Thermal flow meter

Abstract

The present invention provides a thermal flow meter that can accurately measure flow rate at high speed and high precision even at low flow rate. The thermal flow meter (1) of the present invention comprises: a sensor unit (5) that performs heating and temperature detection of a fluid contact portion that contacts a fluid (4) flowing in a flow path (2); and a control unit that controls the heating and temperature detection performed by the sensor unit (5), the control unit measures the temperature rise time until the temperature of the fluid contact portion reaches an upper limit set temperature when the fluid contact portion is heated, and the temperature drop time until the temperature of the fluid contact portion reaches a lower limit set temperature when the heating of the fluid contact portion is stopped, and converts the measured flow velocity into a flow rate value based on the temperature rise time and the temperature drop time to perform flow rate measurement.

Description

Technical Field

[0001] The present invention relates to a thermal flow meter for measuring the flow rate of a fluid based on a temperature rise time and a temperature drop time of a fluid contact portion. Background Art

[0002] Conventionally, as a flow meter for measuring the flow rate of a fluid by detecting the temperature of the fluid, there is known a flow meter described in the following Patent Document 1, for example. The flow meter is composed of a heat source arranged in a flow path and a temperature detection element arranged downstream of the heat source. The flow rate is measured by causing the heat source to generate heat, detecting the temperature of the fluid flowing in the flow path by the temperature detection element, and measuring the flow rate of the fluid by the time difference from the time when the heat source is driven to generate heat to the time when the detected temperature reaches a maximum value.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2002-214015 Summary of the invention

[0006] Technical issues

[0007] However, according to the above-mentioned conventional flow meter, there is the following problem: in the case of a low flow rate where the flow velocity of the fluid is slow, the time until the fluid heated by the heat source reaches the downstream temperature detection element becomes longer, the heat release from the fluid increases, and it is difficult to detect the time when the detection temperature reaches the maximum value.

[0008] The present invention has been made in view of such a problem, and an object of the present invention is to provide a thermal flow meter capable of accurately measuring a flow rate at high speed and high precision even at a low flow rate.

[0009] Technical Solution

[0010] In order to solve the above-mentioned problem, the thermal flow meter of the present invention is characterized in that it comprises: a sensor part that performs heating and temperature detection of a fluid contact part that contacts a fluid flowing in a flow path; and a control part that controls the heating and temperature detection performed by the sensor part, wherein the control part measures the temperature rise time until the temperature of the fluid contact part reaches an upper limit set temperature when the fluid contact part is heated, and the temperature drop time until the temperature of the fluid contact part reaches a lower limit set temperature when the heating of the fluid contact part is stopped, and calculates a time ratio based on the temperature rise time and the temperature drop time, and converts the measured flow velocity into a flow value based on the calculated time ratio to perform flow measurement. It should be noted that the fluid contact part refers to a liquid contact part that contacts a liquid when the fluid is a liquid, and a gas contact part that contacts a gas when the fluid is a gas.

[0011] In addition, the thermal flow meter of the present invention is characterized in that it includes: a sensor part, which heats and detects the temperature of a fluid contact part that contacts the fluid flowing in the flow path; and a control part, which controls the heating and temperature detection performed by the sensor part, the control part measures the temperature rise time until the temperature of the fluid contact part reaches an upper limit setting temperature when the fluid contact part is heated, and the temperature drop time until the temperature of the fluid contact part reaches a lower limit setting temperature when the heating of the fluid contact part is stopped, and calculates the difference between the temperature rise time and the temperature drop time, and converts the measured flow velocity into a flow value based on the calculated difference to perform flow measurement.

[0012] In addition, the present invention is characterized in that, in the thermal flow meter composed of the above structure, the sensor part is constructed to include: a heat pipe, which is arranged in the flow path; a temperature measuring resistor, which is arranged in the heat pipe and performs heating and temperature detection of the fluid contact part; a transformer, which applies a pulse current inversely proportional to the resistance value of the temperature measuring resistor; and a reference resistor, which generates a pulse voltage inversely proportional to the resistance value of the temperature measuring resistor by the pulse current flowing through the transformer.

[0013] Furthermore, the present invention is characterized in that, in the thermal flow meter having the above configuration, the control unit supplies the sensor unit with a superimposed current obtained by adding a detection pulse current for temperature detection to an applied current for heating.

[0014] In addition, the present invention is characterized in that, in the thermal flow meter composed of the above structure, the control unit stops heating the fluid contact part at the time when the temperature of the fluid contact part reaches the temperature increase setting level, and starts heating the fluid contact part at the time when the temperature of the fluid contact part reaches the temperature decrease setting level.

[0015] Technical Effects

[0016] According to the thermal flow meter of the present invention, by adopting the above-mentioned structure, there is an effect that accurate flow rate measurement can be performed at high speed and high precision even at a low flow rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is an overall view showing the structure of a thermal flow meter according to one embodiment of the present invention.

[0018] Figure 2 : is a block diagram showing the structure of the control unit of the flow meter.

[0019] Figure 3This is a graph showing the relationship between the output voltage and time when the first resistor of the flow meter is heated (when the upper limit setting temperature is reached).

[0020] Figure 4 This is a graph showing the relationship between the output voltage and time when the heating of the first resistor of the flow meter is stopped (when the lower limit setting temperature is reached).

[0021] Figure 5 Schematic diagram showing the concept of the time ratio conversion method of the flow meter.

[0022] Figure 6 It is a graph which shows the characteristics of (A) temperature rise time, (B) temperature drop time, and (C) time ratio of this flow meter.

[0023] Figure 7 This is a flowchart showing the process of measuring the flow rate using the flow meter.

[0024] Figure 8 This is a flowchart showing data processing for flow rate conversion of the flow meter.

[0025] Fig. 9 This is a block diagram showing the configuration of an equivalent circuit of the sensor unit of the flow meter.

[0026] Fig.10 : is a block diagram showing an example of using a transformer in the sensor section of the flow meter.

[0027] Fig.11 : is a graph showing the relationship between time and temperature corresponding to the flow velocity and flow rate at the start of heating of the flow meter.

[0028] Fig.12 This is a graph showing the optimization of the setting levels of the temperature increase and temperature decrease of the flow meter.

[0029] Explanation of symbols

[0030] 1: Thermal flow meter

[0031] 2: Flow path

[0032] 3: Piping

[0033] 4: Fluid

[0034] 5: Sensor part

[0035] 6: Main body

[0036] 7: Heat pipe

[0037] 8: The first resistor

[0038] 9: Second resistor

[0039] 10: Control Department

[0040] 11: Apply current generation circuit

[0041] 12: Detection signal generation circuit

[0042] 13: Reference resistance

[0043] 14: Amplifier circuit

[0044] 15: AD conversion unit

[0045] 16: Operation unit

[0046] 17: Transformer DETAILED DESCRIPTION

[0047] Hereinafter, a mode for carrying out the present invention will be described in detail with reference to the drawings.

[0048] like Figure 1 As shown in the overall diagram of the embodiment, the thermal flow meter 1 of the present embodiment is detachably mounted on the pipe 3 constituting the flow path 2, and measures the flow rate of the fluid 4 flowing in the flow path 2 based on the time until the set temperature is reached. The thermal flow meter 1 is composed of a sensor unit 5 provided inside the pipe 3, and a main body 6 which is integrated with the sensor unit 5 and provided outside the pipe 3.

[0049] In the hollow heat pipe (SUS pipe in this embodiment) 7, the sensor unit 5 has two temperature measuring resistors (first resistor 8 and second resistor 9) built in as temperature measuring elements. The temperature measuring resistor is made of metals such as platinum, nickel, copper, etc. with good temperature characteristics and little change over time, and is a sensor that measures the temperature by detecting the resistance value that changes with temperature. The first resistor 8 performs both heating and temperature detection of the fluid contact portion that contacts the fluid 4, and is arranged on the inner bottom surface of the heat pipe 7. The second resistor 9 only performs temperature detection of the fluid contact portion, and is arranged above the inner side surface away from the inner bottom surface of the heat pipe 7 as a position not affected by the heating of the first resistor 8.

[0050] The main body 6 includes a control unit 10 (see Figure 2 ), it also has a display unit (not shown) for displaying the measurement results of the flow rate, an operation unit (not shown) for the user to perform input operations, and the like.

[0051] like Figure 2 As shown in the block diagram of , the control unit 10 includes an applied current generating circuit 11 , a detection signal generating circuit 12 , a reference resistor 13 , amplifier circuits 14 , 14 , . . . , AD (Analog-Digital) converters 15 , 15 , . . . , and a calculation unit 16 .

[0052] The applied current generating circuit 11 is connected to the calculation unit 16 and the first resistor 8, and is a circuit for generating an applied current for heating the first resistor 8 based on an applied current amount control signal output from the calculation unit 16. The applied current generating circuit 11 supplies the applied current for heating to the first resistor 8 if it receives an applied current conduction control signal from the calculation unit 16 during heating, and stops supplying the applied current for heating to the first resistor 8 if it receives an applied current shutdown control signal from the calculation unit 16 during heating.

[0053] The detection signal generating circuit 12 is a circuit connected to the operation unit 16, the first resistor 8, and the second resistor 9, and generates a detection signal for detecting the temperature of the first resistor 8 and the second resistor 9. The detection signal generating circuit 12 supplies a detection pulse current for temperature detection to the first resistor 8 and the second resistor 9 if it receives a detection pulse on control signal from the operation unit 16 when detecting the temperature, and stops supplying the detection pulse current for temperature detection to the first resistor 8 and the second resistor 9 if it receives a detection pulse off control signal from the operation unit 16 when stopping the temperature detection.

[0054] The reference resistor 13 is connected between the input side of the amplifier circuit 14 and the ground, and supplies the inverse of the applied detection pulse current to the amplifier circuit 14 .

[0055] The amplifier circuit 14 is connected to the first resistor 8 and the second resistor 9 and converts the resistance value corresponding to the measured temperature outputted from the first resistor 8 and the second resistor 9 into a voltage. The converted voltage is amplified by the operational amplifier of the amplifier circuit 14 and outputted to the AD converter 15.

[0056] The AD converter 15 converts the analog data of the voltage output from the amplifier circuit 14 into digital data, and outputs the digital data to the calculation unit 16 .

[0057] The calculation unit 16 is composed of an MCU (Micro Control Unit), and performs various calculations based on digital data indicating the measured temperatures of the first resistor 8 and the second resistor 9. The calculation unit 16 has a timer, and measures the temperature rise time [ms] until the upper limit setting temperature is reached, and the temperature drop time [ms] until the lower limit setting temperature is reached by multiplying the count and the clock cycle. In addition, the calculation unit 16 measures the flow rate [m / s] based on the measured temperature rise time [ms] and temperature drop time [ms], and calculates the flow value [L / min] based on the flow rate [m / s] and the pipe diameter [mm].

[0058] In this embodiment, a superimposed current (dual-pulse current) driving method is adopted to apply a detection pulse current to the applied current to the first resistor 8. Therefore, by adding the detection pulse current for temperature detection to the applied current for heating, the temperature of the fluid contact portion can be always detected with the same accuracy even when the first resistor 8 is not heated.

[0059] Next, in the thermal flow meter 1 of the present embodiment, an outline of a method of converting a flow rate will be described.

[0060] exist Figure 3 and Figure 4 2 shows the relationship between the output voltage (temperature) and time when a superimposed current (double pulse current) obtained by adding a detection pulse current to an applied current is applied to the first resistor 8 . Figure 3 The time taken for the fluid contact portion to gradually rise in temperature from the temperature (liquid temperature) at the start of application to the upper limit set temperature when the current is supplied to the first resistor 8 (during heating) is compared for each flow rate. As can be clearly seen from the figure, if the flow rate is small, the temperature rise time until the upper limit set temperature is reached is short, and as the flow rate increases, the heat is lost by the fluid, so it can be seen that the temperature rise time until the upper limit set temperature is reached becomes longer. The temperature rise time is converted into a flow value by the calculation unit 16.

[0061] In this regard, Figure 4 The time taken for the fluid contact portion to gradually drop in temperature from the temperature at the time of stopping the application of current to the first resistor 8 and reach the lower limit set temperature is compared for each flow rate. As can be clearly seen from the figure, if the flow rate is small, the temperature drop time until reaching the lower limit set temperature is long, and as the flow rate increases, the heat is lost by the fluid, so it can be seen that the temperature drop time until reaching the lower limit set temperature becomes shorter. The temperature drop time is converted into a flow value by the calculation unit 16.

[0062] Next, in the thermal flow meter 1 of the present embodiment, a description will be given of a way of thinking about flow rate conversion based on the time ratio conversion method.

[0063] like Figure 5 As shown in the figure, for the temperature rise level (t), the temperature rise time (x) until reaching the upper limit setting temperature and the temperature drop time (y) until dropping to the lower limit setting temperature are different according to the flow rate. Therefore, in order to convert the flow rate into a flow rate value by utilizing the time difference, the ratio of the respective times, i.e., the time ratio, is calculated as follows.

[0064] Time ratio (z) = temperature rise time (x) / temperature drop time (y)

[0065] Figure 6 This is a graph that graphs the temperature rise time (x), temperature drop time (y), and time ratio (z). The vertical axis of the graph is set to flow rate [L / min], and the horizontal axis is set to temperature rise time [ms] in (A), the horizontal axis is set to temperature drop time [ms] in (B), and the horizontal axis is set to time ratio "function" in (C). It is clear from these graphs that if the approximate curve is observed, the characteristic difference is revealed according to the flow rate and flow rate, so the thermal flowmeter 1 of this embodiment calculates the temperature rise time (x), the temperature drop time (y), and the time ratio (z) to convert the flow rate into a flow value.

[0066] Next, a description will be given of a flow rate measurement process in the thermal flow meter 1 according to the present embodiment.

[0067] <Confirmation of initial conditions>

[0068] like Figure 7 As shown in the flowchart, in the operation unit 16, a detection pulse conduction control signal is output to the detection signal generating circuit 12. Then, the detection signal generating circuit 12 applies a detection pulse current (1mA / 100KHz) for resistance value detection to the first resistor 8 and the second resistor 9 (step 1). In addition, the reverse of the detection pulse current applied to the first resistor 8 and the second resistor 9 is applied to the reference resistor 13. Next, in the operation unit 16, the difference in amplitude between the first resistor 8 and the reference resistor 13 is measured and converted into temperature (step 2). In addition, for the second resistor 9, the same measurement as the first resistor 8 is performed and converted into temperature (step 3). Next, in the operation unit 16, the reaching level value of the temperature rise and the reaching level value of the temperature drop are calculated based on the voltage of the second resistor 9 (step 4).

[0069] <Measurement of temperature rise time>

[0070] In the calculation unit 16, an applied current amount control signal and an applied current conduction control signal are output to the applied current generating circuit 11, and a detection pulse conduction control signal is output to the detection signal generating circuit 12. Then, a superimposed current (double pulse current) obtained by adding a detection pulse current to the applied current is applied to the first resistor 8 (step 5). From this point in time, the timer starts to measure time and counts the elapsed time. Next, in the calculation unit 16, the level of the first resistor 8 is measured (step 6). Here, in the calculation unit 16, it is determined whether the level of the first resistor 8 has reached the rising value (step 7). If the rising value is reached, the temperature rise time (x) is measured based on the elapsed time from the start of heating (yes in step 7). If the rising value is not reached, the level of the first resistor 8 continues to be measured until the rising value is reached (no in step 7).

[0071] <Measurement of temperature drop time>

[0072] In the operation unit 16, an applied current shutoff control signal is output to the applied current generating circuit 11. Then, the supply of the applied current applied by the applied current generating circuit 11 to the first resistor 8 is stopped (step 8). From this point in time, the timer is used to start time measurement and the time elapsed is counted. Next, in the operation unit 16, the level of the first resistor 8 is measured (step 9). Here, in the operation unit 16, it is determined whether the level of the first resistor 8 has reached a drop value (step 10). If the drop value is reached, the temperature drop time (y) is measured based on the elapsed time from the time when the heating is stopped (yes in step 10). If the drop value is not reached, the level of the first resistor 8 is continuously measured until the drop value is reached (no in step 10).

[0073] <Conversion and correction of flow rate (flow rate)>

[0074] In the calculation unit 16, the time ratio (z) is calculated based on the measured temperature rise time (x) and temperature drop time (y) (step 11). Then, the measured flow velocity is converted into a flow rate value based on the calculated time ratio (z) and corrected (step 12).

[0075] <Confirmation condition reset>

[0076] The same processing as the above <Confirmation of Initial Conditions> is performed, and the temperature (liquid temperature) of the second resistor 9 is measured in the calculation unit 16 (step 13). In addition, the temperature (liquid temperature) of the first resistor 8 is also measured in the same manner (step 14). Next, the calculation unit 16 calculates the reaching level value of the temperature rise and the reaching level value of the temperature drop based on the voltage of the second resistor 9, and resets the set value (step 15). Then, the measurement is repeated in the order of <temperature rise time measurement> <temperature drop time measurement> <flow rate (flow rate) conversion and correction> (step 16).

[0077] Next, in the thermal flow meter 1 of the present embodiment, data processing for flow rate conversion performed by the calculation unit 16 will be described.

[0078] like Figure 8 As shown in the flowchart, in the calculation unit 16, the arrival time of the temperature rise time (x) and the temperature drop time (y) is measured every 2 Hz or more (0.5 seconds or less) as a cycle. In addition, the flow rate is converted according to the arrival time, and the time ratio (z) is calculated and the flow rate 0L detection is performed. The time ratio (z) is directly used for detection so as not to be affected (delayed) by the digital filter.

[0079] Then, in the calculation unit 16, the noise is removed from the input signal by a digital filter, and the deviation is reduced. After the deviation is reduced, the flow rate is corrected by a digital filter, and the measured values ​​are synthesized to average the respective flow rates. Finally, the averaged flow rate is filtered out by a digital filter, and the deviation is reduced to be the flow rate value.

[0080] Fig. 9 1 is a diagram showing the configuration of an equivalent circuit of the sensor unit 5 in the thermal flow meter 1 of the present embodiment. Fig. 9 As shown, the equivalent circuit of the sensor unit 5 is represented by the thermal resistance (Rt) of the heat pipe (SUS pipe) 7 that transfers heat, the heat capacity (Ct) of the heat pipe (SUS pipe) 7 that accumulates heat, and the thermal resistance (Rc) of the fluid 4 that transfers heat to the fluid 4 when viewed from the first resistor 8. Therefore, by changing the flow rate of the fluid 4, the thermal resistance (Rc) of the fluid 4 changes, and the heat generation and heat release characteristics of the first resistor 8 change.

[0081] Fig.10 1 is a diagram showing an example of using a transformer in the sensor unit 5 in the thermal flow meter 1 of the present embodiment. Fig.10 As shown, when a pulse voltage (V1) for detecting the resistance value is applied to the sensor unit 5, a pulse current (I) inversely proportional to the resistance value of the first resistor 8 flows through the transformer 17. The same pulse current (I) flows through the inductor on the opposite side due to the magnetic field generated by the current flowing through the transformer (inductor) 17. The pulse current (I) flows through the reference resistor (Rref) 13, generating a pulse voltage (V2) inversely proportional to the resistance value of the first resistor 8. By inputting the pulse voltage (V2) to the amplifier circuit 14, the resistance value of the first resistor 8 can be detected with good accuracy regardless of whether the applied voltage for heating is turned on or off.

[0082] Finally, the optimization of the set temperature during measurement in the thermal flow meter 1 of the present embodiment will be described.

[0083] In the thermal flow meter 1 of this embodiment, Fig.11 is a graph showing the relationship between time and temperature corresponding to the flow velocity and flow rate at the start of heating, Fig.12 is a diagram showing the optimization of the setting levels for temperature rise and temperature drop. Fig.11As shown, the initial time of heating start (the part surrounded by the dotted line in the figure) is the temperature rise time of the fluid contact part in contact with the fluid and the response time of the temperature measuring resistor (first resistor 8), and no temperature difference due to the difference in flow velocity and flow rate is found. For example, in the case where the response time of the platinum temperature measuring resistor Pt100 (new JIS) is approximately 0.2 [s], since the time of 0.2 [s] + α does not cause the temperature difference due to the difference in flow velocity and flow rate, the temperature range where the flow rate difference does not occur is not used for measurement from the perspective of responsiveness and accuracy improvement.

[0084] Therefore, if Fig.12 As shown, the setting levels of temperature rise and temperature drop are optimized, and the heating time that is unrelated to the flow rate and flow rate is reduced during measurement. That is, the control unit 10 repeatedly controls the temperature measurement by turning off the current application of the heating temperature measuring resistor (stopping heating) at the time point when the temperature of the fluid contact part reaches the temperature rise setting level, and turning on the current application of the heating temperature measuring resistor (starting heating) at the time point when the temperature of the fluid contact part reaches the temperature drop setting level. In this way, by repeatedly turning on / off the current application when the setting level is reached, the measurement time can be shortened and the responsiveness of the flow measurement can be improved. In addition, by increasing the number of measurements, the accuracy of the flow measurement can also be improved.

[0085] As described above, according to the thermal flowmeter 1 of this embodiment, by applying a superimposed current (double pulse current) obtained by adding a detection pulse current to an applied current to a temperature measuring resistor, measuring the temperature rise time (x) and the temperature drop time (y) to calculate the time ratio (z), and converting the measured flow velocity into the flow rate based on the calculated time ratio (z), it is possible to accurately measure the flow rate at high speed and high precision even at a low flow rate. In addition, by adopting the so-called triangulation detection method (Triangulation) that detects the temperature rise at three points such as 1) the heating start point, 2) the temperature rise point, and 3) the temperature drop point, it is possible to easily detect the flow rate 0L, and the linearity of the temperature rise time and flow rate conversion can be improved, so that more accurate flow measurement can be performed.

[0086] It should be noted that in the above-mentioned embodiment, the temperature rise time (x) and the temperature drop time (y) are measured to calculate the time ratio (z), and the calculated time ratio (z) is converted into a flow value to measure the flow rate, but it can also be replaced by calculating the difference (Δ) between the temperature rise time (x) and the temperature drop time (y), converting the calculated difference (Δ) into a flow value to measure the flow rate.

Claims

1. A thermal flow meter, characterized in that: have: a sensor portion that performs heating and temperature detection of a fluid contact portion that contacts the fluid flowing in the flow path; and a control unit that controls heating and temperature detection by the sensor unit, The control unit measures a temperature rise time until the temperature of the fluid contact part reaches an upper limit setting temperature when the fluid contact part is heated, and a temperature drop time until the temperature of the fluid contact part reaches a lower limit setting temperature when the heating of the fluid contact part is stopped, and calculates a time ratio based on the temperature rise time and the temperature drop time, and converts the measured flow velocity into a flow value based on the calculated time ratio to perform flow measurement.

2. A thermal flow meter, characterized in that: have: a sensor portion that performs heating and temperature detection of a fluid contact portion that contacts the fluid flowing in the flow path; and a control unit that controls heating and temperature detection by the sensor unit, The control unit measures a temperature rise time until the temperature of the fluid contact part reaches an upper limit setting temperature when the fluid contact part is heated, and a temperature drop time until the temperature of the fluid contact part reaches a lower limit setting temperature when the heating of the fluid contact part is stopped, and calculates a difference between the temperature rise time and the temperature drop time, and converts the measured flow velocity into a flow value based on the calculated difference to perform flow measurement.

3. The thermal flow meter according to claim 1 or 2, characterized in that: The sensor unit is configured to include: a heat conducting pipe, which is arranged in the flow path; a temperature measuring resistor, which is arranged in the heat conducting pipe and performs heating and temperature detection on the fluid contacting part; A transformer that applies a pulse current inversely proportional to the resistance value of the temperature measuring resistor; as well as A reference resistor generates a pulse voltage inversely proportional to the resistance value of the temperature measuring resistor by the pulse current flowing through the transformer.

4. The thermal flow meter according to claim 1 or 2, characterized in that: The control unit supplies the sensor unit with a superimposed current obtained by adding a detection pulse current for temperature detection to an applied current for heating.

5. The thermal flow meter according to claim 1 or 2, characterized in that: The control unit stops heating the fluid contact portion when the temperature of the fluid contact portion reaches a temperature increase setting level, and starts heating the fluid contact portion when the temperature of the fluid contact portion reaches a temperature decrease setting level.

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