JP2020008508A - Thermal flowmeter - Google Patents

Abstract

To easily detect passage of air bubbles moving with a liquid flowing in pipeline.SOLUTION: A thermal flowmeter comprises an air bubble detection part 8, and on the basis of variation of a heater command value MV, detects passage of air bubbles moving with a liquid flowing in pipeline 2. For example, the flowmeter periodically obtains a variation ratio γ of the heater command value MV, determines whether the variation ratio γ is a reduction ratio α or an increase ratio β, and measures, as variation time T, time until the increase ratio β becomes equal to or greater than a threshold βth after the reduction ratio α becomes equal to or greater than a threshold αth, then compares the measured variation time T with a permissible variation time Tth, determines that the air bubbles have passed if the measured variation time is shorter than the permissible variation time Th. The flowmeter may detect the passage of air bubbles on the basis of variation of a sensor output S, and may set the permissible variation time Tth to be a constant value, and may set it according to a flow rate v.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. 11 is a view for explaining 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. 12 is a view 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 a 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

  In such a thermal type flow meter, when the fluid is a liquid, if the liquid contains bubbles, the thermal conductivity is reduced by the bubbles, and the measured value is such that the flow rate is apparently reduced. If it is possible to detect whether or not bubbles are contained in the liquid flowing through the pipe, it is useful to be able to distinguish it from apparent fluctuations in the flow rate. It is relatively easy to determine that there is no liquid in the pipe, that is, that the pipe is empty, because the heat conductivity is smaller than a certain value. However, the bubbles move together with the liquid flowing in the piping, and the time required to pass through the sensor unit is several tens to several hundreds of milliseconds. For this reason, the heat conductivity cannot be determined at a certain threshold, and it is difficult to detect the passage of bubbles.

  The present invention has been made in order to solve such a problem, and an object of the present invention is to provide a thermal flow meter capable of easily detecting the passage of bubbles moving with liquid flowing in a pipe. To provide.

  In order to achieve such an object, the present invention provides a pipe (2) configured to flow a liquid to be measured, and a heater (2) installed in the pipe and configured to receive power and generate heat. 3), a temperature sensor (4) installed upstream of the heater and configured to detect the temperature of the liquid, and a heating temperature (TRh) of the heater and a temperature sensor detected from a change in the resistance value of the heater. The temperature difference (TRh-TRr) from the temperature (TRr) of the liquid detected by the above is obtained, a heater command value (MV) that makes this temperature difference a constant value is obtained, and the heater is supplied to the heater based on the heater command value. A controller (5) configured to control the supplied electric power, and a sensor output (a value corresponding to the state of heat diffusion in the liquid when the temperature difference is controlled by the controller to be a constant value). Output as S) Output section (6, 11) configured as described above, a flow rate calculation section (7) configured to obtain the flow rate (Q) of the liquid flowing through the pipe based on the sensor output from the sensor output section, and a heater And a bubble detector (8) configured to detect passage of bubbles moving with the liquid flowing through the pipe based on a change in the command value (MV) or the sensor output (S).

  According to the present invention, the passage of bubbles moving together with the liquid flowing through the pipe is detected based on a change in the heater command value or the sensor output. For example, a fluctuation rate (γ) of the heater command value or the sensor output is periodically obtained, and it is determined whether the obtained fluctuation rate is a decreasing rate (α) or an increasing rate (β). After the threshold value becomes equal to or more than the first threshold value (αth), the time until the rate of increase becomes equal to or more than the second threshold value (βth) is measured as a variation time (T), and the measured variation time is predetermined. By comparing with the allowable fluctuation time (Tth), it is determined that the bubble has passed when the measured fluctuation time is shorter than the allowable fluctuation time.

  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 passage of bubbles moving with the liquid flowing in the pipe is detected based on the fluctuation of the heater command value or the sensor output. The rate of change is determined periodically, and it is determined whether the rate of change is the rate of decrease or the rate of increase. After the rate of decrease is greater than or equal to the first threshold, the rate of increase is greater than or equal to the second threshold. Easily detect the passage of air bubbles that move with the liquid flowing in the pipe by measuring the time until the fluctuation as the fluctuation time and determining that the air bubbles have passed if this fluctuation time is shorter than the allowable fluctuation time. It is possible to do.

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 diagram illustrating a change in the heater command value MV when a bubble passes. FIG. 3 is a flowchart illustrating a processing operation performed by the bubble detector in the thermal flow meter according to the first embodiment. FIG. 4 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. 5 is a diagram showing the relationship between the fluctuation time T and the flow velocity v of the liquid flowing in the pipe. FIG. 6 is a flowchart illustrating a processing operation performed by the bubble detector in the thermal flow meter according to the second embodiment. FIG. 7 is a diagram showing an example corresponding to Embodiment 1 when the present invention is applied to scheme 2. FIG. 8 is a diagram showing an example corresponding to Embodiment 2 when the present invention is applied to Method 2. FIG. 9 is a diagram illustrating an example (an example corresponding to the first embodiment) in which the passage of a bubble is detected based on a change in the sensor output S. FIG. 10 is a diagram illustrating an example (an example corresponding to the second embodiment) in which the passage of a bubble is detected based on a change in the sensor output S. FIG. 11 is a view for explaining 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. 12 is a view 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 a heater.

  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, a control unit 5, a power measuring unit (sensor output unit) 6, a flow rate calculating unit 7, and a bubble detecting unit 8.

  The pipe 2 is made of, for example, glass, and flows a liquid 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 5 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 liquid flowing through the pipe 2 as TRr. The water temperature sensor 4 is installed at a position where the heater 3 is not affected by the heat by increasing the distance between the heater 3 and the water temperature sensor 4 to some extent. The liquid temperature TRr detected by the water temperature sensor 4 is sent to the control unit 5.

  The control unit 5 includes a PID control calculation unit 51 and a control output unit 52. The PID control calculation unit 51 receives the heat generation temperature TRh of the heater 3 detected from the change in the resistance value of the heater 3 and the temperature TRr of the liquid from the water temperature sensor 4 as inputs, and calculates the temperature of the heat generation temperature TRh and the temperature TRr of the liquid. A difference (TRh-TRr) is obtained, and a heater command value MV is generated such that the temperature difference becomes a constant value (for example, 10 ° C.). The control output unit 52 receives the heater command value MV from the PID control calculation unit 51 and controls the power supplied to the heater 3 based on the heater command value MV.

  The power measuring unit 6 measures the power P supplied to the heater 3 when the temperature difference (TRh-TRr) is controlled to be a constant value by the control unit 5, and outputs the measured power P to a sensor output. (Value corresponding to the state of thermal diffusion in the liquid) is sent to the flow rate calculation unit 7 as S.

  The flow rate calculation unit 7 converts the sensor output S (supply power P) from the power measurement unit 6 into a flow rate value using a preset flow rate conversion formula, thereby obtaining the flow rate of the liquid flowing through the pipe 2 ( Find the instantaneous flow rate) Q.

  The bubble detection unit 8 includes a fluctuation rate calculation unit 81, a fluctuation time measurement unit 82, an allowable fluctuation time storage unit 83, and a fluctuation time comparison unit 84. In the bubble detecting section 8, the fluctuation rate calculation section 81 receives the heater command value MV from the PID control calculation section 51 as an input, and changes the fluctuation rate (gradient) γ of the heater command value MV in a predetermined cycle (for example, 3 to 3). (6 msec cycle). The fluctuation time measuring section 82 receives the fluctuation rate γ obtained by the fluctuation rate calculating section 81 as input, determines whether the fluctuation rate γ is the decreasing rate α or the increasing rate β, and determines whether the decreasing rate α is a predetermined threshold value. After the time becomes equal to or more than αth, the time until the increase rate β becomes equal to or more than the predetermined threshold value βth is measured as the fluctuation time T.

  FIG. 2 illustrates a change in the heater command value MV when an air bubble passes. When bubbles pass along with the movement of the liquid flowing through the pipe 2, the heater command value MV temporarily decreases and then immediately increases. The fluctuation time measuring unit 82 measures, as the fluctuation time T, a time interval from when the heater command value MV decreases to when it increases when the bubble passes.

  The variation time comparison unit 84 receives the variation time T measured by the variation time measurement unit 82 as an input, compares the variation time T with the allowable variation time Tth stored in the allowable variation time storage unit 83, and performs measurement. If the fluctuation time t is shorter than the allowable fluctuation time Tth, it is determined that the bubble has passed.

  In this embodiment, a predetermined value (constant value) is stored in the permissible fluctuation time storage unit 83 as the permissible fluctuation time Tth when the bubble passes. The allowable fluctuation time Tth stored in the allowable fluctuation time storage unit 83 is determined by repeating an experiment.

  FIG. 3 shows a flowchart of a processing operation performed by the bubble detection unit 8. The bubble detection unit 8 repeats the processing operation shown in this flowchart at a constant cycle.

In the bubble detection unit 8, the fluctuation rate calculation unit 81 fetches the heater command value MV from the PID control calculation unit 51 (Step S101), and calculates the fluctuation rate γ of the heater command value MV (Step S102). The fluctuation time measuring unit 82 determines whether the fluctuation rate γ is a fluctuation rate in a decreasing direction or a fluctuation rate in an increasing direction (step S103).
(Step S108).

  When air bubbles pass along with the movement of the liquid flowing through the pipe 2, as shown in FIG. 2, the heater command value MV temporarily decreases and then immediately increases. Therefore, when air bubbles pass, the variation rate γ of the heater command value MV becomes a variation rate in a decreasing direction, and then changes to a variation rate in an increasing direction. That is, when the bubble passes, in step S103, the rate of change in the decreasing direction is checked first, and then the rate of change in the increasing direction is checked.

  When the fluctuation rate γ of the heater command value MV is a fluctuation rate in the decreasing direction, the fluctuation time measuring unit 82 sets the fluctuation rate γ as the reduction rate α (step S104). Then, the reduction rate α is compared with the threshold value αth (step S105). When α ≧ αth (YES in step S105: point t1 shown in FIG. 2), it is confirmed that the fluctuation time T is not being counted. Then (NO in step S106), measurement of the fluctuation time T is started (step S107).

  If the fluctuation rate γ of the heater command value MV is a fluctuation rate in the increasing direction, the fluctuation time measuring unit 82 sets the fluctuation rate γ as the increase rate β (step S108). Then, the increase rate β is compared with the threshold value βth (step S109). If β ≧ βth is satisfied (YES in step S109: point t2 shown in FIG. 2), the measurement of the fluctuation time T ends (step S110). ).

  The fluctuation time comparison unit 84 compares the fluctuation time T after the completion of the time measurement with the allowable fluctuation time Tth (step S111), and when T <Tth (YES in step S111), determines that the bubble has passed. (Step S112).

[Embodiment 2]
FIG. 4 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 flow meter 1A of the first embodiment, the allowable fluctuation time Tth is set to a constant value. However, the fluctuation time T measured by the fluctuation time measuring unit 82 is inversely proportional to the flow velocity v of the liquid flowing in the pipe 2 (see FIG. 5). For this reason, when the allowable fluctuation time Tth is a constant value, the allowable fluctuation time Tth may be too long or too short depending on the flow velocity v of the liquid, and the bubble may not be detected properly.

  Therefore, in the thermal type flow meter 1B of the second embodiment, the air flow detecting unit 8 is provided with the flow velocity calculating unit 85 and the allowable fluctuation time calculating unit 86, and the allowable fluctuation time Tth to the fluctuation time comparing unit 84 is set in the pipe 2. As a value inversely proportional to the flow velocity v of the flowing liquid, the influence of the flow velocity v is eliminated.

  That is, in the thermal flow meter 1B according to the second embodiment, the flow velocity calculating section 85 calculates the flow velocity v of the liquid flowing in the pipe 2 from the flow rate (instantaneous flow rate) Q. Is sent to the allowable fluctuation time calculation unit 86, the allowable fluctuation time calculation unit 86 calculates the allowable fluctuation time Tth as a value inversely proportional to the flow velocity v, and sends the obtained allowable fluctuation time Tth to the fluctuation time comparison unit 84.

  FIG. 6 shows a flowchart corresponding to FIG. As shown in this flowchart, the thermal flow meter 1B of the second embodiment calculates the flow velocity v of the liquid flowing in the pipe 2 after finishing measuring the fluctuation time T (step S110) (step S110). S113), the allowable fluctuation time Tth is calculated as a value inversely proportional to the calculated flow velocity v of the liquid (step S114). If T <Tth (YES in step S111), it is determined that the air bubble has passed (step S112). .

  In the above-described first and second embodiments, the case where the present invention is applied to the method (method 1) in which the electric power P supplied to the heater 3 is used as the sensor output S has been described. The present invention may be applied to a system (system 2) in which the temperature difference (TRu-TRd) is used as the sensor output S. FIG. 7 shows an example corresponding to the first embodiment when the present invention is applied to method 2, and FIG. 8 shows an example corresponding to the second embodiment when the present invention is applied to method 2.

  In the thermal flow meters 1C and 1D shown in FIGS. 7 and 8, an upstream temperature sensor (temperature measuring element) 9 for detecting the temperature TRu of the liquid on the upstream side of the heater 3, and the temperature of the liquid on the downstream side of the heater 3 A downstream temperature sensor (temperature measuring element) 10 for detecting TRd 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) 11 is provided for the upstream temperature sensor 9 and the downstream temperature sensor 10.

  The temperature difference calculation unit 11 controls the heater when the control unit 5 controls the power supplied to the heater 3 so that the temperature difference (TRh-TRr) between the heat generation temperature TRh and the liquid temperature TRr becomes a constant value. The temperature difference (TRu-TRd) between the temperature TRu of the liquid on the upstream side and the temperature TRd of the liquid downstream of the heater 3 (TRu-TRd) is calculated, and the calculated temperature difference (TRu-TRd) is calculated. As a sensor output (a value corresponding to the state of thermal diffusion in the liquid) S to the flow rate calculator 7.

  The flow rate calculation unit 7 converts the sensor output S (temperature difference (TRu-TRd) between the upstream and downstream of the heater 3) from the temperature difference calculation unit 11 into a flow rate value using a preset flow rate conversion equation. Thus, the flow rate Q of the liquid flowing through the pipe 2 is obtained.

  The bubble detection unit 8 captures the heater command value MV from the PID control calculation unit 51, and detects the passage of bubbles moving with the liquid flowing in the pipe 2 based on the variation of the captured heater command value MV.

  Further, in the above-described embodiment, the passage of bubbles is detected based on the change in the heater command value MV. However, the passage of bubbles may be detected based on the change in the sensor output S. FIG. 9 shows an example corresponding to the first embodiment as a thermal flow meter 1E, and FIG. 10 shows an example corresponding to the second embodiment as a thermal flow meter 1F.

  In the above-described embodiment, the flow rate calculation unit 7 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.

[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 to 1F): thermal flow meter, 2: pipe, 3: heater, 4: water temperature sensor, 5: control unit, 6: power measurement unit, 7: flow rate calculation unit, 8: bubble detection unit, 9 ... Upstream temperature sensor, 10: downstream temperature sensor, 11: temperature difference calculation unit, 51: PID control calculation unit, 52: control output unit, 81: fluctuation rate calculation unit, 82: fluctuation time measurement unit, 83: allowable fluctuation time storage Unit, 84: fluctuation time comparison unit, 85: flow velocity calculation unit, 86: allowable fluctuation time calculation unit.

Claims (5)

A pipe configured to flow the liquid to be measured,
A heater installed on the pipe, configured to generate heat by receiving supply of electric power,
A temperature sensor installed upstream of the heater and configured to detect a temperature of the liquid;
A temperature difference between the heat generation temperature of the heater detected from a change in the resistance value of the heater and the temperature of the liquid detected by the temperature sensor is determined, and a heater command value such that the temperature difference becomes a constant value is determined. A control unit configured to control electric power supplied to the heater based on a heater command value;
A sensor output unit configured to output, as a sensor output, a value corresponding to a state of heat diffusion in the liquid when the temperature difference is controlled to be a constant value by the control unit;
A flow rate calculation unit configured to determine the flow rate of the liquid flowing through the pipe based on the sensor output from the sensor output unit,
And a bubble detector configured to detect passage of bubbles moving with the liquid flowing through the pipe based on a change in the heater command value or the sensor output. The thermal flow meter according to claim 1,
The air bubble detector,
A fluctuation rate calculator configured to periodically calculate the fluctuation rate of the heater command value or the sensor output,
It is determined whether the fluctuation rate calculated by the fluctuation rate calculation unit is a reduction rate or an increase rate. A variable time measuring unit configured to measure time as variable time,
Compare the fluctuation time measured by the fluctuation time measurement unit and a predetermined allowable fluctuation time, and determine that the bubble has passed when the measured fluctuation time is shorter than the allowable fluctuation time. A thermal flow meter, comprising: a variable time comparing unit configured. The thermal flow meter according to claim 1,
The air bubble detector,
A fluctuation rate calculator configured to periodically calculate the fluctuation rate of the heater command value or the sensor output,
It is determined whether the change rate calculated by the change rate calculation unit is a decrease rate or an increase rate. After the decrease rate is equal to or more than the first threshold, the change rate until the increase rate becomes equal to or more than the second threshold is determined. A variation time measurement unit configured to measure time as variation time,
Compare the fluctuation time measured by the fluctuation time measurement unit and the allowable fluctuation time determined according to the flow rate of the liquid flowing in the pipe, and when the measured fluctuation time is shorter than the allowable fluctuation time, the bubble And a fluctuating time comparison unit configured to determine that the flow has passed. 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 liquid 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.

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