JPH085433A - Mass flow sensor and infrared ray gas analyzer using the sensor - Google Patents

Abstract

PURPOSE:To provide a mass flow sensor with high sensitivity and zero point stability by installing a heating element at a position, which is a middle point of a pair of heat sensitive elements and parted spatially from them and forming a temperature field having sharp temperature gradient between the heat sensitive elements and the heating element. CONSTITUTION:A mass flow sensor is composed of at least a pair of heat sensitive elements (123, 124), (125, 126) whose long sides are set face to face mutually and a heating element 150 and the heat sensitive elements (123, 124); (125, 126) and the heating element 150 form one temperature field. The coupled heat sensitive elements (123, 124), (125, 126) are positioned at thermally symmetric positions in the temperature field in no wind blowing condition and the heating element 150 is installed in parallel to the long sides of the coupled heat sensitive elements (123, 124) (125, 126) on a second plane parted from a first plane including the coupled heat sensitive elements (123, 124), (125, 126) at a prescribed distance while having an air layer between them.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass flow sensor for measuring the flow velocity of a very small amount of gas, and particularly to the qualitative and quantitative analysis of various component gases contained in the measured gas using this mass flow sensor. The present invention relates to the structure of an absorption type infrared gas analyzer.

[0002]

2. Description of the Related Art Absorption-type infrared gas analyzers perform qualitative and quantitative analysis of the measurement gas based on the amount of infrared rays absorbed by the analyte gas contained in the measurement gas. Well, because of its high measurement sensitivity, it is widely used in various fields as a gas analyzer.

First, taking the double beam type absorption type infrared gas analyzer which has been conventionally used as an example, its schematic structure and its operating principle will be described with reference to FIG. Figure
In FIG. 13, 3 is an infrared light source, and the infrared light flux emitted from this light source 3 becomes intermittent light of a specific frequency by the rotary chopper 2. After that, the infrared light flux is distributed in two directions by the light distribution cell 4. One infrared light flux is guided as a measurement light beam 11 to the measurement cell 13, and the other infrared light flux is guided as a reference light beam 12 to the reference cell 14. The measurement cell 13 is provided with light transmission windows 5 and 6, and the measurement gas containing the component gas to be analyzed is introduced and discharged through the introduction pipes 17 and 18. The measuring light beam 11 is a measuring cell in this measuring cell 13.
Infrared absorption occurs according to the concentration of the component gas to be analyzed in the measurement gas flown in 13.

Similarly, the reference cell 14 is also provided with light transmitting windows 7 and 8 and is filled with a gas having no absorbing function for infrared rays, for example, nitrogen gas. The measurement light beam 11 that has passed through the measurement cell 13 and the reference light beam 12 that has passed through the reference cell 14 are guided to the gas filled detector 20, respectively. This detector 20 has a first expansion chamber 15 and a second expansion chamber 16, and the expansion chambers 15 and 16 are provided with light transmission windows 9 and 10, respectively. It is filled. In the first expansion chamber 15, the measurement light beam 11 is transmitted through the light transmission window 9.
And the reference ray 12 enters the second expansion chamber 16 through the light transmission window 10. In the detector 20, the first expansion chamber 15
Has a gas passageway 19 that connects the second expansion chamber 16 with
Due to the pressure fluctuation difference caused by the absorption of the infrared rays at 15 and 16, a gas flow corresponding to the content of the analyte gas in the measurement gas is produced.

An example of a mass flow sensor for detecting this gas flow is shown in FIG. FIG. 11A shows a state (a part A) in which the mass flow sensor 21 is attached to the gas passage 19 of the gas-filled detector 20, and an enlarged view of the part A is shown in FIG.
It is shown in (B). FIG. 11C shows a front view of the heat sensitive elements 23 and 24 inside the mass flow sensor.
Is made of a conductive metal or a conductive ceramic whose resistance changes greatly with temperature, and in the illustrated example, a zigzag band-shaped resistor (hatched portion is formed by alternately removing rectangular nickel foil with thin lines. ) Is composed. Both of the heat sensitive elements 23 and 24 face each other with a gap therebetween to form the mass flow sensor 21 shown in FIG. 11 (B).

FIG. 11 (B) shows the heat sensitive elements 23, 24 of FIG. 11 (C).
Shows a side cross-sectional view cut at the center, the shaded portion shows a portion of the foil in which the conductive metal or conductive ceramic having a large temperature change of the above-mentioned resistance value is connected in a zigzag shape, the thermal element 23 on the left side, the right side The heat sensitive element 24 is arranged close to the.
The central portion of each of the heat sensitive elements 23, 24 is an opening, and a gas flow based on the pressure difference between the first expansion chamber 15 and the second expansion chamber 16 flows in the direction of the arrow shown through the opening. . These thermosensitive elements 23 and 24 are combined with fixed resistors 27 and 28,
A Wheatstone bridge circuit is configured, and a current is applied from a power supply 30. The heat sensitive elements 23 and 24 are close to each other such that the heat sensitive elements 23 and 24 themselves are heated and raised to a temperature higher than the ambient temperature by the power source 30 applied to the bridge circuit, and the heat sensitive elements 23 and 24 are thermally coupled to each other. It is located in the position

With such a structure, when a gas flow is generated due to the pressure difference between the first and second expansion chambers, the gas flow causes the temperature distribution of the heat-sensitive elements 23 and 24 to change from the solid line a to the dotted line b in the windless state. The change in temperature can be detected by the Wheatstone bridge circuit and can be detected as the pressure difference between the two expansion chambers. FIG. 10 illustrates another example of the mass flow sensor. In Fig. 10 (A), the gas filled detector 20
Is a first expansion chamber 15, a second expansion chamber 16, both expansion chambers 15, 16
And a gas passage 19 communicating with
Two pairs of heat-sensitive elements (123, 124) and (125, 126) are integrally formed as a mass flow sensor 120 in the section. The measurement light beam 11 and the reference light beam 12 that enter the expansion chambers 15 and 16 cause a pressure fluctuation difference in the expansion chambers 15 and 16 due to the absorption amount of the infrared light rays in the expansion chambers 15 and 16, and based on this pressure fluctuation difference, the gas passage At 19 a gas flow corresponding to the content of the analyzed gas in the measuring gas is produced. Figure 10 (B), (C)
Is a mass flow sensor 12 disposed in the B portion of the gas passage 19.
It is a detailed view of 0, (B) of FIG. 10 is a top view, (C) of FIG. 10 is a front view. In Figure 10 (B), the mass flow sensor
120 is a pair of thermosensitive elements (123,124) and (125,126)
The two pairs of heat-sensitive elements are composed of a Wheatstone bridge circuit, and a current is applied from the power source 30.

In the illustrated example, the thin film thermosensitive elements 123 to 126 are formed on the silicon substrate 122 by a thin film manufacturing method, and the thin film thermosensitive elements (123,124) and (125,126) forming a pair are vertically arranged in two pairs. Further, as shown in FIG. 10C, the back surfaces of the thin film thermal sensors 123 to 126 of the silicon substrate 122 are etched to reduce the heat capacity and prevent the heat transfer and diffusion, and are fixed to the base 121. ing. The mass flow sensor 120 with this structure can be used for thin-film thermal elements (123,125) and thin-film thermal elements.
(124, 126) are respectively connected to the opposite sides of the Wheatstone bridge, and the thin film thermal sensors 123 to 126 are heated by the current from the bridge power supply 30 and the heat sensitive elements (1
The thermal coupling at (23,124) and (125,126) forms a temperature field having a locally high temperature distribution. The weak gas flow flowing through the gas passages 19 and on the upper surfaces of the thin film heat sensitive elements 123 to 126 causes a change in the resistance value of the thin film heat sensitive elements 123 to 126, and this temperature change causes an imbalance of the bridge. A weak gas flow can be detected as the pressure difference between both expansion chambers by amplifying the balanced signal.

FIG. 12 shows a flow velocity sensor disclosed in Japanese Patent Publication No. 5-7659. A side sectional view is shown in FIG. 12 (A), and a top view is shown in FIG. 12 (B). In FIG. 12A, 323 and 324 are heat detection sensors and 350 is a heater, which are composed of the heat detection sensor 323 and half of the heater 350 on the thin film members 333 and 334, and the heat detection sensor 324 and the other half of the heater 350, respectively. In addition, an air space 330 is provided below the thin film members 333 and 334. In FIG. 12B, the heater 350 is
It is heated to a constant temperature that increases by 00 ℃. Most of the heat transfer from the heater 350 is through the ambient air, including the air space 330, and in the absence of air flow, the heat sensitive sensors 323,324 heat to an average temperature of about 140 ° C. Figure 12
As shown in (B), since the heat detection sensors 323 and 324 are arranged symmetrically with respect to the heater 350, when the air flow rate is 0, the temperatures of these two sensors are the same, There is no difference in resistance between the two sensors.

When there is an air flow, for example, the heat detection sensor 323 located upstream of the air flow is cooled because heat is carried away by the air flow toward the heater 350, and the heat detection sensor located downstream. 324 is heated by the air flow from heater 350. The resulting difference in resistance between the heat sensitive sensors 323 and 324 results in a difference in voltage and the flow velocity is measured.

[0011]

In the conventional infrared gas analyzer, it is difficult to obtain sufficient sensitivity when measuring a low concentration gas, for example. Therefore, when measuring a low-concentration gas, the sensitivity of the detector has been obtained by elongating the measuring cell length or increasing the radiation intensity of the infrared light source. However, in the case of such a method, in the former case, the cell length becomes long and the configuration of the optical system becomes large. Further, in the latter case, since the light source temperature of the infrared light source becomes high, it is necessary to take measures against heat by a heat dissipation mechanism or the like, and in any case, the structure of the infrared gas analyzer becomes large and the cost becomes high.

The present invention has been made in view of the above points, and an object thereof is to solve the above-mentioned problems and improve the detection sensitivity of a mass flow sensor arranged at a communication port of a detector. It is an object of the present invention to provide an inexpensive infrared gas analyzer detector that enables stable measurement even when measuring a low-concentration gas using an infrared gas analyzer using a sensor.

[0013]

In order to achieve the above object, in the invention according to claim 1 of the present application, at least one pair of heat-sensitive elements whose long sides are opposed to each other, and a heating element, The heat-sensitive element and the heating element form one temperature field, the pair of heat-sensitive elements are arranged at a target position where the temperature field is thermally in a windless state, and the heating element is a first plane including the pair of heat-sensitive elements. Second distance from the air through the air layer
Shall be arranged parallel to the long side of the heat-sensitive element paired on the plane.

In the invention according to claim 2, the heat sensitive element is provided on the thin film insulating layer formed on the first semiconductor substrate whose back surface is etched, and the heat sensitive element is a conductive metal whose resistance value largely changes with temperature. Or a conductive ceramic, the heating element is spatially separated from the thin-film insulating layer, and the gas flow in the gas passage is orthogonal to the long side of the pair of heat-sensitive elements. Shall flow between and.

In the invention according to claim 3 of the present application, the heating element is formed on the second thin film insulating layer formed on the second semiconductor substrate whose back surface is etched. Further, in the invention according to claim 4 of the present application, a zigzag band-shaped element made of either a conductive metal or a conductive ceramic having a large change in resistance value with temperature and formed by alternately removing rectangular thin plates with thin wires. A pair of heat-sensitive elements that are opposed to each other through a gap, and a heating element that is interposed in the gap of the pair of heat-sensitive elements. It is assumed that the heat-sensitive elements forming a temperature field and forming a pair are arranged at a target position where the temperature field is thermally in a windless state.

In the invention according to claim 5, the heating element having the same shape as the heat sensitive element is arranged in the same direction as the heat sensitive element. Further, in the invention according to claim 6 of the present application, the heating element is a linear heating wire, and is arranged in a direction orthogonal to the bending direction of the zigzag band thermal element. Further, in the invention according to claim 7 of the present application, an infrared light source, a measuring cell for introducing a measurement gas, receiving an infrared light flux from the light source, and performing infrared absorption while the infrared light flux is passing through the measurement gas, and two tanks. A detector for receiving at least one tank an infrared light flux that has passed through the measurement cell, and the two tanks have an infrared incident window and a measurement target gas enclosed in the tank and contained in the measurement gas. In an infrared gas analyzer that has a gas that absorbs infrared rays in the same infrared absorption wavelength band, and that detects the difference in the amount of infrared light entering each tank as a pressure difference caused by infrared absorption by the enclosed gas, 7. A gas flow communicating between tanks, and a gas flow generated in the gas path is detected by a pressure difference between two tanks arranged in the gas path and generated by infrared absorption. It shall comprise a mass flow sensor according to claim.

Further, in the invention according to claim 8 of the present application, it is assumed that the two tanks for measuring the difference in the amount of infrared light are arranged vertically to the traveling direction of the infrared light. In the invention according to claim 9 of the present application, the two tanks for measuring the difference in the amount of infrared light are arranged in series with respect to the traveling direction of the infrared light. In the invention according to claim 10 of the present application, the current flowing through the heat-sensitive element is controlled based on the output of the temperature sensor provided in the infrared gas analyzer.

According to the eleventh aspect of the present invention, the current flowing through the heat sensitive element is controlled based on the output of the pressure sensor that measures the pressure in the measuring cell of the infrared gas analyzer.

[0019]

With the above structure, at least one of the long sides of the elements for efficiently exchanging heat with the surrounding gas is arranged to face each other.
A pair of heat sensitive elements are arranged on the thin film insulating layer at appropriate intervals,
Further, the heating elements are arranged in parallel with these heat-sensitive elements at positions that are spatially separated and at equal intervals, and a temperature field is formed between the heat-sensitive elements and the heating elements. Appropriate current flows through the heat-sensitive element and the heating element, the gas around the heat-sensitive element and the heating element is heated and the temperature rises, and a temperature field is formed around the heating element so as to have a sharp temperature gradient. . When there is no ambient gas flow in the temperature field formed by the heat-sensitive element and the heating element, the temperature effect on the pair of heat-sensitive elements is the same, and the resistance value of the pair of heat-sensitive elements having large resistance temperature change characteristics. Are equal, and the detection voltage obtained from the bridge circuit is zero. On the other hand, when a weak gas flow that moves the peripheral gas in a direction orthogonal to the long sides of the pair of heat-sensitive elements occurs, the temperature field formed by the heat-sensitive element and the heating element due to this gas flow causes They move in the flow direction and have different temperature effects on the pair of heat-sensitive elements. That is, a difference occurs in heat transfer resistance from the temperature field to the heat-sensitive element, a temperature difference occurs between the heat-sensitive elements, and a weak gas flow can be detected.

Further, by forming the heat sensitive element and the heating element on the thin film insulating layer in which the back surface of the semiconductor substrate is etched, the heat capacity can be reduced, and the fluctuation of the temperature field due to the weak gas flow can be highly sensitive. It is possible to provide a stable mass flow sensor which can be detected and which can be mass-produced by utilizing the semiconductor manufacturing technology, which can easily make the distance between the heat sensitive element and the heating element constant and which has little variation.

Further, a pair of zigzag band-shaped heat-sensitive elements are spatially separated and face-to-face arranged, and a heating element is interposed in the void portion forming the pair so that an appropriate current is applied to the heat-sensitive element and the heating element. , At a high temperature in the void surrounded by the heat sensitive elements, a temperature field of interest is formed in both heat sensitive elements centering on the heating element. When there is no flow of the peripheral gas in the direction orthogonal to the surface of the heat sensitive element, the temperature effect on the pair of heat sensitive elements is the same, and the detection voltage obtained from the bridge circuit is zero. On the other hand, when a weak gas flow that moves the peripheral gas in a direction orthogonal to the surface of the heat-sensitive element is generated, this temperature field moves in the gas flow direction due to the gas flow, and the temperature applied to the pair of heat-sensitive elements is increased. The impact is different. That is, there is a difference in heat transfer resistance from the temperature field to the heat-sensitive element, which causes a temperature difference between the heat-sensitive elements. As the heating element, an element having the same shape as the heat sensitive element is used.
By arranging in the same direction in the middle of the individual heat sensitive elements,
A target high-temperature field can be efficiently formed in the gap between the heat-sensitive elements, and a weak gas flow does not obstruct the gas passage, effectively flows through the mass flow sensor, and improves detection sensitivity. Can be achieved.

Further, it has an infrared light source, a measurement cell, and two tanks in which a gas absorbing infrared rays in the same infrared absorption wavelength band as the gas to be measured is enclosed, and at least one of the infrared light flux transmitted through the measurement cell is provided. In the infrared gas analyzer, which has a detector for receiving light in each of the tanks, and detects the difference in the amount of infrared light entering each tank as a pressure difference caused by infrared absorption by the enclosed gas, a gas that communicates between the two tanks. By disposing the high-sensitivity mass flow sensor in the passage, it is possible to detect with high sensitivity the gas flow generated in the gas passage due to the weak pressure difference between the two tanks caused by infrared absorption.

Further, by arranging the two tanks in parallel with the traveling direction of infrared rays, it can be applied to a double beam infrared gas analyzer. Further, by arranging two tanks in series with respect to the traveling direction of infrared rays, it can be applied to a single-beam infrared gas analyzer. Further, by controlling the current flowing through the heating element based on the output of the temperature sensor provided in the infrared gas analyzer, the temperature dependence of the sensitivity of the heat sensitive element can be compensated.

Further, by controlling the current flowing through the heating element based on the output of the pressure sensor for measuring the pressure in the measurement cell of the infrared gas analyzer, the pressure fluctuation (density fluctuation) in the measurement cell causes It is possible to compensate the fluctuation of the infrared absorption amount of.

[0025]

1 is a block diagram of a mass flow sensor 120A formed on a semiconductor substrate according to an embodiment of the present invention, FIG. 2 is a block diagram of a mass flow sensor 120B according to a second embodiment, and FIG. FIG. 4 is a configuration diagram of the mass flow sensor 120C of the embodiment, FIG. 4 is a temperature distribution diagram for explaining the operation of the mass flow sensor 120C, FIGS. 5 and 6 are configuration diagrams of the mass flow sensors 22 and 22A of other embodiments, and FIG. Fig. 8 is a configuration diagram of the main part of the application to a beam infrared gas analyzer, Fig. 8 is a configuration diagram of the main part of an application to a single beam infrared gas analyzer, and Fig. 9 is a diagram when the compensation by a pressure sensor is applied to the double beam infrared gas analyzer. FIG. 14 is a schematic configuration diagram of the same, and the same members corresponding to those in FIGS. 10, 11, and 13 are denoted by the same reference numerals.

FIG. 1 is an improvement of the conventional mass flow sensor 120 shown in FIG. 10. The main differences from FIG. 10 will be mainly described below. 1 (A) is shown in FIG. 10 (B)
1 is a top view of the mass flow sensor 120A corresponding to FIG.
10B shows a front view of the mass flow sensor 120A corresponding to FIG. 10C, and FIG. 1C shows a side view. In FIG. 1, a mass flow sensor 120A is formed on a thin film insulator in which the back surface of a semiconductor substrate 122 is etched, and has two pairs of heat sensitive elements (123,124), (125,126) with their long sides facing each other.
And these heat-sensitive elements arranged opposite to each other (123,124), (125,126)
The heating element 150 is disposed at an intermediate position of the above and is spatially separated from the semiconductor substrate 122 in parallel with the long side of the heat-sensitive element facing the semiconductor element.

The weak gas flow flows in a direction orthogonal to the heating element 150, and the high temperature temperature field composed of the heating element 150 and two pairs of heat-sensitive elements is moved by the weak gas flow, and the upstream side and The temperature difference is generated between the heat-sensitive elements on the downstream side, and this weak gas flow is detected. Now, assume that the gas flow is flowing from left to right. Thermal element (1
23,125), the temperature drops due to this weak gas flow,
The temperature of the heat sensitive elements (124, 126) rises due to the gas flow heated by the heating element 150. This temperature difference is detected by the Wheatstone bridge, and a weak gas flow can be detected. In the Wheatstone bridge circuit, one terminal of the heat sensitive element (123,124) is the + side terminal of the power source 30, and one terminal of the heat sensitive element (125,126) is the − side terminal of the power source 30, and the heat sensitive element (123,125) and Thermal elements (124,126) are connected across the Wheatstone bridge circuit. This connection is the same as in FIG. In FIG. 1, the heating element 15
0 and two pairs of heat sensitive elements (123,124), (125,126), the space area is composed of heating element 150 and two pairs of heat sensitive elements (123,124),
(125,126) is configured as a high-temperature temperature field having a sharp temperature gradient by heating with (125,126), the heat-sensitive element (123,124) of FIG.
By heating only (125,126) and (123,12)
4) The heat capacity is further increased as compared with the high temperature field formed around (125,126) and the amount of temperature change given to the detection element by the same weak gas flow increases, which helps increase the detection sensitivity.

FIG. 2 shows a mass flow sensor 12 of the second embodiment.
0B, the difference from Fig. 1 is a pair of thermal elements (123,124)
A mass flow sensor 120B is configured by the heating element 150 and the heating element 150, and a bridge circuit is configured by adding resistors 127 and 128 to the outside. Heating element 150 and a pair of thermal elements
By heating with (123,124), it is configured as a high temperature field having a steep temperature gradient, and the amount of temperature change given to the detection element by the weak gas flow increases, which helps increase the detection sensitivity.

FIG. 3 is a structural diagram in which a heating element as a third embodiment is assembled to the mass flow sensor 120C.
(A) shows a bottom view of the heating element, and (B) of FIG. 3 shows that of FIG.
Shows the front view of the mass flow sensor 120C corresponding to (B),
FIG. 3C shows a side view. FIG. 3 shows that, in place of the heating element 150 of the embodiment of FIGS. 1 and 2, the back surface of the second semiconductor substrate 151 is etched and removed in the gas flow direction on the thin film insulating layer, and the direction orthogonal to the etched and removed direction is shown. A heating element 150A is formed on. The heating element 150A is then connected to the spacing piece 152.
Through the heat sensitive elements (123,124), (125,1) of the semiconductor substrate 122.
26) The gas passage for the gas flow is formed so as to face the surface on which the gas flows. The heating element 150A includes a pair of heat-sensitive elements (123,124), (12
It should be placed parallel to the long side of 5,126) and at the target position.

Since the heating element 150A is formed on the thin film insulating layer in which the back surface of the semiconductor substrate 151 is etched, a high temperature state can be generated with low power and the semiconductor manufacturing technology is utilized to achieve high mass productivity. Further, the distance between the detection element and the heating element can be easily made constant, and a stable mass flow sensor 120C with little variation can be provided. 4 is shown in FIG.
FIG. 11 is a temperature distribution diagram for explaining the operation of the mass flow sensor 120C, which is an arrangement corresponding to the front view of FIG. The heat sensitive elements 123, 124 are arranged at intervals X on the thin film insulating layer in which the back surface of the first semiconductor substrate 122 is etched.
4 shows a state in which a heating element in which the back surface of the second semiconductor substrate 151 is etched is arranged at a target position so as to face 4 and a gas flow passage is formed. The sensing elements 123, 124 and the heating element 150A are heated by an appropriate electric current to form a temperature map distribution as shown in the gas passage. At zero gas flow, this temperature map distribution is of interest, and the weak gas flow causes the temperature map distribution in this gas passage to move efficiently. Further, the influence of the temperature distribution in the passage on the heat-sensitive elements 123, 124 is considered to be the volumetric transfer of the heat transfer due to the radiant heat from the temperature at each point in the entire space. There is a change in temperature, that is, the detection sensitivity increases.

In the absence of the heating element shown in FIG. 10, when the temperature of the nearby gas is raised by the self-heating of the heat-sensitive element, the higher the gas temperature is, the more the temperature-sensitive element is affected by the non-linearity of the temperature coefficient of resistance of the heat-sensitive element. The instability of the zero point increases due to the slight imbalance of the resistance value of. By using the heating element 150 or 150A, the gas temperature can be set to a predetermined high temperature temperature distribution without extremely raising the temperature of the heat sensitive element, and a stable mass flow sensor 120A to 120C with high sensitivity and little variation is provided. You can

Consider a case where the heat sensitive elements 323 and 324 and the heating element 350 are formed on the same thin film insulating layer as shown in FIG. A heating element 35 is provided in the middle of the thermal element 323,324.
When there is 0, the temperature effect on each heat sensitive element 323,324 is
There are indirect radiant heat due to the temperature distribution from the gas passage and direct radiant heat from the heating element 350 itself to the heat-sensitive elements 323,324, and the hottest heating element 350 and heat-sensitive element 323,324 are present.
Since it is the closest distance to, rather than indirect radiant heat,
Direct radiant heat is stronger, and the amount of heat obtained by the heat-sensitive elements 323 and 324 by indirect thermal coupling is coupled to the heating element 350 by direct thermal coupling impedance and acts in the direction of decreasing the detection sensitivity. According to the present invention shown in FIGS. 1 to 3, since the direct thermal coupling with the heating element 350 is separated via the air gap, the direct thermal coupling impedance can be increased and the detection sensitivity can be prevented from lowering. it can. Further, by separating the heating element 350 from the heat-sensitive elements 323 and 324 via the air gap, the interval between the heat-sensitive elements 323 and 324 can be selected to be an optimum value that gives the highest detection sensitivity.

FIG. 5 shows the prior art mass flow sensor of FIG.
21 is a modification of the heat sensitive elements 23 and 24, and a heating element 150B having the same shape as the heat sensitive element is inserted between the heat sensitive elements 23 and 24.
The heating element 150B and the heating element 150B form a high temperature field, the gas body heated to a high temperature by a weak gas flow efficiently moves, and a temperature difference is effectively generated between the heat sensitive elements 23 and 24. Gas flow is detected. In FIG. 6, the heating element 150B of FIG. 5 is used as a rod-shaped heating element 150C and is arranged in a direction perpendicular to the zigzag grid. Since the high temperature temperature field composed of the heat sensitive elements 23 and 24 and the heating element 150C is around the rod-shaped heating element,
The detection sensitivity is slightly inferior, but the structure is simple.

Next, the mass flow sensor (22, 22A,
An example in which (120A to 120C) is used in an infrared gas analyzer will be described. Since the mass flow sensors (22, 22A, 120A to 120C) in the infrared gas analyzer have the same function, the mass flow sensor 22 will be described as a representative example. FIG. 7 shows an embodiment in which the mass flow sensor 22 is used in a double beam infrared gas analyzer. In FIG. 7, reference numeral 3 denotes an infrared light source, and an infrared light flux emitted from the light source 3 becomes intermittent light by the rotary chopper 2 and is distributed by a light distribution cell 4 in two directions. One infrared ray bundle is guided as a measuring ray 11 to the measuring cell 13, and the other infrared ray bundle is a reference ray.
It is led to the reference cell 14 as 12. The measurement cell 13 is provided with light transmission windows 5 and 6, and the measurement gas containing the component gas to be analyzed is introduced and discharged through the introduction pipes 17 and 18. The measuring light beam 11 receives infrared rays in the measuring cell 13 according to the concentration of the analyte gas in the measuring gas flowing in the measuring cell 13.

Similarly, the reference cell 14 has a light transmission window 7,
8 is provided, and a gas that does not absorb infrared rays is enclosed. Measuring light beam 11 transmitted through the measuring cell 13,
The reference light beam 12 transmitted through the reference cell 14 and the reference cell 14 are guided to the detector 20, respectively. This detector 20 includes a first expansion chamber 15 and a second expansion chamber 15.
And an expansion chamber 16, each expansion chamber 15, 16 is a light transmission window 9,
10 is provided and is filled with the same type of gas as the component gas to be analyzed. The measurement light beam 11 enters the first expansion chamber 15 through the light transmission window 9, and the reference light beam 12 enters the second expansion chamber 16 through the light transmission window 10.

In the detector 20, the first expansion chamber 15 and the second expansion chamber
16 and a gas passage 19 that connects the expansion chambers 15 and 16 to each other. The mass flow sensor of the present invention is provided in the portion A of the gas passage 19.
22 will be deployed. In the illustrated example, the mass flow sensor 22 of FIG.
Has a heating element 15 of the same shape in the middle of the thermal element (23, 24).
It is configured with 0B. The measurement light beam 11 and the reference light beam 12 that have entered the expansion chambers 15 and 16 cause a pressure fluctuation difference in the expansion chambers 15 and 16 due to the absorption of infrared rays in the expansion chambers 15 and 16, and the measurement is performed in the gas passage 19 based on the pressure fluctuation difference. A gas flow corresponding to the content of the gas to be analyzed in the gas is generated, and this weak gas flow is detected by the mass flow sensor 22.

FIG. 8 shows an embodiment in which the mass flow sensor 22 of the present invention is used in a single beam infrared gas analyzer. In FIG. 8, 3 is an infrared light source, and the infrared light flux emitted from this light source 3 becomes intermittent light by the rotary chopper 2 and enters the measurement cell 13 as a measurement light beam 11. The measurement cell 13 is provided with light transmission windows 5 and 6, and the measurement gas containing the component gas to be analyzed is introduced and discharged through the introduction pipes 17 and 18. The measuring light beam 11 is in this measuring cell 13
Infrared absorption is received according to the concentration of the component gas to be analyzed in the measurement gas that has flowed through. The measuring light beam 11 transmitted through the measuring cell 13 is incident on the detector 20.

In this detector 20, the first expansion chamber 15A and the second expansion chamber 16A are arranged in series with respect to the traveling direction of the measurement light beam 11, and the measurement light beam 11 transmitted through the measurement cell 13 passes through the light transmission window 9. The measurement light beam 11 that has entered the first expansion chamber 15A, which is the front chamber, passes through the first expansion chamber 15 and further enters the second expansion chamber 16A, which is the rear chamber, via the light transmission window 10A. Each expansion chamber 15A, 1
6A is filled with the same type of gas as the component gas to be analyzed. In the first expansion chamber 15A, which is the anterior chamber, most of the infrared rays of the wavelength component with a high infrared absorption coefficient in the wavelength-infrared absorption characteristics of the enclosed gas are absorbed here, and the second expansion chamber 16A absorbs the infrared rays as described above. The remaining infrared rays from which the wavelength components having a high coefficient have been removed are incident, and the infrared rays of the wavelength components having a medium infrared absorption coefficient corresponding to the concentration of the gas to be analyzed are absorbed. The mass flow sensor 22 of the present invention is provided in the portion A of the gas passage 19 that connects the expansion chambers 15A, 16A with the difference in the amount of infrared absorption light absorbed by the expansion chambers 15, 16. In the illustrated example, the mass flow sensor 22 of FIG. 5 is provided with a heating element 150B having the same shape in the middle portion of the heat sensitive elements (23, 24). In the embodiment of FIG. 7, the gas passage 19
8 communicates in the left-right direction, and the gas flow also flows in the left-right direction, but in the embodiment of FIG. 8, the gas passage 19 communicates in the up-down direction, and the gas flow also flows in the up-down direction.

In FIG. 7, the temperature sensor 200 provided in the detector 20 measures the internal temperature of the detector 20, and the heating element 15
By maintaining and controlling the heating operating temperature of 0 (A, B, C) to the optimum temperature by the voltage controller 210, it is possible to compensate for the temperature dependence of sensor sensitivity (generally, the higher the temperature, the higher the gain). A stable infrared gas analyzer can be obtained. Further, the temperature sensor 200 is not limited to the inside of the detector 20, and if the temperature near the detector 20 can be measured,
It may be located anywhere inside the infrared gas analyzer.

Further, while FIG. 7 shows an example in which the detection characteristic is corrected with respect to the internal temperature change, in FIG. 9, a pressure sensor 220 is provided in the measuring cell 13 to measure the pressure in the measuring cell 13. By doing so, an example of sensitivity compensation by the atmospheric pressure fluctuation of the infrared gas analyzer will be shown. In this type of infrared gas analyzer, one end of the measurement cell is open to the atmosphere, so in the prior art, the density of the gas to be measured fluctuated due to atmospheric pressure fluctuations, which caused an error in the sensor output. The pressure in the cell 13 is measured, and the heating element 150 (A, B,
Sensitivity compensation can be performed by controlling the heating operation temperature of C) to the optimum temperature by the voltage controller 210. For example, when the detection output of the detection unit 20 decreases due to the pressure drop in the measurement cell (the density of the measured gas decreases), the output of the voltage controller 210 is increased to increase the heating operating temperature of the heating element 150 (A, B). Is increased to increase the detection output of the detection unit 20 and compensate the fluctuation of the measurement value depending on the pressure change in the measurement cell, whereby a stable infrared gas analyzer can be obtained.

The above-mentioned double beam infrared gas analyzer,
In the embodiment of the single-beam infrared gas analyzer (FIGS. 7, 8 and 9), the mass flow sensor 22 was described in the embodiment of FIG. 5, but the thin-film thermal detectors 123 to 126 are integrated in FIG. The mass flow sensor 120 of the embodiment of FIGS.
A, 120B, 120C may be used.

[0042]

As described above, according to the structure of the present invention, in order to efficiently exchange heat with the surrounding gas, the mass flow sensor is composed of at least one pair of heat sensitive element and heating element, The thermosensitive element and the heating element form one temperature field, and the paired thermosensitive elements are arranged such that the temperature field is thermally arranged at a target position in a windless state, and the heating element includes a paired thermosensitive element. Direct radiation from the heating element to the heat-sensitive element is reduced by arranging the heat-sensitive element in parallel with the long side of the pair of heat-sensitive elements on the second plane separated by a predetermined distance from the plane. In addition, by forming a temperature field with a steep temperature gradient between the heat-sensitive element and the heating element, the influence of the temperature distribution due to indirect radiation heat from the gas passage can be detected with high sensitivity, and the gas flow can be detected with high sensitivity. can do. Due to the weak gas flow flowing in this temperature field, according to the experimental data, the detection sensitivity is increased from several times to more than ten times as compared with the case without the heating element, and the zero point when the gas flow is zero. It is possible to provide a stable and highly sensitive mass flow sensor with increased stability. As a result, by applying this mass flow sensor to an infrared gas analyzer, stable measurement can be performed even in the measurement of low-concentration gas, and a compact and inexpensive infrared gas analyzer can be provided.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a mass flow sensor formed on a semiconductor substrate according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a mass flow sensor according to a second embodiment.

FIG. 3 is a configuration diagram of a mass flow sensor according to a third embodiment.

FIG. 4 is a temperature distribution diagram for explaining the operation of the mass flow sensor.

FIG. 5 is a configuration diagram of a mass flow sensor of another embodiment.

FIG. 6 is a configuration diagram of a mass flow sensor of another embodiment.

FIG. 7 is a schematic diagram of a main part of application to a double beam infrared gas analyzer.

FIG. 8 is a schematic diagram of a main part applied to a single-beam infrared gas analyzer.

FIG. 9 is a schematic configuration diagram when pressure sensor compensation is applied to a double-beam infrared gas analyzer.

FIG. 10 is a block diagram of a conventional mass flow sensor.

FIG. 11 is a block diagram of another conventional mass flow sensor.

FIG. 12 is a configuration diagram of a flow sensor according to a conventional technique.

FIG. 13 is a schematic diagram of a main part of a double-beam infrared gas analyzer according to a conventional technique.

[Explanation of symbols]

 1 Motor 2 Rotating Chopper 3 Infrared Light Source 4 Light Distribution Cell 5-10, 9A Light Transmission Window 11 Measurement Ray 12 Reference Ray 13 Measurement Cell 14 Reference Cell 15, 15A 1st Expansion Chamber 16, 16A 2nd Expansion Chamber 19 Gas Passage 21, 22, 120A to 120C Mass flow sensor 23, 24 Thermal element 27, 28 Fixed resistance 30 Bridge power supply 121 units 122, 151 Semiconductor substrate 123 to 126 Thermal element 150, 150A, 150B, 150C Heating element 200 Temperature sensor 210 Voltage controller 220 Pressure sensor 320 Substrate 323,324 Thermal sensor 328 Insulation layer 350 Heater 330 Air space 333,334 Thin film member

Claims (11)

[Claims] 1. A thermosensitive element comprising at least one pair of thermosensitive elements whose long sides are opposed to each other, and a heating element, wherein the thermosensitive element and the heating element form one temperature field and form a pair of thermosensitive elements. The element is disposed in a position where the temperature field is thermally symmetrical in a windless state, and the heating element is on a second plane separated by a predetermined distance from the first plane including the pair of heat-sensitive elements via the air layer. The mass flow sensor, wherein the mass flow sensor is arranged parallel to the long side of the pair of heat-sensitive elements. 2. The mass flow sensor according to claim 1, wherein the heat-sensitive element is provided on a thin film insulating layer formed on the first semiconductor substrate whose back surface is etched, and the heat-sensitive element has a large change in resistance with temperature. Made of a conductive metal or a conductive ceramic, the heating element is spatially separated from the thin-film insulating layer, and the gas flow in the gas passage is in a direction orthogonal to the long side of the pair of heat-sensitive elements. In addition, the mass flow sensor is characterized in that it flows between the heat sensitive element and the heating element. 3. The mass flow sensor according to claim 1, wherein the heating element is formed on a second thin film insulating layer formed on a second semiconductor substrate whose back surface is etched. A mass flow sensor characterized in that 4. A zigzag band-shaped element, which is made of a conductive metal or a conductive ceramic having a large change in resistance value with temperature and is formed by alternately removing thin rectangular plates with thin wires, through a gap. A pair of heat-sensitive elements facing each other, and a heating element interposed in a space of the pair of heat-sensitive elements, wherein the pair of heat-sensitive elements and the heating element form one temperature field. The mass flow sensor, wherein the heat sensitive elements forming the pair and the temperature field are thermally arranged at a target position in a windless state. 5. The mass flow sensor according to claim 4, wherein a heating element having the same shape as the heat sensitive element is arranged in the same direction as the heat sensitive element. 6. The mass flow sensor according to claim 4, wherein the heating element is composed of a linear heating wire, and is arranged in a direction orthogonal to the bending direction of the zigzag band thermosensitive element. Sensor. 7. An infrared light source and a measurement gas are introduced to receive an infrared light flux from the light source,
The infrared light flux includes a measurement cell that absorbs infrared light while passing through the measurement gas, and a detector that has two tanks and receives the infrared light flux that has passed through the measurement cell into at least one tank. The two tanks each have an infrared incident window and a gas that is enclosed in the tank and that absorbs infrared rays in the same infrared absorption wavelength band as the measurement target gas contained in the measurement gas. In an infrared gas analyzer for detecting a difference in the amount of infrared light incident on a pressure difference caused by infrared absorption by the enclosed gas, a gas passage communicating between the two tanks, and the infrared passage arranged in the gas passage. The mass flow sensor according to any one of claims 1 to 6, which detects a gas flow generated in the gas passage due to a pressure difference between two tanks caused by absorption. Infrared gas analyzer which is characterized. 8. The infrared gas analyzer according to claim 7, wherein the two tanks for measuring the difference in the amount of infrared light are arranged perpendicularly to the traveling direction of the infrared light. Gas analyzer. 9. The infrared gas analyzer according to claim 7, wherein the two tanks for measuring the difference in the infrared light amount are arranged in series with respect to the traveling direction of the infrared light. Total. 10. The infrared gas analyzer according to any one of claims 7 to 9, wherein the current flowing through the heat sensitive element is controlled based on the output of a temperature sensor provided in the infrared gas analyzer. An infrared gas analyzer characterized by the following. 11. The infrared gas analyzer according to claim 7, wherein the current flowing through the heat sensitive element is the output of a pressure sensor for measuring the pressure in the measuring cell of the infrared gas analyzer. An infrared gas analyzer characterized by being controlled based on