The present application is based on and claims priority to Japanese Application No. 2017-147621, filed Jul. 31, 2017, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a thermal flowmeter that measures the temperature of a fluid at two points, namely, an upstream point and a downstream point, of a pipe, controls a heater so as to keep the difference between the temperatures at the two points constant, and calculates the flow rate of the fluid from a power that is supplied to the heater.
A thermal flowmeter according to the related art has a structure in which a heater and temperature sensors are disposed on the exterior of a pipe that constitutes a flow passage for a liquid so that the heater and the temperature sensors are not directly exposed to the liquid to avoid, for example, corrosion due to the liquid (see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-532099). In the case of such a structure, the heater and the temperature sensors are not directly exposed to the liquid, and therefore, responsiveness to changes in the flow rate of the liquid is lower than that attained with a structure in which a heater and temperature sensors are directly exposed to a liquid. This results in an increased error in the calculated flow rate, which is an issue.
The present disclosure has been made to address the above-described issue and provides a thermal flowmeter and a flow rate compensation method with which responsiveness to changes in the flow rate can be improved.
A thermal flowmeter according to an aspect of the present disclosure includes a pipe, a first thermal resistive element, a second thermal resistive element, a control unit, a power measurement unit, a temperature difference gradient calculation unit, a power compensation unit, and a flow rate calculation unit. The pipe is configured to allow a measurement target fluid to flow therethrough. The first thermal resistive element is disposed on the pipe and configured to sense a first temperature of the measurement target fluid. The second thermal resistive element is disposed on the pipe downstream relative to the first thermal resistive element and configured to sense a second temperature of the measurement target fluid. The control unit is configured to output a voltage with which the second temperature is kept higher than the first temperature by a predetermined value and cause the second thermal resistive element to generate heat. The power measurement unit is configured to measure a power that is supplied to the second thermal resistive element. The temperature difference gradient calculation unit is configured to calculate a gradient of a temperature difference between the second temperature and the first temperature. The power compensation unit is configured to compensate the power measured by the power measurement unit on the basis of the gradient of the temperature difference and a known value of the power measured when no fluid is present in the pipe. The flow rate calculation unit is configured to calculate a flow rate of the measurement target fluid on the basis of the power compensated by the power compensation unit.
In the thermal flowmeter described above, the power compensation unit compensates the power only in a case where an absolute value of the gradient of the temperature difference is larger than a predetermined temperature dead-band parameter; and the flow rate calculation unit calculates the flow rate of the measurement target fluid on the basis of the power compensated by the power compensation unit in a case where the absolute value of the gradient of the temperature difference is larger than the temperature dead-band parameter, and calculates the flow rate of the measurement target fluid by using the power measured by the power measurement unit as is in a case where the absolute value of the gradient of the temperature difference is equal to or smaller than the temperature dead-band parameter.
A flow rate compensation method for a thermal flowmeter according to an aspect of the present disclosure is a flow rate compensation method for a thermal flowmeter, the thermal flowmeter including a pipe allowing a measurement target fluid to flow therethrough, a first thermal resistive element disposed on the pipe and sensing a first temperature of the measurement target fluid, and a second thermal resistive element disposed on the pipe downstream relative to the first thermal resistive element and sensing a second temperature of the measurement target fluid. The method includes: a first step of outputting a voltage with which the second temperature is kept higher than the first temperature by a predetermined value and causing the second thermal resistive element to generate heat; a second step of measuring a power that is supplied to the second thermal resistive element; a third step of calculating a gradient of a temperature difference between the second temperature and the first temperature; a fourth step of compensating the power measured in the second step on the basis of the gradient of the temperature difference and a known value of the power measured when no fluid is present in the pipe; and a fifth step of calculating a flow rate of the measurement target fluid on the basis of the power compensated in the fourth step.
According to an aspect of the present disclosure, the power measured by the power measurement unit is compensated on the basis of the gradient of the temperature difference and a known value of the power measured when no fluid is present in the pipe, and the flow rate of the measurement target fluid is calculated on the basis of the compensated power. Therefore, even if the first and second thermal resistive elements are not directly exposed to the measurement target fluid, responsiveness to changes in the flow rate can be improved, and an error in a flow rate measurement can be decreased.
Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings.
The thermal resistive elements 2a and 2b are each formed on a silicon wafer. The thermal resistive element 2a is formed on a surface of the silicon wafer and is adhered to the pipe 1 such that the surface faces the external wall of the pipe 1. In this manner, the thermal resistive element 2a is fixed to the pipe 1. The thermal resistive element 2b is also fixed in a similar manner to the thermal resistive element 2a. In the example illustrated in
Now, the operations of the thermal flowmeter according to this embodiment are described.
The temperature obtaining units 3a and 3b respectively obtain the temperatures TRr and TRh of the fluid that flows through the pipe 1 (step S100 in
The subtractor 4 subtracts the temperature TRr of the upstream fluid from the temperature TRh of the downstream fluid (step S101 in
The PID control arithmetic unit 5 calculates an operation amount with which the temperature difference ΔT=TRh−TRr calculated by the subtractor 4 is kept constant (a set point in control and, for example, 10° C.) (step S102 in
The control output unit 6 applies a voltage to the thermal resistive element 2b in accordance with the operation amount calculated by the PID control arithmetic unit 5 to generate heat (step S103 in
The process from step S100 to step S103 is performed in each control cycle until the thermal flowmeter stops operating (Yes in step S104 in
Q=V2/Rh (1)
Accordingly, the power Q that is needed to keep the temperature TRh of the downstream fluid higher than the temperature TRr of the upstream fluid by a predetermined value can be obtained.
The temperature difference ΔT between the temperature TRh of the downstream fluid and the temperature TRr of the upstream fluid is controlled by the control unit 11 (the subtractor 4, the PID control arithmetic unit 5, and the control output unit 6) so as to be kept constant as described above; however, the instantaneous value of the temperature difference ΔT is not necessarily kept constant due to changes in the flow rate of the fluid. Therefore, the temperature difference gradient calculation unit 8 calculates the gradient (which is the slope of the temperature difference and is expressed as a time derivative) ΔT′ of the temperature difference ΔT (step S201 in
ΔT′=dΔT/dt=d(TRh−TRr)/dt (2)
If the absolute value |ΔT′| of the temperature difference gradient ΔT′ calculated by the temperature difference gradient calculation unit 8 is larger than a predetermined temperature dead-band parameter PRM_DT_DEAD (Yes in step S202 in
Qcomp=Q−PRM_D×(Q−Qempty)×ΔT′ (3)
In equation (3), PRM_D is a predetermined coefficient, and Qempty is a known power Q measured when the pipe 1 is filled with air and no fluid is present therein at all. The power Qempty can be determined in advance by operating the thermal flowmeter as described with reference to
If the absolute value |ΔT′| of the temperature difference gradient ΔT′ is larger than the temperature dead-band parameter PRM_DT_DEAD, the flow rate calculation unit 10 converts the value of the power Qcomp compensated by the power compensation unit 9 to a flow rate value by using a predetermined flow rate conversion property equation f to thereby calculate the flow rate F of the measurement target fluid (step S204 in
F=f(Qcomp) (4)
On the other hand, if the absolute value |ΔT′| of the temperature difference gradient ΔT′ calculated by the temperature difference gradient calculation unit 8 is equal to or smaller than the temperature dead-band parameter PRM_DT_DEAD (No in step S202), the power compensation unit 9 outputs the value of the power Q measured by the power measurement unit 7 as is without compensation (step S205 in
If the absolute value |ΔT′| of the temperature difference gradient ΔT′ is equal to or smaller than the temperature dead-band parameter PRM_DT_DEAD, the flow rate calculation unit 10 converts the value of the power Q output from the power compensation unit 9 without compensation to a flow rate value by using the flow rate conversion property equation f to thereby calculate the flow rate F of the measurement target fluid (step S206 in
F=f(Q) (5)
The process from step S200 to step S206 is performed at predetermined intervals until the thermal flowmeter stops operating (Yes in step S207 in
Now, the principle of power compensation made by the power compensation unit 9 is described. In a thermal steady state, the following equation approximately holds.
(TRh−TRr)×k(FV)=(Q−Qempty) (6)
Here, k(FV) is a heat transfer rate based on the flow velocity FV of the fluid. According to the principle of the thermal flowmeter, when the thermal resistive element 2b (heater) is caused to generate heat so that the temperature difference ΔT=(TRh−TRr) is kept constant (10° C. in this embodiment), the instantaneous flow rate of the fluid can be obtained from the power Q that is supplied to the thermal resistive element 2b.
However, in a case where the flow rate abruptly changes, a delay occurs in a response of the measured flow rate value to the changing flow rate even if heater control by the control unit 11 is accelerated. This is because the heat transfer rate k(FV) changes slowly relative to the changing flow rate. This delay is due to the thermal properties of the pipe 1 and the fluid and is not improved by PID control.
Therefore, in this embodiment, the power Q that is needed to keep the temperature difference ΔT=(TRh−TRr) constant is compensated as follows. First, the following equation is obtained from equation (6).
k(FV)=(Q−Qempty) (7)
From equation (7), the power Qcomp, which is obtained by compensating the power Q, can be approximated as follows.
Δk=(Q−Qempty)ΔT′/ΔT2 (8)
Qcomp=ΔT×k+Δk×ΔT (9)
Here, Δk represents a change in the heat transfer rate k(FV). If the absolute value |ΔT′| of the temperature difference gradient ΔT′ is larger than the temperature dead-band parameter PRM_DT_DEAD, equation (3) is obtained from equation (8) and equation (9). Here, the temperature dead-band parameter PRM_DT_DEAD is a constant that is determined on the basis of the effective resolution of control of the temperature difference ΔT by the control unit 11. As the temperature dead-band parameter PRM_DT_DEAD, for example, the absolute value |ΔT′| of the temperature difference gradient ΔT′ obtained when an error relative to the set point (10° C. in this embodiment) of the temperature difference ΔT goes beyond an allowance due to a delay in changing of the heat transfer rate k(FV) in response to the changing flow rate needs to be specified.
Further, the coefficient PRM_D in equation (3) is a value that is determined in advance and depends mainly on the physical form of the thermal flowmeter and the properties (mainly, the heat transfer rate k(FV)) of the fluid.
On the other hand, the case where the absolute value |ΔT′| of the temperature difference gradient ΔT′ is equal to or smaller than the temperature dead-band parameter PRM_DT_DEAD indicates a steady state where the flow rate of the fluid changes to a small degree, and the second term on the right side of equation (3) becomes sufficiently small. Therefore, the power Q measured by the power measurement unit 7 need not be compensated.
The principle of power compensation made by the power compensation unit 9 is as described above.
As described above, in this embodiment, if the absolute value |ΔT′| of the temperature difference gradient ΔT′ is equal to or smaller than the temperature dead-band parameter PRM_DT_DEAD (in a steady state where the flow rate changes to a small degree), the flow rate is calculated by using the power Q measured by the power measurement unit 7 as is. If the absolute value |ΔT′| of the temperature difference gradient ΔT′ is larger than the temperature dead-band parameter PRM_DT_DEAD (in a state where the flow rate abruptly changes), the flow rate is calculated by compensating the power Q. As described above, in this embodiment, the arithmetic method is switched in the steady state and the state where the flow rate abruptly changes. Therefore, responsiveness to changes in the flow rate can be improved, and an error in a flow rate measurement can be decreased.
In the thermal flowmeter according to this embodiment, at least the subtractor 4, the PID control arithmetic unit 5, the temperature difference gradient calculation unit 8, the power compensation unit 9, and the flow rate calculation unit 10 can be implemented by using a computer including a central processing unit (CPU), a memory, and an external interface and a program that controls these hardware resources. The CPU performs the processes described in this embodiment in accordance with the program, which is stored in the memory, to implement the flow rate compensation method for the thermal flowmeter of the present disclosure.
The present disclosure is applicable to thermal flowmeters.
Number | Date | Country | Kind |
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2017-147621 | Jul 2017 | JP | national |
Number | Name | Date | Kind |
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6125695 | Alvesteffer | Oct 2000 | A |
6681625 | Berkcan | Jan 2004 | B1 |
Number | Date | Country |
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1330764 | Jan 2002 | CN |
105283737 | Jan 2016 | CN |
107923780 | Apr 2018 | CN |
2003-106886 | Apr 2003 | JP |
2003-532099 | Oct 2003 | JP |
Entry |
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Combined Chinese Office Action and Search Report dated Feb. 21, 2020 in Chinese Patent Application No. 201810816576.7 (with English translation of Category of Cited Documents), 7 pages. |
Number | Date | Country | |
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20190033110 A1 | Jan 2019 | US |