JPH11160120A - Mass flow sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mass flow sensor capable of detecting a mass flow of a fluid accurately even if a mass flow meter and a mass flow controller are installed in any attitude or ambient temperature is varied. SOLUTION: A first heat sensitive resistor 21 and a second heat sensitive resistor 22 as sensor coils are wound on one vertical part 13b of a thin tube 13 which is installed vertically in reverse U-shape on a body block 1 of a mass flow meter or a mass flow controller and through which fluid flows in a state of insulating these resistors from each other, and a third heat sensitive resistor 23 is wound as a heater coil on the other vertical part 13c. Then the sensor coils 21 and 22 and the heater coil 23 are driven by drive circuits independent from each other so that the amount of heating from both coils 21 and 22 is equal to that from the coil 23.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass flow sensor used for a mass flow meter for measuring a mass flow rate of a fluid or a mass flow controller for measuring a mass flow rate of a fluid and controlling the fluid flow rate.

[0002]

2. Description of the Related Art FIG. 5 shows a mass flow meter generally used in the prior art.
Is a main body block having a fluid inlet 2 formed at one end and a fluid outlet 3 formed at the other end, and a fluid flow passage 4 connecting the fluid inlet 2 and the fluid outlet 3 therein. The fluid flow path 4 is provided with a bypass element 5 having a constant flow rate characteristic.

[0005] Reference numeral 7 denotes a sensor fixing base provided on the upper part of the main body block 1. This sensor fixing base 7 includes:
A sleeve 10 having a hole 9 communicating with the flow path 4 in the main body block 1 through the communication passage 8 is provided detachably. 1
1 is a sealing member. Reference numeral 12 denotes a mass flow sensor, which is connected to the sleeve 10 by a method such as resistance welding.
A thin tube 13 as a measurement flow path which is provided upright in the inverted U-shape in the sensor fixing base 7 and the main body block 1, and is wound around the outer periphery of a central horizontal portion 13 a of the thin tube 13.
And one thermal resistor 14 and 15. The heat-sensitive resistors 14 and 15 are selected to have the same heat-sensitive characteristics such as a temperature coefficient.

Reference numeral 16 denotes a hermetic terminal provided on the upper surface of the sensor fixing base 7 by resistance welding or the like, and the thermal resistors 14 and 15 are connected to a bridge circuit (not shown) via lead pins of the hermetic terminal 16. Reference numeral 17 denotes a sensor case for accommodating and covering the sensor section 12, the hermetic terminal 16, and the like. The main body block 1, the sleeve 10, the thin tube 13, etc. are made of stainless steel,
Made of metal with excellent corrosion resistance, such as nickel and Kovar.

[0005]

By the way, in the mass flow meter having the above-mentioned structure, as shown in FIG. 6A, the mass flow sensor usually has two heat-sensitive resistors 14 and 15 at the same height. 12 is mounted in a horizontal state. In the mass flow sensor 12 installed in such a horizontal posture, the two heat-sensitive resistors 14 of the thin tube 13,
Since the central portion 13a around which the coil 15 is wound is horizontal and there is no positional relationship between the two thermal resistors 14, 15, heat convection occurs between the thermal resistors 14, 15. There is no.

However, due to the configuration of the piping system or the installation space of the mass flow meter, FIG.
As shown in (B), the mass flow sensor 12 must be installed in a vertical posture such that the central portion 13a of the thin tube 13 around which the two thermal resistors 14 and 15 are wound is vertical. There is. In such a case, one thermal resistor 14 is positioned higher than the other thermal resistor 15, and heat convection occurs inside and outside the thin tube 13. In this case, heat convection outside the thin tube 13 can be prevented by providing a heat insulating material outside the sensor 12. However, the heat convection inside the thin tube 13 prevents the heat-sensitive resistors 14 and 15 as sensor coils. Are thermally interfered with each other and the zero point changes, so that an error occurs in the flow rate measurement result.

[0007] In order to avoid the inconvenience of mounting the vertical position inside the thin tube 13, it has been attempted to increase the differential pressure of the fluid flowing through the thin tube 13 by reducing the inner diameter of the thin tube 13. However, the measurement error could not be reduced so much.

[0008] Such a problem, that is, the influence of the attitude of the mass flow sensor 12 is also occurring in a mass flow controller having a configuration in which a fluid control valve is added to a mass flow meter.

In order to solve the above-mentioned problems, for example, as shown in Japanese Patent Application Laid-Open No. Hei 7-27582 (this is a patent application filed by the present applicant), heat-sensitive resistors 14 and 15 as sensor coils are used. In addition to the above, it has been attempted to provide two heat-sensitive resistors as heat sources to reduce the heat convection and eliminate measurement errors caused by the heat convection.

According to the mass flow sensor disclosed in the above publication, the inconvenience of mounting in the vertical position can be solved in a considerable point as compared with the previous mass flow sensor, but the two heat-sensitive resistors as heat sources can be eliminated. If the ambient temperature changes due to flow rate detection,
Since it is difficult to equalize the amount of heat, a flow rate detection error depending on the posture may occur.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-mentioned matters, and has as its object the accuracy of the mass flow rate of a fluid regardless of the mounting posture of a mass flow meter or a mass flow controller and the change in ambient temperature. It is an object of the present invention to provide a mass flow sensor capable of detecting the mass flow.

[0012]

In order to achieve the above object, a mass flow sensor according to the present invention is provided with an inverted U in a main body block of a mass flow meter or a mass flow controller.
A first thermosensitive resistor and a second thermosensitive resistor are wound around one vertical portion of a thin tube, which is provided in a vertical shape and through which fluid flows inside, as a sensor coil while being insulated from each other. The third thermal resistor is wound as a heater coil, and the sensor coil and the heater coil are driven by drive circuits independent of each other so that the amounts of heat generated by the two coils are equal.

The driving circuit includes a constant current circuit and a constant temperature circuit.

In the above mass flow sensor, the heat generated in the first and second thermal resistors as the sensor coils cancels the heat generated in the third thermal resistor as the heater coil. No heat convection occurs in the coil, and the zero point shift due to the mounting posture of the mass flow meter or the mass flow controller is eliminated. Further, since the circuit for driving the heater coil is provided independently of the circuit for driving the sensor coil, the temperature on the heater coil side can be changed with respect to the temperature change in the sensor coil. Even if the temperature changes, a flow rate detection error due to the posture does not occur.

[0015]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a mass flow sensor 2 according to the present invention.
In this figure, reference numerals 13b and 13c denote vertical portions on both sides of a horizontal portion 13a of the inverted U-shaped thin tube 13, and a first thermal resistor as a sensor coil is provided on the vertical portion 13b on the fluid inflow side. 21 and the second thermal resistor 22 are wound in a state in which they are insulated from each other, and a third thermal resistor 23 as a heater coil is provided on the vertical portion 13c on the fluid outflow side.
Is wound. In addition, these heat-sensitive resistors 21 to 2
As for No. 3, those having the same heat-sensitive characteristics such as temperature coefficient are used, and the total winding width of the sensor coils 21 and 22 and the winding width of the heater coil 23 are made equal.

The first thermal resistor 21 and the second thermal resistor 22 as the sensor coil and the third thermal resistor 23 as the heater coil are connected to different drive circuits.

FIG. 2A shows an example of the flow rate detecting section 24 including the first thermal resistor 21 and the second thermal resistor 22. The flow detection unit 24 shown in this figure is configured as a constant current circuit. That is, the first thermal resistor 21 and the second
The thermal resistors 22 are connected in series with each other,
A bridge circuit 27 is formed together with the bridge resistors 25 and 26. 28 to 31 are connection points between adjacent sides of the bridge circuit 27,
The connection points 28 and 30 on one diagonal line are a transistor 32, an operational amplifier 33, a reference power supply 34,
4 is connected to a constant current power supply 37 composed of resistors 35 and 36 for appropriately dividing the voltage of the voltage V.4.

More specifically, node 28 is connected to the emitter of transistor 32 and node 30 is connected to resistor 38
And is connected to one input terminal of the operational amplifier 33. The connection point 39 between the resistors 35 and 36 is connected to the other input terminal of the operational amplifier 33. The output side of the operational amplifier 33 is connected to the base of the transistor 32.

The connection point 2 of the bridge circuit 27
9 and 31 are connected to a terminal 41 via an amplifier circuit 40, and the output of the bridge circuit 27 is configured to be output between the terminal 41 and the grounded terminal 42.

FIG. 2B shows an example of the heater section 43 including the third thermal resistor 23. The heater section 43 shown in this figure is also configured as a constant current circuit. That is, the transistor 4 is connected to both ends of the third thermal resistor 23.
4. An operational amplifier 45, a reference power supply 46, and a constant current power supply 50 including resistors 47, 48, and 49 for appropriately dividing the voltage of the power supply 46 are connected.

More specifically, one end of the third thermal resistor 23 is connected to the emitter of the transistor 44, the other end is grounded via the resistor 51, and connected to one input terminal of the operational amplifier 45. I have. The other input terminal of the operational amplifier 45 is connected via a movable contact 52 to a resistor 48 interposed between the resistors 47 and 49. The output side of the operational amplifier 45 is a transistor 4
4 are connected to the base.

By the way, in the mass flow sensor having the above-mentioned structure, as shown in FIG. 3A, the horizontal portions 13a of the thin tubes 13 are made vertical, that is, the heat-sensitive resistors 21 and 22 as the sensor coils are connected. In the state in which the wound portion 13b is horizontal (the portion 13c in which the thermal resistor 23 as a heater coil is wound is also horizontal), no thermal convection occurs in the thermal resistors 21 and 22. In step, the zero balance in the flow rate detector is adjusted.

As shown in FIG. 3B, when the portion 13b is made vertical (the portion 13c is also made vertical), when the heat-sensitive resistor 23 is not generating heat (heater off), Thermal convection occurs in the resistors 21 and 22, and the zero point in the flow rate detector shifts.

Therefore, as shown in FIG. 3C, in a state where the portion 13b is made vertical (the portion 13c is also made vertical), the heat-sensitive resistance is set so that the output at the flow rate detecting section becomes zero. The body 23 is caused to generate heat (heater is turned on), so that the amount of heat Ws generated on the side of the heat-sensitive resistors 21 and 22 and the amount of heat Wh generated on the heat-sensitive resistor 23 are equal to each other.

As described above, the amount of heat Ws generated on the side of the heat-sensitive resistors 21 and 22 serving as sensor coils and the amount of heat Wh generated on the heat-sensitive resistor 23 serving as a heater coil.
Are always equal to each other.
3 is operated. By doing this,
The heat amount Ws is canceled by the heat amount Wh, and no heat convection occurs in the heat-sensitive resistors 21 and 22 as the sensor coils regardless of the mounting posture of the mass flow meter.

In the above configuration, the circuit 43 for driving the thermal resistor 23 as a heater coil is replaced with the circuit 2 for driving the thermal resistor 21 and 22 as a sensor coil.
4 is provided independently of the heat-sensitive resistor 21,
The temperature on the side of the thermal resistor 23 can be changed in response to the temperature change at 22, so that even if the ambient temperature changes, a flow rate detection error due to the posture does not occur.

In the above-described embodiment, the constant current circuit is used as a circuit for driving the thermal resistors 21, 22, and 23, respectively.
A constant temperature circuit as disclosed in detail in Japanese Patent Publication No. 9893 and Japanese Patent Publication No. 5-23605 may be used.

That is, FIG. 4A shows an example of the flow detecting section 53 including the first thermal resistor 21 and the second thermal resistor 22. In this figure, reference numerals 54 and 55 denote constant temperature circuits provided in parallel with each other. Each of the constant temperature circuits 54 and 55 includes a bridge including the first thermal resistor 21 and the second thermal resistor 22 as bridge components. It comprises circuits 56 and 57, transistors 58 and 59, and operational amplifiers 60 and 61. 62 is a bridge circuit 56,5
7 is an amplifier that takes the difference between the outputs, 63 and 64 are output terminals. Here, 65 to 70 are bridge resistors.

FIG. 4B shows an example of the heater section 71 including the third thermal resistor 23. The heater section 71 shown in this figure is also configured as a constant temperature circuit. That is, the constant temperature circuit includes a bridge circuit 72 including the third thermal resistor 23 as a bridge component, a transistor 73, and an operational amplifier 74. 75 to 77 are bridge resistors. One end of the third thermal resistor 23 is connected to one input terminal of the operational amplifier 74, and the other end is grounded. The other input terminal of the operational amplifier 74 is connected to a resistor 78 interposed between the resistors 76 and 77 via a movable contact 79. The output side of the operational amplifier 74 is connected to the base of the transistor 73.

The mass flow sensor according to the present embodiment also has the same function and effect as the mass flow sensor according to the above embodiment, and therefore a detailed description thereof will be omitted.

[0031]

As described above, according to the present invention, the thermal resistor as the sensor coil and the thermal resistor as the heater coil are arranged in a parallel positional relationship with each other.
Since the amounts of heat generated in both coils are made equal, thermal convection generated in the sensor coil is skillfully canceled. Therefore, the zero point shift due to the mounting posture of the mass flow meter or the mass flow controller is eliminated.

Since the sensor coil and the heater coil are driven by circuits of the same kind but independent of each other, even if the ambient temperature changes, the heater coil is not affected by the temperature change in the sensor coil. The temperature on the side can be changed, and even if the ambient temperature changes, a flow rate detection error due to the posture does not occur. Therefore,
Highly accurate flow rate detection can always be performed without being affected by the ambient temperature or posture.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of a mass flow sensor according to the present invention.

FIG. 2 is a diagram showing an example of an electric circuit incorporating the mass flow sensor.

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

FIG. 4 is a diagram showing another example of an electric circuit incorporating the mass flow sensor according to the present invention.

FIG. 5 is a diagram showing a mass flow meter incorporating a conventional mass flow sensor.

FIG. 6 is a diagram for explaining a defect of a conventional mass flow sensor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Main body block, 13 ... Thin tube, 13b, 13c ... Vertical part, 20 ... Mass flow sensor, 21 ... 1st thermal resistor,
22 ... second thermal resistor, 23 ... third thermal resistor, 37,
50, 54, 71... Drive circuits.

Claims (1)

[Claims] 1. A first thermo-sensitive resistor and a second thermo-sensitive resistor as sensor coils in one vertical portion of a thin tube which is set upright in an inverted U-shape on a main body block of a mass flow meter or a mass flow controller and through which a fluid flows. Are wound in a state where they are insulated from each other, and a third heat-sensitive resistor is wound around another vertical portion as a heater coil, and the sensor coil and the heater coil are driven by independent driving circuits to generate heat in both coils. A mass flow sensor characterized by being driven to be equal.