JPH0324973B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a thermal mass flowmeter using a thermistor. BACKGROUND TECHNOLOGY Conventionally existing hot wire flowmeters wind a heat-generating resistance wire such as platinum around a capillary tube, and utilize the fact that the amount of heat dissipated in accordance with the flow rate of fluid flowing inside the tube causes a change in the resistance value of the resistance wire. It measures the flow rate. However, the disadvantages of this type of flowmeter are that the resistance wire has a low resistance-temperature coefficient, the operating temperature range is relatively high, and there are many failures due to moisture and dust. On the other hand, the thermistor current meter is expected to be a highly sensitive flow meter because the temperature coefficient of the thermistor is about 10 to 15 times that of normal metals and the operating temperature is low, but in reality it is used for industrial measurement. Practical use has not progressed that much. This is due to the fact that measurement errors due to variations in individual temperature characteristics, which are the essential problems of thermistors, have not been sufficiently resolved, and also because the thermistor (the tip of the thermistor generally protrudes in the transverse direction of the flow path) in the flow path is contaminated with dirt. This is due to the fact that these substances adhere to the surface and form a film that interferes with the flow. Purpose of the Invention The present invention aims to provide a new thermistor-type thermal mass flowmeter that solves the problem of flow path interference such as the conventional capillary flow path and thermistor protruding flow path, and the problem of variations in thermistor temperature characteristics. be. Structure of the Invention Briefly, in order to achieve the above object, the present invention includes a sensor block having a flow path for a test fluid, and a sensor block having a tip facing upstream in the flow path and an axis thereof facing the flow direction. A glass bead-encapsulated thermistor for flow rate detection is arranged to match the direction of the flow path, and a side end of the thermistor for flow rate detection is coaxially attached to the main body fitted in the flow path with a fluid flow gap. a rectifying element formed by penetrating a central axis hole surrounding the central shaft hole and a plurality of rectifying holes arranged in parallel around the central axis hole, and a thermistor that is of the same type as the flow rate detection thermistor and has similar resistance-temperature characteristics. , a temperature compensation thermistor whose tip is close to the flow path and is housed in a small chamber communicating with the flow path, and the mass of the fluid by calibrating and calculating the difference in resistance between the flow rate detection thermistor and the temperature compensation thermistor. This constitutes a thermal mass flowmeter characterized by being equipped with an arithmetic processing circuit for indicating flow rate. In the above configuration, the rectifier element communicates a part of the fluid flow introduced into the flow path of the sensor block to its shaft hole at an accurate splitting ratio, so the flow rate detection thermistor in contact with the rectifier element communicates with the mass flow rate and temperature of the fluid. Heat is radiated accordingly, and the resistance value becomes a value representing the equilibrium temperature. On the other hand, since the temperature compensation thermistor exhibits a resistance value depending on the fluid temperature, if the effective difference between the resistance values of both thermistors is calculated by the arithmetic processing circuit, an accurate mass flow rate of the fluid can be obtained. Embodiments Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing a basic embodiment according to the construction principle of the present invention. In this embodiment, a sensor block 1 serving as the main body of the flowmeter is a rectangular parallelepiped made of metal such as aluminum or acrylic or other resin material. A fluid flow path is made up of an extending vertical flow path portion 3 and a horizontal flow path portion 5 having a fluid outlet 4 that curves into contact with the upper end of the flow path portion 3 in the block 1, extends laterally, and opens on the right side of the figure. It is formed. The flow rate detection thermistor 6 and the temperature compensation thermistor 7 are glass bead-encapsulated thermistors having substantially the same or similar characteristics, and the former 6 penetrates the upper end wall 8 of the sensor block 1 and hangs down into the vertical flow path 3. - The latter 7 also penetrates the upper end wall 8 and is close to the vertical channel 3.
In parallel, it protrudes into a small chamber 10 formed in the left side wall 9 of the figure. The small chamber 10 communicates with the channels 3 and 5 through a small hole 11. The bases of the upper ends of the thermistors 6 and 7 are fixed to the upper ends of the blocks by nuts 12 and 13, respectively, and an O-ring 14 is inserted under the nuts. According to the invention, a rectifying element 15 that cooperates with the thermistor 6 for detecting the flow rate is fitted into the flow path 3 of the sensor block. According to the cross-sectional shape of the flow path 3, the rectifying element 15 is made of a cylindrical body whose outer peripheral surface fits into the inner wall of the flow path with a cylindrical shape in this case, and the tip (heat-sensitive part) of the thermistor 6 for detecting the flow rate is arranged with a margin. That is, a central shaft hole 16 is provided to form an annular gap with the thermistor tip to ensure stable fluid flow, and the shaft holes 16 are arranged in parallel to each other on a central circle around the central shaft hole 16. A rectifying hole 17 consisting of a plurality of annular flow channels is formed through the flow passage. In the above configuration, when appropriate fluid supply pipes and discharge pipes (not shown) are connected to the fluid inlet 2 and outlet 4 of the sensor block 1 and the test fluid is passed through the flow channels 3 and 5, the axis of the rectifying element 15 is The hole 16 has an accurate diversion ratio (the ratio of the shaft hole opening area of the rectifying element to the total opening area) to the total flow rate of the flow path 3. However, the shaft hole opening area is the cross-sectional area of the peripheral gap at the tip of the thermistor. The tip of the thermistor 6, which self-generates heat through a minute flow (.), absorbs heat in accordance with the mass flow rate (and low temperature) of this minute flow. On the other hand, if we consider the case where the thermistor 6 is placed in a flow path where such a rectifying element does not exist, we will find that the flow paths 3 and 5 are bent into an L shape near the thermistor tip, and that the thermistor 6 In comparison, turbulent flow may occur due to the large diameter of the channel, or even if the flow is orderly and laminar, the flow on the surface of the thermistor accurately represents the flow state of the entire channel 3 due to the relative thinness of the thermistor. It is thought that there will be no need to do so. In this sense, it can be seen that the rectifying element of the present invention plays a very important role. Ultimately, the fluid conditioned by the rectifying element 15 removes heat from the thermistor 6 at a rate related to its specific heat and density, so that the change in resistance provides a signal as a function of mass flow rate. Therefore, it is considered that the following formula, which is known as a general formula for mass flow rate, clearly holds true in the present invention as well. M=E/Jcpθ M; mass flow rate J; work equivalent of heat cp; constant pressure specific heat θ of fluid; i. and b. Temperature difference E: Energy imparted to the fluid It should be noted here that the thermistors 6 and 7 have the same resistance value under the initial conditions and work with the same constant current and the same amount of heat.
This means that E in the above equation is the same for both thermistors. In this embodiment, the diameter of the axial hole flow path in which the thermistor 6 is installed is 2.
~3φ, the peripheral rectifying hole flow path is 0.5 to 3.0φ,
These rectifying holes 17 are provided in the most appropriate number depending on the flow rate, and function so that conditions such as flow velocity and heat dissipation in the shaft hole portion remain the same over a wide flow rate range. FIG. 3 shows another embodiment of the mass flowmeter of the present invention. In this embodiment, the flow meter includes four blocks connected in cascade: from right to left in the figure, an inlet block 20, a sensor block 21, a thermistor support block 22, and a control block 23. The inlet block 20 has a filter 20b inserted in the middle part of its through passage 20a, and the rear end is connected to the sensor block 2.
By screwing into the opening 1, the rectifying element 15 inserted in the middle of the opening is fixed. The thermistor support block 22 supports the flow rate detection thermistor 6 on its own central axis so that its tip protrudes into the opening of the sensor block 21, and also supports the thermistor 6 in a small chamber 22a arranged perpendicular to the central axis at the rear of the block 22. It supports a temperature compensating thermistor 7 whose tip protrudes. Regarding the thermistor 6, there is a lead wire extraction hole 24, and regarding the small chamber 22a of the thermistor 7, there is a port 25 opened on the rear end surface 22b of the block.
are formed respectively. Furthermore, four through holes 26 are formed in the sensor support block 22, parallel to the thermistor 6 on the central axis at equal intervals, and continuous to the opening of the sensor block 21 and the entrance chamber 23a of the control block 23. The control block 23 is formed with an inlet chamber 23a communicating with the through hole 26 and port 25 of the thermistor support block 22, and an outlet side passage chamber 23b opposite to this, and these are connected to the upper side of the block by respective vertical passages. The control room 27 is connected to the control room 27. In this case, the rear vertical flow path 28 opens in the bottom protrusion at the center of the control chamber 27, and the flow rate of the entire flowmeter can be controlled according to the proximity of the control piece 29 mounted from above. . In addition, control piece 29
The piece has a recess corresponding to the protrusion opening on its tip surface, and a driving solenoid 30 is arranged around the piece body. In the embodiment shown in FIG.
When dust removal and the like are carried out by means b, and the flow rate is appropriately controlled by the position of the control piece in the control section, it functions in the same manner as in the first embodiment. A circuit for calculating the fluid flow rate from the resistance change of the thermistor 6 having its tip exposed in the shaft hole 16 of the rectifying element and the thermistor 7 arranged in parallel thereto is shown in FIG. In FIG. 4, a flow velocity thermistor 6 and a temperature compensation thermistor 7 are connected in series to a constant current circuit 31, and both ends of each thermistor 6, 7 are connected to each input terminal of a corresponding differential amplifier 32, 33. . The outputs of these amplifiers 32 and 33 are connected to the negative input and positive input of a third differential amplifier 34, respectively. This amplifier 34
The output has a magnitude corresponding to the mass flow rate of the fluid. In this case, a zero adjuster 35 consisting of bias adjusting means and a span adjuster 36 consisting of amplification factor adjusting means are connected to the operational amplifier 34. In the above circuit, the thermistors 6 and 7 generate heat in response to the flowing current, and are maintained at a temperature (and thus a resistance value) that is in equilibrium with their own atmosphere. now,
Assuming that the resistance-temperature characteristics of thermistors 6 and 7 are equal, the first thermistor 6 will have substantially the same temperature as the second thermistor 7 if the fluid flow rate (mass flow rate) is zero, and if it is not zero, The fluid is cooled according to the flow velocity, and therefore, the difference in resistance between the two represents the flow velocity of the fluid. However, since the resistance-temperature characteristics of the two actually differ slightly, in order to determine the fluid flow rate from the value corresponding to the resistance value as described above, the bias and amplification factor of the corresponding amplifiers 32 and 33 must be The difference in their characteristics must be compensated for by appropriate adjustment. As a result, the two signals input to the operational amplifier 34 appear to represent resistance values corresponding to different temperatures in two thermistors having the same characteristics. It is clear that the operational amplifier 34, through the operation of the zero regulator 35 and the span regulator 36, accurately indicates the fluid flow rate flowing into the flow path of the sensor block 1 or 21 according to the set and calibrated scale. FIG. 5 is a graph showing pressure-flow characteristics in the mass flowmeter shown in FIG. 1 of the present invention. The test conditions in this case are to install a constant flow control valve upstream of the flowmeter of the present invention, connect a pressure gauge and a throttle valve downstream, and connect a standard flowmeter to the outlet that is open to the atmosphere to measure the flow rate. During measurement, the pressure was varied from 0 to 5 kg/cm ² using a throttle valve. Actual flow rate adopted as parameter: 0.2, 0.4, 0.6/
It can be seen that at min, the indicated values of the standard flowmeters almost accurately represent the actual flow rates. FIG. 6 is a graph showing the temperature-flow rate characteristics of the mass flowmeter shown in FIG. 1 of the present invention. The test conditions in this case are to place the flowmeter of the present invention in a constant temperature chamber, calibrate it to the actual flow rate value at 20℃, and then measure the change in the output signal as the flow rate measurement value as the temperature changes from 5℃ to 40℃. It is plotted on the vertical axis. During this time, the actual flow rates are 1, 5, 9/
Measurements were made by flowing N2 gas at a constant flow rate of min, but as shown in each graph, the flow rate indication value drops slightly as the temperature rises. It can be seen that there is no problem in practical use. FIG. 7 is a graph showing the response speed characteristics of the mass flowmeter shown in FIG. 1 of the present invention.
The test conditions in this case were to set N2 gas at a constant flow rate of 500 ml/min, and measure the response speed of the rise when rapidly opening and closing a solenoid valve for the flowmeter of the present invention and a conventional capillary flowmeter. and compared. The full scale of the recorder is 1000ml/
The chart speed was 60 cm/min, the ambient temperature was 20°C, and the constant differential pressure constant flow valve was set at 500 ml/min. The results show that the recorder of the present invention provides normal measurements within a few seconds after N2 flow (compared to 12-13 seconds for conventional devices). Figures 8 and 9 show actual flow rates of 0 to 1.0, respectively.
The relationship between flow rate and output signal, a so-called calibration curve, was confirmed for /min and 0 to 200ml/min. The measurement conditions and measurement results are as follows.

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[Table] From these results, the characteristic curve is 0 to 0.5/
Although there is some curvature in the min range,
No irregularities were observed, indicating that meter scale settings etc. can be performed extremely easily at flow rates of approximately 1/min or less. Effects of the Invention As described above, the present invention provides an accurate and stable mass flow meter, and in particular, the rectifying element that serves to form a stable fluid flow path to be tested is removable and can be attached or removed as needed. It is very practical because it can be removed and cleaned or replaced with a new one.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing a basic embodiment of the present invention, FIG. 2 is a perspective view of a rectifying element used in the flowmeter of the embodiment, and FIG. 3 is a longitudinal sectional view showing another embodiment. 4 is a diagram showing an example of the electric circuit of the flow meter of the present invention, FIG. 5 is a graph showing the pressure-flow characteristics in the flow meter of the present invention, FIG. 6 is a graph showing the temperature-flow characteristics, FIG. 7 is a graph showing response speed characteristics, and FIGS. 8 and 9 are graphs showing flow rate-output signal characteristics. 1, 21...Sensor block, 6...Thermistor for flow rate detection, 7...Thermistor for temperature compensation, 1
5... Rectifying element, 16... Shaft hole, 17...
Rectifier hole.

Claims (1)

[Scope of Claims] 1. A sensor block having a flow path for a fluid to be tested, and a flow rate device arranged in the flow path with its tip facing upstream and with its axis aligned with the flow path direction. A glass bead-enclosed thermistor for detection, a central axis hole coaxially surrounding the tip of the flow rate detection thermistor with a fluid flow gap in the main body fitted into the flow path, and a central axis hole parallel to the periphery thereof. a rectifying element penetrating through a plurality of rectifying holes arranged in a straight line, and a thermistor having the same type and similar resistance-temperature characteristics as the flow rate detection thermistor, the tip of which is close to the flow path and has a small hole. a temperature compensation thermistor housed in a small chamber communicated with the temperature compensation thermistor, and an arithmetic processing circuit for calibrating and calculating the resistance difference between the flow rate detection thermistor and the temperature compensation thermistor to indicate the mass of the fluid. Features of thermal mass flowmeter.