JP2010117159A - Micro-flow meter and micro-flow measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a micro-flow meter which is inexpensive and highly accurate with a simple structure, whose necessity of maintenance is low, and moreover which is capable of comparatively-extensive micro-flow measurement, and to provide a micro-flow measuring method. <P>SOLUTION: The micro-flow meter includes: a channel for making a fluid pass through; a heater 4 for heating the fluid in the channel; a temperature detecting section 5 for detecting the temperatures of the fluid before and after being heated on the more downstream side than the heater 4; and an information processor 54 for acquiring a flow rate from the detected temperatures obtained by the temperature detecting section 5. The information processor 54 includes a memory device for storing beforehand a calibration function representing the relation between the amount of temperature increase of the fluid and its flow rate, and an arithmetic processor for acquiring the amount of temperature increase from the detected temperatures obtained by the temperature detecting section 5, and acquiring a flow rate corresponding to this amount of temperature increase from the calibration function. The calibration function has a first region where the relation between the amount of temperature increase and the flow rate is a positive linear relation, and a second region where the relation between the amount of temperature increase and the flow rate is a negative linear relation on the larger flow-rate side than the first region. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a micro flow meter and a micro flow measurement method, and more particularly to a micro flow meter and a micro flow measurement method that can be used for measurement of a very small flow rate.

  The measurement of the flow rate is indispensable in the industrial field, and many detection methods have been devised so far. However, for micro flowmeters of 1 ml / min or less, which are in demand for semiconductor manufacturing and microfluidics, etc., conventionally, expensive and complicated structures have been used.

  For example, a means using a laser beam such as a laser Doppler velocimeter (LDV) can be cited as a means for measuring a minute flow rate (see, for example, Patent Document 1). However, such a measuring apparatus using a laser beam is expensive and has a problem that the apparatus size tends to be large.

  Moreover, as an inexpensive micro flow rate measuring means, for example, a measuring device described in Patent Document 2 can be cited. In the flow rate measuring method described in Patent Document 2, the fluid heated on the upstream side is used as a temperature marker, and the flow velocity is measured from the movement time of the marker. Of course, if the flow velocity is obtained, the flow rate can be obtained by multiplying the cross-sectional area of the flow path, but there is a problem that the heating fluid is difficult to serve as a marker due to the influence of thermal diffusion in the minute flow rate region.

Further, as related technologies for further expanding the measurement flow rate range, there are flow meters described in Patent Literature 3 and Patent Literature 4.
In the flowmeter described in Patent Document 3, a large flow rate measurement channel having a large cross-sectional area and a small flow rate measurement channel having a small cross-sectional area are connected in series, and a plurality of flow velocity sensors are provided for each channel. By providing a flow velocity sensor unit formed as described above, a flow rate in a wide flow range is measured.
Further, in the flow meter described in Patent Document 4, a thermal flow meter and a Karman vortex flow meter are provided in the fluid, and measurement is mainly performed by a thermal flow meter in a low flow rate region, and mainly in a high flow rate region. Measurements are made with a flow meter.
However, in these Patent Documents 3 and 4, since a plurality of flow rate sensors or a plurality of types of flow meters are used, the cost of parts increases, the structure is complicated and the size is easily increased, and maintenance management is difficult. For example, it is not practical to apply the measurement to a very small flow rate of 0 to 10 mL / min.

Japanese Patent No. 3279116 JP 56-43559 A JP-A-9-68448 JP 2007-57452 A

  The present invention has been made in view of the above-mentioned conventional circumstances, and the problem is that a simple flow rate, high accuracy, less maintenance, and a micro flow rate capable of measuring a relatively wide range of micro flow rates. It is to provide a meter and a method for measuring a minute flow rate.

Technical means for solving the above problems include a flow path for allowing fluid to pass through, a heater for heating the fluid in the flow path, and detecting the temperature of the fluid before and after heating downstream of the heater. A micro flowmeter comprising a temperature detection unit and an information processing device for obtaining a flow rate from a temperature detected by the temperature detection unit, wherein the information processing device has a calibration function indicating a relationship between a fluid temperature rise amount and a flow rate. A pre-stored storage device, and an arithmetic processing unit that obtains a temperature rise amount of the fluid from the temperature detected by the temperature detection unit and obtains a flow rate corresponding to the temperature rise amount from the calibration function, wherein the calibration function is: A first region in which the relationship between the temperature rise amount and the flow rate is a positive linear relationship, and a second region in which the relationship between the temperature rise amount and the flow rate is a negative linear relationship on the larger flow rate side than the first region. It is characterized by having.
According to this configuration, when the temperature of the fluid before and after heating is detected by the temperature detection unit, the information processing apparatus obtains the temperature rise amount of the fluid from the detected temperature, and calculates the flow rate corresponding to the temperature rise amount from the calibration function. Ask. As a calibration function, a first region having a positive linear relationship in the relationship between the temperature rise amount and the flow rate, and a second region having a negative linear relationship in the relationship between the temperature rise amount and the flow rate on the larger flow rate side than the first region. Therefore, it is possible to measure a minute flow rate in a relatively wide range.

Further technical means includes means for measuring a relationship between an elapsed time from the start of heating by the heater and a temperature detected by the temperature detection unit, and the information processing device is a time point when the detected temperature becomes maximum. The first region in the calibration function is used on the condition that the elapsed time is greater than a preset reference time, and the first function in the calibration function is used on the condition that the elapsed time is smaller than the reference time. Two regions are used.
According to this configuration, when the elapsed time at the time when the detected temperature becomes maximum is longer than the reference time, the first region in the calibration function is used, and the flow rate corresponding to the temperature increase amount is obtained. Further, when the elapsed time at the time when the detected temperature becomes maximum is smaller than the reference time, the second region in the calibration function is used, and the flow rate corresponding to the temperature increase amount is obtained. That is, when a calibration function having the first region and the second region is used, two flow values correspond to a single temperature increase value. According to the configuration, the two flow values Of these, one flow rate value corresponding to the actual flow rate can be selected easily and accurately.

In a further technical means, the information processing device calculates a flow rate on a boundary line between the first region and the second region in the calibration function on the condition that the elapsed time is substantially the same as the reference time. The flow rate of the fluid passing through the flow path is a feature.
According to this configuration, when the elapsed time when the detected temperature reaches the maximum is substantially the same as the reference time, the flow rate on the boundary line between the first region and the second region in the calibration function is Identified as the flow rate of fluid passing through.

In a further technical means, the information processing apparatus stores in advance a function indicating the relationship between the elapsed time and the flow rate, and on the boundary line between the first region and the second region in the calibration function based on this function. An elapsed time corresponding to the flow rate is obtained, and this elapsed time is set as the reference time.
According to this configuration, the information processing apparatus obtains the elapsed time corresponding to the flow rate on the boundary line between the first region and the second region in the calibration function from the function indicating the relationship between the elapsed time and the flow rate. Therefore, the reference time for determining which one of the first region and the second region is used can be set to an accurate value according to the calibration function, thereby improving the accuracy of flow rate measurement. Can do.

In a further technical means, a heat diffusing means for urging heat diffusion of the fluid heated by the heater is provided in the flow path upstream of the temperature detection unit. .
According to this configuration, since the thermal diffusion of the fluid heated by the heater is promoted before reaching the temperature detection unit, the temperature of the fluid in the vicinity of the temperature detection unit is made uniform and measured. A linear linear relationship can be obtained between the increase amount and the flow rate. As a result, a minute flow rate can be obtained with high accuracy by relatively simple data processing.

In a further technical means, the heat diffusing means is one of a widening pipe whose cross-sectional area is increased in the fluid flow direction and a narrow pipe which is reduced in cross-sectional area in the fluid flow direction. Alternatively, both are provided.
According to this configuration, thermal diffusion can be effectively promoted by one or both of the expansion tube and the narrowing tube.

In a further technical means, the heat diffusing means is connected with a narrowed pipe that reduces the cross-sectional area in the fluid flow direction at the downstream end of the expansion pipe that increases the cross-sectional area in the fluid flow direction. It is characterized by becoming.
According to this configuration, the relationship between the temperature rise amount and the flow rate can be made linear and linear, particularly in a wide range of very small flow rates, by using the expansion tube and the narrowing tube. Accurate flow measurement is possible.

  In a further technical means, a flow path for allowing the fluid to pass through, a heater for heating the fluid in the flow path, a temperature detection unit for detecting the temperature of the fluid before and after heating downstream of the heater, A micro flow rate measuring method for measuring a flow rate of a fluid in the flow path using an information processing device that obtains a flow rate from a temperature detected by the temperature detection unit, wherein the information processing device detects a fluid temperature increase amount and a flow rate. A step of preliminarily storing a calibration function indicating the relationship of the above, a step in which the information processing device obtains a temperature rise amount of the fluid from a temperature detected by the temperature detection unit, and a temperature rise amount obtained by the information processing device in the step A flow rate corresponding to the first function is obtained from the calibration function, and the calibration function includes a first region in which the relationship between the temperature rise amount and the flow rate is a positive linear relationship, and a larger flow rate side than the first region. With temperature rise Wherein the relationship between the amount of a second region which is a negative linear relationship.

Since the present invention is configured as described above, the following effects can be obtained.
Since it is composed of a heater, a temperature detector, and an information processing device (for example, an inexpensive microprocessor), it has a simple structure, is inexpensive, highly accurate, and requires little maintenance.
In addition, since a calibration function composed of a first region having a positive linear relationship and a second region having a negative linear relationship is used, a relatively wide range of minute flow rate measurement is possible.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the drawings are schematic, and may include portions different from the actual ones, and may include portions having different dimensional relationships and ratios between the drawings.
Further, the embodiment described below exemplifies an apparatus and a method for embodying the technical idea of the present invention, and the arrangement and shape of each component are not limited to the following. The technical idea of the present invention can be variously modified within the scope of the claims.

First, a micro flow meter according to an embodiment of the present invention will be described with reference to FIG.
This micro flow meter includes an upstream flow path 1 that allows fluid to pass through, a thermal diffusion means 2 that is connected downstream of the upstream flow path 1, and a downstream side of the thermal diffusion means 2. A downstream flow path 3 provided, a heater 4 for heating the fluid in the flow path composed of the flow paths 1 and 3 and the heat diffusion means 2, and the fluid before and after heating on the downstream side of the heater 4 A temperature detection unit 5 that detects the temperature and an information processing device 54 that obtains the flow rate of the fluid from the temperature detected by the temperature detection unit 5 are provided as main components.

Each of the upstream flow path 1 and the downstream flow path 3 is a straight pipe (that is, a straight pipe) having a circular cross-sectional shape, and is formed so as to allow fluid (liquid or gas) to pass therethrough. is there. In the present embodiment, the upstream flow path 1, the heat diffusing means 2, and the downstream flow path 3 constitute a substantially straight flow path extending in the vertical direction, and the fluid in the flow path is directed in the direction of gravity. It is made to circulate in the opposite direction.
Here, “substantially reverse” means not only when the direction is 180 degrees different from the direction of gravity but also including a direction including a vector component in the direction opposite to the direction of gravity (for example, obliquely upward).
In addition, according to the example of illustration, the internal diameter of the upstream flow path 1 and the downstream flow path 3 is substantially the same diameter.

  The thermal diffusion means 2 has a function of thermally diffusing the fluid heated by the heater 4 (making the temperature uniform). This action makes it possible to apply the present flow meter to a model (a temperature rise amount and a flow rate have a linear relationship) to be described later.

  According to the illustrated example, the heat diffusing means 2 is connected to a spread pipe 2a that gradually increases its cross-sectional area in the fluid flow direction, and to the downstream end of the spread pipe 2a, and the fluid flow direction. And a narrowed tube 2b that gradually reduces its cross-sectional area.

According to a preferred example of the present embodiment, the expansion pipe 2a has a diffuser shape whose flow sectional area gradually expands at a linear ratio as it goes downstream.
Further, the narrowing pipe 2b has a reducer shape whose flow cross-sectional area gradually decreases at a linear ratio as it goes downstream.

  The upstream flow path 1, the heat diffusion means 2, and the downstream flow path 3 are made of acrylic or other synthetic resin material, and may be configured by joining components together or as an integral member. May be.

The heater 4 is a heating element such as a nichrome wire, for example, and includes a ring portion 4a formed in an annular shape and a conductive wire 4b connected to the ring portion 4a as illustrated in FIG.
The heater 4 is disposed at least upstream of the temperature detection unit 5, and according to a preferred example of the present embodiment, in order to obtain a good heat diffusion action by the heat diffusion unit 2, Located near the entrance.

The ring portion 4a is arranged so that its axis is substantially parallel to the fluid flow direction (center line of the heat diffusion means 2).
Moreover, this ring part 4a is arrange | positioned so that the heating position with respect to the fluid may become the approximate center in the virtual surface (circulation cross section) orthogonal to the fluid flow direction. That is, the axis of the ring portion 4a and the center line of the heat diffusing means 2 are substantially coincident. As a result, the ring portion 4a is arranged in the vicinity of the position that becomes the maximum flow velocity point of the fluid.

As is well known, when the Reynolds number is about 2000 or less, the flow of fluid in the straight pipe becomes a Poiseuille flow as shown in FIG. 3 regardless of the flow state at the straight pipe inlet. As shown in FIG. 3, the Poiseuille flow is a laminar flow having a parabolic flow velocity distribution in which the flow velocity distribution does not change in the flow direction. In particular, in the case of a pipe flow having a small Reynolds number Re, it is known that a Poiseuille flow is obtained with a very short approach distance L. The approach distance L is obtained by the following equation.
L = 0.065Re × d
However,
Re≡Ud / γ: Reynolds number d: Diameter of the flow path (m)
U: Average pipe flow velocity (m / s)
γ: Kinematic viscosity coefficient of fluid (m 2 / s)
It is.

  Since the Poiseuille flow is well known from the past, it will not be described any more, but in relation to the present embodiment, what is particularly important is a parabolic velocity distribution as shown in FIG. That is, the flow velocity of the fluid becomes maximum near the center of the cross section of the flow path where the heater 4 is located. Accordingly, the Poiseuille flow, which is an axially symmetric flow, can be heated axisymmetrically by the heater 4 provided in the vicinity of the center of the flow path, and diffusion also occurs symmetrically. Can do.

The structure for generating heat from the heater 4 will be described. The heater 4 is heated for a time th by the square wave voltage supplied from the pulse voltage generator 41.
FIG. 4 is an example of an apparatus used for pulse heating the heater 4. This device includes a heater 4, a pulse voltage generator 41, a constant voltage power supply device 42, a current control resistor circuit 43, and the like. Although the pulse voltage generator is not a feature of the present invention and will not be described in detail, for example, an astable multivibrator circuit using a timer IC is operated by the constant voltage power supply device 42 to change the magnitude of the resistance in the circuit. Thus, the standby time t1 between the pulse energization and the pulse energization time th are adjusted.
Further, when the output from the pulse voltage generator 41 is constant, the current intensity is controlled using the current control resistor circuit 43 shown in FIG. 4, and the heating amount of the heater 4 can be adjusted. Yes. In this way, the heater 4 can be energized at an arbitrary interval, and the fluid can be pulse-heated. Note that by connecting the constant voltage power source to the information processing device 54, not only the collection of temperature measurement data from the temperature detection unit 5 but also the control of the heater 4 can be performed by the information processing device 54.

  The heating fluid heated by the heater 4 moves along the flow while performing thermal diffusion. And the temperature measurement is performed by the temperature detection part 5 in the downstream, and the temperature rise ΔT before and after heating is measured.

FIG. 5 shows an example of the temperature detection unit 5, and in particular, a configuration using a thermocouple 5 a is used in order to configure it at a low cost.
The temperature detection unit 5 includes a thermocouple 5a, a thermocouple output amplifier circuit 52, a low-pass filter circuit 53, an A / D conversion board (not shown), a data storage medium (not shown), and a data processing unit (see FIG. The thermocouple 5a output is amplified by the thermocouple output amplifier circuit 52 and taken into the information processing apparatus 54 via the low-pass filter circuit 53.

  Further, the information processing device 54 obtains a temperature rise amount of the fluid from a storage device that stores in advance a calibration function, a reference time, an arithmetic processing program, and the like, which will be described later, and a temperature detected by the temperature detection unit 5, What is necessary is just the structure equipped with the arithmetic processing apparatus (CPU etc.) which calculates | requires a corresponding flow volume from a calibration function. The information processing apparatus 54 may be a computer, for example, but can be an inexpensive electronic circuit using a microprocessor or the like.

  In some embodiments, the distance L between the heater 4 and the temperature detection unit 5 may be close. In such a case, the heater 4, which is a non-insulated nichrome wire heater, and the measurement site of the thermocouple 5 a are in a close positional relationship, and during the energization of the heater, between the heater and the thermocouple 5 a. Leakage current is generated although it is weak. Since the output of the thermocouple 5a itself is small and the gain of the amplifying device is large, even if the leakage current is weak, the output of the device during and immediately after energizing the heater is greatly affected. In particular, the influence of leakage current becomes significant in water having a higher conductivity than air. Therefore, in a preferred example of the present embodiment, as a countermeasure against leakage current, the measurement site of the thermocouple 5a is covered with the insulating tape 51, and the potential of the thermocouple 5a is changed to Nichrome during energization by pulse energization and during standby. A circuit 55 (see FIG. 5) is provided to match the average potential of the line heater (heater 4).

  Next, a micro flow rate measuring method using the micro flow meter having the above configuration will be described. In the micro flow meter, it is assumed that fluid flows in the direction of the arrow in FIG. Then, a part of the fluid is pulse-heated by the heater 4 provided near the entrance of the heat diffusing means 2. Then, the heated fluid (hereinafter referred to as a heated fluid) is made uniform in temperature while being thermally diffused in the heat diffusing means 2, and moves downstream by riding on the flow.

  Then, the heated fluid reaches the temperature detection unit 5 (specifically, the thermocouple 5a) provided on the downstream side of the heater 4, and the temperature is measured. The information processing device 54 calculates a temperature increase amount ΔT from the temperature of the fluid before and after heating, and calculates a flow rate Q corresponding to the temperature increase amount ΔT as a calibration function (fluid temperature increase amount ΔT and fluid temperature). It is obtained from a calibration curve showing the relationship with the flow rate Q).

  As described above, since the flow rate measuring device according to the present invention has a very simple configuration, it can be manufactured at low cost. Estimating the manufacturing cost on the assumption that this flow meter is used as a single unit, for example, the manufacturing cost of the heater 4 (the manufacturing cost of the pulse generation circuit and the constant voltage power source: ~ \ 2000), the manufacturing cost of the temperature sensor ( Considering thermocouple and amplifier for amplification: ~ \ 500, DC voltmeter for output measurement: ~ \ 2000), one flow meter can be manufactured within 5000 yen.

  Next, a calibration function (calibration curve) showing the relationship between the fluid temperature rise amount ΔT and the fluid flow rate Q will be described. Since this calibration function is based on the following relational expression derived by the present inventor, this relational expression will be described first. Hereinafter, the qualitative relationship between the flow rate Q and the fluid temperature increase ΔT will be described based on a simplified flow system.

  If the heating amount Δq by the heater 4 is constant, the magnitude of the temperature rise ΔT is that heat is transmitted from the start of heating by the heater 4 until the heated fluid reaches the temperature detection unit 5. It is inversely proportional to the total volume Vh of the fluid (heat shall be evenly distributed within the same volume) (Equation (1)). However, heat diffusion outside the measuring tube and the measuring tube shall not be considered. Here, ρ represents the density of the fluid, and Cp represents the constant pressure specific heat.

ここで、モデルの単純化のため以下の(i)〜(iii)を条件とする。

Here, for simplification of the model, the following conditions (i) to (iii) are used.

(i) Replace the model of the thermal diffusion means 2 with a model of a straight tube having the same volume (see FIG. 6). Here, the same volume means that a fluid having an average flow velocity Um in the pipe passes through the same length when Δt is a time required to pass through the length L (distance between the heater 4 and the temperature detection unit 5). The length of the straight tube having an inner diameter that requires Δt is L ′, and the volume of the heat diffusion means 2 having the length L and the volume of the straight tube having the length L ′ is equal.
Hereinafter, it is assumed that the measurement tube is a straight tube having a length L ′ between the heater 4 and the temperature detection unit 5, and this result is applicable to the model of the thermal diffusion means 2 having the length L.
(ii) The flow velocity Um is uniformly in the pipe cross section. That is, the formation of the velocity boundary layer due to the viscosity in the tube and the acceleration due to the buoyancy of the fluid accompanying the rise in the temperature of the fluid are not considered.
(iii) Heating is performed uniformly over the entire installation cross section of the heater 4. The actual ring part 4a of the heater 4 has a finite width in the axial direction and the cross-sectional direction of the tube. Here, the heater that uniformly heats the entire cross-section of the cross-section of the ring part 4a having a thickness of 0 Is assumed.

  FIG. 6 is a model of Vh derived from the above conditions. The order of Vh depends on h0 and δT and is expressed by equation (2). Here, h0 indicates the length of the fluid that passes through the cross section of the ring portion 4a during heating by the heater 4, and δT indicates the length by which the heat given to the fluid by the heater 4 is diffused by heat conduction.

ｈ0は、加熱器４による加熱時間tｈ間に、加熱器４の設置された断面を流速Umで通過する流体の軸方向の長さであり、式(3)で与えられる。

h0 is the axial length of the fluid that passes through the section where the heater 4 is installed at the flow velocity Um during the heating time th by the heater 4, and is given by equation (3).

δTは加熱流体が温度検出部５に到達した時点での、熱拡散の長さである（図７参照）。加熱流体は主流に乗って移動し（図７（a））、加熱流体の移動中に熱伝導によって流れ方向への熱拡散が生じ（同図（b））、加熱器４による加熱開始からΔt[s]後に加熱流体は温度検出部５の熱電対５ａに到達する（同図（c））。このときまでに熱拡散が進んだ長さをδTとする。

δT is the length of thermal diffusion when the heated fluid reaches the temperature detector 5 (see FIG. 7). The heated fluid moves along the main stream (FIG. 7 (a)), and thermal diffusion occurs in the flow direction due to heat conduction during the movement of the heated fluid (FIG. 7 (b)), and Δt from the start of heating by the heater 4 [s] After that, the heated fluid reaches the thermocouple 5a of the temperature detector 5 ((c) in the figure). Let ΔT be the length of thermal diffusion that has occurred so far.

δTのオーダー評価は、無限静止空間中に置かれた点熱源からの熱拡散の式（4）より、式(5)で与えられる。なお、時間の代表量には、解析を容易にするため、流速Uｍの流体が加熱器４−熱電対５ａ間L’を移動する時間Δt(=L’/Um)を用いている。

The order evaluation of δT is given by Equation (5) from Equation (4) of thermal diffusion from a point heat source placed in an infinite static space. In order to facilitate analysis, the time Δt (= L ′ / Um) during which the fluid having a flow velocity Um travels L ′ between the heater 4 and the thermocouple 5a is used as the representative amount of time.

式(2)にから分かるように、Vhのオーダーは、（h0＋２δT）のオーダーに比例する。また、式（2），（3），（5）からは、Vhのオーダーは、流量Qが十分に小さい領域ではδTの影響が支配的に、流量Qが十分大きな領域ではh0の影響が支配的になることがわかる。すなわち、流量Qが十分大きな領域では、その分流速も速くなるため、加熱流体の移動時間Δtが短くなり、結果としてδTも小さくなる。一方、流量Qが小さな領域（微少流量領域）では、加熱流体の移動時間Δtが長くなるため、δTが大きくなる。

As can be seen from equation (2), the order of Vh is proportional to the order of (h0 + 2δT). Also, from equations (2), (3), and (5), the order of Vh is dominated by the effect of δT in the region where the flow rate Q is sufficiently small, and the effect of h0 in the region where the flow rate Q is sufficiently large. You can see that That is, in a region where the flow rate Q is sufficiently large, the flow velocity is increased accordingly, so that the moving time Δt of the heated fluid is shortened, and as a result, ΔT is also decreased. On the other hand, in a region where the flow rate Q is small (a minute flow region), the moving time Δt of the heated fluid becomes long, so ΔT increases.

From Equations (1) to (4), the order of ΔT is a function of Um (Equation (6)). Where a, b, c, a ', b'
, c 'is a constant.

となることが判る。 From equation (6)

It turns out that it becomes.

となる流速Umで温度上昇量ΔTの極大値が生ずる。

The maximum value of the temperature rise ΔT occurs at the flow velocity Um.

  FIG. 8 shows the relationship of Q−ΔT according to the equation (6). As for the relationship between Q and ΔT, the first region where the flow rate is relatively small has an upward relationship, and the second region where the flow rate is relatively large has a downward relationship. In the present embodiment, a minute flow rate is measured using a calibration function (calibration curve) in a range from the first region to the second region.

  As described above, since the influence of thermal diffusion is dominant in the minute flow rate region, it is difficult to realize a conventional method in which the heating fluid is used as a temperature marker and the flow velocity is obtained from the moving time. On the other hand, in the micro flowmeter of the present embodiment, since the model represented by the above equation (6) is applied also utilizing the temperature equalization effect by thermal diffusion, the thermal diffusion length δT is There is no inconvenience due to becoming dominant. The above is the basic principle of the micro flowmeter of the present embodiment.

  Next, a method for creating a calibration function (calibration curve) will be described. The calibration function can be created by using an apparatus as shown in FIG. The apparatus shown in FIG. 9 is obtained by adding a constant flow rate device 61 capable of generating a constant flow rate to the configuration shown in FIG.

An example of the constant flow rate generator 61 is shown in FIG. In the example shown here, the fluid at a constant flow rate uses a traverse device with a speed reducer that pushes out the liquid in the syringe at a constant speed. First, the motor 62 with a speed reducer is driven by an external power source (not shown). The rotation obtained by driving the motor 62 is converted into a linear motion of the pedestal 64 by the traverse device 63.
The pedestal 64 is provided with a plurality of syringes, and the syringe 65 also moves linearly as the pedestal 64 moves linearly (movement in the direction of the arrow 66). This linear movement of the syringe 65 pushes the piston of the syringe and pushes the fluid at a constant speed. Furthermore, the flow rate can be adjusted according to the type and number of syringes used. In FIG. 10, as an example, the pedestal 64 includes a total of four syringes.

  For example, when the motor speed is 60 rev / min, the reduction gear reduction ratio is 12.5: 1, and the traverse device pitch is 3 mm / rev, a linear motion of 14.4 mm / min is obtained. When a 1 mL plastic medical syringe (inner diameter: 4.5 mm) is used as the syringe, the flow rate can be adjusted in units of 0.25 mL / min for a 5 mL (inner diameter: 13 mm) 1.9 mL / min unit.

  In order to create a calibration function (calibration curve), first, a known flow rate generated by the constant flow rate generator 61 is passed through the upstream flow path 1. Then, at the position of the heater 4, the Poiseuille flow shown in FIG. 3 is obtained, and the flow velocity becomes maximum near the center where the heater 4 is located. Subsequently, the density of the fluid (heated fluid) in the heated portion that is pulse-heated by the heater 4 is smaller than that before heating. Then, the flow of the heated portion is accelerated in the direction opposite to gravity due to the buoyancy based on the density difference from the surrounding non-heated fluid. The heated fluid moves along the flow, passes through the thermal diffusion means 2, and reaches the temperature detection unit 5. The temperature of the heated fluid is measured in the temperature detection unit 5.

What is measured here is only the temperature of the heated fluid (the temperature increase ΔT can be calculated from this temperature). Therefore, the relationship with the actual fluid flow rate Q (or flow velocity) is unknown.
Therefore, by changing the flow rate Q with the constant flow rate generator 61, measuring the temperature rise ΔT for each flow rate, and plotting the measurement results, a calibration function (calibration curve) showing the relationship between them can be obtained. it can.

Hereinafter, an example of a more specific procedure for creating a calibration function (a calibration curve) will be described in detail.
First, by controlling the constant flow rate generating device 61, the fluid is allowed to flow into a plurality of different flow rate conditions (for example, 0, 0) in the flow channel composed of the upstream flow channel 1, the heat diffusion means 2, and the downstream flow channel 3. .25, 0.5, 0.75, 1.00, 1.25, 1.50, 1.90, 3.90, 5.90 mL / min).

Next, heating of the fluid by the heater 4 is started, and a discrete time series output of the temperature detection unit 5 is taken for each flow rate.
More specifically, the temperature detection by the temperature detection unit 5 is performed at predetermined minute intervals from the time when the heating by the heater 4 is started, for example, as shown in FIGS. 11 (A) to 11 (C). The relationship between the elapsed time Δt from the start of heating by the vessel 4 and the detected temperature T by the temperature detector 5 is measured and stored in a storage device (not shown).

Then, when the detected temperature T reaches the maximum, a temperature increase ΔT before and after heating the fluid is obtained. The time when the detected temperature T becomes maximum is the time when the amount of change in the detected temperature T by the temperature detector 5 becomes 0 (in other words, the slopes of the curves in FIGS. 11A to 11C are substantially horizontal). At that point).
Here, the temperature increase ΔT that is the difference between the detected temperature T immediately before the start of heating and the detected temperature T at the time when the temperature reaches the maximum will be referred to as a heating diffusion temperature increase.

  Then, for example, a calibration function (calibration curve) shown in FIG. 13 is obtained from the correlation between the heating diffusion temperature rise amount ΔT and the plurality of different known flow rates Q. (Not shown).

The procedure for obtaining the calibration function may be artificially performed as a preliminary measurement before the flow measurement, but as another example, it may be automatically executed by a control program provided in the micro flow meter. It is also possible to do. That is, in the other example, the information processing device 54 is caused to function by a program stored in a storage device (not shown), and the constant flow rate generating device 61, the heater 4, the temperature detection unit 5 and the like are received by a command from the information processing device 54. The calibration function may be derived from the relationship between the known flow rate Q supplied from the constant flow rate generator 61 and the temperature rise amount based on the temperature detected by the temperature detection unit 5.
As another example, the calibration function can be obtained by theoretical calculation.

As shown in FIG. 13, the calibration function (calibration curve) obtained as described above includes a first region in which the relationship between the heating diffusion temperature rise amount ΔT and the flow rate Q is a positive linear relationship, and the first region. The second region has a negative linear relationship between the heating diffusion temperature rise amount ΔT and the flow rate Q on the larger flow rate side.
Therefore, as shown in FIG. 13, two flow rates Q1 and Q2 correspond to a single heating diffusion temperature rise amount ΔT1. Next, which of the two flow rates Q1 and Q2 is specified as the flow rate of the fluid actually flowing through the flow path will be described.

  8 and 13, the flow rate Qc changes the flow rate Q dependency of the heating diffusion temperature rise amount ΔT, and the time lapse Δtc from when heating by the heater 4 is started until the detected temperature T of the temperature detecting unit 5 reaches the maximum temperature. (Δt = Δtc when Q = Qc). This Δtc discriminates whether the output value corresponding to the two flow rate Q values of the first region and the second region is the first region or the second region with respect to the same heating diffusion temperature increase ΔT at the time of the flow rate measurement. This is a reference time (hereinafter, this Δtc is referred to as a reference time).

  FIG. 11 shows three regions: a first region of Q <Qc, a boundary region of Q≈Qc (elapsed time from the start of heating indicating ΔT at this time Δtc), and a second region of Q> Qc in FIG. , The relationship between the fluid temperature T and the elapsed time Δt is shown. FIG. 11A shows a case where Δt> Δtc, FIG. 11B shows a case where Δt≈Δtc, and FIG. 11C shows a case where Δt <Δtc.

The difference between the flow rate Q1 and the flow rate Q2 in the first region and the second region (see FIG. 13) can be clearly distinguished by the elapsed time Δt from the start of heating until the fluid temperature reaches the maximum value T.
Specifically, since the flow rate Q is small in the first region (Q <Qc, where Qc is the flow rate when [mL / min], the dependence of ΔT varies with this flow rate as a boundary), the maximum temperature after heating Is longer (see FIG. 11A), whereas in the second region, the flow rate Q is larger (Q> Qc), so the elapsed time Δt is shorter (FIG. 11C). )

  Next, a specific procedure in which the micro flowmeter according to the present embodiment determines the above-described two flow rates Q1 and Q2 based on the temperature detected by the temperature detection unit 5 will be described with reference to the flowchart shown in FIG. .

  As a precondition, the calibration function is stored in advance in a storage device (not shown) of the information processing apparatus 54, and fluid is circulated through the upstream flow path 1, the thermal diffusion means 2, and the downstream flow path 3. State.

In the above state, the micro flowmeter of the present embodiment first takes in the discrete time series of the output of the temperature detection unit 5 and heats the fluid by the heater 4 for a predetermined time (th) (step 1).
More specifically, in step 1, temperature detection is performed by the temperature detection unit 5 at a predetermined minute time interval from at least the start of heating by the heater 4, for example, FIGS. 11 (A) to (C). As shown in FIG. 5, the relationship between the elapsed time Δt from the start of heating by the heater 4 and the detected temperature T by the temperature detecting unit 5 is measured and stored.

Next, in step 2, the heating diffusion temperature rise amount ΔT1 and the elapsed time Δt are obtained from the discrete time series.
More specifically, when the detected temperature T reaches the maximum, the temperature increase ΔT before and after heating the fluid is obtained, and this temperature increase ΔT is set as the heating diffusion temperature increase ΔT. When the detected temperature T reaches the maximum, the slope of each curve in FIGS. 11A to 11C becomes substantially horizontal when the amount of change in the detected temperature T by the temperature detector 5 becomes 0 (for example, FIGS. Time).

  Next, in step 3a, it is determined whether or not the elapsed time Δt when the detected temperature T reaches the maximum is substantially the same as a preset reference time tc. The process proceeds to step 4a. Otherwise, the process proceeds to step 3b.

The reference time tc may be a value obtained experimentally or theoretically in advance and stored in the information processing apparatus 54, but may be obtained from data obtained by the calibration function creation procedure described above.
That is, in the preliminary measurement for creating the calibration function, the function shown in the graph of FIG. 12 is created from the relationship between the known flow rate Q and the elapsed time Δt from the start of heating to the maximum detected temperature T. This function is stored as a mathematical formula, for example. Then, an elapsed time Δt corresponding to the flow rate Qc on the boundary line between the first region and the second region in the calibration function is obtained from this function, and this elapsed time Δt may be set as the reference time Δtc.

Here, the function shown in the graph of FIG. 12 is theoretically described. This function is the relationship between the elapsed time Δt from the start of heating by the heater 4 to the heating diffusion temperature increase ΔT and the flow rate Q (Δt ~ Q).
Δt is evaluated for the order of Δtdiff and Δtconv in the mainstream direction with respect to the convection transfer time (Δtconv) based on the heating diffusion time (Δtdiff) and the mainstream average flow velocity (Um).
Δtdiff ～ L2 / α [From diffusion equation] (9)
Δtconv ~ L / Um = AL / Q (10)
Where α is the thermal diffusivity, L is the distance between the heater 4 and the temperature detector 5, and A is the cross-sectional area of the tube (assuming constant).
The above time is estimated for the thermal diffusion means 2. α = 0.15x10-4 m2 / s (water flow), L = 21x10-3 m / s (length of thermal diffusion means 2), Um = 2.4x10-3 m / s (when Q = 1 mL / min) Is substituted into Δtdiff and Δtconv, Δtdiff << Δtconv.
It can be seen that Δt may be Δtconv.
That is, the relationship between Δt to Q is a hyperbola indicated by a solid line in FIG. 12 from the relationship of Expression (10). (This curve passes through (ΔtC, QC).)

  Returning to the specific procedure of the flow rate measurement according to the present embodiment and continuing the description, in step 4a, the flow rate Qc (flow rate on the boundary line between the first region and the second region in the calibration function) is the measurement target. Stored as fluid flow rate Q.

  In step 3b, it is determined whether or not the elapsed time Δt at the time when the detected temperature T becomes maximum is greater than a preset reference time tc, and if so, the process proceeds to the next step 4b. If not, the process proceeds to step 4c.

  In step 4b, using the first region in the calibration function (see FIG. 13), a flow rate Q1 corresponding to the heating diffusion temperature rise amount ΔT1 is obtained, and this flow rate Q1 is stored as the flow rate Q of the fluid to be measured. .

  Further, in step 4c, using the second region in the calibration function (see FIG. 13), a flow rate Q2 corresponding to the heating diffusion temperature rise amount ΔT1 is obtained, and this flow rate Q2 is stored as the flow rate Q of the fluid to be measured. .

  In step 5, the flow rate Q obtained in the above step is output as an electrical signal and displayed on a display unit (not shown) such as a liquid crystal display or output to a printer.

Next, an experimental example in which flow measurement is actually performed using the micro flow meter and micro flow measurement method of the present embodiment will be described.
In this experiment, the thermal diffusing unit 2 having the above-described configuration was used as Example 1, and the thermal diffusing unit 2 ′ consisting of only a narrowed tube was used as Example 2.

The experimental conditions of this experiment are as follows.
(Example 1 (thermal diffusion means 2: see FIG. 14))
-Inner diameter of heat diffusion means 2: 3 mm
-Maximum inner diameter of the enlarged portion of the thermal diffusion means 2: 6 mm
-Outer diameter of heat diffusion means 2: 3 mm
-Overall length L of the thermal diffusion means 2: 21 mm
・ Length of expansion tube 2a: 9mm
・ Length of narrow tube 2b: 12mm
・ Flow rate: 0, 0.25, 0.5, 0.75, 1.00 mL / min
(Example 2 (thermal diffusion means 2 ′: see FIG. 15))
・ Inner diameter of the inlet part: 6mm
-Inner diameter of outlet part: 3mm
Narrow tube length L: 18mm
Flow rate: 0, 0.25, 0.5, 0.75, 1.00, 1.25, 1.50, 1.90, 3.90, 5.90 mL / min
(Common conditions)
-Heater 4: Nichrome wire (element diameter 0.12mm)
・ Diameter D = 0.5mm of the ring part 4a of the heater 4, length in the flow direction: 3mm
・ Output of heater 4: 1.7W (heating time th = 1.8s, heating amount Δq = 3J)
・ Measuring fluid: Water temperature: Approximately 19 ℃

  As can be seen from the graph shown in FIG. 16, in Example 1 using the thermal diffusion means 2 composed of the expansion pipe 2 a and the narrowing pipe 2 b, Q−ΔT in the range of the flow rate Q from 0 to about 2.6 (mL / min). Is a straight linear relationship that rises to the right (a relationship expressed by a linear function), and when the flow rate Q is about 2.6 (mL / min) or higher, it is a substantially linear relationship that goes to the right. . This closely approximates the result of the model analysis shown in FIG. By using this as a calibration function (calibration curve), the actual fluid flow rate can be accurately measured from the heating diffusion temperature rise amount ΔT. That is, if the relationship between the two is substantially linear, the value between the measurement points when the calibration function is created can be accurately interpolated. In addition, the experimental results shown in FIG. 16 indicate that according to the present embodiment (Example 1), it is possible to measure the flow rate in a wide range of very small flow rates from 0 to about 6 (mL / min).

  In Example 1, the experimental example shown in FIG. 16 shows a slightly non-linear relationship at 1.0 to 2.0 (mL / min), but the inclination angles of the expansion tube 2a and the narrowing tube 2b. Further, it is possible to correct to a more linear relationship by adjusting the overall length dimension of the thermal diffusion means 2, the inner diameter dimension of each part, and the like.

  On the other hand, in the graph of the heat diffusing means 2 ′ with only a narrowed tube shown as the second embodiment, the relationship of Q−ΔT is almost linearly increasing in the range of the flow rate Q from 0 to about 1.0 (mL / min). This shows a linear relationship. For Example 2, the amount of temperature increase with a flow rate Q of 1.0 (mL / min) or higher was not measured, but from the results of the model analysis described above, approximately the same as in Example 1, It is considered that a linear relationship of increasing right is obtained in the relationship of Q−ΔT. However, when it is assumed that the flow rate is measured in a relatively wide flow rate range (for example, a range of 0 to about 6 (mL / min)), the measurement accuracy in Example 1 is expected to be improved over that in Example 2. .

Therefore, according to the present embodiment, the thermal diffusion means 2 including the expansion tube 2a and the narrowing tube 2b, the heater 4 such as a nichrome wire heater, the temperature detection unit 5 such as a thermocouple, and the information processing device 54 such as a microprocessor. Thus, a micro flow meter and a micro flow rate measuring method with high accuracy and less need for maintenance can be realized by using only relatively inexpensive and simple structure parts.
In addition, since a calibration function composed of a first region having a positive linear relationship and a second region having a negative linear relationship is used, a relatively wide range of minute flow rate measurement is possible. It is suitable for measuring a minute flow rate of 10 mL / min.

In addition, according to the said embodiment, although the thermal diffusion means 2 which connected the expansion pipe 2a and the narrowing pipe | tube 2b continuously was used as a particularly preferable aspect, as another example of this thermal diffusion means 2, it was mentioned above. A heat diffusion means 2 ′ (narrowed tube), a straight tube, a tube made of only the expanded tube 2a, a tube of other shape, or a tube formed by appropriately combining these plural types of tubes It is also possible to do.
Moreover, according to the said embodiment, although the expansion pipe 2a and the narrowing pipe 2b were set as the structure which each changes a flow cross-sectional area gradually by a linear ratio, as another example, a flow cross-sectional area is made into predetermined | prescribed curvature. Thus, a configuration that gradually changes, a mode in which the flow cross-sectional area is changed in stages, and the like are conceivable.

Furthermore, as another example of the thermal diffusion means 2, it is also possible to adopt a mode in which the heated fluid is thermally diffused by the influence of buoyancy by making the flow direction of the fluid in the flow path substantially opposite to the direction of gravity. It is.
Furthermore, as another example of the heat diffusing means 2, a plate (for example, an orifice plate) having at least one through hole in the flow path is provided so as to be substantially orthogonal to the flow direction, and the heat diffusion is performed by reducing the flow velocity by the plate. It is also possible to make it an aspect to promote.

It is a schematic diagram which shows an example of the micro flowmeter of this invention. It is the enlarged view which looked at the ring part of the heater to the direction of a central axis. It is a schematic diagram explaining a Poiseuille flow. It is a block diagram which shows the apparatus structure of a heater. It is a block diagram which shows the apparatus structure of a temperature detection part. It is a schematic diagram for demonstrating the basic principle of this invention. It is a schematic diagram for demonstrating the basic principle of this invention. It is a graph which shows the relationship between heating diffusion temperature rise amount and flow volume. It is a schematic diagram which shows an example of the apparatus for calibration function preparation. It is a schematic diagram which shows an example of a fixed flow generator. It is a graph which shows the relationship between the elapsed time after a heating start, and the temperature of a fluid, (A) shows the case where elapsed time is in reference time, (B) shows the case where elapsed time is substantially reference time. , (C) shows the case where the elapsed time exceeds the reference time. It is a graph which shows the relationship between the elapsed time until detection temperature becomes the maximum, and flow volume. It is a graph which shows an example of a calibration function (calibration curve). 3 is a schematic diagram illustrating an example of a thermal diffusion unit used in Example 1. FIG. 6 is a schematic diagram illustrating an example of a thermal diffusion unit used in Example 2. FIG. It is a graph which shows the result of having experimentally measured the relationship between the heating diffusion temperature rise amount and the flow rate. It is a flowchart which shows an example of the flow measurement procedure using the micro flowmeter which concerns on this invention.

Explanation of symbols

1: Upstream channel 2: Thermal diffusion means 2a: Expansion tube 2b: Narrow tube 3: Downstream channel 4: Heater 5: Temperature detection unit 54: Information processing device

Claims (8)

A flow path through which the fluid passes, a heater that heats the fluid in the flow path, a temperature detection unit that detects the temperature of the fluid before and after heating on the downstream side of the heater, and a temperature detected by the temperature detection unit A micro flowmeter equipped with an information processing device for obtaining a flow rate from
The information processing device stores in advance a calibration function indicating the relationship between the fluid temperature rise amount and the flow rate, and obtains the fluid temperature rise amount from the temperature detected by the temperature detection unit, and corresponds to the temperature rise amount. An arithmetic processing unit for obtaining a flow rate from the calibration function,
The calibration function is a first region in which the relationship between the temperature rise amount and the flow rate is a positive linear relationship, and the relationship between the temperature rise amount and the flow rate is a negative linear relationship on the larger flow rate side than the first region. A micro flowmeter having a second region. Means for measuring the relationship between the elapsed time from the start of heating by the heater and the temperature detected by the temperature detector;
The information processing apparatus uses the first region in the calibration function on the condition that the elapsed time at the time when the detected temperature becomes maximum is larger than a preset reference time, and the elapsed time is 2. The micro flowmeter according to claim 1, wherein the second region in the calibration function is used on condition that the time is shorter than a reference time.   The information processing apparatus passes the flow rate on the boundary line between the first region and the second region in the calibration function through the flow path on the condition that the elapsed time is substantially the same as the reference time. 3. The micro flow meter according to claim 2, wherein the flow rate is a fluid.   The information processing apparatus stores in advance a function indicating the relationship between the elapsed time and the flow rate, and from this function, an elapsed time corresponding to the flow rate on the boundary line between the first region and the second region in the calibration function is calculated. 4. The micro flowmeter according to claim 2, wherein the elapsed time is determined as the reference time.   5. The thermal diffusion means for promoting thermal diffusion of a fluid heated by the heater is provided in the flow path upstream of the temperature detection unit. 1. A micro flow meter according to item 1.   The heat diffusing means includes any one or both of a widening pipe that expands a cross-sectional area in the fluid flow direction and a narrow pipe that reduces the cross-sectional area in the fluid flow direction. The micro flowmeter according to claim 5.   The heat diffusing means is formed by connecting a narrow pipe that reduces the cross-sectional area in the fluid flow direction to the downstream end of the expansion pipe that increases the cross-sectional area in the fluid flow direction. The micro flowmeter according to claim 5. A flow path through which the fluid passes, a heater that heats the fluid in the flow path, a temperature detection unit that detects the temperature of the fluid before and after heating on the downstream side of the heater, and a temperature detected by the temperature detection unit A flow rate measurement method for measuring a flow rate of a fluid in the flow path using an information processing device for obtaining a flow rate from
A step in which the information processing apparatus stores in advance a calibration function indicating a relationship between the temperature rise amount of the fluid and the flow rate;
The information processing device obtaining a temperature rise amount of the fluid from the temperature detected by the temperature detector;
A step of obtaining a flow rate corresponding to the temperature increase obtained in the step by the information processing device from the calibration function,
The calibration function is a first region in which the relationship between the temperature rise amount and the flow rate is a positive linear relationship, and the relationship between the temperature rise amount and the flow rate is a negative linear relationship on the larger flow rate side than the first region. And a second flow rate measurement method.

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