Thermal flowmeter, and flow rate calculation method

Abstract

To minimize a calculation error of a flow rate in a case where a flow rate of a liquid is small to expand a lower limit value of a measurable flow rate range.SOLUTION: A flow rate calculation method is provided that comprises: time measurement steps S103 and S104 of measuring first arrival time T1 until a liquid heated by heating resistor arrives at a first temperature detection resistor, and measuring second arrival time T2 until the liquid heated by the heating resistor arrives at a second temperature detection resistor; and calculation steps S108, S110 and S111 of calculating a flow rate of the liquid flowing through a measurement pipe on the basis of the first arrival time T1 and second arrival time T2. The calculation step is configured to calculate the flow rate of the liquid on the basis of both the first arrival time T1 in a case where first flow rate FL1 is larger than a first prescribed flow rate and the second arrival time T2, and, when the first flow rate FL1 is equal to or less than the first prescribed flow rate, calculate the flow rate of the liquid on the basis of the first arrival time T1, not using the second arrival time T2.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a thermal flow meter and a flow rate calculation method. [Background technology]

[0002] Conventionally, a thermal flow meter has been known that includes a temperature detection board having a heating resistor and a temperature detection resistor formed on the detection surface along an axis, and calculates the flow rate of a liquid from the time difference between when the liquid is heated by the heating resistor and when the heated liquid is detected by the temperature detection resistor (see, for example, Patent Document 1). Patent Document 1 discloses that a control board calculates the flow rate of a liquid flowing through a measuring tube from the time when the heating resistor is instantaneously heated and the time when the two temperature detection resistors subsequently detect the temperature of the heated liquid, and calculates the flow rate of the liquid from the flow rate and the cross-sectional area of ​​the measuring tube.

[0003] The thermal flow meter disclosed in Patent Document 1 has a first temperature-detecting resistor arranged downstream of a heating resistor in the direction of liquid flow, and a second temperature-detecting resistor arranged further downstream of the first temperature-detecting resistor. When the thermal flow meter disclosed in Patent Document 1 is used, for example, a first flow rate can be calculated from the timing when the heating resistor is instantaneously heated and the timing when the first temperature-detecting resistor detects the temperature of the heated liquid, a second flow rate can be calculated from the timing when the heating resistor is instantaneously heated and the timing when the second temperature-detecting resistor detects the temperature of the heated liquid, and the first flow rate and the second flow rate can be averaged to calculate the flow rate of the liquid. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-131323 Summary of the Invention [Problem to be solved by the invention]

[0005] When the flow rate of the liquid flowing through the measuring tube is relatively high, the temperature difference between the temperature of the heated liquid reaching the first temperature detection resistor and the temperature of the heated liquid reaching the second temperature detection resistor is small, and the first flow rate and the second flow rate are approximately the same value, so the flow rate of the liquid can be appropriately calculated by averaging the first flow rate and the second flow rate.

[0006] When the flow rate of the liquid flowing through the measuring tube is relatively low, the temperature of the heated liquid that reaches the second temperature detecting resistor is lower than the temperature of the heated liquid that reaches the first temperature detecting resistor, and the flow rate of the liquid cannot be calculated appropriately from the first flow rate and the second flow rate. This is because, when the flow rate of the liquid is relatively low, the heat applied to the liquid from the heating resistor propagates through the measuring tube and the liquid before it reaches the second temperature detecting resistor.

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a thermal flow meter and a flow rate calculation method that can reduce flow rate calculation errors when the liquid flow rate is relatively low and expand the lower limit of the measurable flow rate range. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention employs the following means.

[0009] A thermal flow meter according to one aspect of the present invention includes a measuring tube having an inlet through which a liquid flows and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path extending along an axis; a temperature detection board on which a heating resistor, a first temperature detection resistor, and a second temperature detection resistor are formed at intervals on a detection surface along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal to the temperature detection board for heating the heating resistor; a voltage detection unit that detects a first output voltage corresponding to the temperature of the first temperature detection resistor and a second output voltage corresponding to the temperature of the second temperature detection resistor; a time measurement unit that measures a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detection resistor, and measures a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detection resistor based on the voltage signal and the second output voltage; and a calculation unit that calculates the flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, wherein the calculation unit calculates the flow rate of the liquid based on both the first arrival time and the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid based on the first arrival time without using the second arrival time when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

[0010] According to a thermal flow meter according to one aspect of the present invention, when the flow rate of the liquid is greater than the first predetermined flow rate, the flow rate of the liquid is calculated based on both the first arrival time and the second arrival time, which reduces errors in calculating the flow rate compared to when the flow rate is calculated without using either the first arrival time or the second arrival time.

[0011] Furthermore, with a thermal flow meter according to one aspect of the present invention, when the flow rate of the liquid is equal to or less than the first predetermined flow rate, the flow rate of the liquid is calculated based on the first arrival time without using the second arrival time. This reduces calculation errors in the flow rate of the liquid caused by heat applied to the liquid from the heating resistor propagating through the measuring tube and the liquid before reaching the second temperature detecting resistor. Therefore, with a thermal flow meter according to one aspect of the present invention, it is possible to reduce calculation errors in the flow rate when the flow rate of the liquid is relatively low and expand the lower limit of the measurable flow rate range.

[0012] In a thermal flow meter according to one aspect of the present invention, the calculation unit may be configured to calculate the flow rate of the liquid by equally weighting the first flow rate calculated from the first arrival time and the second flow rate calculated from the second arrival time when the flow rate of the liquid is greater than a second predetermined flow rate that is greater than the first predetermined flow rate, and to calculate the flow rate of the liquid by weighting the first flow rate more heavily than the second flow rate when the flow rate of the liquid is greater than the first predetermined flow rate but equal to or less than the second predetermined flow rate.

[0013] According to the thermal flow meter of this configuration, when the flow rate of the liquid is greater than the second predetermined flow rate, the flow rate of the liquid is calculated by equally weighting the first flow rate calculated from the first arrival time and the second flow rate calculated from the second arrival time. Therefore, it is possible to reduce errors in calculating the flow rate compared to when the flow rate is calculated from only one of the first flow rate calculated from the first arrival time and the second flow rate calculated from the second arrival time.

[0014] Furthermore, with the thermal flow meter of this configuration, when the flow rate of the liquid is greater than the first predetermined flow rate and equal to or less than the second predetermined flow rate, the flow rate of the liquid is calculated by weighting the first flow rate greater than the second flow rate, which reduces calculation errors in the flow rate of the liquid caused by heat applied to the liquid from the heating resistor propagating through the measuring tube and the liquid before reaching the second temperature detecting resistor.

[0015] In the thermal flow meter having the above configuration, the calculation unit may calculate the flow rate of the liquid by gradually increasing the weighting of the first flow rate at a constant rate of increase compared to the weighting of the second flow rate and gradually decreasing the weighting of the second flow rate at a constant rate of decrease compared to the weighting of the first flow rate as the flow rate of the liquid calculated by the calculation unit decreases from the second predetermined flow rate to the first predetermined flow rate.

[0016] According to the thermal flow meter of this embodiment, as the flow rate of the liquid calculated by the calculation unit decreases and approaches the first predetermined flow rate from the second predetermined flow rate, the weighting of the first flow rate gradually increases at a constant rate of increase compared to the weighting of the second flow rate, and the weighting of the second flow rate gradually decreases at a constant rate of decrease compared to the weighting of the first flow rate, thereby making it possible to appropriately suppress fluctuations in the flow rate calculation results.

[0017] A thermal flow meter according to one aspect of the present invention includes a measuring tube having an inlet through which a liquid flows and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path extending along an axis; a temperature detection board having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal for heating the heating resistor to the temperature detection board; a voltage detection unit that detects a first output voltage corresponding to the temperature of the first temperature detection resistor and a second output voltage corresponding to the temperature of the second temperature detection resistor; and a flow meter that detects whether the liquid heated by the heating resistor is heated to the first temperature detection resistor based on the voltage signal and the first output voltage. The liquid flow rate measuring device includes a time measuring unit that measures a first arrival time until the liquid reaches a temperature detecting resistor, and measures a second arrival time until the liquid heated by the heating resistor reaches the second temperature detecting resistor based on the voltage signal and the second output voltage, and a calculation unit that calculates the flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, wherein when the flow rate of the liquid is greater than a first predetermined flow rate, the calculation unit calculates the flow rate of the liquid by assigning a predetermined weight to a first flow rate calculated from the first arrival time and a second flow rate calculated from the second arrival time, and when the flow rate of the liquid is equal to or less than the first predetermined flow rate, the calculation unit calculates the flow rate of the liquid by weighting the first flow rate more than the predetermined weight.

[0018] According to a thermal flow meter of one aspect of the present invention, when the flow rate of the liquid is equal to or less than a first predetermined flow rate, the flow rate of the liquid is calculated by weighting the first flow rate more heavily than the predetermined weighting. This reduces calculation errors in the flow rate of the liquid caused by heat applied to the liquid from the heating resistor propagating through the measuring tube and the liquid before reaching the second temperature-detecting resistor. Therefore, the thermal flow meter of one aspect of the present invention reduces calculation errors in the flow rate when the flow rate of the liquid is relatively low, thereby expanding the lower limit of the measurable flow rate range.

[0019] A flow rate calculation method according to one aspect of the present invention is a method for calculating a flow rate of a liquid using a thermal flow meter, the thermal flow meter comprising: a measuring tube having an inlet through which the liquid flows in and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path extending along an axis; a temperature detection board having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal for heating the heating resistor to the temperature detection board; and a voltage detection unit that detects a first output voltage corresponding to a temperature of the first temperature detection resistor and a second output voltage corresponding to a temperature of the second temperature detection resistor, a time measurement step of measuring a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detection resistor based on the voltage signal and the second output voltage, and measuring a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detection resistor based on the voltage signal and the second output voltage; and a calculation step of calculating the flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, wherein the calculation step calculates the flow rate of the liquid based on both the first arrival time and the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid based on the first arrival time without using the second arrival time when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

[0020] According to a flow rate calculation method according to one aspect of the present invention, when the flow rate of the liquid is greater than a first predetermined flow rate, the flow rate of the liquid is calculated based on both the first arrival time and the second arrival time, thereby reducing errors in calculating the flow rate compared to when the flow rate is calculated without using either the first arrival time or the second arrival time.

[0021] Furthermore, according to a flow rate calculation method according to one aspect of the present invention, when the flow rate of the liquid is equal to or less than the first predetermined flow rate, the flow rate of the liquid is calculated based on the first arrival time without using the second arrival time. This reduces calculation errors in the flow rate of the liquid caused by heat applied to the liquid from the heating resistor propagating through the measuring tube and the liquid before reaching the second temperature detecting resistor. Therefore, according to the flow rate calculation method according to one aspect of the present invention, it is possible to reduce calculation errors in the flow rate when the flow rate of the liquid is relatively low.

[0022] A flow rate calculation method according to one aspect of the present invention is a method for calculating a flow rate of a liquid using a thermal flow meter, the thermal flow meter comprising: a measuring tube having an inlet through which the liquid flows in and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path extending along an axis; a temperature detection board having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit outputting a voltage signal for heating the heating resistor to the temperature detection board; and a voltage detection unit detecting a first output voltage corresponding to a temperature of the first temperature detection resistor and a second output voltage corresponding to a temperature of the second temperature detection resistor, The method includes a time measurement step of measuring a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detection resistor, and measuring a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detection resistor based on the voltage signal and the second output voltage, and a calculation step of calculating the flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, wherein the calculation step calculates the flow rate of the liquid by assigning a predetermined weight to a first flow rate calculated from the first arrival time and a second flow rate calculated from the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid by weighting the first flow rate more heavily than the predetermined weight when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

[0023] Furthermore, according to a flow rate calculation method according to one aspect of the present invention, when the flow rate of the liquid is equal to or less than a first predetermined flow rate, the flow rate of the liquid is calculated by weighting the first flow rate more heavily than the predetermined weighting. This reduces calculation errors in the flow rate of the liquid caused by heat applied to the liquid from the heating resistor propagating through the measuring tube and the liquid before reaching the second temperature detecting resistor. Therefore, according to the flow rate calculation method according to one aspect of the present invention, calculation errors in the flow rate when the flow rate of the liquid is relatively low can be reduced. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a thermal flow meter and a flow rate calculation method that can reduce flow rate calculation errors when the liquid flow rate is relatively low and expand the lower limit of the measurable flow rate range. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a vertical cross-sectional view of a thermal flow meter according to an embodiment of the present invention. [Figure 2] FIG. 2 is a partially enlarged view of a sensor unit shown in FIG. [Figure 3] 3 is an end view of the sensor unit shown in FIG. 2 taken along the arrow AA. [Figure 4] 4 is a plan view of the sensor substrate shown in FIG. 3 as viewed from the detection surface side. [Figure 5] 2 is a block diagram showing a control configuration of a control board shown in FIG. 1. FIG. [Figure 6] 10 is a graph showing the relationship between the flow rate of the liquid calculated by the calculation unit and the amplitude of the first differential voltage and the second differential voltage. [Figure 7] 1 is a flowchart showing a flow rate calculation method in which the thermal flow meter according to one embodiment of the present invention calculates the flow rate of a liquid. [Figure 8] 10 is a graph showing the relationship between the flow rate of the liquid calculated by the calculation unit and the first coefficient and the second coefficient used in the calculation formula. [Figure 9] 10 is a graph showing a modified example of the relationship between the flow rate of the liquid calculated by the calculation unit and the first coefficient and the second coefficient used in the calculation formula. DETAILED DESCRIPTION OF THE INVENTION

[0026] A thermal flow meter 100 according to one embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a vertical cross-sectional view of a thermal flow meter 100 according to one embodiment of the present invention.

[0027] The thermal flow meter 100 of this embodiment is a thermal flow meter that measures the flow rate of a liquid by heating the liquid flowing through an internal flow path and detecting the temperature of the heated liquid. The thermal flow meter 100 of this embodiment is suitable for measuring minute flow rates of, for example, 4 μL / min to 350 μL / min. As shown in FIG. 1 , the thermal flow meter 100 of this embodiment includes a sensor unit 10, a control board 20, a relay board 30, an upper case 40, and a bottom case 50.

[0028] 1, the sensor unit 10 allows liquid flowing in from an inlet 10a connected to an external pipe (not shown) to flow out from an outlet 10b connected to the external pipe (not shown), heats the liquid flowing through an internal flow path 10c, and outputs a voltage corresponding to the temperature of the heated liquid to a control board 20. Details of the sensor unit 10 will be described later.

[0029] The control board 20 is a device that outputs a voltage signal for heating the liquid to the sensor unit 10, receives a voltage signal corresponding to the temperature of the heated liquid from the sensor unit 10, and calculates the flow rate of the liquid flowing through the sensor unit 10. Details of the control board 20 will be described later.

[0030] The relay board 30 is a board that relays signals for transmitting and receiving various signals between the control board 20 and an external device (not shown). A cable 200 is connected to the relay board 30 to transmit and receive various signals to and from the external device (not shown).

[0031] The upper case 40 is a member that serves as a housing on the upper side of the thermal flow meter 100, and houses the control board 20 inside. The bottom case 50 is a member that serves as the housing on the lower side of the thermal flow meter 100, and houses the sensor unit 10 inside. With the sensor unit 10 inserted into the bottom case 50, a stopper 70 is inserted between the bottom case 50 and the sensor unit 10 from the inlet 10a side of the sensor unit 10.

[0032] Similarly, with the sensor unit 10 inserted into the bottom case 50, the stopper 70 is inserted between the bottom case 50 and the sensor unit 10 from the outlet 10b side of the sensor unit 10. The sensor unit 10 is fixed to the bottom case 50 by the stopper 70. Fastening holes 50a are formed in the bottom surface of the bottom case 50, and the bottom case 50 is fixed to an installation surface (not shown) by fastening bolts (not shown) inserted from below the installation surface.

[0033] Next, the sensor unit 10 will be described in detail with reference to Fig. 2 to Fig. 4. Fig. 2 is a partially enlarged view of the sensor unit 10 shown in Fig. 1. Fig. 3 is an end view of the sensor unit 10 shown in Fig. 2 as seen from the arrow AA. Fig. 4 is a plan view of the sensor substrate 12 shown in Fig. 3, viewed from the detection surface 12A side.

[0034] As shown in FIG. 2, the sensor unit 10 has a measurement pipe 11, a sensor board 12 (temperature detection board), a nut 15, an inflow side body 16, and an outflow side body 17.

[0035] The measuring tube 11 is a tube having an inlet 11a through which a liquid flows in and an outlet 11b through which the liquid that has flowed in from the inlet 11a flows out. As shown in FIG. 3, the measuring tube 11 has an internal flow path 10c that is circular in cross section and extends along the axis X. The measuring tube 11 is made of glass (such as quartz glass with a high silicon dioxide content), for example. The measuring tube 11 may also be made of another tube that is resistant to corrosion by alkaline liquids.

[0036] As shown in FIG. 4, the sensor substrate 12 is a glass (e.g., quartz glass with a high silicon dioxide content) substrate on which a temperature detection resistance wire 12e, a temperature detection resistance wire 12c, a heating resistance wire (heating resistor) 12a, a temperature detection resistance wire (first temperature detection resistor) 12b, and a temperature detection resistance wire (second temperature detection resistor) 12d are formed on a detection surface 12A at intervals along an axis X in a liquid flow direction FD.

[0037] The detection surface 12A is a flat surface that extends along the axis X. The heating resistance wire 12a and the temperature detection resistance wires 12b, 12c, 12d, and 12e are each formed by depositing a metal film such as platinum on a glass substrate.

[0038] The liquid flowing through the measuring tube 11 flows along the axis X in the flow direction FD, which is from left to right in Fig. 4. Therefore, when the heating resistance wire 12a is heated instantaneously, the heated liquid flows along the axis X and reaches the position of the temperature detection resistance wire 12b, and then reaches the position of the temperature detection resistance wire 12d. The control board 20 can measure the temperatures of the temperature detection resistance wire 12b and the temperature detection resistance wire 12d by detecting a voltage signal corresponding to the electrical resistance values ​​of the temperature detection resistance wires 12b and 12d, which change with temperature.

[0039] The center position of the sensor substrate 12 in the direction of the axis X is located closer to the outlet 11b than an intermediate position equidistant from both the inlet 11a and the outlet 11b of the measuring tube 11. In addition, the position at which the heating resistance wire 12a is located on the sensor substrate 12 is located closer to the outlet 11b than an intermediate position equidistant from both the end of the sensor substrate 12 on the inlet 11a side and the end of the sensor substrate 12 on the outlet 11b side.

[0040] The distance on the axis X from the inlet 11a of the measuring tube 11 to the heating resistance wire 12a is longer than the distance on the axis X from the outlet 11b of the measuring tube 11 to the heating resistance wire 12a. By doing so, a long distance is ensured from the inlet 11a of the measuring tube 11 to the heating resistance wire 12a, and the liquid can be heated after turbulence and the like of the liquid that has flowed into the inlet 11a of the measuring tube 11 has been sufficiently reduced.

[0041] 4, the detection surface 12A is formed with a wiring pattern 12f connected to one end of the heating resistance wire 12a and a wiring pattern 12g connected to the other end of the heating resistance wire 12a. The detection surface 12A also is formed with a wiring pattern 12h connected to one end of the temperature detection resistance wire 12b, a wiring pattern 12i connected to the other end of the temperature detection resistance wire 12b, and a wiring pattern 12j connected to one end of the temperature detection resistance wire 12d. The other end of the temperature detection resistance wire 12d is connected to the wiring pattern 12i.

[0042] The detection surface 12A is also formed with a wiring pattern 12k connected to one end of the temperature detection resistance wire 12c, a wiring pattern 12l connected to the other end of the temperature detection resistance wire 12c, and a wiring pattern 12m connected to one end of the temperature detection resistance wire 12e. The other end of the temperature detection resistance wire 12e is connected to the wiring pattern 12l. The wiring patterns 12f, 12g, 12h, 12i, 12j, 12k, 12l, and 12m are each formed by depositing a metal film such as platinum on a glass substrate.

[0043] Ends of the wiring patterns 12f, 12g, 12h, 12i, 12j, 12k, 12l, and 12m are respectively joined to metal wiring patterns 60f, 60g, 60h, 60i, 60j, 60k, 60l, and 60m arranged on a flexible substrate (external connection terminal) 60 formed from a film-like resin. The wiring patterns 60f, 60g, 60h, 60i, 60j, 60k, 60l, and 60m of the flexible substrate 60 are each electrically connected to the control substrate 20.

[0044] As shown in Fig. 3, the upper side of the cross section of the measuring tube 11 taken along a plane perpendicular to the axis X at the position where the sensor substrate 12 is bonded is substantially circular. Of the outer peripheral surfaces of the measuring tube 11, the surface that faces the detection surface 12A of the sensor substrate 12 is a flat surface 11c. On the other hand, the cross section of the measuring tube 11 taken along a plane perpendicular to the axis X at a position where the sensor substrate 12 is not bonded is circular. The flat surface 11c of the measuring tube 11 is disposed so as to face the detection surface 12A of the sensor substrate 12. The detection surface 12A is bonded to the flat surface 11c of the measuring tube 11 along the axis X with an adhesive.

[0045] The distance D1 from the detection surface 12A of the sensor substrate 12 to the inner circumferential surface 10d of the internal flow path 10c is shorter than the distance D2 from the top 11d of the measurement tube 11 to the inner circumferential surface 10d of the internal flow path 10c. This is because by making the distance D1 from the detection surface 12A of the sensor substrate 12 to the inner circumferential surface 10d of the internal flow path 10c shorter than the distance D2, the thermal conductivity from the heating resistance wire 12a to the liquid is improved and the temperature detection characteristics of the temperature detection resistance wires 12b and 12d are improved. The distance D1 is preferably 0.2 mm or less.

[0046] 2, the inlet-side body 16 is a member into which the inlet 11a of the measuring pipe 11 is inserted and which has a connecting flow path 16a formed therein that is circular in cross section. The outlet-side body 17 is a member into which the outlet 11b of the measuring pipe 11 is inserted and which has a connecting flow path 17a formed therein that is circular in cross section.

[0047] The nut 15 consists of an inlet-side nut 15a attached to the inlet-side body 16 and an outlet-side nut 15b attached to the outlet-side body 17. The inlet-side nut 15a is a cylindrical member that is inserted along the outer circumferential surface of the measuring tube 11 closer to the outlet 11b than the inlet-side body 16.

[0048] Next, the control board 20 will be described in detail with reference to Fig. 5. Fig. 5 is a block diagram showing the control configuration of the control board 20 shown in Fig. 1. As shown in Fig. 5, the control board 20 has a first voltage detection unit 21, a second voltage detection unit 22, a third voltage detection unit 23, a fourth voltage detection unit 24, a voltage output unit 25, a time measurement unit 26, and a calculation unit 27.

[0049] The voltage output unit 25 outputs a voltage signal for heating the heating resistance wire 12a to the sensor substrate 12 via the wiring patterns 60f, 60g of the flexible substrate 60. The voltage output unit 25 outputs a voltage signal to the heating resistance wire 12a so as to cyclically repeat a heating period in which the heating resistance wire 12a is heated and a non-heating period in which the heating resistance wire 12a is not heated.

[0050] For example, the voltage output unit 25 outputs a voltage signal so that the heating period is shorter than the non-heating period. The heating period is set at a ratio of less than 0.5 to one cycle, which is the sum of the heating period and the non-heating period. The ratio of the heating period to one cycle may also be set at less than 0.4.

[0051] The first voltage detection unit 21 detects a first output voltage Vo1 that corresponds to the temperature of the temperature detection resistance wire 12b. The first voltage detection unit 21 supplies a current of a constant current value to the temperature detection resistance wire 12b and detects the first output voltage Vo1 that corresponds to the resistance value of the temperature detection resistance wire 12b, which changes with temperature.

[0052] The second voltage detection unit 22 detects a second output voltage Vo2 that corresponds to the temperature of the temperature detection resistance wire 12d. The second voltage detection unit 22 supplies a current of a constant current value to the temperature detection resistance wire 12d and detects the second output voltage Vo2 that corresponds to the resistance value of the temperature detection resistance wire 12d, which changes with temperature.

[0053] The third voltage detector 23 detects a third output voltage Vo3 corresponding to the temperature of the temperature detection resistor wire 12c. The third voltage detector 23 supplies a constant current to the temperature detection resistor wire 12c and detects the third output voltage Vo3 corresponding to the resistance value of the temperature detection resistor wire 12c, which changes with temperature.

[0054] The fourth voltage detection unit 24 detects a fourth output voltage Vo4 that corresponds to the temperature of the temperature detection resistance wire 12e. The fourth voltage detection unit 24 supplies a current of a constant current value to the temperature detection resistance wire 12e and detects the fourth output voltage Vo4 that corresponds to the resistance value of the temperature detection resistance wire 12e, which changes with temperature.

[0055] The time measurement unit 26 measures a first arrival time T1, which is the time it takes for the liquid heated by the heating resistance wire 12a to reach the first temperature-detection resistance wire 12b, based on the voltage signal output by the voltage output unit 25 and the first output voltage Vo1 detected by the first voltage detection unit 21. The time measurement unit 26 also measures a second arrival time T2, which is the time it takes for the liquid heated by the heating resistance wire 12a to reach the second temperature-detection resistance wire 12d, based on the voltage signal output by the voltage output unit 25 and the second output voltage Vo2 detected by the second voltage detection unit 22.

[0056] It is preferable that the time measurement unit 26 measures the first arrival time T1 based on a first differential voltage DV1 (Vo1-Vo3) obtained by subtracting the third output voltage Vo3 detected by the third voltage detection unit 23 from the first output voltage Vo1 detected by the first voltage detection unit 21. This eliminates the influence of heat transferred from the heating resistance wire 12a to the measurement tube 11 to the temperature detection resistance wire 12c via the measurement tube 11 without being transferred to the liquid, thereby improving measurement accuracy.

[0057] The temperature detection resistance wires 12b and 12c are arranged equidistant from the heating resistance wire 12a. The third output voltage Vo3 detected by the temperature detection resistance wire 12c located upstream of the heating resistance wire 12a in the flow direction FD corresponds to the heat transferred from the heating resistance wire 12a to the measuring tube 11 that is not transferred to the liquid but is transferred to the temperature detection resistance wire 12b via the measuring tube 11. Therefore, by measuring the first arrival time T1 based on the first differential voltage DV1, the influence of the heat transferred to the temperature detection resistance wire 12c via the measuring tube 11 can be eliminated.

[0058] Similarly, when measuring the second arrival time T2, the time measurement unit 26 preferably measures the second arrival time T2 based on a second differential voltage DV2 (Vo2-Vo4) obtained by subtracting the fourth output voltage Vo4 detected by the fourth voltage detection unit 24 from the second output voltage Vo2 detected by the second voltage detection unit 22. By doing so, it is possible to eliminate the influence of heat that is not transferred to the liquid but is transferred to the temperature detection resistance wire 12d via the measurement tube 11 out of the heat transferred from the heating resistance wire 12a to the measurement tube 11, thereby improving measurement accuracy.

[0059] The temperature detection resistance wire 12d and the temperature detection resistance wire 12e are arranged equidistant from the heating resistance wire 12a. The fourth output voltage Vo4 detected by the temperature detection resistance wire 12e located upstream of the heating resistance wire 12a in the flow direction FD corresponds to the heat transferred from the heating resistance wire 12a to the measuring tube 11 that is not transferred to the liquid but is transferred to the temperature detection resistance wire 12d via the measuring tube 11. Therefore, by measuring the second arrival time T2 based on the second differential voltage DV2, the influence of the heat transferred to the temperature detection resistance wire 12d via the measuring tube 11 can be eliminated.

[0060] The time measurement unit 26, for example, performs a Fourier transform on the time-series signal of the first differential voltage DV1 and calculates a first arrival time T1 from the coefficient of the fundamental wave (a sine wave having the same frequency as the voltage signal output by the voltage output unit 25) after the Fourier transform. The time measurement unit 26 also performs a Fourier transform on the time-series signal of the second differential voltage DV2 and calculates a second arrival time T2 from the coefficient of the fundamental wave after the Fourier transform.

[0061] The time measurement unit 26, for example, performs a Fourier transform on the time-series signal of the first differential voltage DV1 and calculates an amplitude VA1 of the fundamental wave after the Fourier transform. The amplitude VA1 has a value corresponding to the amount of heat of the liquid transferred to the temperature detection resistance wire 12b. The time measurement unit 26 also performs a Fourier transform on the time-series signal of the second differential voltage DV2 and calculates an amplitude VA2 of the fundamental wave after the Fourier transform. The amplitude VA2 has a value corresponding to the amount of heat of the liquid transferred to the temperature detection resistance wire 12d.

[0062] The calculation unit 27 calculates the flow rate of the liquid flowing through the measuring pipe 11 based on the first arrival time T1 and the second arrival time T2 measured by the time measurement unit 26. The calculation unit 27 switches the calculation formula for calculating the flow rate of the liquid depending on the flow rate currently calculated by the calculation unit 27. Below, the operation of the calculation unit 27 to calculate the flow rate of the liquid by switching the calculation formula depending on the flow rate will be described.

[0063] 6 is a graph showing the relationship between the flow rate of the liquid calculated by the calculation unit 27 and the amplitude of the first differential voltage DV1 and the second differential voltage DV2. In FIG. 6, the first region AR1 is a region where the flow rate is equal to or greater than 4 μL / min and less than a first predetermined flow rate PFL1 [μL / min]. The second region AR2 is a region where the flow rate is equal to or greater than the first predetermined flow rate PFL1 [μL / min] and less than a second predetermined flow rate PFL2 [μL / min]. The third region AR3 is a region where the flow rate is equal to or greater than the second predetermined flow rate PFL2 [μL / min]. For example, the first predetermined flow rate PFL1 is 8 μL / min, and the second predetermined flow rate PFL2 is 20 μL / min. The first predetermined flow rate PFL1 and the second predetermined flow rate PFL2 may be set to any other value.

[0064] Note that the flow rate in the graph shown in Figure 6 is different from the flow rate calculated by the calculation unit 27 of this embodiment, and is a flow rate measured by another type of calibration flow meter (e.g., a Coriolis flow meter) that can accurately measure the flow rate of the liquid flowing through the internal flow path 10c in any of the flow rate regions of the first region AR1, the second region AR2, and the third region AR3.

[0065] 6, in the third region AR3 (particularly in the region of approximately 50 μL / min or more), the difference between the amplitude VA1 of the fundamental wave after the Fourier transform of the first differential voltage DV1 and the amplitude VA2 of the fundamental wave after the Fourier transform of the second differential voltage DV2 is small. On the other hand, in the first region AR1 and the second region AR2, the amplitude VA1 is larger than the amplitude VA2, and the difference between the amplitudes VA1 and VA2 increases as the liquid flow rate decreases.

[0066] As described above, the difference between the amplitude VA1 and the amplitude VA2 becomes large in the first region AR1 and the second region AR2 because, when the flow rate of the liquid is relatively low, the heat applied to the liquid from the heating resistance wire 12a propagates into the measuring tube 11 and the liquid before reaching the temperature detection resistance wire 12d. Therefore, in this embodiment, the calculation formula used by the calculation unit 27 to calculate the flow rate is switched depending on whether the flow rate calculated by the calculation unit 27 is in the first region AR1, the second region AR2, or the third region AR3.

[0067] Next, a flow rate calculation method by which the thermal flow meter 100 of this embodiment calculates the flow rate of a liquid will be described. Fig. 7 is a flowchart showing a flow rate calculation method by which the thermal flow meter 100 according to one embodiment of the present invention calculates the flow rate of a liquid. Each process in the flowchart shown in Fig. 7 is executed by a control program stored in the control board 20.

[0068] In step S101, the voltage output unit 25 of the control board 20 outputs a voltage signal to the heating resistance wire 12a to heat the liquid flowing through the measuring tube 11. The voltage output unit 25 outputs a voltage signal to the heating resistance wire 12a so as to cyclically repeat a heating period in which the heating resistance wire 12a is heated and a non-heating period in which the heating resistance wire 12a is not heated.

[0069] In step S102, the first voltage detection unit 21 of the control board 20 detects a first output voltage Vo1 between the wiring pattern 60h connected to the temperature detection resistance wire 12b and the wiring pattern 60i. The second voltage detection unit 22 of the control board 20 detects a second output voltage Vo2 between the wiring pattern 60j connected to the temperature detection resistance wire 12d and the wiring pattern 60i. The third voltage detection unit 23 of the control board 20 detects a third output voltage Vo3 between the wiring pattern 60k connected to the temperature detection resistance wire 12c and the wiring pattern 60l. The fourth voltage detection unit 24 of the control board 20 detects a fourth output voltage Vo4 between the wiring pattern 60m connected to the temperature detection resistance wire 12e and the wiring pattern 60l.

[0070] In step S103, the time measurement unit 26 of the control board 20 performs a Fourier transform on the time series signal of the first differential voltage DV1 obtained by subtracting the third output voltage Vo3 from the first output voltage Vo1, and calculates the first arrival time T1 from the coefficient of the fundamental wave (a sine wave with the same frequency as the voltage signal output by the voltage output unit 25) after the Fourier transform.

[0071] In step S104, the calculation unit 27 of the control board 20 calculates the first flow rate FL1 of the liquid based on the first arrival time T1 using the following calculation formula (A). FL1=S·D1 / T1 (A) Here, S[m 2 ] is the cross-sectional area of ​​the internal flow path 10c of the measuring tube 11, and D1 [m] is the distance from the heating resistance wire 12a to the temperature detection resistance wire 12b.

[0072] Instead of the above calculation formula (A), the following calculation formula (A1) may be used. FL1=a0+a1 / T1+a2 / T1 2 +a3 / T1 3 +a4 / T1 4 (A1) Here, a0, a1, a2, a3, and a4 are each coefficients calculated from the relationship between multiple flow rates measured in advance with a calibration flow meter and the first arrival time T1. The above calculation formula (A1) approximates the first flow rate FL1 with a fourth-order polynomial of the first arrival time T1, but approximation with a second-order, third-order, fifth-order or higher polynomial is also possible.

[0073] In step S105, the time measurement unit 26 of the control board 20 performs a Fourier transform on the time series signal of the second differential voltage DV2 obtained by subtracting the fourth output voltage Vo4 from the second output voltage Vo2, and calculates the second arrival time T2 from the coefficient of the fundamental wave (a sine wave with the same frequency as the voltage signal output by the voltage output unit 25) after the Fourier transform.

[0074] In step S106, the calculation unit 27 of the control board 20 calculates the second flow rate FL2 of the liquid based on the second arrival time T2 using the following calculation formula (B). FL2=S·D2 / T2 (B) Here, D2 [m] is the distance from the heating resistance wire 12a to the temperature detection resistance wire 12d.

[0075] Instead of the above formula (A), the following formula (B1) may be used. FL2=b0+b1 / T2+b2 / T2 2 +b3 / T2 3 +b4 / T2 4 (B1) Here, b0, b1, b2, b3, and b4 are coefficients calculated from the relationship between multiple flow rates measured in advance with a calibration flow meter and the second arrival time T2. The above calculation formula (B1) approximates the second flow rate FL2 with a fourth-order polynomial of the second arrival time T2, but approximation with a second-order, third-order, fifth-order or higher polynomial is also possible.

[0076] In step S107, the calculation unit 27 of the control board 20 determines whether the first flow rate FL1 calculated in step S104 is equal to or less than the first predetermined flow rate PFL1, and if the answer is YES, the process proceeds to step S108, and if the answer is NO, the process proceeds to step S109.

[0077] In steps S108, S110, and S111, the calculation unit 27 calculates the flow rate FL of the liquid using the following calculation formula (C). FL=C1·FL1+C2·FL2 (C) Here, the first coefficient C1 and the second coefficient C2 are values ​​that, when added together, become 1, and indicate the weighting of the first flow rate FL1 calculated from the first output voltage Vo1 output from the temperature detection resistance wire 12b and the second flow rate FL2 calculated from the second output voltage Vo2 output from the temperature detection resistance wire 12d when calculating the flow rate FL.

[0078] In step S108, the calculation unit 27 of the control board 20 calculates the flow rate FL of the liquid using the following calculation formula (1). FL=FL1 (1) The above calculation formula (1) is obtained by setting the first coefficient C1 to 1 and the second coefficient C2 to 0 in calculation formula (C). Calculation formula (1) is an equation in which the weighting of the flow rate calculated from the second output voltage Vo2 output from the temperature detection resistance wire 12d is set to 0.

[0079] In step S109, the calculation unit 27 of the control board 20 determines whether the latest calculated liquid flow rate is less than or equal to the second predetermined flow rate PFL2, and if YES, proceeds to step S110, and if NO, proceeds to step S111.

[0080] In step S110, the calculation unit 27 of the control board 20 calculates the flow rate FL of the liquid using the following calculation formula (2). FL = C1 FL1 + C2 FL2 (2) The above calculation formula (2) is calculated by changing the first coefficient C1 and the second coefficient C2 in calculation formula (A) to be greater than 0 and less than 1. Calculation formula (2) is a formula that weights each of the flow rate calculated from the first output voltage Vo1 output from the temperature detection resistance wire 12b and the flow rate calculated from the second output voltage Vo2 output from the temperature detection resistance wire 12d.

[0081] In step S111, the calculation unit 27 of the control board 20 calculates the flow rate FL of the liquid using the following calculation formula (3). FL=0.5·FL1+0.5·FL2 (3) In the above calculation formula (3), the first coefficient C1 and the second coefficient C2 are each set to 0.5. The calculation formula (3) is a formula that equally weights the flow rate calculated from the first output voltage Vo1 output from the temperature detection resistance wire 12b and the flow rate calculated from the second output voltage Vo2 output from the temperature detection resistance wire 12d.

[0082] After calculating the flow rate FL of the liquid in steps S108, S110, and S111, the control board 20 transmits an output signal indicating the calculated flow rate FL to an external device via the relay board 30, and ends the processing of this flowchart. The control board 20 continuously calculates the flow rate FL of the liquid by repeatedly executing the processing of steps S101 to S111 above.

[0083] Fig. 8 is a graph showing the relationship between the liquid flow rate calculated by the calculation unit 27 and the first coefficient C1 and second coefficient C2 used in calculation formulas (1), (2), and (3). As shown in Fig. 8, when the flow rate calculated by the calculation unit 27 is in the first region AR1, the first coefficient C1 is fixed at 1 and the second coefficient C2 is fixed at 0. When the flow rate calculated by the calculation unit 27 is in the third region AR3, the first coefficient C1 and the second coefficient C2 are each fixed at 0.5.

[0084] On the other hand, when the flow rate calculated by the calculation unit 27 is in the second region AR2, the first coefficient C1 and the second coefficient C2 are each greater than 0 and less than 1. The first coefficient C1 gradually increases at a constant rate as the flow rate of the liquid calculated by the calculation unit 27 decreases from the second predetermined flow rate PFL2 (20 μL / min in FIG. 8) to the first predetermined flow rate PFL1 (8 μL / min in FIG. 8). The second coefficient C2 gradually decreases at a constant rate as the flow rate of the liquid calculated by the calculation unit 27 decreases from the second predetermined flow rate PFL2 to the first predetermined flow rate PFL1.

[0085] 8, the first coefficient C1 and the second coefficient C2 may be modified when the flow rate calculated by the calculation unit 27 is in the second region AR2. FIG. 9 is a graph showing a modified example of the relationship between the flow rate of the liquid calculated by the calculation unit 27 and the first coefficient C1 and the second coefficient C2 used in the calculation formula. As shown in FIG. 9, when the flow rate calculated by the calculation unit 27 is in the second region AR2, the first coefficient C1 may be increased in stages as the flow rate of the liquid calculated by the calculation unit 27 decreases from the second predetermined flow rate PFL2 to the first predetermined flow rate PFL1. The second coefficient C2 may be decreased in stages as the flow rate of the liquid calculated by the calculation unit 27 decreases from the second predetermined flow rate PFL2 to the first predetermined flow rate PFL1.

[0086] As described above, when the first flow rate FL1 is greater than the first predetermined flow rate PFL1, the calculation unit 27 calculates the flow rate of the liquid based on both the first arrival time T1 and the second arrival time T2 in steps S110 and S111. On the other hand, when the flow rate of the liquid is equal to or less than the first predetermined flow rate PFL1, the calculation unit 27 calculates the flow rate of the liquid based on the first arrival time T1 without using the second arrival time T2.

[0087] When the first flow rate FL1 is greater than the first predetermined flow rate PFL1, the calculation unit 27 calculates the liquid flow rate FL by assigning a predetermined weight to the first flow rate FL1 calculated from the first arrival time T1 and the second flow rate FL2 calculated from the second arrival time T2, and when the first flow rate FL1 is equal to or less than the first predetermined flow rate PFL1, the calculation unit 27 assigns a weight to the first flow rate FL1 greater than the predetermined weight to calculate the liquid flow rate FL.

[0088] Furthermore, when the first flow rate FL1 is greater than a second predetermined flow rate PFL2 that is greater than the first predetermined flow rate PFL1, the calculation unit 27 calculates the flow rate of the liquid by equally weighting the first flow rate FL1 calculated from the first arrival time T1 and the second flow rate FL2 calculated from the second arrival time T2. On the other hand, when the first flow rate FL1 is greater than the first predetermined flow rate PFL1 but equal to or less than the second predetermined flow rate PFL2, the calculation unit 27 calculates the flow rate FL of the liquid by weighting the first flow rate FL1 more than the second flow rate FL2.

[0089] The actions and effects achieved by the thermal flow meter 100 of the present embodiment described above will be described.

[0090] According to the thermal flow meter 100 of this embodiment, when the flow rate of the liquid is greater than the first predetermined flow rate PFL1, the flow rate FL of the liquid is calculated based on both the first arrival time T1 and the second arrival time T2. Therefore, it is possible to reduce errors in calculating the flow rate FL compared to when the flow rate is calculated without using either the first arrival time T1 or the second arrival time T2.

[0091] Furthermore, according to the thermal flow meter 100 of this embodiment, when the flow rate FL of the liquid is equal to or less than the first predetermined flow rate PFL1, the flow rate FL of the liquid is calculated based on the first arrival time T1 without using the second arrival time T2. This reduces the calculation error of the flow rate FL of the liquid caused by the heat applied to the liquid from the heating resistance wire 12a propagating through the measuring tube 11 and the liquid before reaching the second temperature detection resistance wire 12d, thereby making it possible to expand the lower limit of the measurable flow rate range.

[0092] According to the thermal flow meter 100 of this embodiment, when the flow rate of the liquid is greater than the second predetermined flow rate PFL2, the flow rate FL of the liquid is calculated by equally weighting the first flow rate FL1 calculated from the first arrival time T1 and the second flow rate FL2 calculated from the second arrival time T2. Therefore, it is possible to reduce errors in calculating the flow rate FL compared to when the flow rate FL is calculated from only one of the first flow rate FL1 calculated from the first arrival time T1 and the second flow rate FL2 calculated from the second arrival time T2.

[0093] Furthermore, according to the thermal flow meter 100 of this embodiment, when the flow rate of the liquid is greater than the first predetermined flow rate PFL1 and equal to or less than the second predetermined flow rate PFL2, the flow rate FL of the liquid is calculated by weighting the first flow rate FL1 greater than the weighting the second flow rate FL2. This makes it possible to reduce calculation errors in the flow rate of the liquid caused by the heat applied to the liquid from the heating resistance wire 12a propagating through the measuring tube 11 and the liquid before reaching the second temperature detection resistance wire 12d.

[0094] According to the thermal flow meter 100 of this embodiment, when the liquid flow rate FL calculated by the calculation unit 27 decreases and approaches the first predetermined flow rate PFL1 from the second predetermined flow rate PFL2, the first coefficient C1 increases and the second coefficient C2 decreases. Therefore, the calculation error of the liquid flow rate caused by the heat applied to the liquid from the heating resistance wire 12a propagating into the measuring tube 11 and the liquid before reaching the second temperature detection resistance wire 12d can be reduced in accordance with the decrease in the flow rate FL.

[0095] According to the thermal flow meter 100 of this embodiment, as the liquid flow rate FL calculated by the calculation unit 27 decreases and approaches the first predetermined flow rate PFL1 from the second predetermined flow rate PFL2, the first coefficient C1 gradually increases at a constant rate of increase and the second coefficient C2 gradually decreases at a constant rate of decrease, so that fluctuations in the calculation result of the flow rate FL can be appropriately suppressed. [Explanation of symbols]

[0096] 10 Sensor section 10a Inlet 10b Outlet 10c Internal flow path 10d Inner surface 11 Measuring tube 11a Inlet 11b Outlet 11c flat surface 12 Sensor board 12A Detection surface 12a Heating resistance wire 12b, 12c, 12d, 12e Temperature detection resistance wire 12f, 12g, 12h, 12i, 12j, 12k, 12l, 12m wiring pattern 20 Control board 21 First voltage detection unit 22 Second voltage detection unit 23 Third voltage detection unit 24 Fourth voltage detection unit 25 Voltage output section 26 Time measurement section 27 Calculation section 30 Relay board 40 Upper Case 50 bottom case 50a fastening hole 60 Flexible PCB 60f, 60g, 60h, 60i, 60j, 60k, 60l, 60m wiring pattern 100 Thermal flow meter AR1 1st area AR2 2nd area AR3 3rd area D1,D2 distance DV1 First differential voltage DV2 Second differential voltage FD flow direction X axis

Claims

1. a measuring tube having an inlet through which a liquid flows and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path formed therein and extending along an axis; a temperature detection substrate having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal for heating the heating resistor to the temperature detection board; a voltage detection unit that detects a first output voltage corresponding to the temperature of the first temperature detection resistor and a second output voltage corresponding to the temperature of the second temperature detection resistor; a time measurement unit that measures a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detecting resistor based on the voltage signal and the first output voltage, and that measures a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detecting resistor based on the voltage signal and the second output voltage; a calculation unit that calculates a flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, The calculation unit calculates the flow rate of the liquid based on both the first arrival time and the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid based on the first arrival time without using the second arrival time when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

2. 2. The thermal flow meter according to claim 1, wherein the calculation unit calculates the flow rate of the liquid by equally weighting a first flow rate calculated from the first arrival time and a second flow rate calculated from the second arrival time when the flow rate of the liquid is greater than a second predetermined flow rate that is greater than the first predetermined flow rate, and calculates the flow rate of the liquid by weighting the first flow rate more heavily than the second flow rate when the flow rate of the liquid is greater than the first predetermined flow rate and equal to or less than the second predetermined flow rate.

3. 3. The thermal flow meter according to claim 2, wherein the calculation unit calculates the flow rate of the liquid by gradually increasing the weighting of the first flow rate at a constant rate of increase compared to the weighting of the second flow rate and gradually decreasing the weighting of the second flow rate at a constant rate of decrease compared to the weighting of the first flow rate as the flow rate of the liquid calculated by the calculation unit decreases from the second predetermined flow rate to the first predetermined flow rate.

4. a measuring tube having an inlet through which a liquid flows and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path formed therein and extending along an axis; a temperature detection substrate having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal for heating the heating resistor to the temperature detection board; a voltage detection unit that detects a first output voltage corresponding to the temperature of the first temperature detection resistor and a second output voltage corresponding to the temperature of the second temperature detection resistor; a time measurement unit that measures a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detecting resistor based on the voltage signal and the first output voltage, and that measures a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detecting resistor based on the voltage signal and the second output voltage; a calculation unit that calculates a flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, The calculation unit calculates the flow rate of the liquid by assigning a predetermined weight to a first flow rate calculated from the first arrival time and a second flow rate calculated from the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid by assigning a weight to the first flow rate greater than the predetermined weight when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

5. A flow rate calculation method for calculating a flow rate of a liquid using a thermal flow meter, comprising: The thermal flow meter is a measuring tube having an inlet through which a liquid flows and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path formed therein and extending along an axis; a temperature detection substrate having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal for heating the heating resistor to the temperature detection board; a voltage detection unit that detects a first output voltage corresponding to the temperature of the first temperature detection resistor and a second output voltage corresponding to the temperature of the second temperature detection resistor, a time measurement step of measuring a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detecting resistor based on the voltage signal and the first output voltage, and measuring a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detecting resistor based on the voltage signal and the second output voltage; a calculation step of calculating a flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, The calculation step calculates the flow rate of the liquid based on both the first arrival time and the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid based on the first arrival time without using the second arrival time when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

6. A flow rate calculation method for calculating a flow rate of a liquid using a thermal flow meter, comprising: The thermal flow meter is a measuring tube having an inlet through which a liquid flows and an outlet through which the liquid flowing in from the inlet flows out, and having an internal flow path formed therein and extending along an axis; a temperature detection substrate having a heating resistor, a first temperature detection resistor, and a second temperature detection resistor formed on a detection surface at intervals along a flow direction from the inlet to the outlet; a voltage output unit that outputs a voltage signal for heating the heating resistor to the temperature detection board; a voltage detection unit that detects a first output voltage corresponding to the temperature of the first temperature detection resistor and a second output voltage corresponding to the temperature of the second temperature detection resistor, a time measurement step of measuring a first arrival time required for the liquid heated by the heating resistor to reach the first temperature detecting resistor based on the voltage signal and the first output voltage, and measuring a second arrival time required for the liquid heated by the heating resistor to reach the second temperature detecting resistor based on the voltage signal and the second output voltage; a calculation step of calculating a flow rate of the liquid flowing through the measuring tube based on the first arrival time and the second arrival time, The calculation step calculates the flow rate of the liquid by assigning a predetermined weight to a first flow rate calculated from the first arrival time and a second flow rate calculated from the second arrival time when the flow rate of the liquid is greater than a first predetermined flow rate, and calculates the flow rate of the liquid by weighting the first flow rate more heavily than the predetermined weight when the flow rate of the liquid is equal to or less than the first predetermined flow rate.

Images