JP2020008338A - Thermal flow meter - Google Patents

Abstract

To allow an accurate measurement of a flow rate even in an environment in which there is a difference between the temperature of a fluid passing through a pipe and the ambient temperature.SOLUTION: An ambient temperature sensor 5 is set in a position free from the influence of the temperature of a fluid, and the temperature TRr of the fluid detected by a water temperature sensor 4 is corrected on the basis of an ambient temperature TRe near the water temperature sensor 4 detected by the ambient temperature sensor 5. If the temperature of the fluid after the correction is Tw, the correction coefficient is k, and the relation of Tw=TRr+k(TRr-TRe) is established, the temperature Tw of the fluid after the correction is determined.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a thermal flow meter that measures the flow rate of a fluid flowing through a pipe by utilizing the action of heat diffusion in the fluid.

  2. Description of the Related Art Conventionally, techniques for measuring the flow rate and flow velocity of a fluid flowing through a flow channel have been widely used in the industrial and medical fields and the like. There are various types of devices for measuring a flow rate and a flow velocity, such as an electromagnetic flow meter, a vortex flow meter, a Coriolis flow meter, and a thermal flow meter, and they are used properly according to applications.

  The thermal flow meter has an advantage that it can detect gas, has essentially no pressure loss, and can measure a mass flow rate. In addition, a thermal flow meter capable of measuring the flow rate of a corrosive liquid by forming a flow path from a glass tube is also used (see Patent Documents 1 and 2). Such a thermal flow meter for measuring the flow rate of a liquid is suitable for measuring a very small flow rate.

  The thermal flow meters include a method in which the power supplied to the heater is used as a sensor output (method 1) and a method in which the temperature difference between the upstream and downstream of the heater is used as the sensor output (method 2). For example, in a case where the fluid is water and the flow rate of the water is measured, the electric power supplied to the heater is controlled so that the heater temperature becomes a constant temperature such as + 10 ° C. with respect to the water temperature. The power or the temperature difference between the upstream and downstream of the heater is used as a sensor output (a value corresponding to the state of thermal diffusion in the fluid), and the flow rate of water is determined from the sensor output.

[Method 1]
FIG. 3 is a diagram illustrating the principle (method 1) of a thermal flow meter that measures the flow rate of a fluid from the power supplied to a heater. In this method 1, a water temperature sensor (temperature measuring element) 101 and a heater (heat generation / temperature measurement element) 102 are installed in a pipe 100 through which a fluid to be measured flows, and a temperature (heat generation) detected from a change in the resistance value of the heater 102 is measured. The power P supplied to the heater 102 is controlled such that the temperature difference between the temperature (TRh) TRh and the temperature (water temperature) TRr detected by the water temperature sensor 101 becomes a constant value (TRh-TRr = Const). At this time, since the flow rate Q of the fluid and the power P supplied to the heater 102 have a relationship of Q∝P, the flow rate Q can be calculated from the power P supplied to the heater 102.

[Method 2]
FIG. 4 is a diagram for explaining the principle (method 2) of a thermal flow meter that measures the flow rate of a fluid from the temperature difference between the upstream and downstream of the heater. According to this method 2, a water temperature sensor (temperature measuring element) 101, a heater (heat generation / temperature measuring element) 102, an upstream temperature sensor (temperature measuring element) 103, and a downstream temperature sensor ( The temperature difference between the temperature (heat generation temperature) TRh detected from the change in the resistance value of the heater 102 and the temperature (water temperature) TRr detected by the water temperature sensor 101 is a fixed value (TRh-TRr = (Const) is controlled. At this time, the temperature difference (TRu−TRd) between the temperature TRu of the fluid detected by the upstream temperature sensor 103 and the temperature TRd of the fluid detected by the downstream temperature sensor 104 has a relationship of Q∝ (TRu−TRd). The flow rate Q can be calculated from the temperature difference (TRu-TRd) between the upstream and downstream of the heater 102.

  In the above-described method 1, the electric power P supplied to the heater 102 is used as a sensor output, and in the above-described method 2, the temperature difference (TRu-TRd) between the upstream and downstream of the heater 102 is used as the sensor output. Here, when the sensor output is S, it is known that the sensor output S is simply represented by the following equation (1).

S = (A + B · μ ^1/2 ) · ΔT (1)

  In the equation (1), A and B are constants determined by the area of the water temperature sensor 101 and the heater 102, the thermal conductivity of the fluid, the density of the fluid, the viscosity of the fluid, the heat capacity, etc., μ is the flow rate, and ΔT is the The heating temperature (heating temperature from water temperature).

JP 2006-010322 A JP-T-2003-532099

  However, when there is a difference between the temperature of the water flowing through the pipe 100 and the ambient temperature (the ambient temperature near the water temperature sensor 101), the water temperature sensor 101 can accurately determine the temperature of the water flowing through the pipe 100 due to the influence of the ambient temperature. Cannot be detected.

  Therefore, even if the power supplied to the heater 102 is controlled so that the temperature difference between the heat generation temperature TRh detected from the change in the resistance value of the heater 102 and the temperature TRr detected by the water temperature sensor 101 becomes a constant value, the actual power is controlled. The heating temperature ΔT from the water temperature cannot be kept at a constant value, which affects the sensor output S and makes it impossible to measure the flow rate accurately.

  That is, when the actual water temperature (true water temperature) of the water flowing through the pipe 100 is Tr, the temperature TRr detected by the water temperature sensor 101 due to the influence of the ambient temperature becomes lower than the actual water temperature Tr, Or high values. When the temperature TRr detected by the water temperature sensor 101 becomes lower than the actual water temperature Tr, the heating temperature ΔT from the actual water temperature becomes smaller. When the temperature TRr detected by the water temperature sensor 101 becomes higher than the actual water temperature Tr, the heating temperature ΔT from the actual water temperature becomes large.

  For example, when there is no temperature difference between Tr = 23 ° C. and ambient temperature = 23 ° C., the temperature TRr detected by the water temperature sensor 101 is 23 ° C., which is equal to the actual water temperature Tr. In this case, assuming that a constant value (Const) for controlling the power supplied to the heater 102 is 10 ° C., the heating temperature ΔT is 10 ° C. (see FIG. 5).

  On the other hand, for example, when Tr = 23 ° C., ambient temperature = 5 ° C., and the ambient temperature is lower than the actual water temperature Tr, the temperature TRr detected by the water temperature sensor 101 is lower than the actual water temperature Tr such as 22.9 ° C. Even lower temperatures. Therefore, even if the heating temperature TRh is controlled to be higher by 10 ° C. than TRr, the heating temperature ΔT actually becomes 9.9 ° C. (see FIG. 6), and the sensor output S decreases. .

  Further, for example, when Tr = 23 ° C. and ambient temperature = 45 ° C. and the ambient temperature is higher than the actual water temperature Tr, the temperature TRr detected by the water temperature sensor 101 is higher than the actual water temperature Tr, such as 23.1 ° C. Temperature. Therefore, even if the heating temperature TRh is controlled to be higher by 10 ° C. than TRr, the heating temperature ΔT actually becomes 10.1 ° C. (see FIG. 7), and the sensor output S increases. .

  FIG. 8 shows the relationship between the sensor output S and the flow rate Q. In the figure, a characteristic I is obtained when there is no temperature difference between the actual water temperature and the ambient temperature (ΔT = 10 ° C.), and a characteristic II is obtained when the ambient temperature is lower than the actual water temperature (ΔT = 9.9 ° C.). III shows the case where the ambient temperature is higher than the actual water temperature (ΔT = 10.1 ° C.). As described above, since the relationship between the sensor output S and the flow rate Q changes due to the change in the ambient temperature, the flow rate Q obtained from the sensor output S also changes, and accurate flow rate measurement cannot be performed.

  The present invention has been made in order to solve such a problem, and an object of the present invention is to accurately measure a flow rate even in an environment where there is a temperature difference between a temperature of a fluid flowing through a pipe and an ambient temperature. It is to provide a possible thermal flow meter.

  In order to achieve such an object, the present invention provides a pipe (2) configured to allow a fluid to be measured to flow, and a heater (2) installed in the pipe and configured to generate heat when supplied with electric power. 3) a temperature sensor (4) installed at a position that is not affected by the heat of the heater upstream of the heater and configured to detect the temperature (TRr) of the fluid; An ambient temperature sensor (5) that is installed at a position that is not located and configured to detect an ambient temperature (TRe) near the temperature sensor; and an ambient temperature that detects the temperature of the fluid detected by the temperature sensor. A fluid temperature correction unit (6) configured to make correction based on the temperature of the heater (TRh) detected from a change in the resistance value of the heater, and a fluid temperature (Tw) corrected by the fluid temperature correction unit. Temperature difference (T h-Tw), and a control unit (7) configured to control electric power supplied to the heater so that the temperature difference becomes a constant value, and control is performed by the control unit so that the temperature difference becomes a constant value. A sensor output section (8) configured to output a value corresponding to a state of thermal diffusion in the fluid when the heat is output as a sensor output (S), and flows through the pipe based on the sensor output from the sensor output section. A flow rate calculator (9) configured to determine the flow rate of the fluid.

  In the present invention, an ambient temperature sensor is installed at a position that is not affected by the temperature of the fluid, and the temperature of the fluid detected by the temperature sensor is corrected based on the ambient temperature near the temperature sensor detected by the ambient temperature sensor. I do. For example, the temperature of the fluid detected by the temperature sensor is TRr, the ambient temperature detected by the ambient temperature sensor is TRe, the temperature of the fluid after correction is Tw, the correction coefficient is k, and Tw = TRr + k (TRr-TRe). The temperature Tw of the subsequent fluid is determined.

  Then, a temperature difference between the heat generation temperature of the heater and the temperature of the corrected fluid is obtained, and the power supplied to the heater is controlled so that the temperature difference becomes a constant value, which corresponds to the state of heat diffusion in the fluid at this time. The measured value (power supplied to the heater or temperature difference between the fluid upstream and downstream of the heater) is used as a sensor output, and the flow rate of the fluid flowing through the pipe is determined based on the sensor output.

  This allows the corrected fluid temperature (Tw) to approach the actual fluid temperature (Tr) so that accurate flow rate measurement can be performed even in an environment where there is a temperature difference between the temperature of the fluid flowing through the pipe and the ambient temperature. It becomes possible.

  In the above description, as an example, components on the drawings corresponding to the components of the invention are indicated by reference numerals with parentheses.

  As described above, according to the present invention, the ambient temperature sensor is installed at a position not affected by the temperature of the fluid, and the temperature sensor is set based on the ambient temperature near the temperature sensor detected by the ambient temperature sensor. Since the temperature of the fluid to be detected is corrected, the temperature of the corrected fluid is made closer to the actual temperature of the fluid, so that even if the temperature of the fluid flowing through the piping differs from the ambient temperature, accurate flow rate can be obtained. Can be measured.

FIG. 1 is a block diagram showing a configuration of a main part of a thermal flow meter according to Embodiment 1 of the present invention. FIG. 2 is a block diagram showing a configuration of a main part of a thermal flow meter according to Embodiment 2 of the present invention. FIG. 3 is a diagram illustrating the principle (method 1) of a thermal flow meter that measures the flow rate of a fluid from the power supplied to a heater. FIG. 4 is a diagram for explaining the principle (method 2) of a thermal flow meter that measures the flow rate of a fluid from the temperature difference between the upstream and downstream of the heater. FIG. 5 is a diagram illustrating a case where the temperature TRr detected by the water temperature sensor is equal to the actual water temperature Tr. FIG. 6 is a diagram illustrating a case where the temperature TRr detected by the water temperature sensor becomes lower than the actual water temperature Tr due to the influence of heat from the ambient temperature. FIG. 7 is a diagram illustrating a case where the temperature TRr detected by the water temperature sensor becomes higher than the actual water temperature Tr due to the influence of heat from the ambient temperature. FIG. 8 is a diagram illustrating a relationship between the sensor output S and the flow rate Q.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Embodiment 1]
FIG. 1 is a block diagram showing a configuration of a main part of a thermal flow meter 1 (1A) according to Embodiment 1 of the present invention. The thermal flow meter 1A is realized by hardware including a processor and a storage device, and a program for realizing various functions in cooperation with the hardware, and includes a pipe 2 and a heater (heat generation / temperature measuring element). 3, a water temperature sensor (temperature measuring element) 4, an ambient temperature sensor (temperature measuring element) 5, a water temperature correcting section 6, a control section 7, a power measuring section (sensor output section) 8, and a flow rate calculating section 9. And

  The pipe 2 is made of, for example, glass, and flows a fluid to be measured (in this example, water). The heater 3 is installed on the outer wall of the pipe 2, and receives heat from the control unit 7 to generate heat.

  The water temperature sensor 4 is installed on the outer wall of the pipe 2 on the upstream side of the heater 3 and detects the temperature of the fluid flowing through the pipe 2 as TRr. The water temperature sensor 4 is installed at a position where the water temperature sensor 4 is not affected by the heat of the heater 3 by keeping a certain distance from the heater 3.

  The ambient temperature sensor 5 is installed at a position that is not affected by the temperature of the fluid outside the pipe 2, and detects the ambient temperature near the water temperature sensor 4 as TRe.

  The water temperature correction unit 6 corrects the fluid temperature TRr detected by the water temperature sensor 4 based on the ambient temperature TRe detected by the ambient temperature sensor 5, and sends the corrected fluid temperature TRr to the control unit 7 as Tw.

In the present embodiment, the water temperature correction unit 6 obtains the corrected fluid temperature Tw from the following equation (2) where the correction coefficient is k.
Tw = TRr + k (TRr-TRe) (2)

  In the equation (2), the correction coefficient k is a constant determined as a value such that the temperature Tw of the fluid after the correction becomes the actual fluid temperature (true temperature) Tr. It is determined by the thermal positional relationship of the temperature sensor 5 and the like. For example, assume that the temperature TRr of the fluid detected by the water temperature sensor 4 when Tr = 23 ° C. and TRe = 45 ° C. is 23.1 ° C. In this case, the correction coefficient k may be determined so that the temperature Tw of the fluid after the correction is Tw = 23.1 + k (23.1-45) = 23 ° C.

  The control unit 7 receives the heat generation temperature TRh of the heater 3 detected from the change in the resistance value of the heater 3 and the corrected fluid temperature Tw from the water temperature correction unit 6 as inputs, and generates the heat generation temperature TRh and the corrected fluid A temperature difference (TRh-Tw) from the temperature Tw is obtained, and the power supplied to the heater 3 is controlled so that the temperature difference becomes a constant value (for example, 10 ° C.).

  The power measurement unit 8 measures the supply power P to the heater 3 when the control unit 7 controls the temperature difference (TRh−Tw) to be a constant value, and outputs the measured supply power P to a sensor output. (Value corresponding to the state of heat diffusion in the fluid) S is sent to the flow rate calculation unit 9.

  The flow rate calculating section 9 converts the sensor output S (supply power P) from the power measuring section 8 into a flow rate value using a preset flow rate conversion formula, thereby obtaining a flow rate Q of the fluid flowing through the pipe 2. Ask for.

  As described above, in the thermal type flow meter 1A of the first embodiment, the ambient temperature sensor 5 is installed at a position not affected by the temperature of the fluid, and the vicinity of the water temperature sensor 4 detected by the ambient temperature sensor 5 is provided. The temperature TRr of the fluid detected by the water temperature sensor 4 is corrected based on the ambient temperature TRe, so that the corrected temperature Tw of the fluid approaches the actual temperature Tr of the fluid, and thus the temperature of the fluid flowing through the pipe 2 is corrected. Even in an environment where there is a temperature difference between the ambient temperature and the ambient temperature, accurate flow rate measurement can be performed.

[Embodiment 2]
FIG. 2 is a block diagram showing a configuration of a main part of a thermal flow meter 1 (1B) according to Embodiment 2 of the present invention. 2, the same reference numerals as those in FIG. 1 denote the same or equivalent components as those described with reference to FIG. 1, and a description thereof will be omitted.

  In the thermal type flow meter 1B of the second embodiment, an upstream temperature sensor (temperature measuring element) 10 for detecting the temperature TRu of the fluid on the upstream side of the heater 3 and a downstream for detecting the temperature TRd of the fluid on the downstream side of the heater 3. A temperature sensor (temperature measuring element) 11 is provided on the outer wall of the pipe 2 with the heater 3 interposed therebetween. Further, a temperature difference calculation unit (sensor output unit) 12 is provided for the upstream temperature sensor 10 and the downstream temperature sensor 11.

  The temperature difference calculating unit 12 controls the power supplied to the heater 3 so that the temperature difference (TRh−Tw) between the heat generation temperature TRh and the corrected fluid temperature Tw becomes a constant value. The temperature difference between the temperature TRu of the fluid on the upstream side of the heater 3 and the temperature TRd of the fluid on the downstream side (temperature difference between the upstream and downstream of the heater 3 (TRu−TRd)) is calculated. The downstream temperature difference (TRu-TRd) is sent to the flow rate calculator 9 as a sensor output (a value corresponding to the state of heat diffusion in the fluid) S.

  The flow rate calculation unit 9 converts the sensor output S (temperature difference between upstream and downstream of the heater 3 (TRu−TRd)) from the temperature difference calculation unit 12 into a flow rate value using a preset flow rate conversion equation. Thus, the flow rate Q of the fluid flowing through the pipe 2 is obtained.

  In this manner, also in the thermal type flow meter 1B of the second embodiment, the ambient temperature sensor 5 is installed at a position which is not affected by the temperature of the fluid, and the vicinity of the water temperature sensor 4 detected by the ambient temperature sensor 5 The temperature TRr of the fluid detected by the water temperature sensor 4 is corrected based on the ambient temperature TRe, so that the corrected temperature Tw of the fluid approaches the actual temperature Tr of the fluid, and thus the temperature of the fluid flowing through the pipe 2 is corrected. Even in an environment where there is a temperature difference between the ambient temperature and the ambient temperature, accurate flow rate measurement can be performed.

  In the above-described embodiment, the flow rate calculation unit 9 converts the sensor output S into a flow rate value using a flow rate conversion formula. However, the value of the flow rate Q corresponding to the sensor output S is registered. The flow rate conversion table may be used, and the value of the flow rate Q corresponding to the sensor output S may be obtained from the flow rate conversion table.

  Further, in the above-described embodiment, the water temperature sensor 4 is installed on the outer wall of the pipe 2, but may be installed on the inner wall of the pipe 2. Even when the water temperature sensor 4 is installed on the inner wall of the pipe 2, the temperature of the portion where the water temperature sensor 4 is installed changes depending on the ambient temperature.

  Further, in the above-described embodiment, the fluid flowing through the pipe 2 is water (liquid), but the fluid flowing through the pipe 2 is not limited to liquid, but may be gas.

[Expansion of Embodiment]
As described above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical idea of the present invention.

  1 (1A, 1B): thermal flow meter, 2: pipe, 3: heater, 4: water temperature sensor, 5: ambient temperature sensor, 6: water temperature correction unit, 7: control unit, 8: power measurement unit, 9 ... Flow rate calculation unit, 10: upstream temperature sensor, 11: downstream temperature sensor, 12: temperature difference calculation unit.

Claims (5)

A pipe configured to flow the fluid to be measured,
A heater installed on the pipe, configured to generate heat by receiving supply of electric power,
A temperature sensor installed at a position upstream of the heater that is not affected by the heat of the heater, and configured to detect a temperature of the fluid;
An ambient temperature sensor installed at a position that is not affected by the temperature of the fluid, and configured to detect an ambient temperature near the temperature sensor;
A fluid temperature correction unit configured to correct the temperature of the fluid detected by the temperature sensor based on the ambient temperature detected by the ambient temperature sensor,
A temperature difference between the heat generation temperature of the heater detected from the change in the resistance value of the heater and the temperature of the fluid corrected by the fluid temperature correction unit is obtained, and the temperature difference is supplied to the heater so as to be a constant value. A control unit configured to control the power;
A sensor output unit configured to output, as a sensor output, a value corresponding to a state of heat diffusion in the fluid when the temperature difference is controlled to be a constant value by the control unit;
A flow rate calculating unit configured to obtain a flow rate of the fluid flowing through the pipe based on a sensor output from the sensor output unit. The thermal flow meter according to claim 1,
The fluid temperature correction unit,
The temperature is detected by the temperature sensor TRr, the ambient temperature detected by the ambient temperature sensor is TRe, the temperature of the fluid after the correction is Tw, and the correction coefficient is k. A thermal type flow meter for determining a temperature Tw of a subsequent fluid.
Tw = TRr + k (TRr-TRe) (A) The thermal flow meter according to claim 2,
The correction coefficient k is
A thermal flow meter, characterized in that the fluid temperature Tw after correction obtained from the equation (A) is set to a value that becomes the actual fluid temperature. The thermal flowmeter according to any one of claims 1 to 3,
The sensor output unit,
The thermal flow meter according to claim 1, wherein the controller outputs the power supplied to the heater as the sensor output when the temperature difference is controlled to be a constant value. The thermal flowmeter according to any one of claims 1 to 3,
The sensor output unit,
A thermal flow meter, wherein the temperature difference between the fluid upstream and downstream of the heater when the temperature difference is controlled to be a constant value by the control unit is output as the sensor output.