JP2004093160A - Thermal flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermal flowmeter in a simple configuration for effectively preventing the adhesion of dust or the like onto the surface of a flow sensor and for maintaining the measurement precision for a long time. <P>SOLUTION: The thermal flowmeter has a flow rate measurement means for measuring the flow rate of fluid that flows back the channel by selectively using a plurality of flow sensors that are assembled into a specific channel such as a low-speed flow sensor, and a high-speed flow sensor. In the thermal flowmeter, the heater element of each flow sensor is simultaneously subjected to heat generation drive regardless of whether the flow sensor is a flow sensor especially used for measuring a flow rate or not, and the natural convection of fluid (gas) by a heat flow is induced to prevent the adhesion of dirt contained in the fluid onto the sensor element surface. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thermal flow meter that can always stably measure a flow rate.
[0002]
[Related background art]
A flow meter used as a gas meter or the like obtains a flow rate of a fluid (gas) flowing through a predetermined fluid passage (gas inlet pipe) using a thermal flow rate sensor as shown in FIG. 3, for example, and integrates this flow rate. Thus, for example, it is configured to obtain the fluid flow rate (gas usage amount) every month.
[0003]
Incidentally, the thermal type flow sensor is basically composed of a pair of temperature sensors composed of a temperature measuring resistor in a flow direction F of a fluid with a heater element Rh composed of a heating resistor provided on a silicon base B interposed therebetween. It has an element structure provided with Ru and Rd. Using the fact that the degree of diffusion (temperature distribution) of the heat generated from the heater element Rh changes due to the flow of the fluid, the flow rate Q of the fluid is detected from the change in the resistance value of the temperature sensors Ru and Rd due to the heat. It is configured so that Incidentally, Rr in the drawing is a temperature sensor comprising a temperature measuring resistor provided at a position distant from the heater element Rh, and is used for measuring the ambient temperature.
[0004]
Recently, a plurality of thermal flow sensors of this type have been used, and the flow rate measured by each of these flow sensors has been averaged to improve the measurement accuracy. The measurement accuracy has been improved by selectively using a high-speed flow sensor for measuring a high flow area.
[0005]
[Problems to be solved by the invention]
However, when the above-described thermal type flow sensor is used for a long period of time, it is unavoidable that the detection sensitivity is reduced due to dust or the like adhering to the sensor surface. This decrease in detection sensitivity causes a decrease in flow rate measurement accuracy, and as described above, poses a problem in obtaining, for example, a fluid flow rate (gas usage amount) every month. Therefore, conventionally, measures such as providing a filter such as a wire mesh upstream of a flow path (pipe) in which the thermal flow sensor is provided have been taken exclusively. However, even if such a filter is provided, there is a limit in preventing dust or the like from adhering to the sensor surface.
[0006]
The present invention has been made in view of such circumstances, and an object of the present invention is to effectively prevent dust and the like from adhering to the surface of a flow sensor and to maintain the measurement accuracy for a long time. An object of the present invention is to provide a thermal flow meter having a simple configuration.
[0007]
[Means for Solving the Problems]
In order to achieve the above-described object, a thermal flow meter according to the present invention has a sensor element surface such as dust contained in a fluid (gas) due to a heat flow generated when a heater element included in a thermal flow sensor is driven to generate heat. Focusing on the phenomenon that adhesion to hardly occurs,
A plurality of flow sensors incorporated in a predetermined flow path, for example, a flow rate of a fluid flowing through the flow path by selectively using a low flow rate sensor for measuring a low flow rate range and a high speed flow rate sensor for measuring a high flow rate range In the thermal type flow meter provided with a flow rate measuring means for measuring the flow rate, particularly, regardless of whether the flow rate sensor is used for flow rate measurement, the heater elements of the respective flow rate sensors are driven to generate heat simultaneously. I have.
[0008]
That is, the thermal type flow meter according to the present invention measures the flow rate of each of the heater elements of a plurality of thermal type flow sensors incorporated in a predetermined flow path such as a gas suction pipe using the thermal type flow sensor. Regardless of whether or not it is driven by heat, natural convection is generated by the heat flow on the surface of each thermal type flow sensor, thereby preventing dust or the like contained in the fluid (gas) from adhering to the sensor element surface. Thus, the measurement accuracy is maintained for a long period of time.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a thermal flowmeter according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of a main portion of a thermal flow meter according to this embodiment. Reference numerals 10 and 20 denote thermal flow sensors having the above-described element structure shown in FIG. Incidentally, in these thermal type flow sensors 10, 20, the high-speed flow rate sensor 10 for measuring a high flow rate region in which the distance L1 between the heater element Rh and the temperature sensors Ru, Rd is set short, and the distance L2 is set long. A low flow rate sensor 20 for measuring a low flow rate region.
[0010]
The thermal flow meter according to this embodiment is provided with the two types of high-speed flow sensor 10 and the low-speed flow sensor 20 described above in a fluid passage such as a gas inlet pipe, and the fluid flowing through the fluid passage. The flow rate is measured by using one of the flow rate sensors 10 and 20 in accordance with the flow rate of (gas).
A CPU (arithmetic processing unit) 30 as a control device includes a control unit 31 that simultaneously drives the heater drive circuits 11 and 21 of the flow rate sensors 10 and 20 at predetermined measurement cycles, and a control unit 31 that controls the flow rate sensors 10 and 20. And a switch (selector) 32 for selectively taking in the sensor outputs Vout1 and Vout2 detected through the detection circuits 12 and 22, respectively. The operation of the switch 32 is controlled by the sensor selecting means 33 according to the flow rate. The switch 32 selects the high-speed flow sensor 10 when the flow rate is high (high speed), and selects the low-speed flow rate sensor when the flow rate is low (low speed). Select 20.
[0011]
The sensor outputs Vout1 and Vout2 from the flow sensors 10 and 20 selectively taken in through the switch 33 in this manner are supplied to the flow rate calculating means 34, and are synchronized with the driving of the flow sensors 10 and 20 by the control unit 31. Then, the flow rate Q corresponding to the sensor outputs Vout1 and Vout2 is calculated. The flow rate Q obtained in this way is given to the sensor selection means 33 and used for selection control of the flow sensors 10 and 20.
[0012]
That is, in the thermal type flow meter configured as described above, the control unit 31 generates heater drive signals H1 and H2 for driving the respective heater elements Rh of the two flow sensors 10 and 20 to generate heat, as shown in FIG. 2, for example. At the same time, the sampling signals S1 and S2 for measuring the flow rate Q by selectively using the sensor outputs Vout1 and Vout2 output from the flow rate sensors 10 and 20, respectively, are synchronized with the heater drive signals H1 and H2. Output selectively. Then, the flow rate calculating means 34 calculates the flow rate Q using the sensor output Vout1 output from the high-speed flow sensor 10 or uses the sensor output Vout2 output from the low-speed flow sensor 20 according to the sampling signals S1 and S2. The flow rate Q is calculated.
[0013]
Here, the sensor output immediately before the heater element Rh is driven to generate heat and the sensor output at the time when the heat generation of the heater element Rh is stabilized are obtained from the sampling signals S1 and S2 synchronized with the heater drive signals H1 and H2, respectively. It has become something. By obtaining sensor outputs in the on / off state of the heater element Rh in this way, high-precision flow measurement with zero correction of the flow sensors 10 and 20 is realized.
[0014]
Thus, according to the thermal type flow meter configured as described above, even when the flow rate measurement is performed using the plurality of thermal type flow sensors 10 and 20 selectively, the thermal type flow sensors 10 and 20 may be used. Since the heater elements Rh are simultaneously driven to generate heat, a natural convection of the fluid (gas) due to the heat flow is generated on the surface of each of the thermal flow sensors 10 and 20 so that dust or the like contained in the fluid (gas) is applied to the sensor element surface. Adhesion can be effectively prevented. In particular, even when the flow rate is measured using only one of the flow rate sensors 10 (20) periodically according to the flow rate, the heater element Rh of the other flow rate sensor 20 (10) is also driven to generate heat. In addition, there is no problem that dust adheres to the surface of the other flow sensor 20 (10) that is not used for flow measurement.
[0015]
Therefore, the surfaces of the plurality of flow sensors 10 and 20 can be kept simple and effective at all times, so that stable and reliable flow measurement can be performed over a long period of time. In addition, since there is no need to periodically remove the plurality of flow sensors 10 and 20 from the gas inlet pipe and clean the surfaces thereof, it is possible to achieve a great effect in practical use such as simplified maintenance. .
[0016]
Note that the present invention is not limited to the above-described embodiment. Here, a thermal type flow meter provided with two flow sensors has been described as an example, but the number of flow sensors incorporated in the flow meter is not particularly limited. In short, the present invention can be implemented with various modifications without departing from the scope of the invention.
[0017]
【The invention's effect】
As described above, according to the present invention, the phenomenon that dust or the like contained in a fluid (gas) hardly adheres to the sensor element surface by driving the heater element of the thermal flow sensor to generate heat is effective. In the case of measuring the flow rate by using multiple flow rate sensors selectively, the heater elements of each of these flow rate sensors are simultaneously driven to generate heat, effectively preventing dust from adhering to the sensor element surface. However, it is possible to realize a thermal flow meter having a simple configuration capable of maintaining the measurement accuracy for a long period of time.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a main part of a thermal flow meter according to an embodiment of the present invention.
FIG. 2 is a diagram showing a relationship between heat generation drive timings of heater elements in a plurality of flow sensors and flow measurement timings selectively using the flow sensors.
FIG. 3 is a diagram showing a schematic element structure of a thermal flow sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 High-speed flow sensor 20 Low-speed flow sensor 31 Control part 32 Switch 33 Sensor selection means 34 Flow rate calculation means

Claims (2)

A plurality of flow sensors each including a heater element, and first and second temperature sensors provided in the flow direction of the fluid with the heater element interposed therebetween;
Heater drive means for simultaneously driving the heater elements of these flow sensors to generate heat; and flow rate measuring means for selectively taking in the output of the flow sensor and measuring the flow rate of the fluid flowing through the flow path. A thermal flow meter characterized by the following. The thermal flow meter according to claim 1, wherein the plurality of flow sensors include a low flow rate sensor for measuring a low flow rate range and a high speed flow rate sensor for measuring a high flow rate range.

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