JPS6173023A - Thermal type mass flowmeter - Google Patents

Abstract

PURPOSE:To obtain an accurate mass flowmeter which has stable performance by forming an annular gap between the center axial hole of a straightening element fitted in a flow passage and the tip of a thermistor for flow rate detection, and securing a stable flow of fluid. CONSTITUTION:The straightening element 15 is fitted in the flow passage 3 of a sensor block 1. The tip part of a glass bead type thermistor 6 for flow rate detection which has its tip directed to the upstream side and is arranged coaxially having its axis in the flow passage direction is inserted into the center axial hole of the element to form a fluid flow gap, and plural strightening holes are formed in its periphery. Thus, part of fluid admitted into the flow passage flows to the axial hole at an accurate flow dividing rate, so the thermis tor 6 for flow rate detection which contacting the fluid radiates heat according to the mass flow rate and temperature of the fluid and the resistance value shows balanced temperature. A thermistor 7 for temperature compensation, on the other hand, shows a resistance value corresponding to the fluid tempera ture, so the effective difference in resistance value between both thermistors is calculated by an arithmetic processing circuit to obtain the accurate mass flow rate of the fluid.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a thermal mass flowmeter using a thermistor.

2. Description of the Related Art 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.

-l The t-mister current meter is expected to be a highly sensitive flow meter because the temperature coefficient of the thermistor is 10 to 15 times that of normal metals, and the operating temperature is low. Practical use for industrial measurement has not progressed 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.

OBJECTS 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 conventional capillary flow paths and thermistor protruding flow paths, and the problem of variations in thermistor temperature characteristics. be.

Structure of the Invention Briefly, in order to achieve the above-mentioned 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 detecting flow rate is arranged to match the direction of the flow path, and a main body fitted into the flow path is connected coaxially with the tip of the thermistor for detecting the flow rate with a fluid flow gap. a rectifying element formed by penetrating a central axis hole surrounding the central axis 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 end is close to the flow path and is housed in a small chamber communicated through a small hole, and a resistance value difference between the flow rate detection thermistor and the temperature compensation thermistor is calibrated and calculated, and the fluid is detected. The present invention constitutes a thermal mass flowmeter characterized in that it is equipped with an arithmetic processing circuit for indicating the mass flow rate of.

In the above configuration, the rectifying element communicates a part of the fluid flow introduced into the flow path of the sensor block to its shaft hole at a precise flow division ratio, so that the flow rate detection thermistor in contact with the rectifying 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, the mass flow rate of the fluid can be accurately 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, the sensor block (1) as the main body of the flowmeter is a rectangular parallelepiped made of metal such as aluminum or acrylic or other resin material, and this block (1) has a fluid inlet ( 2
) and a vertical flow path section (3) extending upward from this flow path section (
A fluid flow path is formed in which the white liquid flows from the upper end in the block (1) of 3) and includes a horizontal flow path portion (5) that extends laterally and has a fluid outlet (4) that opens on the right side of the figure. .

The thermistor for flow rate detection (6) and the thermistor for temperature compensation (7) are substantially mounted or glass bead-encapsulated thermistors with similar characteristics, and the former (6) is the upper end wall ( 8) and hangs down and protrudes into the vertical channel (3), and the latter (7) also passes through the upper end wall (8) and runs close to and parallel to the vertical channel (3) on the left side wall in the figure. (9)
It protrudes into the small chamber α1 formed in .

The small chamber α0 communicates with the small hole αN) and the upward passages (3) and (5). The bases of the upper ends of the thermistors (6) and (7) are each fitted with nuts 02. It is fixed to the upper end of the block by α gloss, and an O-ring α ring is inserted into the lower side of the oak 1-.

According to the present invention, the flow path (3) of the sensor block has a rectifying element αQ that cooperates with the flow rate detection thermistor (6).
is inserted. The rectifying element 09 has a cylindrical outer peripheral surface that fits into the inner wall of the cylindrical flow path according to the cross-sectional shape of the flow path (3). In other words, they are arranged on a concentric circle around the central shaft hole αQ to form an annular gap with the thermistor tip to ensure stable fluid flow, and are parallel to this shaft hole M. A rectifying hole consisting of a plurality of tubular channels is formed through the flow passage.

In the above configuration, appropriate fluid supply pipes and discharge pipes (not shown) are connected to the O fluid inlet (2) and outlet (4) of the sensor block (1), and the test fluid is connected to the flow paths (3) and (5). When passing through the shaft hole α→ of the rectifying element αG, an accurate dividing ratio (ratio of the shaft hole opening area of the rectifying element to the total opening area) with respect to the total flow rate of the flow path (3) is determined. The opening area is the cross-sectional area of the gap around the thermistor tip.) Through which a minute flow passes, the tip of the thermistor (6), which self-heats, absorbs heat according to the mass flow rate (and low temperature) of this minute flow. It is.

On the other hand, when considering the case where the thermistor (6) is placed in a flow path where such a rectifying element does not exist, in this case, near the tip of the thermistor, the flow path (3), (5)
This may result in turbulent flow due to the L-shaped refraction of the flow or the larger flow path diameter compared to the thermistor (6), or even if the flow is ordered and laminar, the relative thinness of the thermistor may cause turbulence. Therefore, it is considered that the flow on the surface does not accurately represent the flow state in the entire channel (3). 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 rectifier element αQ will remove heat from the thermistor (6) at a rate that is related to its specific heat and density, so that the change in resistance provides a signal as a function of the 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.

cp: Specific heat at constant pressure of the fluid θ: Temperature difference between A and the mouth E: Energy given to the fluid The following points should be noted here:
) have the same resistance value under the initial conditions and work with the same constant current and the same amount of heat generation. This means that E in the above equation is the same for both thermistors. In this example, the diameter of the axial hole flow path in which the thermistor (6) is attached is 2 to 3φ, the diameter of the rectification hole flow path in the peripheral area is 0.5 to 3. Depending on the situation, the most appropriate number of these tubes are provided 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 example, the flow meters are cascaded into four
From right to left in the figure, it includes an inlet block (E), a sensor block Q), a thermistor support block (A), and a control block 4. The input robot block can be inserted into the middle of the opening of the sensor block Qυ by inserting the filter (20b) in the middle of the through flow path (20B) and screwing the rear end into the opening of the sensor block Qυ. The fixed rectifier element αυ is fixed. The thermistor support Glock (A) supports the flow rate detection thermistor (6) on its own central axis so that its tip protrudes into the opening of the sensor block Q1), and also supports the thermistor (6) at the rear of the block (A) perpendicular to the central axis. A temperature compensating thermistor (7) whose tip extends into a small chamber (22a) is supported. Thermistor (6
) is formed with an IJ-wire lead-out hole (c), and a small chamber (22a) of the thermistor (7) [with respect to the block rear end face (22b), a port (c) is formed, respectively. Furthermore, the sensor support block (A) has an opening of the sensor block I2υ and an entrance chamber (23a) of the control block holder, which are parallel to the thermistor (6) on the central axis at equal intervals.
Four through holes (c) are formed which are connected to each other. The control block handle has a through hole (
1) and an entrance chamber (23a) communicating with the boat (c);
An outlet side flow path chamber (28b) contrary to this is formed,
These are connected to the control room (A) on the upper side of the block by vertical channels. In this case, the rear vertical flow path opens in the bottom protrusion in the center of the control room, and the flow rate of the entire flowmeter can be controlled according to the proximity of the control piece mounted from above. . The tip of the control piece has a recess that corresponds to the opening of the protrusion, and a drive solenoid (7) is provided around the piece body.
) are located.

In the embodiment shown in FIG. 3 above, the rectifying element 0Q1 and thermistors (6) and (7) are the same as the rectifying element and thermistor in the first embodiment, so the filter (20b) in this embodiment This operates in the same manner as the first embodiment, provided that dust is removed and the flow rate is appropriately controlled by the position of the control piece in the control section.

A thermistor (6) having a tip exposed in the shaft hole OQ of the rectifying element, and a thermistor (7) installed in parallel with the thermistor (6)
The circuit for calculating the fluid flow rate from the resistance change is shown in FIG. In Figure 4, the flow velocity thermistor (6)
and the temperature compensation thermistor (7) are connected in series to the constant current circuit G.
υ, and both ends of each thermistor (6), (7) are connected to each input terminal of the corresponding differential amplifier oz, t;n. The outputs of these amplifiers (A) and (TO) are connected to the negative input and positive input of the third differential amplifier (2), respectively. The output of this amplifier has a magnitude corresponding to the mass flow rate of the fluid. In this case, a zero adjuster (to) consisting of bias adjustment means and a span adjuster (to) consisting of amplification factor adjustment means are connected to operational amplifier (2).

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 the thermistors (6) and (7) are equal, the first thermistor (6) will substantially become the second thermistor (7) if the fluid flow rate (mass flow rate) is zero. ), and if it is not zero, it will be cooled according to the flow velocity, and therefore the difference in resistance value between the two represents the flow rate of the fluid. However, since the resistance-temperature characteristics of the two will actually be in close agreement with each other, in order to determine the fluid flow rate from the value corresponding to the resistance value as described above, it is necessary to use the corresponding amplifier and The difference in characteristics must be compensated for by appropriately adjusting the bias and amplification factor of . As a result, the two signals inputted to the operational amplifier K appear to represent resistance values corresponding to different temperatures in two thermistors having the same characteristics.

Operational amplifier ■ has a zero adjuster (to) and a span adjuster (to)
By operating the set and calibrated scale,
It is clear that it is possible to accurately indicate the fluid flow rate flowing into the flow path of the sensor block (1) or Ql).

FIG. 5 is a graph showing pressure-flow characteristics in the mass flowmeter shown in FIG. 1 of the present invention.

In this case, the test conditions were to install a constant flow control valve upstream of the flowmeter of the present invention, connect a pressure gauge and throttle valve downstream, and connect a standard flowmeter to the outlet that was opened to the atmosphere. While measuring the pressure, adjust the pressure from 0 to 5 kg/
d. The actual flow rate used as a parameter was 0.2.

It can be seen that at 0.4 and 0.611/min, the indicated values of the standard flowmeter almost accurately represent the actual flow rates.

FIG. 6 is a graph showing the temperature-flow characteristics of the mass flowmeter shown in FIG. 1 of the present invention.

The test conditions in this case were to calibrate the flowmeter of the present invention to the actual flow rate value at 20°C in a constant temperature chamber, and then calibrate it to the actual flow rate value as the temperature changed from 5°C to 40°C. The output signal change is plotted on the vertical axis. During this time,
The actual flow rate is N of constant flow rate of 1, 5, 9e/I old n, respectively.
Measurements were made by flowing two gases, and in each graph, the flow rate indication value slightly decreased as the temperature rose, but this does not pose a practical problem for a room temperature change of about 10 degrees in a normal room. Recognize.

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 the N2 gas at a constant flow rate of 500ml/min, and simultaneously measure the response speed of the rise when rapidly opening and closing the solenoid valve for the flowmeter of the present invention and the conventional capillary type flowmeter. This is a comparison. The full scale of the recorder is 1000z//min, the chart speed is 60α/min,
The ambient temperature was 20°C, and a constant pressure differential constant flow valve was set at 500 wtl/min.

In the recorder of the present invention, the result becomes a normal measurement value within a few seconds after N2 flow (in the conventional device, the result is 12 to 13
(It takes several seconds).

Figures 8 and 9 are actual flow rates, 0 to 1.047 m, respectively.
In and 0 to 200 txt/man, the relationship between the flow rate and the output signal, a so-called calibration curve, was confirmed. The measurement conditions and measurement results are as follows.

Flow value-output characteristics I Measurement conditions Measurement results
Unit [・] l/fr11n Flow rate-output characteristics ■ Measurement conditions Measurement results Unit [Hl/m”0] Value 0 From these results, the characteristic curve is O~0.5 (1/min
Although there is some curvature in the range, no irregularities are observed, indicating that meter scale setting can be done very easily at flow rates below Lg/m1nalf.

Effects of the Invention As described above, the present invention provides an accurate and stable mass flowmeter, 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 drawings]

Claims (1)

[Scope of Claims] A sensor block having a flow path for a fluid to be tested, and a flow rate sensor disposed in the flow path with its tip facing upstream and with its own axis coinciding with the direction of the flow path. a glass bead-encapsulated thermistor for use in the flow path, and a central axis hole that coaxially surrounds 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 that extends parallel to the periphery thereof. a rectifying element formed by passing 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 small holes. 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.