JPH0996619A - Catalytic combustion type gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalytic combustion type gas sensor which has a small output at a zero point and can immediately take in gas detection signals to sudden change of ambient temperature, by combining a compensation element of a smaller weight to a gas-detecting element. SOLUTION: A gas-detecting element D and a compensation element C are manufactured beforehand. Weights of the elements D and C are measured. The element C of a smaller weight is combined to the element D to obtain the gas sensor. The temperature rise of the element D is always slow even to sudden rise of ambient temperature and transient zero output during the temperature rise always shifts in the opposite direction to gas detection signal. Accordingly, signals to detect incomplete combustion of a hot water supply system can be taken in earlier than in the prior art. A malfunction is not generated even when the signals are taken in immediately after the hot water supply system was turned on, that is, gas can be detected immediately after the ignition.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a catalytic combustion type gas sensor for detecting a combustible gas.

[0002]

2. Description of the Related Art Conventionally, as a gas detection device for detecting a combustible gas, a gas detection element having a structure in which a resistance temperature detector is covered with a carrier carrying a catalyst and a gas detection device having the same structure and not compatible with the combustible gas are used. Bridge circuits incorporating catalytic combustion gas sensors with active compensating elements are known.

FIG. 5 is a bridge circuit diagram of a catalytic combustion type gas detector. The two fixed resistors R1 and R2 are connected in series, the gas detection element D and the compensation element C are connected in series, and the power source E (voltage Vc) is connected in parallel. The resistance thermometer is composed of a platinum coil or the like, the carrier is composed of porous ceramics such as aluminum oxide or tin oxide, and the catalyst is composed of a noble metal such as platinum or palladium.

When the combustible gas comes into contact with the gas sensing element D that has been energized and preheated by applying the voltage Vc to the bridge circuit, combustion occurs, the temperature of the platinum coil rises, and the electric resistance increases in proportion to the gas concentration. Occurs. Combustion does not occur in the compensating element, and almost no temperature change occurs even if combustible gas comes in. In order to extract this minute change in electrical resistance as a voltage difference, the voltage at the connection point between the two fixed resistors and the voltage at the connection point between the gas detection element and the compensation element are subtracted by the differential amplifier A1, and this voltage difference is output as a bridge output. (It is referred to as a gas sensitivity where the voltage is Vb). Since the bridge output rises in proportion to the gas concentration, the gas concentration can be detected. The bridge output when the gas detection element is not in contact with the gas is called the zero point output.

When this type of gas sensor is used for incomplete combustion detection of exhaust gas from a combustion device, the resistance value of the gas detection element increases with a change in exhaust gas temperature, but the compensation element also increases in the same manner. , The basic principle is that the zero point output does not change. However, when the applicable temperature range is quite wide, or when the detected gas concentration level is in the low range of several hundreds ppm, a slight difference in temperature coefficient of resistance change becomes a level that cannot be ignored for gas sensitivity. Come on.

In order to prevent the gas sensitivity from being affected by the ambient temperature, the ambient temperature is detected using a temperature sensor,
The temperature of the bridge output is corrected for the same gas concentration.
The temperature sensor and the two elements are housed in the same cap so as to be in contact with the atmosphere of the same temperature. Since the zero-point output and the change in the gas sensitivity with respect to temperature have a substantially linear relationship, the correction can be easily performed by the temperature correction circuit added to the bridge circuit. FIG. 6 is a connection diagram of an example of a bridge circuit with a temperature correction circuit. A temperature sensor S and a resistor R3 are serially connected in parallel to a general bridge circuit (FIG. 5) to form a bridge circuit that shares the series connection of the resistors R1 and R2. The output of the gas sensor that has passed through the differential amplifier A1 and the output corresponding to the temperature that has been appropriately amplified through the differential amplifier A2 are subtracted by the differential amplifier A3, and the temperature-corrected gas sensor output is obtained. When the ambient temperature and all elements are in a steady state, the output of this bridge is used as a temperature correction output, and the amplification factor of the differential amplifier A2 is adjusted so that the difference from the zero point output becomes zero.

However, in actual use, it may be exposed to a sudden temperature change. At that time, the output is transiently unbalanced, and the zero point output linearly fluctuating with respect to the normal temperature is output from this line. There may be deviations. In particular, in the case of showing a characteristic that it largely swings in the positive direction, there is a disadvantage that detection cannot be started until it becomes stable because it causes a malfunction when the signal acquisition timing from the gas sensor is early.

This situation will be described below. As the gas detecting element and the compensating element, an element in which a γ-alumina porous carrier was attached to a platinum coil of 60 μφ was used, and the same platinum coil as the gas detecting element was used as a temperature sensor. The gas detection element, the compensating element and the temperature sensor are on the same base, and the catalytic combustion type gas sensor housed in the wire mesh is inserted into the mounting hole provided in the exhaust part of the water heater, and the above-mentioned bridge circuit with temperature compensation circuit is used as the gas detection circuit. The bridge output was measured by energizing with an applied voltage of 0.9V. This is the case where the temperature rise rate is the highest and the most extreme variation appears when the maximum combustion is started from the water heater stopped state.

FIG. 7 shows a typical zero point output (Vb) transient characteristic in the case of a conventional combination of a gas detection element and a temperature compensation element. The curve group 52 is the zero point output, and the curve 51 is the gas temperature of the exhaust portion. This transient phenomenon is because the gas detecting element and the temperature compensating element have different tracking speeds with respect to temperature changes. For example, if the temperature rise rate of the gas detection element exceeds the rise rate of the temperature compensation element when the temperature rises, it transiently appears as an output in the positive direction. Also, when the temperature falls from a high temperature to a low temperature, the deflection direction appears in the opposite direction, but from the viewpoint of the temperature change rate of the gas sensor element, it is much slower than when it rises, so the influence of the deflection range becomes smaller. Therefore, it is not a big problem, but it is necessary to keep it within a certain range.

[0010]

SUMMARY OF THE INVENTION In view of the above drawbacks, an object of the present invention is to provide a catalytic combustion type gas sensor which has a small zero point output and is ready to take in a gas detection signal in response to a sudden temperature change in the atmosphere. It is to be.

[0011]

In order to achieve the above-mentioned object, a gas detection element in which a carrier made of a porous ceramic carrying a catalyst for burning a combustible gas is attached to a resistance temperature detector. And a compensating element which has the same temperature measuring resistance and carrier as the gas detecting element and has the same shape as the gas detecting element and does not carry a catalyst or carries a catalyst different from the gas detecting element. It is assumed that the weight of the compensating element is smaller than the weight of the gas detecting element.

In the catalytic combustion gas sensor, the resistance temperature detector is a platinum coil, and the porous ceramic is γ.
-Alumina, and the weight difference between the compensating element and the gas detecting element is preferably 4 to 18%. In this way, the weight of the compensating element is smaller than that of the gas detecting element, so the heat capacity is small, and heat dissipation (or heat absorption) from the element surface per weight
Is big. Therefore, when both elements are placed in an atmosphere where the temperature changes rapidly, the compensating element can follow the ambient temperature first. When the temperature rises rapidly, the temperature rise rate of the temperature compensation element will surely exceed the gas detection element, so the output will transiently fluctuate in the negative direction.
Even if the detection signal is taken in early, it does not cause a malfunction. On the contrary, when exposed to an atmosphere where the temperature drops, it transiently fluctuates in the positive direction, but in the case of a water heater etc., the temperature fluctuation speed is smaller than when it rises, so
The swing range also becomes smaller.

Although the specific heat and the thermal emissivity differ depending on the material forming the element, since the two elements are formed of the same material, it is easy to experimentally determine the weight difference between the two elements.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION

Example In this example, an example of detecting incomplete combustion in exhaust gas of a water heater will be described. The gas sensing element and the compensating element were manufactured in advance by the conventional manufacturing method, and the weights of the gas sensing element and the compensating element were measured. From this, a gas sensor was manufactured by combining a compensating element with a weight that is 0.2 mg (about 4%) less than the weight of the gas detecting element. The average weight of the gas detecting element and the temperature compensating element including the platinum coil and the carrier is about 5 mg, and the weight of the platinum coil is 1.4 mg.

The gas sensor thus manufactured is incorporated into the bridge with a temperature compensation circuit (FIG. 6), and the bridge output when the maximum combustion is started from the water heater stopped state is carried out by the same method as in the prior art. The transient response characteristics were investigated. In Figure 1, the weight of the compensating element is 4% of the gas sensing element.
It is a graph of the bridge output when there are few. Curve 1
1 is the temperature change of the atmosphere and curve 12 is a typical bridge output. As is clear from Fig. 1, all transient outputs are in the negative direction (direction opposite to the gas detection signal).
Came to appear in.

For comparison, a gas sensor was manufactured with a combination in which the weight of the compensating element was 0.1 mg less than the weight of the gas sensing element (about 2% of the weight of the gas sensing element) with respect to the gas sensing element. I checked. FIG. 2 is a graph of bridge output when the weight of the compensating element is 2% less than the gas sensing element. Curve 21 is the temperature change of the atmosphere and curve 22 is a typical bridge output. Under this condition, similarly, the weight of the compensating element is lighter than that of the gas detecting element, but some transiently appearing in the positive direction are mixed, and the maximum swing width is plus.
There was a 5 mV type, and not all had negative characteristics.

The same applies to the case where the weight difference is 4% (FIG. 1), but even if the weight difference is the same, there is some variation in the transient characteristics. This is due to variations in the density of the porous ceramic of the element carrier, variations in the coating state of the porous ceramic on the platinum wire, and the like. If the maximum fluctuation range is 0.5 mV, this range corresponds to the output for carbon monoxide of 500 ppm, so there is a risk of malfunction when the minimum detection concentration is set to 500 ppm or less.

Next, the case where the temperature change was a temperature decrease was investigated. This is when the maximum combustion state (exhaust temperature approx. 250 ° C) is switched to the minimum combustion state (exhaust temperature approx. 70 ° C) while the water heater is energized. FIG. 3 is a graph of transient characteristics when the temperature of the gas sensor in which the weight of the compensating element is 4% less than that of the gas detecting element is lowered. Curve 31 is the temperature change of the atmosphere and curve 32 is a typical bridge output. As can be seen from FIG. 3, the transient swing direction is the positive direction, which is the opposite of that in FIG. 1, but since the temperature change rate of the atmosphere is not as steep as the temperature rises, the peak height is about 1/5 and the maximum. But the peak is 0.1 mV. This is a concentration of 100 ppm when converted to the output for carbon monoxide gas. Normally, this type of gas sensor is used to detect a carbon monoxide concentration of at least about 500 ppm, so a peak of this level is not a problem.

FIG. 4 is a graph showing the relationship between the weight difference between the gas detecting element and the compensating element and the fluctuation width of the transient characteristic when the maximum combustion is switched to the minimum combustion. Five gas sensors were measured for each weight difference, and the average value, maximum value, and minimum value are shown. For example, in the case of the combination in which the weight of the compensating element is 1.0 mg (20%) heavier than that of the gas detecting element, the average peak height reaches 0.5 mV. this is,
This is equivalent to the output for carbon monoxide of 500 ppm, and it becomes a problem when detecting 500 ppm or more as a gas sensor. In such a case, the upper limit of the weight difference between the compensating element and the gas detecting element is 0.9 mg ( 18%).

[0020]

According to the present invention, since the gas detecting element is combined with the compensating element having a small weight to form a gas sensor, the temperature of the gas detecting element is always slowed down even when the ambient temperature rapidly increases. Since the transitional zero point output in the temperature rising process is always biased in the opposite direction to the gas detection signal, the timing when capturing the signal from the gas sensor to detect the incomplete combustion in the water heater is longer than in the past. It is possible to speed up the operation, and even if the water heater is switched on immediately after it is turned on, no malfunction occurs, and gas detection can be performed immediately after ignition. Therefore, the safety of the water heater can be improved.

[Brief description of drawings]

FIG. 1 is a graph of bridge output when the weight of a compensating element is 4% less than that of a gas sensing element.

FIG. 2 is a graph of bridge output when the weight of the compensating element is 2% less than the gas detecting element

FIG. 3 is a graph of transient characteristics when the temperature of a gas sensor in which the weight of the compensating element is 4% less than that of the gas detecting element when the temperature is lowered.

FIG. 4 is a graph showing the relationship between the weight difference between the gas detection element and the compensation element and the fluctuation width of the transient characteristic when the maximum combustion is switched to the minimum combustion.

FIG. 5: Bridge circuit diagram of catalytic combustion type gas detector

FIG. 6 is a connection diagram of an example of a bridge circuit with a temperature correction circuit.

FIG. 7 is a graph of a typical zero point output (Vb) transient characteristic in the case of a conventional combination of a gas detection element and a compensation element.

[Explanation of symbols]

 D Gas detection element C Compensation element S Temperature sensor R1 resistance R2 resistance R3 resistance E power supply V load A1 differential amplifier A2 differential amplifier A3 differential amplifier

Claims (2)

[Claims] 1. A gas sensing element comprising a temperature sensing resistor to which a carrier made of porous ceramic carrying a catalyst for burning a combustible gas is attached, and the same temperature sensing resistor and carrier as the gas sensing element. It is a catalytic combustion type gas sensor that has the same shape as the gas detection element and is paired with a compensation element that does not support a catalyst or that supports a catalyst different from the gas detection element. Catalytic combustion type gas sensor characterized by being smaller than the weight of the sensing element 2. The catalytic combustion gas sensor according to claim 1, wherein the resistance temperature detector is a platinum coil, the porous ceramic is γ-alumina, and the weight difference between the compensating element and the gas detecting element is 4 to 4. Catalytic combustion type gas sensor characterized by being 18%