JPH095135A - Thermal flowmeter and its measuring element - Google Patents

Abstract

PURPOSE: To provide a thermal flowmeter which uses a measuring element formed on one ceramic substrate, keeps the strength and the responsivity of the element, and prevent a measuring error even when the partial reverse flow of intake air is generated. CONSTITUTION: A heat generating resistance film 3 and a temperature compensating resistance film 4 are formed collectively on a stepped substrate. A wide heat nongenerating part 5c is formed in a heat generating resistance element, a slit is formed in its upstream, and the wide heat nongenerating part 5c is held by a stepped support member 15 so as to be exposed to a fluid. Thereby, a drop in the bending strength of a measuring element itself can be reduced even when the slit is formed, and transient responsivity with reference to a change in a flow velocity can be enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal type flow meter and its measuring element, and more particularly to a thermal type air flow meter suitable for detecting the intake air amount of an internal combustion engine.

[0002]

2. Description of the Related Art As shown in FIG. 3 of Japanese Unexamined Patent Publication No. 59-151020, a conventional thermal type air flow meter is constructed by forming a temperature-dependent resistor on the outer surface of a cylindrical ceramic. The heating resistance element and the temperature compensation resistance element are arranged in different stages, and as shown in FIG. 2 of JP-A-2-67922, a temperature-dependent resistance film is formed on a thin electrically insulating substrate. It is known that the formed flow velocity detecting resistor (: heat generating resistor element) RS and the intake air temperature detecting resistor (: temperature compensation resistor element) RT which are formed independently of each other are arranged on the upstream side in a staggered manner. That is, it is conventionally known that the heating resistance element and the temperature compensation resistance element are arranged in different stages. It is also known to form a plurality of elements on a single substrate.

FIG. 1 of JP-A-5-126613 discloses that a temperature sensor, a compensating resistor and a heating (and measuring) resistor are formed on one electrically insulating substrate, and the compensating resistor is provided by the temperature sensor. Contact the heating resistance from the compensation resistance with the contact surface (:
What is formed in a stepped shape protruding from the connection electrode terminal) is described. Further, the figure discloses that a slit is provided between the compensation resistor and the heating resistor.

Further, in Japanese Patent Application Laid-Open No. 1-185416, an element in which a pair of heating resistors and temperature compensating resistors, which are arranged in the direction of air flow on an electrically insulating substrate, are formed on one substrate ( (Fig. 4 of the publication) and a circuit capable of detecting backflow (Fig. 3 of the publication) are disclosed.

On the other hand, Japanese Patent Laid-Open No. 232524/1990 discloses that
The measurement element is arranged in the sub-flow path housed in the main flow path, and the upstream side inlet opening of the sub-flow path is provided with an air flow stabilizing means consisting of a concave portion provided with an edge projecting to the upstream side, There is disclosed a structure of a thermal air flowmeter for an internal combustion engine in which an inlet opening is located at the bottom of the recess and is eccentric with respect to the main flow path axis.
This configuration is intended to accurately measure the average forward flow of the intake air flow of the internal combustion engine, and is a sub-flow passage configuration that is rather difficult to detect for reverse flow from the engine. However, the sub-flow path configuration is not suitable for a thermal type flow meter that detects a backflow and tries to correct it.

[0006]

However, in the technique disclosed in the above-mentioned Japanese Patent Laid-Open No. 5-126613, the substrate portion provided with the compensation resistor is partially located upstream of the measurement resistor (and the heating resistor). Since they are formed so as to overlap with each other, it is difficult for the airflow to hit the end of the non-heating portion of the heating resistor, and the non-heating portion is hard to be cooled. Therefore, it is necessary to lengthen the slit close to the electrode, and in particular, in order to improve the response, the substrate length of the measurement resistance (heat-generating resistance element) portion desired to be formed with a small width in the flow direction becomes long, which is a problem in terms of strength. Furthermore, since the flow disturbed in the substrate part of the compensation resistance hits the measurement resistance,
The problem of noise arises.

Further, in the element shown in FIG. 4 of the present disclosure example, each pair of resistors is vertically formed, and a heat insulating slit is provided between the resistors. Such a shape as an element realizes a heating resistor element that is desired to be formed with a small width in the flow direction in order to ensure the responsiveness required for backflow detection correction, since each pair of resistors is formed vertically. Inevitably, the length in the longitudinal direction becomes long, which causes a problem in strength. This also causes the slits in the middle to further reduce the strength.

An object of the present invention is to ensure thermal insulation between a heating resistance element and a temperature compensation resistance element, and to make good contact of each element with a fluid flow to realize high responsiveness and output accuracy. The purpose is to increase the strength.

[0009]

The above object is to provide a thin electrical insulating substrate, at least one heating resistance element and a temperature compensation resistance element formed on the surface thereof in a film shape, and these heating resistance element and temperature. A plurality of electrodes formed corresponding to the compensation resistance element, a lead member connecting the heating resistance element and the temperature compensation resistance element to the electrode, and the heating resistance element and the temperature compensation resistance element in the flow direction. In a thermal type flow meter having a measuring element formed on the insulating substrate with a step so that the heating resistor element projects, a substrate having the same width in the flow direction as the substrate portion on which the heating resistor element is formed. A portion of the substrate on which the temperature compensating resistor element film is formed, and a substrate portion that is formed wide at least on the upstream side through a slit, and the heating resistor element are formed at substantially the same stage in the flow direction. An accommodating member for accommodating so as to include the root part of the slit except for a board portion having the same width as the board portion and a board portion formed continuously from the board portion, and holding the accommodating member by a heat-defective conductor member. It is achieved by also using as a member.

[0010]

According to the present invention, the integrated substrate is stepped, and a slit for thermal insulation is provided between the temperature compensating element having a short protruding length and the non-heat generating portion of the heat generating resistor element formed to have a wide width. A short stepped shape formed so that at least the upstream edge portion of the non-heating portion having the same width as the substrate portion on which the film is formed and the wide non-heating portion continuous with the substrate portion are exposed to the fluid flow to be measured. Since it is provided with a length, this portion is well cooled even at the time of low flow velocity, and the temperature change at the time of high flow velocity becomes small. That is, it is possible to reduce the change in the amount of heat transferred from the heat generating portion toward the base of the substrate, and the time for equilibrating the temperature distribution is shortened. On the other hand, with respect to the decrease in bending strength of the measuring element itself due to the provision of the slit, the strength is increased because the root of the slit is housed and this housing member is made of a heat-defective conductor member and is also used as a support structure.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a plan view of an embodiment of a measuring element of a thermal type flow meter of the present invention. The heat generating resistance element 1 includes the heat generating portion 5 of the substrate 5.
a heat-generating resistor element film 3 formed on a, a non-heat-generating portion 5b having the same width as the heat-generating portion 5a and exposing the substrate portion, and a wide non-heat-generating portion formed with a step on the non-heat-generating portion 5b. It consists of the part 5c. The temperature compensation resistance element 2 is composed of a substrate portion 5d on the most upstream side with respect to the measured airflow 100 and an element film 4. These heating resistance element 1 and temperature compensation resistance element 2
Is connected to the connection terminal films 6, 7, 8 and 9 through the lead electrode films 10, 11, 12 and 13. The slit 14 is provided to thermally insulate the heat generating element 1 and the temperature compensating element 2.

The resistance value of the heat generating portion 5a of the heat generating resistance element 1 changes due to the air flow blown to this portion, and this change is converted into an air flow rate by a calculation device (not shown) in the subsequent stage,
The fuel injection charge of the internal combustion engine is controlled based on this air flow rate. Therefore, in order to improve the control accuracy, that is,
In order to improve the response of the internal combustion engine, the heat generating portion 5a
The temperature change must follow the air flow rate change without delay. That is, it is necessary to make the heat generating portion 5a thermally independent. Therefore, in this embodiment, the non-heat generating portion 5b having the same width as that of the heat generating portion 5a and having the exposed substrate portion is provided, and the heat generated by the heat generating portion 5a is generated by cooling this portion. I try not to spread the whole of 1 as much as possible.

On the other hand, the temperature compensating resistance element 2 has a heating portion 5a.
Is provided to correct an error due to temperature, and if it is thermally connected to the heat generating portion 5a, the heat generated by the heat generating portion 5a cannot perform accurate temperature compensation. Therefore, in this embodiment, the slit 14 and the wide non-heat generating portion 5c are provided. If the slit 14 does not exist, the heat conduction of the heat generating part 5a becomes high, which hinders temperature compensation. Further, by making a part of the wide non-heat generating portion 5c an upstream side edge portion which is not covered with the support 5 and is exposed to the air flow as described later, the heat generating portion 5c is formed.
The heat from a is cooled, and the thermal influence on the temperature compensation resistance element 2 is reduced.

By the way, the substrate 5 is made of ceramic such as alumina and has an extremely thin plate thickness of about 0.1 mm in order to increase the response speed. The resistance films 3 and 4 are made of a platinum thin film or the like and are 0.002 m by a method such as sputtering or vapor deposition.
After being collectively deposited on the substrate 5 with a film thickness of m (2 microns) or less, a pattern is formed by a method such as photoetching. Terminal electrodes 6, 7, 8, 9 and extraction electrode 1
In order to reduce the electric resistance, 0, 11, 12, and 13 form a thick film such as a platinum-silver alloy thicker than the platinum thin films 3 and 4 on the platinum thin film by a method such as printing. Although not shown in the figure,
A protective film such as glass is formed on the resistance films 3 and 4 to protect the elements.

Since the substrate 5 is extremely thin as described above, it is weak in strength. Therefore, the portion indicated by the broken line is covered with the support 15 of the substrate 5 formed of the heat-defective conductor member.
The heat-generating portion 5a of the heat-generating resistor element 1, the upstream end portions of the non-heat-generating portion 5b and the wide non-heat-generating portion 5c, and the temperature-compensating resistor element 2 are exposed to the flow 100 at 15a, 1a.
The portion shown as 5b is formed with a step. The wide non-heat generating portion 5c is advantageous in terms of heat to eliminate this portion (having the same width as the non-heat generating portion 5b) to form a slit, but it is wide because it is weak in strength. Thermally, as described above, the problem is solved by exposing a part of the wide non-heat generating portion 5c without covering it and forming an edge portion. Since the base of the slit 14 is fragile, the base of the slit 14 was also covered with the heat-defective conductor member.

The heating resistance element 1 must be fixed to any of the intake systems of the internal combustion engine. However, since the substrate 5 is weak as described above, the heat-defective conductor member covering the substrate 5 is also used as a support member instead of directly fixing the substrate 5.

As described above, in this embodiment, the problem of strength contradictory to the air flow rate detection accuracy is solved while maintaining the air flow rate detection accuracy.

FIG. 2 is a sectional view of an embodiment of the present invention in which the measuring element shown in FIG. 1 is constructed as an air flow meter for an internal combustion engine. The body 101 of the flowmeter that forms the main passage 102 of the intake air 90 and the passage member 10 that forms the L-shaped sub passage 104.
3 forms a flow channel as a flow meter. The heat generating resistance element 1 and the temperature compensating resistance element 2 are arranged in a flow path portion parallel to the flow of the sub-passage 104 via a support member 15 integrated with the circuit unit 106, and the non-heat generating wide substrate of the heat generating element 1 is provided. It is configured to be exposed to the air flow 100 flowing into the sub passage 104 together with the upstream end portion 5c. 107 is an outflow flow from the sub-passage 104. In the passage member 103, an upstream edge of the sub-passage 104 is provided with a projecting edge toward the upstream side with respect to the forward flow 90 over the entire circumference, and an air flow stabilizing means 105 composed of a concave portion is formed.

FIG. 3 is a plan view of a second embodiment of the measuring element of the thermal type flow meter of the present invention. Differences from the embodiment shown in FIG. 1 will be mainly described. The heating resistor element 1 has a forward flow of 10
Two resistance films of the upstream resistance film 3 and the downstream resistance film 23 are formed close to 0. Further, the temperature compensation resistance element 2 also has two resistance films, that is, the upstream resistance film 4 and the downstream resistance film 24. Six connection electrode terminals 26 to 31 are provided in total, and the terminals 26 and 29 are common terminals to which the extraction electrodes of the heating resistance element 1 and the temperature compensation resistance element 2 are connected. The temperature of the upstream resistance film 3 and the temperature of the downstream resistance film 23 are the same as those of a normal constant temperature type thermal flow meter, so that the difference between each temperature is a constant value regardless of the air flow velocity. The element film and the temperature compensating resistance element film are electrically heated by a drive circuit that operates independently. Arrow 10
When there is a flow in the forward direction indicated by 0, the upstream resistance film 3
Since the cooling due to the flow is larger than that of the downstream resistance film 23, the supply current from the drive circuit is larger in the upstream resistance film 3 than in the downstream resistance film 23. On the other hand, arrow 200
In the case of the reverse flow shown in FIG. 5, the cooling due to the flow is larger in the downstream resistance film 23 than in the upstream resistance film 3, and the supply current from the drive circuit is larger in the downstream resistance film 23 in the downstream resistance film 23. Will be larger than Therefore, the direction of the flow can be detected by the difference in the current supplied to the heating resistance films 3 and 23. The output according to the flow rate of each of the upstream resistance film 3 and the downstream resistance film 23 is detected by the voltage comparator in the direction of the flow, and the output is switched by the switch circuit so that the backflow component is not output. Correction such as subtraction of minutes becomes possible. However, in order to perform this accurately, the heating resistance element needs to have high transient response. In order to realize high transient response, it is effective to minimize the heat capacity of the substrate, that is, the plate thickness and the width in the flow direction, reduce the amount of heat transfer to the support, reduce the change in temperature distribution due to the flow velocity, etc. . However, reducing the plate thickness and the width in the flow direction of the substrate causes a problem of lowering the bending strength of the element.
Further, when the heat generating resistance element and the temperature compensating element are formed on one substrate, thermal insulation between them is indispensable for improving accuracy, and therefore it is effective to provide a slit between them.
However, it is understood that this slit also causes a decrease in strength. Therefore, in order to obtain a high transient response, the width of the substrate portion 5a on which the heating resistance element films 3 and 23 are provided in the flow direction is made small, but the portion at which the slit is provided is made wide, that is, 5
By providing the portion c, the strength reduction can be suppressed. Further, in order to reduce the amount of heat transfer to the support portion and the change in temperature distribution due to the flow velocity, it is effective to lower the temperature of the non-heat generating portion of the heating resistance element and reduce the temperature change due to the change in flow velocity. For this reason, the non-heating portion 5b having the same width as the substrate portion 5a on which the heating resistance element films 3 and 23 are provided is made as long as the strength allows, and this portion and the wide non-heating portion 5c are effectively used even at a low flow velocity. It is important to cool down. for that reason,
Similar to that described with reference to FIG. 1, the stepped structure of the support member 15 shown by 15a and 15b is adopted.

FIG. 4 is a plan view of a slightly modified embodiment of the embodiment shown in FIG. The wide non-heat generating portion 5c of the heating resistor element is formed by extending from the lower end edge of the temperature compensating element 2 to the heating resistor element films 3 and 23 side. Thereby, the wide non-heat generating portion 5c of the heat generating resistance element can be cooled more sufficiently.

FIG. 5 is a plan view of an embodiment obtained by further improving the embodiment shown in FIG. The non-heat generating portions 5c and 5e formed wide in the heat generating resistance element 1 are formed substantially uniformly in the upstream and downstream directions, and the resistance films 3 and 23 of the heat generating portion and the connection electrode terminals 26, 27, 28 and 29 are formed. Extraction electrodes 40, 41,
Including the patterns 42 and 43, the substrate portion on the downstream side of the slit 14 on which the heating resistance element 1 is formed is substantially symmetrical in the flow direction with respect to the center of the separation portion 5f of the two film heating resistance films 3 and 23. Has been formed. As a result, the electrical characteristics of the two heating resistance elements 3 and 23 can be made uniform, and the accuracy of backflow detection correction can be improved. Next, another change will be described.

The substrate 5 is cut out from the large substrate by a method such as laser processing. At the corners of the step or slit, the laser temporarily stops due to the change of direction, the temperature of the part rises, and the rate of damage to the substrate increases. Therefore, the damage rate can be reduced by forming the corners 16a and 16b of the step and the slit corner 17 with curved lines such as arcs so that the laser movement is continuous.

FIG. 6 is a plan view of an embodiment in which the shape of the supporting member 18 is slightly changed from that of the measuring element shown in FIG.
As shown by 18a of the support member 18, an arrow 2 indicating a backflow so that both the non-heat generating portions 5c and 5e formed in the heating resistor element 1 and having a wide width in the upstream and downstream directions are exposed to the flow to be measured.
A step is also provided on the 00 side. This is also advantageous in improving the accuracy of backflow detection correction in terms of temperature symmetry.

FIG. 7 is a sectional view of an embodiment of the present invention in which the measuring element shown in FIG. 3 or 5 is constructed as an air flow meter of an internal combustion engine. The body 101 of the flowmeter that forms the main passage 102 of the intake air 90 and the passage member 113 that forms the sub-passage 114 form a flow passage as a flowmeter. The heat-generating resistance element 1 and the temperature-compensating resistance element 2 are arranged in a flow path portion parallel to the flow of the sub-passage 114 via a support member 15 integrated with the circuit unit 116. It is configured to be exposed to the airflow 100 flowing into the auxiliary passage 114 together with the upstream end portion 5c. 117
Is an outflow flow from the auxiliary passage 114. In the passage member 113, the upstream and downstream openings of the sub-passage 114 are provided with edges that project over the forward flow 90 and around the entire downstream, respectively, and air flow stabilizing means 115a and 115b formed of concave portions are formed. There is. An arrow 91 indicates a backflow from the engine, and an arrow 200 indicates an inflow flow into the sub passage 114.

FIG. 8 is a front view seen from the forward flow 90 side in FIG. Corresponding to FIG. 7, 1 is a heating resistance element, 2 is a temperature compensation resistance element, 15 is a support member, 101 is a body,
102 is a main passage and 116 is a circuit unit. A wall surface of the sub passage 114 on the side of the circuit unit 116 is formed in an arc, and an air flow stabilizing means 115a is provided at an inlet portion thereof. The front shape of the downstream air flow stabilizing means 115b is also substantially the same as that of the upstream air flow stabilizing means 115a. By the air flow stabilizing means 115a and 115b, the influence of the flow velocity distribution of the flow is reduced on both the forward flow side and the reverse flow side, and the measurement accuracy is improved.

According to these embodiments described above, the decrease in the bending strength of the measuring element itself due to the provision of the slits can be reduced, the change in the amount of conduction heat from the heat generating portion toward the base of the substrate can be reduced, and the time for balancing the temperature distribution can be reduced. Is shortened, and as a result, improved transient responsiveness to flow velocity fluctuations is achieved. This realizes a thermal type flow meter that enables favorable backflow correction. Further, by forming the substrate portion on the downstream side of the slit, on which the heating resistance element is formed, symmetrically with respect to the center of the separation portion of the two film-shaped heating resistance patterns,
The electrical characteristics of the two heating resistance elements can be equalized, and
By exposing the upstream and downstream ends of the non-heat generating portion of the widely formed heat generating resistance element to the fluid flow, temperature symmetry is obtained to improve the accuracy of backflow detection correction. In addition, by forming the inner corner of the step portion of the non-heat generating portion of the heating resistor element with a continuous curve, it is possible to reduce defects when cutting the substrate using a laser, reduce the cost during element manufacturing, and reduce the strength of the unit. Will also be realized. On the other hand, the influence of the flow velocity distribution of the flow is reduced by arranging the measuring element in the sub-flow passage in which the air flow stabilizing means consisting of the concave portion is formed by providing the edges projecting all around the upstream and downstream openings, thereby reducing the back flow. Improvement of measurement accuracy including correction is achieved. The error due to backflow is as high as 30 to 100%, depending on the engine and the conditions. An error of 5% or less is realized by the backflow correction to which the present embodiment is applied.

[0027]

By improving the responsiveness, it is possible to prevent the strength from decreasing.

[Brief description of drawings]

FIG. 1 is a plan view of a first embodiment of a measuring element of a thermal type flow meter according to the present invention.

FIG. 2 is a sectional view of a thermal type flow meter according to an embodiment of the present invention.

FIG. 3 is a plan view of a second embodiment of the measuring element of the thermal type flow meter of the present invention.

FIG. 4 is a plan view of a third embodiment of the measuring element of the thermal type flow meter of the present invention.

FIG. 5 is a plan view of a fourth embodiment of the measuring element of the thermal type flow meter of the present invention.

FIG. 6 is a plan view of a fifth embodiment of the measuring element of the thermal type flow meter of the present invention.

FIG. 7 is a sectional view of a thermal type flow meter according to an embodiment of the present invention.

FIG. 8 is a front view of the thermal type flow meter according to the embodiment of the present invention shown in FIG.

[Explanation of symbols]

1 ... Heating resistance element, 2 ... Temperature compensation resistance element, 3, 4, 2
3, 24 ... Resistive film, 5 ... Substrate, 5a ... Substrate portion on which the heat generating resistive film is formed, 5b ... Non-heat generating portion of the heat generating resistive element having the same width as the substrate portion on which the heat generating resistive film is formed, 5c, 5e ... The non-heat generating portion of the heat generating resistance element, which is wider than the substrate portion on which the heat generating resistance film is formed, 6, 7, 8, 9, 26, 27, 28, 29,
30, 31 ... Connection electrode terminals, 14 ... Slits, 15, 1
8 ... Support member, 90, 100 ... Forward flow, 91, 200
... Backflow, 101 ... Body, 102 ... Main passage, 10
4, 114 ... Sub passages, 105, 115a, 115b ... Air flow stabilizing means, 106 ... Circuit unit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takehiko Owata 502 Jinritsucho, Tsuchiura-shi, Ibaraki Machinery Research Institute, Hiritsu Manufacturing Co., Ltd. (72) Inventor, Izumi Watanabe, Takata Ibaraki, Ibaraki Prefecture, Takaba 2477, Kashima Yatsu 3 Hitachi Automotive Engineering Co., Ltd. Address 3 Within Hitachi Automotive Engineering Co., Ltd.

Claims (7)

[Claims] 1. A thin electrically insulating substrate, at least one heating resistance element and a temperature compensation resistance element formed on the surface thereof in a film shape, and formed corresponding to these heating resistance element and temperature compensation resistance element. A plurality of electrodes, a lead member for connecting the heating resistance element and the temperature compensation resistance element to the electrodes, and the heating resistance element and the temperature compensation resistance element so that the heating resistance element projects in the flow direction. In a thermal type flow meter having a measuring element formed on the insulating substrate with a step at the step, a substrate portion having the same width in the flow direction as the substrate portion on which the heating resistance element is formed, and the temperature compensation resistance element Substantially in the same flow direction as the substrate part where the film is formed,
Except for a substrate portion formed wide at least on the upstream side through a slit, a substrate portion having the same width as the substrate portion on which the heating resistance element is formed, and a substrate portion formed continuously wide to the slit, the slit A thermal type flow meter in which the housing member is housed so as to include the root part thereof, and the housing member is composed of a heat-defective conductor member and also serves as a holding member. 2. The temperature compensating resistance element film pattern and the heat generating resistance element film pattern are formed adjacent to each other by two pieces, and one temperature compensating resistance element and one heating resistance element are paired. In addition to having a constant temperature drive circuit that is driven independently, it also has a comparison circuit that compares the output sizes of two heating resistance elements that are close to upstream and downstream with respect to the flow, and backflow correction is possible. The thermal type flow meter according to claim 1. 3. A non-heat generating portion of the heat generating resistance element is formed to have a wide width in the upstream and downstream directions, and includes a resistance film pattern of the heat generating portion and a pattern of a lead electrode up to a connecting electrode terminal.
The thermal type flow meter according to claim 2, wherein the substrate portion on the downstream side of the slit, on which the heating resistance element is formed, is formed substantially symmetrically with respect to the center of the separation portion of the two film-shaped heating resistance film patterns. Measuring element. 4. A heat-defective conductor member having steps formed upstream and downstream so that the upstream and downstream ends of the non-heat generating portion of the widened heating resistor element are exposed to the fluid flow to be measured. A thermal flow meter comprising the measuring element according to claim 2 or claim 3. 5. The measuring element for a thermal type flow meter according to claim 1, wherein at least the inner corner of the step portion of the non-heat generating portion of the heat generating resistor element is formed by a continuous curve such as an arc. 6. A three-step shape in which a lower end edge of a wide non-heating portion of the heating resistor element is slightly projected from a lower end edge of the temperature compensation resistor element, and a lower end edge of the wide non-heating portion is also formed. The thermal flow meter according to claim 1, wherein the thermal flow meter is configured to be exposed to a fluid flow to be measured. 7. A main flow path for circulating intake air, and a sub-flow path for circulating a part of the intake air and having a measuring element therein to measure the intake air amount. And forming an air flow stabilizing means consisting of a recess by providing rims projecting around the upstream and downstream openings of the sub-flow path, and providing the upstream and downstream openings of the sub-flow path at the bottom of the recess. The thermal air flow meter according to claim 2, wherein the upstream and downstream openings are eccentrically located on the circuit unit side with respect to the main flow path axis.