JPH04339218A - thermal flow meter - Google Patents

Abstract

PURPOSE:To measure a flow rate without generating a disturbance in a fluid even when the flow rate of the fluid is small. CONSTITUTION:A heater 21 is provided on a pipeline through which a fluid flows, an upstream temperature detection element 22a and a downstream temperature detection means 22b are on the upstream and downstream sides of the heater 21 and an arithmetic circuit part 3 is provided to compute a flow rate of the fluid based on outputs of these temperature detection elements 22a and 22b. With the arithmetic circuit section 3, a heating value of the heater 21 is controlled based on the flow rate of the fluid. The heating value of the heater 21 is controlled with the arithmetic circuit section 3 based on the flow rate of the fluid to check overheating of the fluid thereby allowing the prevention of possible bubbles in the fluid. Thus, a drop in flow rate measuring accuracy can be prevented as caused by the generation of the bubbles thereby enabling the obtaining of very good measurement results. This also can prevent thermal reaction or denaturization of the fluid as caused by the overheating.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal flowmeter that is excellent in controlling fluid temperature.

0002

2. Description of the Related Art Conventionally, a thermal flowmeter is one type of flowmeter that measures the flow rate of fluid flowing through a pipe. This type of thermal flowmeter is equipped with a heater that heats the fluid flowing in the pipe, and temperature detection elements are provided in each of the pipes on the upstream and downstream sides of the heater, and the output from these temperature detection elements is Some devices measure the flow rate of the fluid based on the detection results.

0003

[Problems to be Solved by the Invention] Incidentally, in order to equalize measurement conditions, the above-mentioned heater uses a constant current circuit as a current supply circuit that supplies current to the heater, thereby keeping the heat generation of the heater constant. A heating method is used to heat the fluid, but with this heating method, the flow rate can be measured without any problem if the fluid flow rate is relatively large, but if the fluid has no flow rate or is extremely small, the flow rate can be measured without any problem. When there is only a flow rate (that is, when the temperature difference ΔT between the upstream side and the downstream side across the heater is zero or extremely small), as shown in Figure 7, the fluid The temperature is heated more than necessary (1cc/
(It will be heated to about twice the temperature compared to when there is a flow rate of min or more). As a result, bubbles may be generated in the fluid, making it difficult to accurately measure the flow rate, or causing a thermal reaction or deterioration of the fluid.

[0004] The present invention was made in view of the above circumstances, and an object of the present invention is to provide a thermal flow meter that is capable of extremely accurate flow measurement even when the flow rate of fluid is small. .

[0005]

[Means for Solving the Problems] The thermal flowmeter of the present invention includes a pipe constituting a flow path, a heating section that heats a fluid flowing in the pipe, and an upstream side and a downstream side of the heating section. a temperature detection element provided in each of the pipe lines to detect the temperature of the fluid; an arithmetic circuit unit that calculates the flow rate of the fluid based on the detection result of the temperature detection element; and a control section that controls the amount of heat generated by the heating section based on the flow rate of the fluid.

[0006]

[Operation] According to the thermal flowmeter of the present invention, the heating amount of the heating section that heats the fluid is controlled based on the flow rate of the fluid flowing in the pipe. Therefore, it is possible to prevent the fluid from being overheated, and it is possible to prevent the generation of bubbles, thermal reaction, deterioration, etc. of the fluid due to overheating.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the thermal flowmeter of the present invention will be described below with reference to the drawings. First, a first example will be described. As shown in Figure 1, the thermal flowmeter of the present invention is made of thin metal (
A conduit 1 made of a resin (e.g., stainless steel, etc.) or a resin (e.g., fluororesin, etc.), a sensor section 2 provided in the conduit 1 and entirely covered with a heat insulating material (not shown), and a sensor section 2 from the sensor section 2. It is comprised of an arithmetic circuit section 3 that converts the detected signal into a flow rate. As shown in FIG. 2, the sensor section 2 includes a heater (heating section) 21, and an upstream temperature detection element 22a and a downstream temperature detection element 22b provided on the upstream and downstream sides of the heater 21. There is. The heater 21 is attached closely to the outer periphery of the pipe line 1 so that heat can be efficiently transferred to the fluid flowing inside the pipe line 1, and the heater 21 is attached to the outer circumference of the pipe line 1 in close contact with the upstream temperature detection element 22a and the downstream temperature detection element 22b. are attached in close contact with the pipe line 1, similarly to the heater 21, so that the temperature of the fluid flowing through the pipe line 1 can be accurately detected. Further, the upstream temperature detection element 22a is placed at a distance where the fluid flowing into the sensor section 2 is not affected by heating by the heater 21 so that the reference temperature of the fluid flowing into the sensor section 2 can be accurately detected. It is installed,
The downstream temperature detection element 22b is attached at a position close to the heater 21 so that the temperature of the fluid heated by the heater 21 can be accurately detected.

Next, the configuration of the arithmetic circuit section 3 will be explained with reference to FIG. This arithmetic circuit section 3 includes an upstream temperature detection element 2
The temperature difference Δ of the fluid on the upstream side and the downstream side with the heater 21 in between is determined from the upstream side detected temperature Ta and the downstream side detected temperature Tb detected by the downstream side temperature detection element 2a and the downstream side temperature detection element 22b.
A subtraction circuit 31 that calculates T (ΔT=Tb-Ta) compares and determines the magnitude of the temperature difference ΔT obtained by this subtraction circuit 31 with a preset temperature TE (described later), and determines whether ΔT>TE. Judgment circuit 3 that outputs the judgment result only when
2, the supply current I to the heater 21 is controlled by inputting the determination result from the determination circuit 32, and when the determination result is not input, a preset constant current I0 is supplied to the heater 21 as the supply current I. an arithmetic circuit that calculates the flow rate of the fluid flowing through the conduit 1 based on the temperature difference ΔT obtained by the subtraction circuit 31 and the supply current I supplied from the current supply circuit 33 to the heater 21; 3
It consists of 4.

Here, the arithmetic circuit 34 is connected to the current supply circuit 3
The heating amount of the heater 21 is calculated from the supply current I of No. 3 as q=rI2(
However, the mass flow rate Q (Q=ρν) of the fluid is calculated from the heating amount q and the temperature difference ΔT using the following equation. Q = ρν = κ (q / ΔT) 1.67 ... (1) (where ρ: fluid density, ν: flow velocity, κ: coefficient)

0010
] Furthermore, the above-mentioned set temperature TE is a temperature at which bubbles are not generated in the fluid. In other words, the temperature difference ΔT between the upstream side and the downstream side of the fluid with the heater 21 in between is determined by the flow rate when the supply current supplied from the power supply circuit 33 is set to a constant current I0 to keep the heating amount of the heater 21 constant. As a result of the change, the flow rate becomes a curve that increases up to a certain flow rate and then decreases thereafter, as shown by reference numeral a in FIG. In a small flow rate region where the temperature difference ΔT increases, the temperature in the fluid may be excessively heated by the heat of the heater 21, which may lead to the generation of bubbles or the like in the fluid. Therefore, it is necessary to adjust the supply current I to the heater 21 to eliminate the increase in the temperature difference ΔT of the fluid in the small flow rate region as shown in b in the figure. Then, b in the figure
Let the temperature in the minute flow rate region shown by be the aforementioned set temperature TE. Further, the above equation (1) can be applied in a region where the temperature difference ΔT decreases as the flow rate increases.

Next, as described above, the arithmetic circuit section 3 is provided with a control system (control section) for controlling the amount of heat generated by the heater 21.
The operation of fluid temperature control according to the present invention will be explained with reference to the flowchart shown in FIG. Step SP1 First, constant current I0 and set temperature TE are set in advance. Step SP2: The subtraction circuit 31 calculates a temperature difference ΔT based on the detected temperatures of the upstream temperature detection element 22a and the downstream temperature detection element 22b. Step SP3 Then, the determination circuit 32 compares and determines this temperature difference ΔT with a preset temperature TE. Here, if the temperature difference ΔT is ΔT>TE, the process moves to step SP4,
If not, the process moves to step SP6. Step SP4: The current value of the supply current I output from the current supply circuit 33 is calculated using the following equation. I=I0-A(ΔT-TE) (A: coefficient) Step SP5 The supply current I having the current value obtained in step SP4 above is supplied from the current supply circuit 33 to the heater 21, and the amount of heating in the heater 21 is This prevents the temperature difference ΔT from increasing. Then, the process moves to step SP2 and ΔT is detected again. Step SP6: The supply current I output from the current supply circuit 33 to the heater 21 and the constant current I0 are compared. Here, if the relationship between the supply current I and the constant current I0 is I<I0, then
The process moves to step SP4, and if not, the process moves to step SP7. Step SP7: The supply current I is set to the constant current I0, and the process moves to step SP5, where the constant current I0 is supplied from the current supply circuit 33 to the heater 21 as the supply current I.

As explained above, according to the thermal flowmeter of the first embodiment, the temperature difference Δ
Since T can be set to a preset temperature TE (temperature at which no bubbles are generated in the fluid), it is possible to prevent the generation of bubbles in the fluid in a micro flow rate region. Further, since the temperature of the fluid can be appropriately controlled in the micro flow rate region, responsiveness can be improved. Further, by reducing the current value of the supply current I supplied from the current supply circuit 33, it is possible to reduce power consumption. Note that power consumption can be further reduced by setting the fluid output to zero in the operating state of the determination circuit 32 and automatically setting the output from the current supply circuit 33 to zero.

Next, a second embodiment will be explained. First, the configuration of the arithmetic circuit section 3' of this thermal flowmeter will be explained. This arithmetic circuit unit 3' calculates the temperature difference of the fluid on the upstream side and the downstream side of the heater 21 from the upstream side detected temperature Ta and the downstream side detected temperature Ta detected by the upstream side temperature detection element 22a and the downstream side temperature detection element 22b. a subtraction circuit 41 that calculates ΔT and calculates the flow rate of the fluid based on this temperature difference ΔT;
A comparison circuit 43 that compares and determines the output value S1 output from the subtraction circuit 41 with a set value S2 output from a comparison setting circuit 42 to be described later and outputs comparison data; and a comparison circuit 43 that outputs comparison data. Consists of a switching circuit 46 that selectively connects either a high temperature heating circuit 44 that outputs a current with a high current value or a low temperature heating circuit 45 that outputs a current with a low current value to the heater 21 based on the data. has been done.

Here, the subtraction circuit 41 calculates the flow rate Q of the fluid from the temperature difference ΔT using the following equation. Q=κCpΔT (2) (where κ: constant, Cp: specific heat) Next, as described above, the arithmetic circuit section 3' is provided with a control system (control section) for controlling the amount of heat generated by the heater 21. The operation of fluid temperature control will be explained below. When the fluid flows through the pipe line 1, the temperature of the fluid is detected by the upstream temperature detection element 22a and the downstream temperature detection element 22b, and the temperature difference ΔT is determined by the subtraction circuit 41, and the flow rate Q of the fluid is calculated. Ru. Then, the output value S1 outputted from the subtraction circuit 41 is transferred to the comparison setting circuit 4 by the comparison circuit 43.
2 is compared with the set value S2 outputted from 2, and the comparison data that is the result of the comparison is outputted to the switching circuit 46. Here, the comparison judgment result by the comparison circuit 42 is S1≦S
2, the contact 46a of the switching circuit 46 moves toward the contacts 46b, 46b and comes into contact with the contacts 46b.
, 46b are electrically connected to each other, and a high current is supplied from the high temperature heating circuit 44 to the heater 21 to heat the fluid with a large amount of heat. Further, when the comparison judgment is S1>S2, the contact 46a of the switching circuit 46 is the contact 46c,
46c side and make contact, these contacts 46c, 46c
They are electrically connected to each other, and a low current value is supplied from the low temperature heating circuit 45 to the heater 21 to heat the fluid with a small amount of heat generated.

That is, according to the thermal flowmeter of the second embodiment, when the flow rate of the fluid flowing in the pipe line 1 is zero or less than a predetermined flow rate, the heating of the fluid by the heater 21 is reduced. , when the flow rate is larger than the predetermined flow rate, the heating of the fluid by the heater 21 is increased. Thereby, it is possible to prevent the fluid from being overheated when the fluid flow rate is zero or less than a predetermined flow rate, and it is possible to prevent the generation of bubbles due to the fluid overheating. Therefore, it is possible to prevent the flow rate measurement accuracy from decreasing due to the generation of bubbles, and it is possible to obtain extremely good measurement results. In addition, unnecessary heating can be eliminated, so
It is possible to reduce power consumption, and it is possible to obtain an extremely economical thermal flowmeter.

Note that when the thermal flowmeter of the second embodiment operates in the high-temperature heating circuit 44, the output is larger than when operating in the low-temperature heating circuit 45, so the comparison circuit 43 uses the comparison data. At the time when S2 is output, the comparison setting circuit 42 newly sets the setting value S2 and outputs it to the comparison circuit 43. The comparison circuit 43 then compares and determines the newly set setting value S2 and the output value S1 output from the subtraction circuit 41. Furthermore, if the flow rate output of the fluid is set to zero based on the operating state of the low-temperature heating circuit 45, it can also be used as a zero cut function.

Note that the specific configuration of the thermal flowmeter of the above embodiment is not limited to the embodiment. Furthermore, when detecting a state in which a fluid is flowing, it is of course necessary to supply a larger amount of heat in order to obtain a good S/N ratio, especially if the target is a liquid with a large heat capacity. .

[0018]

[Effects of the Invention] As explained above, according to the thermal flowmeter of the present invention, the following effects can be obtained.

Since the heating amount of the heating section that heats the fluid can be controlled based on the flow rate of the fluid flowing in the pipe, it is possible to prevent the generation of bubbles due to excessive heating of the fluid. Thereby, it is possible to prevent the flow rate measurement accuracy from decreasing due to the generation of air bubbles, and it is possible to obtain extremely good measurement results. Further, it is possible to prevent a thermal reaction or deterioration of the fluid due to excessive heating.

Furthermore, since unnecessary heating can be eliminated, power consumption can be reduced, resulting in an extremely economical thermal flow meter.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a thermal flowmeter of the present invention.

FIG. 2 is a schematic cross-sectional view of the sensor section of the thermal flowmeter of the present invention.

FIG. 3 is a functional block diagram of the thermal flowmeter of the first embodiment.

FIG. 4 is a diagram illustrating the relationship between the flow rate of fluid flowing through the pipe and the temperature difference.

FIG. 5 is a flowchart illustrating a heating control operation of the thermal flowmeter of the first embodiment.

FIG. 6 is a functional block diagram illustrating the configuration of a thermal flowmeter according to a second embodiment.

FIG. 7 is a temperature distribution diagram of fluid heated in a pipe.

[Explanation of symbols]

1 Pipeline 3, 3' Arithmetic circuit section 21 Heater (heating part)

Claims (1)

[Claims] 1. A pipe constituting a flow path, a heating part that heats the fluid flowing in the pipe, and a heating part provided in the pipe on the upstream and downstream sides of the heating part to adjust the temperature of the fluid. a temperature detection element that detects the temperature, a calculation circuit unit that calculates the flow rate of the fluid based on the detection result of the temperature detection element, and a calculation circuit unit that is provided in the calculation circuit unit and calculates the amount of heat generated by the heating unit based on the flow rate of the fluid. 1. A thermal flowmeter comprising: a control section for controlling the flowmeter;