JP2947381B2 - Carbon dioxide sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to carbon dioxide having a novel structure for detecting carbon dioxide gas in the atmosphere in various fields such as combustion control, plant growth management, fermentation management, environmental control and medical diagnosis. Related to sensors. More specifically, a carbon dioxide sensor that coexists with carbon dioxide and has a significantly reduced error in measured values due to an organic gas that interferes with its detection and exhibits stable performance for a long period of time.

[0002]

2. Description of the Related Art Conventionally, measurement of carbon dioxide has been performed in an atmosphere in various fields and used for various controls and managements. Infrared absorption and glass electrode systems have been mainly used for measuring carbon dioxide.However, it has been pointed out that the devices based on these systems are large and require maintenance even in normal measurement. Have been.

On the other hand, for measuring carbon dioxide, a method of measuring a change in electromotive force using a carbon dioxide sensor element using a solid electrolyte, and a method using a carbon dioxide sensor element using a composite oxide are used. Various small, simple and inexpensive carbon dioxide sensors have been developed, such as those that measure the change in electrostatic capacity and those that measure the change in heat of adsorption using a carbon dioxide sensor element using zeolite. I have.

[0004] These sensors are provided with heating means for heating the carbon dioxide sensor element to an operating temperature, and are used by heating the sensor element to a high temperature of about 300 to 700 ° C.

However, the above-mentioned carbon dioxide sensor generally has sensitivity to an organic gas in addition to the carbon dioxide of the gas to be measured in many cases, and when the organic gas coexists with the carbon dioxide, it is difficult to detect the concentration of the carbon dioxide. However, there is a serious problem that accurate detection of carbon dioxide cannot be performed due to the influence of the organic gas.

[0006] Conventionally, it has been known that a gas sensor is provided with a filter as a means for removing the organic gas when the organic gas coexists with the inorganic gas to be detected. For example, Japanese Patent Application Laid-Open No. 1-227951 proposes a carbon monoxide sensor in which the effect of organic gas is prevented by directly sintering zeolite on a detection material of a semiconductor type carbon monoxide sensor element. Such documents state that zeolites act as catalysts for burning organic gases at high temperatures. Means for adsorbing organic gas by providing a filter made of activated carbon has also been proposed.

[0007]

However, when the above-mentioned means for directly sintering zeolite on the surface of the detection material of the sensor element is applied to a carbon dioxide sensor, the carbon dioxide sensor generally has a high temperature of 300 to 700 ° C. If this technology is directly applied, the carbon dioxide sensor has sensitivity to carbon dioxide generated by the combustion of organic gas, so when performing accurate measurement of carbon dioxide in the atmosphere, There is a problem that the organic gas interferes. In addition, when a filter made of activated carbon is applied to a carbon dioxide sensor, the activated carbon filter has the effect of removing organic gas, but also partially adsorbs carbon dioxide together with organic gas, making the response of the sensor extremely slow. There was a big problem that it would. In addition, since activated carbon has a limit in its ability to adsorb organic gas, there has been a problem that the effect of removing organic gas is lost in a relatively short period of time.

Accordingly, a carbon dioxide sensor capable of measuring the carbon dioxide concentration by suppressing the influence of the organic gas stably even during long-term use without significantly lowering the accuracy and responsiveness of the carbon dioxide sensor has been developed. Was desired.

[0009]

The inventor of the present invention has conducted various studies to develop an organic gas filter having such characteristics. As a result, a filter made of zeolite is provided at a distance from a carbon dioxide sensor element, and the sensor element is provided. When the gas to be detected contains an organic gas, the influence of the gas can be effectively suppressed by treating the gas that comes into contact with the gas, and the accuracy and responsiveness of the sensor can be prevented from being reduced. The present inventors have found that a carbon dioxide sensor capable of stably measuring the carbon dioxide concentration even after long-term use has been obtained, and have completed the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited to these accompanying drawings.

FIG. 1 is a perspective view showing a typical embodiment of the carbon dioxide sensor of the present invention, showing a partially broken cross section. FIGS. 2 and 3 are cross-sectional views showing a typical mode of the carbon dioxide sensor element used in the present invention.

In the carbon dioxide sensor of the present invention, a carbon dioxide sensor element 2 provided with a heating means is provided in a casing 4 having an opening 5 for gas intake in a part thereof. A filter 3 made of zeolite is provided between the carbon dioxide sensor element and the opening or the gas intake opening and the carbon dioxide sensor element.

In the present invention, the casing 4 is not particularly limited as long as it has a gas intake opening 5 in part and the other part is sealed. Generally, it is preferable that a carbon dioxide sensor element can be arranged at the bottom and an opening is formed at the top. For example, in the embodiment shown in FIG. 1, the casing 4 includes a carbon dioxide sensor element 2
Is sealed by a bottom plate (sensor base) on which is mounted, and a lid having an opening 5 is provided at the upper part of the cylindrical body. Of course, in addition to the embodiment in which the opening is provided at the top of the casing as shown in FIG. 1, an embodiment in which the opening is provided on the side surface or the bottom of the casing or an embodiment in which two or more casings are provided according to the use environment may be implemented. it can. In addition, as long as the material of the casing 4 can withstand the operating temperature, a known material is used without any particular limitation. Furthermore, the shape and size of the opening provided in the casing are as follows:
What is necessary is just to determine suitably according to the shape and size of a casing.

[0014] The present invention is directed to a carbon dioxide sensor using a carbon dioxide sensor element having a heating means for heating the sensor element to an operating temperature.

Examples of such a carbon dioxide sensor element include those that measure a change in electromotive force using a solid electrolyte (solid electrolyte type), those that measure a change in capacitance using a composite oxide (capacitance type), A known method and a structure having a heating means such as a method of measuring a change in heat of adsorption using zeolite (heat of adsorption) or the like may be employed without any particular limitation.

Hereinafter, the solid electrolyte type carbon dioxide sensor element will be described. Although a solid electrolyte type carbon dioxide sensor element has already been reported in Japanese Patent Application Laid-Open No. 60-256043, a typical structure thereof is shown in FIG. The solid electrolyte 10 includes Na _{1 + X} Zr ₂ Si _X P _3−X O ₁₂ (where x is 0) having ion conductivity of sodium, lithium, or the like.
３3, generally called NASICON), β-Al
(In where x is 0 ≦ x <8, generally referred to as _{LISICON) 2 O 3, β-} Ga 2 O 3, Li 16 _{over 2X Zn X (GeO 4) 4} , Li 4 GeO 4 -Li 3 VO 4 or the like It can be used without limitation, but NASICON, which has high ionic conductivity,
β-Al ₂ O ₃ and LISICON are particularly preferred. Electrodes 9 and 11 are present on both surfaces of the solid electrolyte 10. The electrodes 9 and 11 are made of the same or different conductors. Examples of preferred materials are platinum, gold, silver, palladium,
Noble metals such as rhodium and their oxides, general formula L
a _1-x Sr _x BO ₃ (where B represents an element such as Co, Cu, Fe, Ni, etc., and X is a number of 0.01 to 0.5); Examples thereof include a composite composition in which a metal oxide is mixed. The method for manufacturing the electrodes 9 and 11 is not particularly limited, and generally, a method such as a screen printing method, a sputtering method, an ion plating method, or a vapor deposition method is suitably employed. A metal carbonate layer 12 is formed on the electrode 11, and this metal carbonate can be used without limitation as long as it has a dissociation equilibrium with carbon dioxide gas in the atmosphere. In particular, alkali metal such as Na, K, and Li can be used. And alkaline earth metal carbonates such as Ca, Mg, Ba and the like are used singly or in a mixture at an optional ratio. There is no particular limitation on the method of forming the metal carbonate layer, but a method of impregnating the electrode 11 with an aqueous solution or slurry of the metal carbonate, a method of forming a paste with terpineol or the like, followed by melting after printing, or a method of forming the electrode and the metal carbonate in advance. Is preferably used in a method of mixing, forming a paste, printing and baking.

The above-mentioned sensor element is generally 300 to 700
Used by heating to ° C. Such a heating means may be provided by providing a heat source at a position distant from the gas sensor and radiating the heat. Alternatively, as shown in FIG.
Are laminated, and a Pt paste, for example, is screen-printed in a wave shape to form a heater 6 baked at 1000 ° C., and the heater is connected to a power supply 14 via a conductor 13 and a DC voltage is applied. May be used for direct heating. The mounting position of the heater to the gas sensor may be provided at a position other than the above as long as the position does not hinder the operation of the gas sensor. In FIG. 2, 8 is a sealing material such as glass, and 15 is a voltmeter.

FIG. 3 shows a typical embodiment of a capacitance type carbon dioxide sensor element provided with a heater. Such a sensor element has a structure in which electrodes 21 are laminated on both surfaces of a gas-sensitive body 24, and an alumina substrate 19 is laminated on one of the electrodes in the same manner as in the solid electrolyte type carbon dioxide sensor element. A heater 18 is formed, the heater is connected to a power supply 16 by a conducting wire 17, and direct heating is performed by applying a DC voltage. As a material of the gas-sensitive body, a mixture of CuO and BaTiO ₃ ,
Mixture of PbO and BaTiO ₃ , CuO and BaSnO ₃
, A mixture of Cu and SrTiO ₃ , etc.
It can be obtained by sintering. In addition, as a material of the electrode, a conductive substance such as Ag, Pt, and Pd is generally used. Each electrode is connected to an ammeter 23 by a lead 21 such as Pt, and the capacitance is measured.

In the present invention, the filter 3 made of zeolite has a gap between the gas intake opening of the casing 4 or the gas intake opening and the carbon dioxide sensor element incorporated in the casing. Provided. By providing the filter 3 at an interval from the carbon dioxide sensor element, the phenomenon that the filter is excessively heated by the heating means of the carbon dioxide sensor element can be prevented, and the organic gas is changed to carbon dioxide by the filter, and the measurement is performed. In addition to solving the problem of adversely affecting the water, under such conditions, a surprising effect that the organic gas can be removed to a very high degree is also exhibited.

The filters 3 are preferably provided at intervals so as not to be heated to a temperature exceeding 150 ° C. by the heating means of the carbon dioxide sensor element 2 under use conditions. Generally, such spacing is between 0.5 and 30 mm.

The filter 3 separates the carbon dioxide sensor element from the outside air of the casing. The filter 3 is provided with an opening for taking in gas within a range having an interval from the carbon dioxide sensor element. A mode of providing so as to cover, a mode of providing so as to partition a space between the gas intake opening and the carbon dioxide sensor element, and the like are adopted.

In the present invention, the method for forming the filter 3 is not particularly limited. To illustrate the preferred method, FIG.
As shown in (1), there is a mode in which the holding space formed by the two mesh plates 1 is filled with zeolite, and a mode in which zeolite is directly formed by a zeolite sintered body.
Considering that it is desirable to use a zeolite having a large specific surface area as described later, an embodiment in which the zeolite is filled in the holding space is preferable.

When the thickness of the filter made of zeolite is too small, the effect of removing the organic gas is not sufficient, and when the thickness is too large, the response tends to decrease. Therefore, it is preferable to configure the thickness to be 0.5 to 15 mm.

As the zeolite used in the present invention, known zeolites can be used without any limitation. Generally, when the specific surface area is small, the effect of removing organic gas is reduced and the responsiveness to carbon dioxide tends to be reduced. Therefore, its specific surface area is 300 m ² / g or more, preferably 5 m ² / g.
00m ² / g or more, more preferably 700 meters ² / g or more zeolites are preferably used. Zeolite is A
A natural or synthetic material such as a mold, an X-form, a Y-form, an L-form, a ZSM-5-form, a USY-form, jabasite, erionite, offretite, mordenite, ferrierite, etc. can be used without limitation. The zeolite can arbitrarily change the Si / Al ratio and the cation, but the zeolite used in the present invention is not limited by the Si / Al ratio and the type of the cation.

Of the above zeolites, the type of cation is Na, NH ₄ or H, and the Si / Al ratio is 2 to 140.
Y-type, mordenite, and USY are preferred.
The type of cation is Na, the Y-type zeolite having a Si / Al ratio of 2 to 5 or the type of cation is Na,
USY zeolite having a / Al ratio of 4 to 8 is preferred.

The above-mentioned zeolite may be used in the form of powder or granulated into pellets or beads. However, considering that a specific surface area of 300 m ² / g or more is suitable as an organic gas filter, zeolite is preferably a fine powder having a particle size of 3 μm or less. Granulation of powder is usually carried out by kneading a binder such as kaolinite, attapulgite, montmorillonite, silica, alumina and a caking additive such as poval and ethylene glycol with water, and pelletizing beads using an extrusion granulator. In the case of the above, it can be performed by using a rolling granulator or the like.

When the zeolite is used in the form of powder, the packing density in the holding space varies depending on the zeolite used, but is generally from 0.01 to 2 g / cm ³ .

In the present invention, as described above, the filter 3 is provided at an interval from the carbon dioxide sensor element so as to be 150 ° C. or less, and may be used as long as it is used within the temperature range. Although it can be used at a temperature, it is generally in a temperature range of 20 ° C to 150 ° C, in particular, 30 ° C to 11 ° C.
It is preferable to use in a temperature range of 0 ° C. That is, as described above, the filter made of zeolite is heated at 150 ° C.
When used at such a high temperature, a part of the combustion reaction of the organic gas occurs, and the resulting carbon dioxide is sensed by the carbon dioxide sensor element, making it difficult to accurately measure the carbon dioxide in the atmosphere. On the other hand, at a temperature of 20 ° C. or lower, the effect of removing the organic gas by the filter decreases, and the filter performance tends to decrease in a relatively short time.

As a method for maintaining the filter of the present invention at a constant temperature, a known method can be applied without any particular limitation. As an example of the most preferable method, a method of adjusting the distance between the carbon dioxide sensor element 2 and the organic gas filter 3 and heating by the residual heat from the sensor element is adopted.
In some cases, the filter may be heated to a certain temperature by an external heating means before use.

Examples of the organic gas that can be removed by the filter include vapors of aromatic compounds such as toluene and benzene, vapors of ester compounds such as ethyl acetate and methyl acrylate, vapors of ketones such as acetone and methyl ethyl ketone, and gasoline. There is a wide variety of things such as steam. It is also effective in removing alcohols such as ethanol and methanol.

Another structure of the carbon dioxide sensor of the present invention is as follows.
There is no particular limitation as long as the above configuration is satisfied.

For example, the electrical connection between the sensor element 2 and the outside is generally made by projecting a conductive pin through the casing 4 and electrically connecting the conductive pin to an internal conductor. .

[0033]

The carbon dioxide sensor of the present invention can accurately detect carbon dioxide while selectively removing an organic gas serving as an interfering gas. Further, it is possible to stably exhibit such an effect over a long period of time. Further, the carbon dioxide sensor of the present invention has excellent characteristics in response, and the effect is remarkable when zeolite having a large specific surface area is used.

[0034]

The mechanism by which the carbon dioxide sensor of the present invention exerts the above effects is not clear, but by providing a filter made of zeolite at an interval from the carbon dioxide sensor element, the combustion of organic gas in the filter is prevented. It is considered that the organic gas can be prevented and the specific adsorption affinity of the zeolite to polar molecules acts to remove the organic gas. In general, since inorganic gas has a smaller molecular diameter than organic gas and zeolite has a specific pore diameter, it is estimated that it may be removed by a molecular sieving function due to a difference in molecular size. Furthermore, since zeolite has a function as a catalyst, it is conceivable that the organic gas is changed to another gas that does not affect the carbon dioxide sensor. Further, it is considered that zeolite having a large specific surface area can secure good responsiveness because it has many spaces through which carbon dioxide can sufficiently pass.

[0035]

EXAMPLES The present invention will be described specifically with reference to the following examples, but the present invention is not limited to these examples.

EXAMPLES 1 TO 12 AND COMPARATIVE EXAMPLES 1 AND 2 A solid electrolyte type carbon dioxide sensor element provided with a heater having a structure as shown in FIG. 2 was produced. As the solid electrolyte 10, a sintered body on a disk having a diameter of about 5 mm and a thickness of about 0.7 mm obtained by molding and sintering NASICON powder was used. The electrodes 9 and 11 on both sides were formed by screen printing a gold paste, drying and baking at 700 ° C. A conductive wire 13 made of platinum is attached to the electrodes 9 and 11. On one electrode, lithium carbonate as a metal carbonate 12 having dissociation equilibrium with carbon dioxide was added to terpineol at 5 wt. It was formed by screen printing, drying, and heat-treating at 650 ° C. after paste-forming with a vehicle in which (% by weight)% ethyl cellulose was dissolved. On the other electrode side, a heater 6 formed of a platinum paste on an alumina substrate 7 by screen printing was joined by a sealing material 8 made of glass. The sensor element was heated to 450 ° C. by applying a DC voltage from the power supply 14 to the heater 6 before use.

The above carbon dioxide sensor measures the change in the carbon dioxide concentration in the atmosphere using the voltmeter 15 and the electrodes 9 and 11.
The value was obtained by subtracting the electromotive force in the measured gas from the electromotive force in the atmosphere, and the sensitivity was shown.

A conductive wire 13 made of platinum was attached to a conductive pin provided at the bottom of the casing 4 of FIG. 1 by spot welding. The filter 3 was configured by filling a 4.0 mm thick filling space defined by the mesh plate 1 with various zeolites shown in Table 1.

The above-mentioned filter was provided at a distance of 30, 60, 100 and 130 ° C. by means of the heater 6 of the carbon dioxide sensor element so as to be spaced from the element.

[0040]

[Table 1]

With respect to each of the carbon dioxide sensors configured as described above, in order to see the influence of the organic gas,
The sensor was used for a carbon dioxide concentration of 2%, and as an organic gas, a toluene concentration of 1000 ppm and an ethyl acetate concentration of 1000 p.
It was installed in a test gas having a pm and ethanol concentration of 1000 ppm, and the sensitivity to each gas was measured.

The response was evaluated by replacing the concentration of carbon dioxide in the atmosphere from 350 ppm to 3000 ppm.
The response time is as follows: the electromotive force of the sensor when the atmosphere gas is replaced is 90% of the electromotive force in a carbon dioxide concentration of 3000 ppm.
The time to reach the% value was measured.

The results are shown in Table 2.

[0044]

[Table 2]

As a comparative example, the gas sensitivity was measured in the same manner as above when the activated carbon was used instead of zeolite and when nothing was filled, and the results are also shown in Table 2.

The sensitivity to carbon dioxide gas was the same as that in the case where the zeolite shown in Examples 1 to 12 was used for the organic gas filter, the case where the activated carbon of Comparative Example 1 was used, and the case where nothing was filled in Comparative Example 2. There was almost no change, and it became clear that the filter had no effect on the responsiveness to carbon dioxide gas. On the other hand, when the activated carbon of Comparative Example 1 was filled, the sensitivity to the organic gas was reduced to about 1/4 to about 1/5 of the case where Comparative Example 2 was not filled. In contrast,
In the case where zeolite was used, the sensitivity to organic gas was lower than in the case where the activated carbon of Comparative Example 1 was filled. In particular, when the zeolites of Examples 1 to 4, 8, 9 and 10 to 12 were used, the effect of removing the organic gas was large and particularly good. In general, the organic gas removing effect of zeolite showed a tendency to be more exhibited with respect to toluene and ethyl acetate.

The response time when the activated carbon of Comparative Example 1 was filled was four times or more as compared with the case where no activated carbon was filled in Comparative Example 2, and the response to carbon dioxide was remarkably reduced. On the other hand, in the case of the zeolite having a specific surface area of about 300 to 650 m ² / g as shown in Examples 1, 2, 5 to 8, the response is slower than the case where nothing is filled in Comparative Example 2, The response time was about 1/2 that of the case using activated carbon. Furthermore, when zeolites having a specific surface area of 700 m ² / g or more were used as shown in Examples 3, 4, 9 to 12, it was possible to sufficiently reduce only the ethanol sensitivity without impairing the response.

Examples 13 to 20, Comparative Examples 3 and 4 Capacitance type C provided with a heater as shown in FIG.
An O ₂ sensor was made. CuO and Ba are used for the gas sensitive body 24.
A powder obtained by molding and sintering a powder obtained by mixing TiO ₃ at a molar ratio of 1: 1 was obtained with a diameter of about 5 mm and a thickness of about 0.1 mm.
Using a 7 mm disk, the electrodes 21 were formed by screen printing Ag paste on both sides, drying and baking at 500 ° C. The sensor is formed by screen-printing a Pt paste on an alumina substrate 19 in a wave form, drying and baking at 1000 ° C. to supply a power supply 16 to a heater 18.
And heated to 450 ° C. by applying a DC voltage.

The above-mentioned carbon dioxide sensor detects CO _{2 in the} atmosphere.
In order to detect a change in density as a change in capacitance, the electrode 2
The lead wire 22 made of platinum taken out of the sample No. 1 is connected to an ammeter (LC
(R meter) 23 to measure the capacitance. The measurement conditions are a frequency of 100 Hz, an applied voltage of 1
V. The CO ₂ sensitivity is shown as a value obtained by dividing the capacitance in an atmosphere having an arbitrary CO ₂ concentration by the capacitance in the atmosphere.

The carbon dioxide sensor element was mounted on the casing 4 in the same manner as in the first embodiment. Filter 3
Each zeolite shown in Table 3 was filled in a 4.0 mm thick filling space formed by the mesh plate 1.

The filter was provided with a heater 6 of a carbon dioxide sensor element so as to keep the temperature at 60 and 100 ° C. with the element.

[0052]

[Table 3]

For each of the carbon dioxide sensors configured as described above, in order to see the influence of the organic gas,
The sensor was used for a carbon dioxide concentration of 2%, and as an organic gas, a toluene concentration of 1000 ppm and an ethyl acetate concentration of 1000 p.
It was installed in a test gas having a pm and ethanol concentration of 1000 ppm, and the sensitivity to each gas was measured.

The response was evaluated by replacing the concentration of carbon dioxide in the atmosphere from 350 ppm to 3000 ppm.
The response time is as follows: the electromotive force of the sensor when the atmosphere gas is replaced is 90% of the electromotive force in a carbon dioxide concentration of 3000 ppm.
The time to reach the% value was measured.

Table 4 shows the results.

[0056]

[Table 4]

The carbon dioxide gas sensitivity was almost the same as the case where the organic gas filter was filled with the zeolite of Examples 13 to 20 or the activated carbon of Comparative Example 3 but not filled with anything of Comparative Example 4, and the carbon dioxide gas with the filter was almost unchanged. It was found that there was no effect on carbon gas. The sensitivity to organic gas was obtained when the activated carbon of Comparative Example 3 was filled.
Was reduced to about 1/3 to 1/4 of that without filling. On the other hand, when the zeolite of Example was filled, the sensitivity to organic gas was further reduced, and the organic gas removing effect was higher than that of the case where the activated carbon of Comparative Example 3 was filled.
The effect of removing organic gas was more remarkable for toluene and ethyl acetate.

The response time when the activated carbon of Comparative Example 3 was filled was increased by a factor of 5 or more compared to the case where no activated carbon was filled in Comparative Example 4, and the response to carbon dioxide was reduced. On the other hand, Examples 13, 14, 17, and 19
In the case of the zeolite having a specific surface area of about 300 to 650 m ² / g as shown in (1), the response is slower than that of Comparative Example 14 in which nothing is filled, but the response is delayed from の of that in the case of using activated carbon. It was about 1/3. Example 1
The specific surface area 70 as shown in 5, 16, 18 and 20
When a zeolite of 0 m ² / g or more is used, it is possible to sufficiently reduce only the organic gas sensitivity without impairing the response.

Examples 21 to 23 and Comparative Example 5 Using the carbon dioxide sensors produced in Examples 1, 4 and 8 and Comparative Example 1, long-term stability tests in organic gas were performed. The carbon dioxide sensor was placed in a closed air set to a toluene concentration of 3000 ppm, taken out of the atmosphere at regular intervals, and the sensitivity to a carbon dioxide concentration of 2% and an ethyl acetate concentration of 1000 ppm was measured in the same manner as in Examples 1 to 12. The measurement was performed and the results are shown in Table 5.

[0060]

[Table 5]

In Comparative Example 5 in which activated carbon was used for an organic gas filter, although the sensitivity to carbon dioxide did not change,
The sensitivity to ethyl acetate gradually increased from about 30 days, and showed about three times the initial sensitivity after 100 days.
On the other hand, all the sensors using zeolite as the organic gas filter showed that the sensitivity to carbon dioxide and ethyl acetate remained unchanged even after 100 days had elapsed, and that it was possible to accurately measure atmospheric carbon dioxide over a long period of time. .

[Brief description of the drawings]

FIG. 1 is a perspective view (including a cross section) of a typical organic gas filter of the present invention.

FIG. 2 is a sectional view showing the structure of a typical solid electrolyte type carbon dioxide sensor element used in the present invention.

FIG. 3 is a sectional view showing the structure of a typical capacitive carbon dioxide sensor element used in the present invention.

[Explanation of symbols]

 Reference Signs List 1 mesh plate 2 carbon dioxide sensor element 3 filter 4 casing 5 opening 6, 18 heater 7, 19 alumina substrate 8, 20 sealing material 9, 11, 21 electrode 10 solid electrolyte 12 metal carbonate 13, 17, 22 conductive wire 14 , 16 Power supply 15 Voltmeter 23 Ammeter 24 Gas sensitive body

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01N 27/416 G01N 27/22 G01N 27/406

Claims (1)

(57) [Claims] A carbon dioxide sensor element provided with a heating means is provided in a casing having an opening for gas intake in a part thereof, and the opening for gas intake or the opening for gas intake is provided. A carbon dioxide sensor, wherein a filter made of zeolite is provided between the unit and the carbon dioxide sensor element at a distance from the carbon dioxide sensor element.