JP3393504B2 - Contact combustion type carbon monoxide sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalytic combustion type carbon monoxide sensor. 2. Description of the Related Art A carbon monoxide sensor is mounted on a combustion appliance such as a water heater, and its combustion is monitored by the presence or absence of carbon monoxide in exhaust gas. When incomplete combustion is detected, these combustion appliances are detected. Used to shut down or control combustion conditions. Among these carbon monoxide sensors, the catalytic combustion type carbon monoxide sensor is widely used because it has good sensitivity and can be made compact. The detection element of this catalytic combustion type carbon monoxide sensor has a carrier (usually using alumina) carrying a catalyst active against carbon monoxide in the atmosphere around a thin conducting wire made of platinum or the like. Usually, in order to eliminate the influence on the detection value due to the fluctuation of the ambient temperature, a bridge as schematically shown in FIG. 1 together with a temperature compensating element of the same size having a carrier inactive against carbon monoxide is arranged. It is used after forming a circuit. In the figure, reference numeral 1 denotes a temperature compensation element, and reference numeral 2 denotes a detection element. However, in such a catalytic combustion type carbon monoxide sensor, measurement errors occur due to carbon dioxide, water vapor, and the like, which should have no effect in principle.
FIG. 2A shows the relationship between the sensor output and the carbon monoxide of a general catalytic combustion type carbon monoxide sensor. As can be seen from this figure, this sensor has good linearity with respect to changes in the concentration of carbon monoxide. On the other hand, the sensor output of this sensor with respect to carbon dioxide was examined using sample gas in which carbon dioxide was added to a 20% oxygen-80% nitrogen mixed gas at various concentrations. The result is shown in FIG. FIG. 2B shows that the output of this sensor is reduced by carbon dioxide. That is, carbon dioxide is 5
%, The output drops by about -0.6 V.
When converted according to (a), carbon monoxide is detected as low as about 250 ppm. As described above, when the conventional catalytic combustion type carbon monoxide sensor is used, accurate measurement of carbon monoxide cannot be performed in an environment where carbon dioxide coexists. SUMMARY OF THE INVENTION The present invention prevents the above-mentioned disadvantages of the prior art, and accurately detects carbon monoxide in the atmosphere without being affected by carbon dioxide or water vapor. It is an object of the present invention to provide a catalytic combustion type carbon monoxide sensor that can perform the above-described processes. [0005] That is, the catalytic combustion type carbon monoxide sensor of the present invention has a carrier layer which is active against carbon monoxide around the conducting wire as described in claim 1. A catalytic combustion type carbon monoxide sensor having a detection element and a temperature compensating element having a carrier layer inactive against carbon monoxide around the conductor, wherein the size of the carrier layer of the detecting element is a temperature compensating element. the size of the carrier layer and different Do Ri, the temperature
Both the carrier layer of the compensation element and the carrier layer of the sensing element
And the particle size of the carrier layer of the temperature compensation element is detected.
It has a configuration in which the particle size of the carrier layer of the device is 1.05 times or more and 1.4 times or less . [0006] In the present invention, the carrier layer of the temperature compensation element and the carrier layer of the sensing element are both granular,
It is desirable that the particle size of the carrier layer of the temperature compensation element is 1.05 times or more and 1.4 times or less the particle size of the carrier layer of the sensing element.
In the above description, the grain shape refers to a sphere, an egg shape, a rice grain shape, and a substantially cylindrical shape. Further, the shape of the carrier layer of the temperature compensation element and the shape of the carrier layer of the sensing element are preferably similar to each other.
If these shapes are different, it is difficult to obtain the effects of the present invention. In the present invention, alumina is usually used as the carrier layer material, γ-alumina supporting a catalyst is used for the detection element, and α-alumina is used for the heat compensation element. here,
By subjecting boehmite-type aluminum hydroxide to a heat treatment at 800 to 900 ° C., γ-alumina becomes 1000 to 12
By performing a heat treatment at 00 ° C., α-alumina can be obtained. Note that γ-alumina supporting a catalyst used in the detection element is prepared by supporting a catalyst metal such as palladium, rhodium and platinum by a conventional method such as an impregnation method. Next, a little water and, if necessary, a binder and the like are added to these aluminas and kneaded to form a paste. The paste-like material is obtained by forming the paste around the conductive wire into granules, and then heating and baking. The material of the conductive wire to be used is determined in consideration of oxidation resistance, durability, heat resistance, and price, but it is usually preferable to use a platinum wire. In addition,
By using a platinum wire wound up in a coil shape and forming the paste-like material into a granular shape in the coil portion, the sensitivity can be improved. (Preparation of Sensing Element) An aqueous solution of chloroplatinic acid in which the amount of platinum was adjusted to 5% by weight, an aqueous solution of palladium chloride in which the amount of palladium was adjusted to 5% by weight in hydrochloric acid, and the like The aqueous solutions were mixed in equal amounts to prepare three types of aqueous solutions for supporting the catalyst. Γ-alumina (confirmed to be γ-alumina by X-ray diffraction analysis) obtained by heat-treating boehmite-type aluminum hydroxide at 850 ° C. in each of these aqueous solutions for supporting a catalyst is filtered, taken out, and dried. Thereafter, a slight amount of water is added and kneaded, and the paste is wound into a coil shape so as to form a spherical shape having a diameter of 1 to 2 mm so as to cover the coil portion of a platinum wire (coil portion outer diameter: 0.6 mm) (conductor). After being attached and air-dried, it was energized and baked at around 700 ° C. to obtain three types of sensing elements. (Production of Temperature Compensating Element) The same boehmite-type aluminum hydroxide used for producing the sensing element was used.
A small amount of water was added to α-alumina obtained by heat treatment at 0 ° C. (confirmed to be α-alumina by X-ray diffraction analysis) and kneaded to form a paste, which was then used in a sensing element. A platinum wire (conductive wire) was adhered in a spherical shape so as to cover the coil portion and air-dried. In addition, several kinds of samples having different diameters of the spheres were produced. Thereafter, the platinum wires were energized to generate heat, and were baked at around 700 ° C. to obtain temperature compensation elements. In the same manner, several temperature compensating elements having different particle sizes were produced. (Assembly and Evaluation of a Catalytic Combustion Carbon Monoxide Sensor) Of the sensing elements obtained above, a sensing element carrying palladium and several types of temperature compensating elements having different particle sizes were used as shown in FIG. A bridge circuit as shown in Fig. 7 was formed to obtain a catalytic combustion type carbon monoxide sensor. Then, both elements of the catalytic combustion type carbon monoxide sensor were
When the sensor output for a mixed gas of carbon monoxide and hydrogen was examined while keeping the temperature at 50 ° C., the same result as shown in FIG. 2A was obtained. Further, using a sample gas obtained by adding carbon dioxide to a 20% oxygen-80% nitrogen mixed gas to a concentration of 5% and 10% in the catalytic combustion type carbon monoxide sensor, the sensor sensitivity to carbon dioxide was examined. . The results are shown in FIG. FIG. 3B shows the relationship between the sensor output and the ratio of the particle size of the carrier layer of the temperature compensation element to the particle size of the carrier layer of the detection element when the carbon dioxide concentration was 5%. . At this time, FIG.
When the particle size ratio is 1.05 or more and 1.4 or less according to (a), the effect of 5% carbon dioxide on the sensor output is ± 200 ppm in terms of carbon monoxide concentration. here,
The alarm point of a commonly used water heater is 1000
Since the concentration of carbon monoxide is ppm, if the influence of carbon dioxide is within 200 ppm, it is determined to be sufficient. Similarly, the relationship between the particle size ratio and the sensor output was also examined for a sensing element carrying platinum and a sensing element carrying platinum and palladium. FIG. 4 shows the results. FIG. 4 shows that the particle size ratio was 1.05 even when a sensing element carrying platinum or a sensing element carrying platinum and palladium was used, as in the case of using a sensing element carrying only palladium. 1.4 above
When the ratio is not more than twice, the effect of 5% carbon dioxide on the sensor output is ± 200 ppm in terms of the concentration of carbon monoxide, indicating that the effect of carbon dioxide can be sufficiently reduced. The catalytic combustion type carbon monoxide sensor of the present invention is an excellent sensor capable of accurately detecting carbon monoxide in the atmosphere without being affected by carbon dioxide or water vapor. It is.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a model diagram showing a bridge circuit including a detection element and a temperature compensation element. FIG. 2 (a) is a graph showing the relationship between sensor output and carbon monoxide of a general catalytic combustion type carbon monoxide sensor. (B) It is a graph which shows the relationship of the sensor output with respect to carbon dioxide of the sensor of (a). FIG. 3 (a) is a graph showing the sensor sensitivity to carbon dioxide of a catalytic combustion type carbon monoxide sensor having different particle diameter ratios of the carrier layer of the temperature compensation element and the carrier layer of the sensing element. (In the case of a sensing element carrying palladium). (B) A graph showing the relationship between the ratio of the particle size of the carrier layer of the temperature compensation element to the particle size of the carrier layer of the sensing element and the sensor output (in the case of a sensing element carrying palladium). FIG. 4 shows the relationship between the particle size of the carrier layer of the temperature compensating element and the particle size of the carrier layer of the sensing element when using a sensing element carrying platinum and a sensing element carrying platinum and palladium. 5 is a graph showing a relationship between a particle size ratio and a sensor output. [Description of Signs] 1 Temperature compensation element 2 Detection element

────────────────────────────────────────────────── (5) References JP-A-9-96619 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/00-27/24

Claims (1)

(57) [Claim 1] A detection element having a carrier layer having activity against carbon monoxide around a conducting wire, and a carrier layer having no activity against carbon monoxide around a conducting wire. a catalytic combustion type carbon monoxide sensor including a temperature compensating element having, unlike the size of the carrier layer of the size of the carrier layer of the detecting element is a temperature compensating element <br/>, carrier layer of the temperature-compensating element And carrier for sensing element
The layers are both granular and the temperature compensation element
The particle size of the body layer is at least 1.05 times the particle size of the carrier layer of the sensing element
Catalytic combustion type carbon monoxide sensor according to claim der Rukoto 1.4 times or less.