JPH0862008A - Thermal flow rate sensor - Google Patents

Abstract

PURPOSE: To obtain a thermal flow rate sensor for detecting the presence of flow stably of any liquid having different temperature by connecting a resistor element tar correcting the temperature rise in series with a resistor element for detecting the water temperature configuring a bridge circuit on the outside of a liquid channel. CONSTITUTION: A bridge circuit measuring the current variation of a resistor element 3 for detecting the flow rate comprises a series circuit of a resistor element 2 (resistance R4) for detecting the water temperature and a resistor element 8 (resistance R5) for correcting the temperature rise, an element 3 (resistance R3), and two fixed resistor elements (resistances R1 and R2) disposed on the outside of a fluid channel. In such bridge circuit, the element 2 has same temperature as a fluid F when the fluid F is stationary and the element 3 is heated. Since the elements 2 and 3 have different temperatures, they exhibit slightly different resistances to cause a micro potential difference between V1 and V2 which is corrected by connecting the elements 8 in series with the element 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal type flow sensor for measuring the presence / absence of a liquid flow, and more particularly to a resistance value of a thermal type flow detecting resistance element which is cooled by a flow of a fluid and changes. The present invention relates to a thermal type flow sensor that operates by determining the output voltage obtained from a bridge circuit and that can operate stably even when the liquid temperature changes.

[0002]

2. Description of the Related Art Conventionally, as a thermal type flow rate sensor for detecting a flow of a fluid, a flow meter for sequentially measuring a flow rate and a running water switch for simply detecting the presence or absence of a predetermined flow are used. Then, as a flow meter and a water switch, a heater and a pair of thermocouples are installed in the conduit through which the fluid flows, and the temperature drop of the fluid heated by the heater is measured by the thermocouple, thereby the flow velocity of the liquid and the flow rate of the fluid. A thermal flow meter is used to measure. However, the thermal type flow meter using the thermocouple has a problem that the response speed is slow. Therefore, when a fast response is required, the flow velocity and flow rate are measured by a thermal anemometer. On the other hand, a thermal anemometer is also used as a running water switch.In this case, a resistance element for flow rate detection is placed in a fluid to heat it, and it is cooled by the flow of the fluid to detect a change in the fluid. The resistance value of the resistance element is detected as an output voltage by the bridge circuit, and by comparing the output voltage with a certain threshold value and making a determination,
It is operated as an ON-OFF switch.

That is, in the conventional thermal anemometer,
One of the resistance elements forming the bridge circuit is arranged as a flow rate detecting resistance element in a pipe through which a fluid flows, and a current is constantly supplied to the resistance element to heat it. Here, when the flow velocity of the fluid changes, the amount of heat taken away from the resistance element by the fluid changes, so the resistance value of the flow rate detection resistance element changes. The thermal anemometer measures the flow velocity and the flow rate by detecting the change in the current flowing through the resistance element for flow rate detection whose resistance value has changed as an output voltage from the bridge circuit.
The thermal anemometer has been widely used recently because it can measure the flow rate accurately and with good responsiveness.

However, when the temperature of the fluid passing through the conduit changes, the temperature of the flow detecting resistance element changes even with the same flow rate, and the amount of heat taken away by the fluid from the flow detecting resistance element differs. It will be. Along with this, the output voltage detected by the bridge circuit for the change in the current flowing through the resistance element for flow rate detection whose resistance value has changed also varies depending on the fluid temperature even if the flow rate is the same. That is, there is a problem that an error occurs in the flow rate measurement due to the temperature change of the fluid.
That is, FIG. 4 shows the relationship between the output voltage and the flow rate measured by the conventional one-element thermal flow sensor.
Here, the one-element type thermal type flow rate sensor is one in which only one resistance element for flow rate detection is arranged in a pipeline through which a fluid is flowing.

Using this one-element thermal flow sensor,
The liquid used for the measurement is water. Liquid temperature (water temperature)
Are experimenting with 6 kinds of 0, 10, 20, 30, 40, 50 (° C.). It can be seen that the change in the output voltage with respect to the change (increase) in the flow rate is small at all water temperatures. However, as can be seen from FIG. 4, the value of the output voltage for the same flow rate greatly differs depending on the water temperature, and it is understood that the influence of the liquid temperature on the output voltage is large. Therefore, when measuring the flow rate of liquids with different temperatures, the output voltage changes depending on the temperature of the liquid, and the same output voltage is not applied to all liquids with different temperatures. Unable to set threshold and act as a flush switch.

In order to eliminate the measurement error due to the temperature change of the fluid, in the thermal anemometer, in order to correct the temperature of the flow detecting resistance element forming the bridge circuit, the flow detecting resistance element and the water temperature detecting A resistance element is attached inside the flow path to form a bridge circuit.

[0007]

However, the conventional thermal type flow rate sensor has the following problems. That is, the change in the resistance value of the flow rate detecting resistance element itself caused by the self heating of the flow rate detecting resistance element is not considered. And the state where the bridge circuit is in equilibrium (the state where no current flows to the galvanometer of the bridge circuit)
The bridge circuit was operating in a state in which it was displaced. Therefore, there is a problem in that the operation as the running water switch may malfunction.

That is, FIG. 5 shows the relationship between the output voltage and the flow rate measured by a two-element running water switch using a conventional flow rate detecting element and a water temperature detecting element. The liquid used for measurement using this two-element thermal flow sensor is
It is water. The liquid temperature (water temperature) is 0, 10, 20, 3
We are experimenting with 6 types of 0, 40, 50 (° C). As can be seen from FIG. 5, in this two-element type thermal type flow sensor, the output voltage value for the same flow rate at all water temperatures is higher than that in the one-element type flow type sensor of FIG. It is not as large as in the case of a one-element flow sensor. That is, the effect of the liquid temperature on the output voltage is
It turns out that it's not that big.

However, when measuring the flow rates of liquids having different temperatures, the output voltage slightly changes depending on the temperature of the liquids, and the same output voltage cannot be obtained for all liquids having different temperatures. However, there was a problem that it was not possible to set a certain threshold value and it could not operate as a running water switch. The reason is that the resistance value of the flow rate detecting resistance element itself changes due to self-heating of the flow rate detecting resistance element.

The present invention has been made in order to solve the above-mentioned problems, and the same constant output voltage is detected for all liquids having different temperatures without being affected by the temperature of the liquids. Accordingly, it is an object of the present invention to provide a thermal type flow rate sensor capable of stably detecting the presence or absence of flow.

[0011]

In order to achieve this object, a thermal type flow rate sensor of the present invention is formed to include a flow rate detecting resistance element and a water temperature detecting resistance element arranged in a liquid passage. And a temperature rise correction resistance element connected in series with a resistance element for detecting water temperature which is outside the liquid passage and constitutes a bridge circuit. In the above thermal type flow sensor, when the resistance value of the temperature rise correction resistance element is applied to the bridge circuit and the resistance value of the flow detection resistance element changes due to the temperature rise, the bridge circuit is electrically balanced. It is characterized by a resistance value.

[0012]

The thermal type flow sensor of the present invention having the above structure is arranged in the liquid passage and detects the presence or absence of the flow of the liquid. Here, the resistance element for flow rate detection, which constitutes the bridge circuit, is constantly heated to a temperature higher by about 20 to 30 degrees than the fluid temperature. The flow detecting resistance element heated in the flow path is deprived of heat by the fluid. Here, the amount of heat absorbed by the fluid is proportional to the flow velocity. Then, as the flow velocity increases, heat is taken away, and the temperature of the flow rate detecting resistance element decreases. Then, the resistance value of the flow rate detection resistance element decreases, the voltage division ratio of the bridge in the flow rate detection resistance element changes, and a bridge voltage is generated. The flow velocity is measured by detecting the change in this voltage as the output voltage.

When the temperature of the fluid changes, the resistance element for detecting the water temperature becomes the same temperature as that of the fluid, and the bridge circuit corrects the temperature for the temperature change of the fluid. In addition, a minute change in the resistance value of the flow detection resistance element caused by being heated to a constant temperature was connected in series to the water temperature detection resistance element at a position opposed to the flow detection resistance element of the bridge circuit. When the resistance element for temperature rise correction is heated to a predetermined temperature by energizing the resistance element for flow rate detection, the bridge circuit will be in an equilibrium state, so the error caused by heating the resistance element for flow rate detection is corrected. To be done. As a result, even if the temperature of the fluid changes, the equilibrium state in the resistance element for flow rate detection becomes constant, so the threshold value can be easily taken, and a thermal type flow rate sensor that operates stably can be obtained. it can.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A thermal flow sensor 1 which is an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the thermal type flow sensor 1. In the thermal type flow sensor 1, a water temperature detecting resistance element 2 and a flow rate detecting resistance element 3 are arranged in series in a flow direction of the fluid F so that a detection portion is in contact with the fluid F. There is. Then, the water temperature detecting resistance element 2 and the flow rate detecting resistance element 3
Are covered with a protective cover 4 which is a protective member. A mold material 5 is filled between the water temperature detecting resistance element 2, the flow rate detecting resistance element 3 and the protective cover 4. The molding material 5 fixes the resistance element 2 for water temperature detection, the resistance element 3 for flow rate detection, and the like, and also transfers heat to prevent water drops and moisture from entering from the outside on the air side.

Further, the protective cover 4 is an O for preventing leakage.
It is attached to the body portion 7 via the ring 6. The body portion 7 is a cylindrical tube, and an inlet for the fluid F to flow in and an outlet for the fluid F to flow out are formed to have slightly larger diameters. The fluid F flows in from the left direction and flows out in the right direction. In the present embodiment, the water temperature detecting resistance element 2 and the flow rate detecting resistance element 3 are arranged in series with respect to the flow, but they may be arranged in parallel, or they may be arranged in separate channels. May be.

In the body portion 7, the cross-sectional area of the flow path through which the fluid F flows in the pipe is formed to be constant. Further, inside the body portion 7, the flow of the fluid F is configured to be a laminar flow state. Since the water temperature detecting resistance element 2 and the flow rate detecting resistance element 3 are provided in the substantially center of the flow path, they are in contact with the fluid F at the position where the fluid F has the highest speed. Therefore, the maximum velocity of the fluid F is detected as a thermal type flow sensor.

FIG. 2 shows a bridge circuit for measuring the current change in the flow rate detecting resistance element 3. The bridge circuit includes a water temperature detecting resistance element 2 (resistance value R4) and a temperature rise correction resistance element 8 (resistance value R5) connected in series therewith.
, A flow rate detecting resistance element 3 (resistance value R3), and two fixed resistance elements outside the fluid passage (resistance value R1, resistance value R
2) and. Here, the temperature rise correction resistance element 8 (resistance value R5) is connected in series to the water temperature detection resistance element 2 (resistance value R4) outside the fluid passage.
Two fixed resistance elements (resistance value R1 and resistance value R2)
Are for the flow rate detecting resistance element 3 (resistance value R3) and the water temperature detecting resistance element 2 (resistance value R4), respectively. Further, Vs is a power supply voltage, and Vb is an output voltage.

In the bridge circuit, when the fluid F is not flowing, the water temperature detecting resistance element 2 has the same temperature as the fluid F and the flow rate detecting resistance element 3 is heated. Here, since the water temperature detection resistance element 2 and the flow rate detection resistance element 3 have different temperatures, the resistance values are minutely different, and there is a minute potential difference between V1 and V2. In the present invention, in order to correct this minute potential difference, the temperature rise correction resistance element 8 is connected in series with the water temperature detection resistance element 2.

Next, how to obtain the resistance value of the temperature rise correction resistance element 8 will be described. Formula 1 is a calculation formula for calculating the output voltage Vb detected from the bridge circuit of FIG.

[Equation 1] In the bridge circuit of FIG. 2, the output voltage Vb is obtained by the difference between the voltages V2 and V1 at the connection point of the bridge circuit. However, in practice, the temperature coefficient (T
CR), it is necessary to add the correction. Therefore, the actual output voltage is the corrected corrected output voltage Vb '.
Is calculated by Here, since R1 'and R2' are attached outside the fluid passage, they are corrected by the ambient temperature TR. Also, because R3 'is in the fluid passage,
It is corrected by the water temperature TW and further heated to a predetermined temperature, so the temperature rise TO that is heated by energization is corrected. Further, since R4 'is in the fluid passage, the water temperature T
Corrected by W. Further, since R5 'is attached outside the fluid passage, it is corrected by the ambient temperature TR.

Next, the thermal type flow sensor 1 having the above structure.
The operation of will be described. In this embodiment, the flow rate of water, which is one of the fluids F, is measured. The fluid F flows into the body portion 7 from the left. Inside the body portion 7, the flow of the fluid F is in a laminar flow state. Since the water temperature detecting resistance element 2 and the flow rate detecting resistance element 3 are attached to the central position of the fluid passage, the maximum velocity of the fluid F is detected. Here, the flow rate detecting resistance element 3 is constantly heated to a predetermined temperature. The predetermined temperature is set higher than the temperature of the fluid F by about 20 to 30 degrees.

Then, heat is taken away by the flow rate detecting resistance element 3 and the fluid heated in the flow path. Here, the amount of heat absorbed by the fluid is proportional to the flow velocity. Then, as the flow velocity increases, heat is removed and the temperature of the flow rate detecting resistance element 3 decreases. Then, the resistance value R3 of the flow rate detection resistance element 3 decreases, the voltage division ratio by the resistance values R3 and R1 of the flow rate detection resistance element 3 changes, and V1 decreases. Then, the bridge output voltage Vb is generated. The flow velocity is measured by detecting the change in this voltage as the output voltage. Where fluid F
When the temperature changes, the temperature of the water temperature detecting resistance element 2 changes like the flow rate detecting resistance element 3, and thus the error due to the temperature change of the fluid F is corrected by the bridge circuit. Further, when the flow rate detecting resistance element 3 is heated to a predetermined temperature, the resistance value of the flow rate detecting resistance element 3 slightly changes, but the error due to this change is corrected by the temperature rise correction resistance element 8.

Next, FIG. 3 shows an experimental result of the thermal type flow sensor 1 which is an embodiment embodying the present invention. This Figure 3
It shows the relationship between the flow rate and the output voltage. The horizontal axis represents the flow rate and the vertical axis represents the output voltage. FIG. 3 shows an example in which water is used as the liquid. And the temperature of water is 0, 10, 20, 30, 40, 50 (° C),
6 types of water are used. As can be seen from FIG. 3, compared to the conventional FIG. 4 and FIG. 5, at all water temperatures,
It can be seen that the output voltage values are almost the same. That is, it can be seen that there is almost no influence of the liquid temperature on the output voltage.

This is also confirmed from the value of the corrected output voltage Vb 'in the above equation (1). That is, when obtaining the corrected output voltage Vb ', the corrected resistance values (R1', R2 ', R3', R are added to the equation for calculating the corrected output voltage Vb '.
4 ', R5'), temperature coefficient of resistance (TCR), ambient temperature, etc. are substituted. For example, when calculating the corrected output voltage Vb ′ when the water temperature is 0 ° C., the resistance value of each resistance element (R1 = 100Ω, R2 = 1000Ω,
R3 = 100Ω, R4 = 1000Ω, R5 = 48Ω) and the temperature coefficient of the resistance value of each resistance element (TCR1 =
0.0001, TCR2 = 0.0001, TCR3 =
0.005, TCR4 = 0.005, TCR5 = 0.0
001), ambient temperature = 20 ° C., water temperature = 0 ° C., etc., are substituted into the equation for calculating the corrected resistance value of each resistance element to obtain the corrected resistance value of each resistance element (R1 ′ = R1 ′ =
100.2Ω, R2 '= 1002Ω, R3' = 100
.OMEGA., R4 '= 1000 .OMEGA., R5' = 48.096 .OMEGA.). Furthermore, when these values are substituted into the formula for calculating the corrected output voltage Vb ′ and calculated, Vb ′ = 0.1755
V is calculated.

When calculating the corrected output voltage Vb 'when the water temperature is 50 ° C., the resistance value (R
1 = 100Ω, R2 = 1000Ω, R3 = 100Ω, R
4 = 1000Ω, R5 = 48Ω) and the temperature coefficient of the resistance value of each resistance element (TCR1 = 0.0001, TCR
2 = 0.0001, TCR3 = 0.005, TCR4 =
0.005, TCR5 = 0.0001), ambient temperature = 20 ° C., water temperature = 50 ° C., etc., are substituted into the equations for obtaining the correction resistance value of each resistance element to obtain the correction resistance value of each resistance element ( R1 '= 100.2Ω, R2'
= 1002Ω, R3 ′ = 125Ω, R4 ′ = 1250
Ω, R5 ′ = 48.096Ω) is obtained. Further, by substituting these values into the formula for calculating the corrected output voltage Vb ′, calculation is made to Vb ′ = 0.1395V.

The difference in the corrected output voltage Vb 'between the two when the water temperature is 0 ° C. and 50 ° C. is 0.036 V, which is extremely small. That is, it can be seen that there is almost no influence of the liquid temperature on the output voltage. From this, it can be seen that the output voltage values can be made almost the same for all water temperatures.

As described in detail above, according to the thermal type flow rate sensor 1 of this embodiment, the flow rate detecting resistance element 3 and the water temperature detecting resistance element 2 arranged in the liquid passage are formed. The thermal type flow sensor 1 having the bridge circuit has the temperature rise correction resistance element 8 which is outside the liquid passage and is connected in series with the water temperature detection resistance element 2 which constitutes the bridge circuit. Not only the temperature change of F,
Since the change in the resistance value generated by heating the flow rate detecting resistance element 3 is also corrected, it is possible to obtain an output voltage that is less related to the temperature of the fluid F, easily select an appropriate threshold value, and stabilize. It is possible to obtain the thermal type flow sensor 1 that performs the above operation. Further, since a complicated temperature correction circuit is not required, it is possible to provide the low-cost thermal type flow sensor 1.

Further, in the thermal type flow sensor 1 of this embodiment, when the resistance value of the temperature rise correction resistance element 8 is applied to the bridge circuit and the resistance value of the flow rate detection resistance element 3 changes due to the temperature rise, Since it is a resistance value that electrically balances the bridge circuit, it can be corrected even when it is generated by heating the flow rate detecting resistance element 3 to a predetermined temperature, so that an output voltage that is less related to the temperature of the fluid F is obtained. Therefore, it is possible to obtain the thermal type flow sensor 1 which can easily select an appropriate threshold value and operates stably.

The embodiment of the present invention has been described above.
The present invention is not limited to the above embodiment, but can be applied in various ways. For example, in the present embodiment, the resistance value of the temperature rise correction resistance element 8 is calculated, but the resistance of the temperature rise correction resistance element 8 is set to a variable resistance, and the value of the temperature rise correction resistance element 8 is determined by an experiment. May be. Further, although the running water switch has been described in the present embodiment, it can be used in a thermal type flow meter for the purpose of measuring the flow rate.

[0029]

As is apparent from the above description, the thermal type having the bridge circuit formed by including the flow rate detecting resistance element and the water temperature detecting resistance element arranged in the liquid passage of the present invention. In the flow sensor, outside the liquid passage,
It has a temperature rise correction resistance element connected in series with the water temperature detection resistance element that forms the bridge circuit, so it is generated not only by the temperature change of the fluid but also by heating the flow rate detection resistance element to a specified temperature. However, since it is corrected, it is possible to obtain an output voltage that is less related to the temperature of the fluid, easily select an appropriate threshold value, and obtain a thermal flow sensor that operates stably. Moreover, since a complicated temperature correction circuit is not required, a low-cost thermal type flow sensor can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration of a thermal type flow sensor 1 which is an embodiment of the present invention.

FIG. 2 is a bridge circuit diagram of a flow rate measuring unit of the thermal type flow sensor 1.

FIG. 3 is a data diagram showing an experimental result of the thermal type flow sensor 1.

FIG. 4 is a data diagram showing an experimental result of a conventional one-element type thermal flow meter.

FIG. 5 is a data diagram showing an experimental result of a conventional two-element thermal flow meter.

[Explanation of symbols]

 1 Thermal Flow Sensor 2 Resistance Element for Water Temperature Detection 3 Resistance Element for Flow Rate Detection 4 Protective Cover 8 Temperature Rise Compensation Resistance Element F Fluid

Claims (2)

[Claims] 1. A thermal type flow rate sensor having a bridge circuit formed by including a flow rate detecting resistance element and a water temperature detecting resistance element arranged in a liquid passage, wherein the thermal type flow sensor is outside the liquid passage, A thermal type flow sensor having a temperature rise correction resistance element connected in series with the water temperature detection resistance element which constitutes a bridge circuit. 2. The device according to claim 1, wherein when the resistance value of the temperature rise correction resistance element is applied to the bridge circuit and the resistance value of the flow rate detection resistance element changes due to temperature rise, A thermal flow sensor having a resistance value that electrically balances a bridge circuit.