplifier and the reference voltage, and to input a second feedback signal to the bridge circuit, further comprising the steps of:

reconnecting the differential amplifier to the bridge circuit;

disconnecting the first adjusting amplifier from the bridge circuit; and

second adjusting the bridge circuit so that an output of the amplifier is equal to the reference voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermo-sensitive flow velocity sensor for measuring a flow velocity of gas based on a temperature variation of the gas due to the flow thereof, and a thermo-sensitive flow sensor for measuring a flow rate of gas based on a flow velocity of the gas.

2. Description of the Prior Art

Thermo-sensitive flowmeters are known, wherein a heating resistor or a heating element is disposed in the flow of fluid to detect a flow rate of the fluid from a temperature distribution generated in a direction of the fluid flow, that is, to measure a flow velocity of the fluid by detecting a variation of calories taken away by the fluid based on calorific values of the heating element. In some of these flowmeters, a compensation resistor or a fluid temperature detecting element is provided for correcting an output variation of the flowmeter caused by changes in fluid temperature or ambient temperature as disclosed in, for example, Japanese First (unexamined) Patent Publications Nos. 56-18381 and 61-274222 and others.

In the Publication No. 56-18381, two beams each formed of a thermal oxide layer are provided close to each other on a substrate of Si or the like. On one of the beams is formed a detection element or a heating element temperature detecting element, while a fluid temperature detecting element having the same characteristics as the heating-element temperature detecting element is formed on the other of the beams so as to perform correction of an output variation due to the temperature variation. On the other hand, in the Publication No. 61-274222, a fluid temperature detecting element formed of a patterned metal film is arranged upstream of a heating element formed on an alumina substrate so as to perform correction of an output variation due to the fluid temperature variation using this fluid temperature detecting element.

FIG. 19 shows an example of a temperature-compensated fluid measuring circuit of a background thermo-sensitive flowmeter for measuring a flow velocity of fluid while correcting an output variation due to fluid temperature variation or the like. The shown circuit includes a Wheatstone bridge circuit 1, an amplifying circuit 2 and a buffer 3. The bridge circuit 1 includes a heating-element temperature detecting element Rs, a fluid temperature detecting element Rf, resistors R1 and R2, a pre-set variable resistor VR and a temperature setting resistor Rt. The amplifying circuit 2 is formed by an operational amplifier (differential amplifier). The buffer 3 includes an npn transistor 4 and a heating element Rh connected to an emitter of the transistor 4.

In the measuring circuit shown in FIG. 19, the fluid temperature detecting element Rf monitors a temperature of the fluid, while the heating-element temperature detecting element Rs monitors a temperature of the heating element Rh. The measuring circuit feeds current from the bridge circuit 1 to the heating element Rh via the amplifying circuit 2 and the transistor 4 so as to hold constant a difference in temperature between the fluid as monitored by the detecting element Rf and the heating element Rh as monitored by the detecting element Rs. Accordingly, the flow velocity of the fluid can be measured from radiant quantities of the heating element Rh with the temperature difference between the fluid and the heating-element temperature detecting element Rs being held constant, while the temperature variation of the fluid is compensated.

In the measuring circuit shown in FIG. 19, the temperature difference between the fluid temperature detecting element Rf and the heating-element temperature detecting element Rs is held constant in the following manner.

First, the bridge circuit 1 is balanced or equilibrated using the pre-set variable resistor VR in a state where the temperature setting resistor Rt is short-circuited by closing a switch SW1 and the heating element Rh is disconnected by opening (OFF) a switch SW2. Subsequently, the switch SW1 is opened to connect the temperature setting resistor Rt and the switch SW2 is closed (ON) to connect the heating element Rh, so as to start measurement of the fluid. A set temperature value .DELTA.T between the fluid temperature detecting element Rf and the heating-element temperature detecting element Rs is defined by an equation (1) as follows:

.DELTA.T=Rt/(.alpha..multidot.Rf) (1)

wherein a temperature coefficient of a resistance value of the heating-element temperature detecting element Rs is given by .alpha. (/.degree.C.).

The flow velocity of the fluid is measured by feeding the current to the heating element Rh so as to hold .DELTA.T constant and by deriving an output Vout from a quantity of the current.

However, when the bridge circuit 1 is balanced using the pre-set variable resistor VR, it is possible that the balanced condition of the bridge circuit 1 is impaired with a lapse of time due to a mechanical portion of the pre-set variable resistor VR, to thereby change the set temperature value .DELTA.T. Accordingly, due to an adjusting error or drift in the bridge circuit 1 and/or due to an adjusting error or drift of an offset voltage of the operational amplifier in the amplifying circuit 2, the set temperature value .DELTA.T may change to affect the measured value of the flow velocity.

On the other hand, there has been available a fluid measuring device in which film resistors forming the bridge circuit 1 are laser-trimmed in advance to adjust a balanced condition of the bridge circuit 1. However, since resistance values of the laser-trimmed film resistors are liable to change as total energization time increases, the balanced condition of the bridge circuit 1 may be deteriorated with a lapse of time to thereby change the set temperature value .DELTA.T.

Further, the offset voltage of the operational amplifier connected to the bridge circuit 1 has its own characteristic value and temperature coefficient, which makes it difficult to hold the set temperature value .DELTA.T constant.

There have been further proposed thermo-sensitive flow sensors as disclosed in, for example, Japanese First (unexamined) Patent Publications Nos. 56-18381, 60-142268, 61-274222 and 62-30021, Japanese Second (examined) Utility Model Publication No. 5-35289 and others, wherein a bridge structure is formed on a substrate made of Si or the like to provide a bridge (microbridge) on which are arranged a heating element, a pair of temperature detecting elements (an upstream temperature detecting element and a downstream temperature detecting element) interposing the heating element therebetween, and a fluid temperature detecting element provided on the substrate at a position where a thermal influence from the heating element is not liable to occur, so as to measure a flow rate based on a temperature variation of the gas caused by the flow thereof.

The flow sensor disclosed in the Publication No. 6-230021 will be explained with reference to FIG. 20. An electrically insulating film is formed on a substrate 51. A surface of the substrate 51 is etched so as to form a trench 52 and a microbridge 53 across the trench 52. On the microbridge 53 are provided a heating element Rh at a center portion thereof and a heating-element temperature detecting element Rs adjacent to the heating element Rh. An upstream temperature detecting element Ru and a downstream temperature detecting element Rd are further provided on the microbridge 53 at upstream and downstream sides of the heating element Rh and the detecting element Rs relative to the flow direction of the gas, respectively. Further, a fluid temperature detecting element Rf is provided on the insulating film of the substrate 51 at a position where a thermal influence from the heating element Rh is not liable to occur. The fluid temperature detecting element Rf and the heating-element temperature detecting element Rs are connected to a first bridge circuit 54 which, in turn, is connected to an Rh drive circuit 56 via an operational amplifier 55. On the other hand, the upstream temperature detecting element Ru and the downstream temperature detecting element Rd are connected to a second bridge circuit 57. The first and second bridge circuits 54 and 57 are connected via an A/D converter 58 to a memory computing unit 59 where the flow velocity is derived and outputted.

In the flow sensor shown in FIG. 20, the first bridge circuit 54 monitors a temperature difference between the fluid temperature detecting element Rf and the heating-element temperature detecting element Rs and drives the heating element Rh via the Rh drive circuit 56 so as to hold the temperature difference at a constant. The flow velocity is determined based on an output value f1 from the Rh drive circuit 56. On the other hand, a temperature difference between the upstream temperature detecting element Ru and the downstream temperature detecting element Rd is monitored by the second bridge circuit 57. When an output value f2 from the second bridge circuit 57 becomes 0 (zero), the corresponding value f1 is stored in the memory computing unit 59 as determining that the flow velocity at the time of the output value f2 being zero is zero. The stored value is used for zero-point correction so as to ensure accurate computation of the flow velocity.

FIG. 21 shows another background thermo-sensitive flow sensor. In FIG. 21 on a microbridge formed across a trench of a substrate (not shown) are arranged a heating element Rh, an upstream temperature detecting element located upstream of the heating element Rh relative to the flow direction of the gas, and a downstream temperature detecting element Rd located downstream of the heating element Rh. The shown flow-sensor measures a flow rate of the fluid by detecting a temperature difference between the upstream and downstream temperature detecting elements Ru and Rd.

In FIG. 21, the heating element Rh is driven by an Rh drive circuit 60. The upstream and downstream temperature detecting elements Ru and Rd constitute a Wheatstone bridge circuit 61, along with fixed resistors (reference resistances) R1 and R2. The bridge circuit 61 is balanced or equilibrated while the power supply to the heating element Rh is stopped The balanced condition of the bridge circuit 61 is confirmed by an output voltage V.sub.d-u of a differential amplifier 62. In the balanced condition, a relationship defined by an equation (2) is established as follows:

R1 R.sub.do =R2 Ruo (2)

wherein Ruo represents a resistance value of the upstream temperature detecting element at a reference temperature, and Rdo represents a resistance value of the downstream temperature detecting el