JP4790430B2 - Detection circuit using catalytic combustion type gas sensor - Google Patents

Description

  The present invention relates to a detection circuit used in a high-sensitivity catalytic combustion type gas sensor having a wide detectable concentration range.

  The catalytic combustion type gas sensor is a sensor of a type that uses catalytic combustion of combustible gas by a catalyst and detects a change in temperature of the sensor as a change in sensor resistance value. Since the sensitivity of the sensor has a good proportional relationship with the concentration of combustible gas, a catalytic combustion type gas sensor is used for equipment for measuring and monitoring the gas concentration (for example, a safety device for a gas water heater). Has been.

FIG. 1 is a view for explaining the structure of a conventional catalytic combustion type gas sensor. FIG. 1 is a diagram showing the structure of a catalytic combustion type gas sensor as a sensing element. The sensing element has a structure in which, for example, a platinum wire coil having a diameter of about 20 μm is spherically wrapped with alumina as a carrier. A catalyst (for example, a noble metal such as platinum or palladium) is supported on the surface of the support.
Such a contact combustion type gas sensor is manufactured by dropping a carrier on a coil and attaching it to the coil. After firing, the catalyst is further applied to the surface of the carrier and then fired.

  FIG. 2 is a diagram showing a circuit using a conventional constant voltage power source of a measuring apparatus using a contact combustion type sensor. In addition to serving as a heater for heating the sensor, the platinum coil also serves as a thermometer that captures changes in temperature due to contact combustion of combustible gas. For this reason, the resistance value of the detection element E1 also changes with respect to temperature changes other than gas catalytic combustion, for example, ambient temperature and wind changes. A temperature compensation element E2 is used to compensate for this. The temperature compensation element E2 is preferably the same in temperature characteristics as the sensing element E1, and therefore, the same platinum coil as the sensing element E1 is obtained by sintering alumina that does not carry a catalyst. The measurement principle by the circuit of FIG. 2 is as follows.

  As shown in FIG. 1, when an air containing a flammable gas is brought into contact with the sensing element E1 enclosing a platinum coil with alumina supporting platinum or palladium having a high catalytic activity for an oxidation reaction of a flammable gas, A combustible gas and oxygen in the air react (catalytic combustion reaction) on the catalyst to generate reaction heat (combustion heat). This reaction heat is proportional to the concentration of the combustible gas, and the resistance value of the platinum coil increases accordingly. For this reason, the resistance value of the platinum coil increases in proportion to the concentration of the combustible gas in the air. In order to convert this into an electric quantity, as shown in FIG. 2, a bridge circuit having two sides of the detection element E1 and the temperature compensation element E2 (the other sides are fixed resistors R1 and R2) is used. The detection element E1 and the compensation element E2 are constantly supplied with a current of about 100 mA, and are maintained at a temperature necessary for the combustible gas to cause a catalytic combustion reaction. Since the electric resistances of the detection element E1 and the temperature compensation element E2 are set to be equal, the bridge circuit is kept in balance in air containing no flammable gas, and no potential difference is generated between A and B. . On the other hand, when there is a flammable gas in the air, the temperature of the sensing element E1 rises due to the contact combustion, and the electrical resistance increases, so that a potential difference occurs between A and B. Since this potential difference changes in proportion to the combustible gas concentration, the concentration of the combustible gas in the air can be known from this potential difference.

However, the conventional catalytic combustion type gas sensor has the following problems.
First, since the carrier covers the cylindrical coil in a spherical shape, the distance between the coil and the surface of the sphere varies depending on the position of the carrier, so that when the coil is energized to burn the combustible gas, the surface temperature Vary considerably. Therefore, not all surface temperatures can be maintained at the combustion temperature of the combustible gas to be detected, and gases other than the combustible gas to be detected are combusted, and good combustible gas selectivity cannot be obtained.
For example, when a platinum catalyst is used, the platinum catalyst causes catalytic combustion with carbon monoxide gas at about 160 ° C., and catalytic combustion with hydrogen gas at about 200 ° C., so that temperature unevenness occurs on the surface of the carrier, resulting in a partial temperature. If the gas is too high, contact combustion may occur with hydrogen gas that is not a combustible gas to be detected, and gas selectivity is poor.

Secondly, since the carrier is made spherical, the mass with respect to the surface area of the carrier is increased, and the heat capacity of the sensor is increased. When the heat capacity is large, the entire sensor rises when the combustible gas contacts the catalyst and burns. The heating speed becomes slow and the detection response becomes poor. In particular, the gas detection sensitivity in the low concentration region is deteriorated, and the detection range on the low concentration side is narrowed. For example, conventionally, the measurement of the concentration of carbon monoxide gas on the low concentration side is limited to the measurement up to about 0.03% (300 ppm).
Thirdly, although the surface area supporting the catalyst can be increased to some extent by making fine pores on the alumina surface of the support, for high concentration gas exceeding the combustion capacity of the catalyst supported on the surface Combustion is saturated, and the detection range on the high concentration side is limited. For example, conventionally, in the measurement of the concentration of carbon monoxide gas, only measurement up to about 0.3% (3000 ppm) was possible.

Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-121402) discloses a high sensitivity with excellent gas selectivity by making the winding pitch at both ends of the coil denser than the winding pitch at the center. A catalytic combustion gas sensor is disclosed.
In this sensor, a cylindrical carrier is attached to a coil, and a catalyst layer is provided on the surface of the carrier. The surface area can be secured also on the inner surface of the hollow region of the cylinder, the contact area between the combustible gas and the catalyst is improved, and the sensitivity of the sensor is increased. Further, since the distance between the coil and the carrier surface can be made substantially constant, an improvement in gas selectivity can be expected.

JP 2003-121402 A

  However, the catalytic combustion gas sensor manufactured by the manufacturing method of Patent Document 1 also has the same problems as the spherical catalytic combustion gas sensor described above. That is, since the mass with respect to the surface area of the carrier is still large, the heat capacity is also large, and there is a limit in improving the sensitivity and the detection concentration range in the low concentration region. Further, since the heat capacity is still large, it takes a long time to raise the temperature, and temperature unevenness is likely to occur on the surface of the carrier, making it difficult to obtain good gas selectivity. Also, in the high concentration region, the contact area increases by the area of the inner surface of the cylindrical carrier, so that the detectable concentration range is slightly expanded compared to the spherical contact combustion type gas sensor, but in practical use. The conventional spherical contact combustion type gas sensor is not so different.

  The catalytic combustion type gas sensor used in the present invention is a catalytic combustion type gas sensor in which a carrier supporting a catalyst is attached to a nickel-based conductor, and the concentration of the combustible gas is detected by catalytic combustion of the combustible gas by the catalyst. The catalyst is supported in a state of being mixed with the carrier, and a plurality of pores are formed on the surface of the carrier so that the catalyst inside the carrier can come into contact with the combustible gas. 1 to 0.5 mm. The main configuration of a detection circuit suitable for this catalytic combustion type gas sensor is as follows.

(1) A detection circuit using two catalytic combustion type gas sensors in which a catalyst carried on a carrier having a large number of holes is attached to a nickel-based conductor with a thickness of 0.1 to 0.5 mm,
A constant current circuit C2 arranged in series and one catalytic combustion gas sensor S1, a constant current circuit C3 arranged in series and the other catalytic combustion gas sensor S2 are arranged in parallel,
Detectors V1 and V2 for detecting the respective voltages of the two catalytic combustion gas sensors S1 and S2 are provided, and a mixed combustion gas is detected by comparing the two detected voltages. Detection circuit.
(2) The detection circuit using the catalytic combustion type gas sensor according to (1), wherein the nickel-based conductor has a hollow coil shape.
(3) A combustible gas sensor comprising a detection circuit using the contact combustion gas sensor described in (1) or (2) .

The present invention can realize a detection circuit that detects the ability of a highly sensitive gas sensor. In particular, by incorporating a constant current circuit in the detection circuit, the detection output gain can be increased, and the sensitivity of the sensor can be sufficiently extracted.
In particular, the gas sensor unit is supported in a thin film state with the carrier mixed with the catalyst, and a plurality of holes are formed in the carrier surface so that the combustible gas can contact the catalyst inside the carrier. The area to be performed is greatly increased, and the detection range on the high concentration side is dramatically expanded. In addition, since the carrier made of a porous material is attached to the conducting wire in a thin layer, the heat capacity of the carrier is greatly reduced, the sensitivity in the low concentration region is improved, and the detection range on the low concentration side is widened. Further, due to the decrease in the heat capacity, the temperature unevenness of the surface temperature of the carrier does not occur, and good gas selectivity can be obtained.
In particular, by using a nickel-based conductor that does not have a catalytic reaction as the conductive wire, a non-side reaction does not occur on the compensating element side, and the sensitivity of the sensor can be exhibited with high accuracy.
In addition, by using the magnetization of nickel, workability such as assembly is excellent.

  In particular, when used for detection of carbon monoxide gas, the catalytic combustion type gas sensor of the present invention can reduce noise on the compensation element side by using a nickel-based conducting wire that does not have a catalytic reaction, and is approximately 0.002% ( 20ppm) to 7% (70,000ppm) wide concentration measurement range. Despite being a small and simple sensor, it is a large-scale infrared and gas chromatographic fixed type concentration measurement that requires a large power source. Measurement in the same concentration range as the device is possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to the scope of the present embodiment.

<Contact combustion type gas sensor>
The catalytic combustion type gas sensor according to the embodiment of the present invention is supported in a state in which a catalyst is mixed in a carrier adhering to a conducting wire, and the carrier has a plurality of holes communicating from the surface to the inside. Many of the holes formed in the carrier penetrate therethrough and are preferably made porous.

  Compared to the case where the catalyst is supported only on the surface of the carrier, the catalyst is also present inside the carrier, and the catalyst inside the carrier comes into contact with the combustible gas entering from a plurality of holes, thereby supporting the carrier. The area in which the catalyst can be catalytically burned increases dramatically. In addition, since the air permeability is ensured by the large number of holes, the inflow of combustible gas and oxygen into the inside of the hole, and the discharge of carbon dioxide from the inside of the hole by catalytic combustion are efficiently performed. For this reason, the measurable range is greatly expanded even in the high concentration range that could not be measured due to combustion saturation. For example, in the detection of carbon monoxide gas, conventionally, the upper limit of the measurable concentration is about 0.5% (5000 ppm), but it can be measured up to about 7% (70000 ppm).

  In addition, since the carrier attached to the conductive wire is thin and porous, the mass is small, the heat capacity of the carrier is reduced, and the sensitivity in a low concentration region can be improved. In particular, as described above, in addition to the increase in the surface area where the catalyst can be combusted, the mass of the carrier adhering to the conducting wire is small, so that a significant sensitivity improvement effect in a low concentration region can be obtained. For example, in the detection of carbon monoxide gas, conventionally, the lower limit of the measurable concentration is about 0.03% (300 ppm), but it can be measured to about 0.005% (50 ppm). .

<About nickel conductor>
Temperature coefficient, to the extent practical temperature of 100 to 300 ° C., compared to platinum ^{13.6~21.0 (x10 -8 Ω · m)} , nickel is ^{10.3~22.5 (x10 -8 Ω · m)} , the temperature coefficient ratio Is 21.0 / 13.6 = 1.54 for platinum and 22.5 / 10.3 = 2.18 for nickel. Compared with platinum, nickel has a temperature coefficient ratio of 2.18 / 1.54 = 1.41, which is more than 40% gain. Moreover, although there is a catalytic reaction in the platinum wire itself, it is not in nickel, and this is ideal to react only with the catalyst coated as a sensor, but there is also a risk of reacting with the platinum wire. In particular, it is inconvenient that the dummy compensation element reacts because it becomes an unstable element. In addition, nickel is magnetized, has good workability, has a good compatibility with other substances, is inexpensive, and can withstand temperatures up to about 700 ° C., so it is also used as a temperature sensor and is suitable as an electrical material. By using nickel as a conductor material, an element excellent in sensitivity and responsiveness is obtained.

  Specifically, for example, in a conventional example in which a catalyst is added in a beat shape (rice granular shape) to a platinum coil, the mass of the sensor of the present invention is about 10 times larger than that of the sensor of the present invention. Thus, the responsiveness can be increased by about 10 times. Specifically, the beat-shaped conventional example is stabilized in 10 to 20 seconds, while the hollow coil heater of the present invention is stabilized in 1 to 2 seconds. Furthermore, compared with what formed the catalyst on the support | carrier surface of the hollow coil which adhered the phase body disclosed to patent document 1, the sensitivity increase of 2 to 3 times with the catalyst was able to be confirmed by experiment by the mass difference.

<Carrier>
The carrier is usually alumina (Al ₂ O ₃ ), but silica (SiO ₂ ), zinc oxide (ZnO), or the like may be used.

<Catalyst>
The catalyst is selected from platinum (Pt), palladium (Pd), rhodium (Rd) and the like according to the type of combustible gas to be detected.
For example, platinum (Pt) is used for detecting the concentration of carbon monoxide gas. The coil temperature for burning carbon monoxide gas in the platinum (Pt) catalyst is about 160 ° C. In the platinum catalyst, the coil temperature for burning hydrogen gas is 200 ° C., which is relatively close to the temperature of carbon monoxide gas. In the catalytic combustion type gas sensor of the present embodiment using a platinum catalyst, since the carrier is attached to the coil wire in a thin film form, the distance between the coil wire and the surface of the carrier becomes closer and becomes almost constant. There is no variation in temperature on the surface of the carrier, and good carbon monoxide gas selectivity can be obtained. Therefore, the concentration of the carbon monoxide gas can be accurately detected without burning the hydrogen gas even in a gas mixture of the carbon monoxide gas and the hydrogen gas.

  Palladium (Pd) is used for hydrogen gas concentration detection. In the palladium catalyst, the coil temperature for burning hydrogen gas is about 150 ° C., whereas the coil temperature for burning carbon monoxide gas is about 180 ° C. Also in this case, although the coil temperatures of hydrogen gas and carbon monoxide gas are relatively close, in the catalytic combustion type gas sensor of the embodiment of the present invention using palladium, good hydrogen gas selectivity can be obtained.

<Baking>
In the firing, heat is applied from the outside in an air (oxygen) atmosphere, and at the same time, the coil is energized to heat the coil, thereby heating from the inside. Firing is performed under conditions necessary for separating the electrodeposition resin. Since nickel is oxidized at 700 ° C. or higher, the ambient temperature is 500 to 700 ° C., the coil applied voltage is 3 to 5 V, and the firing time is 10 minutes or longer. When the above conditions are met, the electrodeposition resin is not sufficiently combusted and the characteristics of the catalytic combustion type gas sensor of the present invention cannot be exhibited.
<Electrodeposition resin>
The electrodeposition resin is, for example, a mixture of vinyl acetate and an acrylic acid alkyl ester (acrylic resin).

<About catalytic combustion type gas sensor>
Hereinafter, the structure and manufacturing method of the catalytic combustion type gas sensor in the embodiment of the present invention will be described in detail.

  FIG. 3 is an external perspective view showing an example of the structure of the catalytic combustion type gas sensor in the embodiment of the present invention. As shown in FIG. 3, the catalytic combustion gas sensor 1 according to the embodiment of the present invention has a structure in which a carrier 3 supporting a catalyst is attached to a coil wire 2 such as a nickel nickel wire in a thin film shape. The carrier 3 adheres like a garment to the coil wire 2 like a thin film, and the central portion of the coil becomes hollow. When the winding pitch of the coil wire is small and narrower than the thickness of the thin film, the carriers attached to the adjacent coil wires 2 can be brought into contact with each other to form a hollow cylindrical shape.

  Further, the conducting wire can be folded back into a flat triangular wave or the like to form a flat shape. Such a contact combustion type gas sensor configured in a thin plate shape is suitable for application to a part whose thickness is limited, such as application to a printed circuit board. Moreover, as long as it is a flat thin plate shape, the shape is not limited to a triangular wave shape, and may be a rectangular wave shape, for example.

  FIG. 4 is a view for explaining the characteristics of the catalytic combustion type gas sensor according to the embodiment of the present invention. FIG. 4 is a cross-sectional view of a catalytic combustion type gas sensor manufactured by the electrodeposition method according to the embodiment of the present invention, and an enlarged view of a dotted line portion is also shown. FIG. 4 illustrates a contact combustion type gas sensor configured by attaching a carrier 3 to a coil wire 2 in a cylindrical shape.

In the gas sensor according to the present invention, the resin part is removed and the carrier is ceramicized in a porous state by firing after thinly adhering to the coil in a mixed state of the electrodeposition resin, the carrier and the catalyst. . The catalyst is exposed on the outer surface of the carrier and the inner surface of the pores, and the reaction area is large. Moreover, since it is reduced in weight by the porous material, and the holes are continuous and penetrated, air and gas can be easily passed through and the combustibility is improved, so that the sensitivity is improved.
The schematic diagram is shown in FIG. (A) shows a state in which the carrier 3 is attached to the coil 2 in a thin layer. (B) is a partially enlarged view showing a state in which the electrodeposition resin is removed by firing and is in a porous state in which a large number of holes 5 and penetrating terms 6 exist. Since the catalyst 4 is distributed almost uniformly in the carrier, a large number of the catalysts 4 are exposed on the outer surface and the inner surface of the pores. By adopting such a structure, a catalyst with a sufficient reaction amount can be ensured even if it is thinned, so that the film thickness of the support is 0.1 to 0.5 mm. The thinner the temperature, the more sensitive the temperature is to the conductor, and the sensitivity is improved. However, due to production limitations such as film thickness control, about 0.1 mm is currently a practical value.
In addition, since the sensor disclosed in the conventional patent document 1 has electrode resin and a carrier attached, and then baked after applying the catalyst to the surface, it is necessary to apply a thick coating to ensure a sufficient amount of catalyst. The thickness of the carrier was a practical value of 1 mm or more.

  In addition, by reducing the film thickness, a large number of holes formed by separation of the electrodeposition resin are formed at a relatively deep position inside the carrier during firing, and the through holes are formed. It becomes easier to form. Furthermore, by reducing the film thickness, the mass of the carrier itself can be reduced, and the heat capacity of the carrier can be lowered, so that the sensitivity in the low concentration region can be improved. According to the experiments by the inventors of the present invention, the preferred film thickness is 0.1 mm to 0.5 mm. Compared to the conventional case, it can be reduced to half or less. In addition, in order to make the combustible gas in the gas to be measured contact combustion with the catalyst inside the carrier and to obtain a wide detection concentration range, the hole formed on the surface of the carrier by separation of the electrodeposition resin is about 10 μm to It is desired to have a diameter of 60 μm.

<Sensor manufacturing method>
FIG. 5 is a diagram for explaining a method of manufacturing the catalytic combustion type gas sensor in the present embodiment. The catalytic combustion type gas sensor of the present invention is manufactured as follows using an electrodeposition coating method.

First, as shown in FIG. 5A, a fine nickel (Ni) coil wire of about 15 μm to 30 μm is formed. A coil wire prepared in advance may be prepared.
Subsequently, as shown in FIG. 5B, the coil wire is immersed in an electrodeposition solution in which a carrier, a catalyst, and an electrodeposition resin are mixed. A coil wire is used as a cathode, a predetermined voltage is applied for a predetermined time, and a carrier in which a catalyst and an electrodeposition resin are mixed is electrodeposited on the coil wire. The voltage is preferably applied intermittently.

  Since the film thickness changes according to the integrated power amount, the electrodeposition time is adjusted so that the integrated power amount becomes a predetermined amount. It has been found that when the film thickness is 0.1 mm to 0.5 mm (more preferably, 0.15 to 0.35 mm), the film has the above-described remarkable characteristics. For this reason, the integrated electric energy that is within the range of the film thickness is obtained, and the electrodeposition time that becomes the integrated electric energy is determined for the set current value and voltage value. In the embodiment described below, the integrated power amount for making the film thickness about 0.15 to 0.35 mm is about 0.6 to 2.0 mW. If the electrodeposition time is too long, the film thickness becomes too thick, and if the time is short, the amount of adhesion is not constant and unevenness occurs.

  The electrodeposition liquid is an aqueous solution containing a catalyst, a carrier, and an electrodeposition resin, and the respective components are mixed at a predetermined ratio. About the component ratio (weight ratio) of each component, it is preferable that electrodeposition resin: (catalyst + support | carrier) is 60: 40-85: 15. By relatively increasing the ratio of the electrodeposition resin that is oxidized during baking to become pores (up to about 85%), a large number of through-holes and relatively large and deep holes can be formed. The catalyst can be used effectively.

  When the electrodeposition conditions (time, voltage, etc.) are the same, if the amount of electrodeposition resin is small (less than 60%), the mass after firing does not decrease sufficiently, and the heat capacity increases, causing a decrease in sensitivity. Further, when the amount of the electrodeposition resin is too much (over 85%), the mass is reduced and the heat capacity is also reduced, so that the sensitivity is improved. Problems arise. That is, if the ratio of the pore portion to the volume of the carrier becomes too large, the amount of oxygen (air layer) on the surface of the carrier increases and the temperature of the surface of the carrier becomes too high when a relatively high concentration of combustible gas is burned. . For example, when a carbon monoxide gas concentration is detected from a gas in which hydrogen gas and carbon monoxide gas are mixed using a platinum (Pt) catalyst, the coil temperature for burning the carbon monoxide gas in the platinum catalyst is: The coil temperature for burning hydrogen is 200 ° C., which is relatively close to about 160 ° C. Therefore, when a contact combustion type gas sensor is used for detecting the concentration of carbon monoxide gas, if the temperature of the support surface rises too much, there is a risk of reaching the combustion temperature of hydrogen, and a gas in which carbon monoxide gas and hydrogen are mixed. (In general, when gas using hydrocarbons as fuel, such as city gas, is incompletely burned), hydrogen is also burned near the surface, leading to poor carbon monoxide gas detection selectivity. .

<Specific examples of gas sensors>
Next, an example of the catalytic combustion type gas sensor of the present invention will be shown.

(1) Coil: A nickel (Ni) wire having a diameter of 18 μm and a 21-turn winding pitch of 0.1 mm was used.

(2) Electrodeposition liquid composition: An aqueous solution containing a catalyst, a carrier, and an electrodeposition resin.
Catalyst: 8.5%, support (alumina): 6.0%, electrodeposition resin: 55.7%, water: 29.8%
Electrodeposition resin: A mixture of vinyl acetate and alkyl acrylate (acrylic resin).
Catalyst: A mixture of platinum (Pt), chromium oxide (Cr ₂ O ₃ ) and copper oxide (CuO), and the constituent ratio is 1: 0.5: 0.5 in molar ratio.

(3) Electrodeposition method: A voltage was applied intermittently. An AC voltage with a maximum voltage of 20 V and a frequency of 50 Hz was applied, the current value was 20 mA, and three sample sensors with different application times were created. As described above, the difference in application time is the difference in accumulated electric energy (mW), and the thickness of the carrier attached to the coil is substantially proportional to the accumulated electric energy.

(4) Electrodeposition conditions: Table 1 shows.

(5) Firing conditions:
Atmospheric temperature: 500 ° C
Coil applied voltage: 3V (coil temperature 300 ° C)
Baking time: 10 minutes (6) Sensor sensitivity characteristic: The sensitivity characteristic on the low concentration side is lower than that of the sensor 3 having a relatively thick film thickness. It was confirmed that the sensitivity was greatly improved even in the region of 0.03% (300 ppm) or less, which could not be measured in the past, and measurement was possible even at a concentration of about 0.005% (50 ppm).

<About detection circuit>
In a general detection method, the reaction catalyst is applied to the detection coil in a beat shape, the reaction gas is added to the catalyst in a state in which a constant power is applied and the self-heat is generated up to the base temperature. On the other hand, a non-reactive substance is similarly applied to a dummy coil, which is a compensation element, and power is applied to cause self-heating to the same base temperature to form both of them as a bridge circuit, and a temperature difference obtained by reaction heating is extracted as a voltage difference. is there. Incidentally, the catalyst side may be referred to as a first temperature sensor and the non-catalyst side as a second temperature sensor. This second temperature sensor is commonly referred to as a reference or dummy sensor, but hereinafter referred to as a dummy. This dummy means a compensating element. A combination of both of these sensors is called a sensor housing, and gas is generally injected through an activated carbon filter.

<(1) Conventional detection circuit>
A typical conventional example shows a schematic circuit in FIGS. FIG. 6 shows an example of a circuit based on the constant current feedback compensation bridge method, and FIG. 7 shows an example of a circuit based on the constant current simple parallel compensation method. E is a constant-voltage power supply, D is a compensation element (dummy) coil that does not have a catalytic reaction and performs a compensation function, S is a contact combustion type sensor coil, a detection element, and a structure in which a measurement gas is applied to both. is there. The electrical condition at no gas equilibrium is basically s = d. R1 and R2 are resistors, mV is an output meter, and G is a measurement gas.
The circuit equilibrium condition shown in FIG.
When d and s are effective resistances of the dummy coil D and the sensor S, the following equation is established.
R1 / R2 = d / s
In the circuit shown in FIG. 7, R1 / d = R2 / s is established.
The difference between the two is that in the circuit of FIG. 6, the dummy D and the sensor S are in series and the same current flows, and the complementary current feedback system that interferes with each other increases the dynamic range and increases the linearity. The circuit of FIG. 7 is a circuit in which the dummy coil D and the sensor S are assembled in parallel to eliminate mutual interference. The sensitivity is low and the sensitivity is good, and the voltage range is wide by selecting R1 and R2, but the current consumption is doubled.

In the circuit adjustment method of FIG. 6, the terminal voltage of the sensor S is monitored, and the power supply E is adjusted to a voltage that reaches the base temperature obtained from the prior data. Finally, the output meter mV is driven to zero with the variable resistor R2. In this circuit, since the terminal voltages of the dummy coil D and the sensor S are the same, the least square method is established and the maximum sensitivity is obtained. The output characteristic at the time of a large signal shows the S curve characteristic peculiar to the bridge circuit.
In the circuit of FIG. 6, the power source E is set so that the terminal voltage of the sensor “S” is a base temperature voltage, and zero is driven by a resistor R1 in which the terminal voltage of the dummy D is variable. Also in this circuit, if the terminal voltages of S and the resistor R2 are set to be the same, the least square method is applied as in the circuit of FIG. 6, and it can be regarded as a linear output with maximum sensitivity near the equilibrium.
Both circuits use a compensation dummy coil D, but the same metal with the same temperature coefficient is coated with a non-reactive substance, the mass is adjusted and immersed in the same gas atmosphere, and mainly compensates for the gas temperature and ambient temperature It is. Of course, if it is possible to physically and electrically compensate, it is considered that this dummy element is not necessary, but at present, compensation in a low concentration region is difficult.

<(2) Bridge-type circuit with constant current of the present invention>
FIGS. 6 and 7 are constant voltage types because the power source E is a constant voltage power source. The biggest disadvantage of these is that when immersed in a high-concentration gas, the temperature rises due to reaction heating, and thus the resistance value rises. As a result, the current value of both coils decreases, resulting in a decrease in sensitivity. It will be.

The constant current method proposed here makes the current of this decrease constant, and as a result, increases the output signal. In the experiment, an output more than twice that of the circuit of FIG. 6 was confirmed. An increase in signal output is advantageous for all detections.
8 and 9 are schematic diagrams of constant current type circuits. If d, s, c1, and c2 are effective resistances of the respective elements, the equilibrium condition is as shown in FIG.
R1 / R2 = d / s,
In the circuit shown in FIG. 9, c1 / d = c2 / s. Since C, C1, and C2 are constant current sources, the effective resistance can be defined by Ohm's law. However, since the output impedance is infinite, the current value is constant even if the power supply E fluctuates. Therefore, the power source E does not need to be particularly stable. Incidentally, the output impedance of the constant voltage source is theoretically zero.

  In the circuit shown in FIG. 8, if R1, R2 >> d, c are set, it can be ignored without two constant current sources as in the circuit of FIG. In the circuit of FIG. 8, each coil is connected in series. However, since it is a constant current source, current feedback as in the circuit of FIG. 6 is not established. Therefore, only the amount of change in the resistance of the sensor S is output in the circuits shown in FIGS. In the circuit shown in FIGS. 6 and 7, the current decreases as the resistance increases, and the output decreases as “voltage V = current I × resistance R” according to Ohm's law. As described above, the constant current method is a circuit having a feature that a maximum signal can be extracted. This circuit is very useful for weak signal changes like the sensor of the present invention.

<(3) Compensation method and composite detection method>
Next, the meaning and method of compensation and composite detection will be mainly described.
(3) -1 Stability Compensation In the catalytic combustion type gas detection system of the present invention, the base temperature is set by the detection gas and the catalyst. For this reason, the fluctuation of the base temperature affects the accuracy in a low concentration gas. Therefore, in general, a balance is made with a non-catalyst heater coil having the same temperature coefficient for stabilization. These are mainly for the compensation of the detecting element itself such as the incoming gas temperature and ambient temperature, and it is also necessary to compensate with a third temperature sensor for correcting the electronic circuit and other temperature coefficients. Naturally, since it is performed by a microcomputer, various methods can be taken. In any case, the enhancement of the output by the constant current method is very useful for the stability and the low concentration range.

(3) -2 Composite detection method An example of a circuit for searching for a flammable gas mixture is shown.
In the circuit of FIG. 10, the mixed gas can be detected by detecting the element voltage changes v1 and v2 of the catalyst elements S1 and S2, multiplying by the proportional constant, and adding between the two elements without a bridge configuration.
In addition, the selectivity of the detection gas can be enhanced by subtracting in reverse from the specific gas of the mixed gas and the characteristics of the catalyst. When expressed simply, V = k | V ₁ ± V ₂ | (k = proportional constant). In addition, improvement in accuracy can be expected by forming a bridge with a single element as in the circuit of FIG. 8 and calculating with a plurality of bridge circuits. In this case, the degree of freedom in setting the base temperature of each element increases, and the utilization range of the catalyst also increases. In any case, when a plurality of bridge openings are used, a dedicated temperature sensor is required for ambient temperature compensation.
(3) -3 Characteristics of specifications for household alarm devices Household fire alarms, which have been legalized today, position the detection of carbon monoxide as useful. It is said that the monitoring of carbon monoxide gas generated at the time of incomplete combustion before a fire is in principle superior to the current fire alarm mainly monitoring the rise in room temperature of the heat generated by the fire. In short, early detection is possible because it can be detected in the state of being hit before the fire.

Now, home use is in line with measuring instruments in terms of mass productivity, cost and maintenance. In other words, it is impossible to take it home to the manufacturer and perform maintenance, so it is necessary to do it on site. Further, if the pairing of the sensor coil and the dummy coil is exchanged at the same time, the cost increases.
In the circuit shown in FIG. 10, S1 is used as a sensor coil to provide compatibility, and S2 is used as a reference to fix the circuit. With special operation when the power switch is turned on, the balance amount is automatically measured in the auto tuning mode, data is collected, the sensitivity difference of the suncer is measured in advance, and it is designed to be set in hardware and software. It is exchangeable. Further, if accuracy at high density setting is not required, the reference in S2 can be replaced with a simple temperature sensor.

<(4) Output characteristics of detection circuit>
(4) -1 Output Characteristics Table 2 shows output measurement data when the sensor 2 shown in Table 1 is applied to the constant voltage circuit shown in FIG. 6 and the constant current circuit shown in FIG.
The combination of sensor and reference was the same, and the base temperature was also set.

  In the example of Table 2, it can be seen that the output voltage is about twice as high in the constant current circuit than in the constant voltage circuit. In the strong concentration region of 50,000 ppm (5%) or more, the output of the constant voltage circuit of FIG. 6 started to decrease by the least square method, and an output decrease of about 10% was observed at 100,000 ppm (10%). These are seen as least-squares S-curves at the bridge. However, in the constant current circuit of FIG. 8, it was confirmed that those curves were not seen and extended in a straight line. This is because the constant current circuit is simply proportional to the resistance. As described above, the constant current circuit is a detection method excellent in output and linearity.

<(5) Calibration method>
(5) -1 General calibration method In general, the sensor housing is integrated with the sensor and the dummy so that the sensor housing can be replaced at the same time. For calibration, adjust the sensitivity to zero with standard air (generally clean air) and adjust the sensitivity to the specified value. For those equipped with ambient temperature fluctuation correction, standard air or specified concentration gas, for example, measure the fluctuation amount at 0 ° C, 25 ° C, 50 ° C, and set the correction value. Calibration is required.

(5) -2 Calibration method for home alarm devices Home alarm devices are difficult to calibrate for take-out each time in terms of quantity and cost. Therefore, there is a method in which the reference side is fixed, only the sensor is replaced, “zero” is auto-tuning by software, and the sensitivity is obtained by encrypting the measured value for each sensor and inputting it with a digital switch or the like. The same applies to a system that eliminates the reference and substitutes it with another temperature element.

<(6) Constant current>
There is a constant current diode by a field effect transistor as a constant current circuit element, but no other constant current element is found as a commercial element. The constant current diode is a fixed type with a maximum of about 10 mA and has a large temperature coefficient. When used in a gas sensor, the coil cannot be used because fine adjustment is required at about 100 mA.
Therefore, we propose a circuit that can withstand these.
As an example thereof, a circuit diagram shown in FIG. 11 is a commonly used operational amplifier, and the feedback circuit shows a constant current characteristic. FIG. 11 shows an example in which a single power supply operational amplifier “OP” is used, and the current of the feedback resistors “Ra, Rb” can be set by the variable resistor “VR”. The bias current “Iin−” of the inverting input is large and about several μA to few pA. Therefore, the feedback circuit current Ira >> bias current Iin− (“>>” indicates that the difference is very large) and can be ignored. Therefore, the feedback circuit has a constant current characteristic.
In the circuit diagram shown in FIG. 12, a power transistor “TR” is inserted in order to enhance the output current of the circuit diagram of FIG. The sensor S is inserted into the resistor Rb, and the resistor Ra is inserted to the same extent as the internal resistance Ra of the sensor. When the coil current is finely adjusted by VR, constant current driving can be realized.
The circuits shown in FIGS. 11 and 12 assume a stabilized power supply. However, if the “+” terminal of the operational amplifier “OP” is stabilized, a stable constant current can be secured even with an unstable power supply. That is, a constant current can be supplied even if the power source fluctuates.

<6 Overall evaluation and usefulness>
Now, as a comprehensive evaluation, the sensitivity comparison between the conventional beat type and the ultra-thin hollow thin-film coil type of the present invention is more than twice, and the temperature coefficient ratio of platinum to nickel is 1.4 times higher. The conversion with a constant current circuit is slightly more than twice, and since the fusion with other substances is good, improved development of the catalyst has progressed and the sensitivity more than twice has been experimentally found. As a result, the sensitivity increase of 11 times (2 × 1.4 × 2 × 2 = 111.2) is improved. This is because in the conventional product which is a bead type sensor, carbon monoxide (CO) is limited to 200 ppm, but the sensor using the nickel-based conductor of the present invention and the detection circuit using the constant current circuit By using this, a stable sensitivity up to about 20 ppm can be secured.
Because of its good fusion with nickel wire, the range of utilization of the catalyst is widened, and it is possible to detect carbon monoxide (CO) and hydrogen (H ₂ ) alone compared to the conventional type, which is more sensitive to combustible gases. Become. These are characteristics that are not available in the ready-to-use instruments currently on the market.

Schematic diagram of conventional bead-shaped catalytic combustion gas sensor Conventional circuit diagram example The schematic perspective view which shows the example of the contact combustion type gas sensor of this invention Schematic diagram showing a porous carrier containing the catalyst of the present invention The schematic diagram showing the method of manufacturing the catalytic combustion type gas sensor of the present invention Example of conventional constant voltage detection circuit Example of conventional constant voltage detection circuit Example of constant current detection circuit of the present invention Example of constant current detection circuit of the present invention Example of composite constant current detection circuit of the present invention Example of constant current element Example of constant current element

Explanation of symbols

1: catalytic combustion type gas sensor 2: coil wire 3: carrier 4: catalyst 5: hole 6: through hole

Claims (3)

A detection circuit using two catalytic combustion gas sensors in which a catalyst supported on a carrier having a large number of holes is attached to a nickel-based conductor to a thickness of 0.1 to 0.5 mm,
A constant current circuit C2 arranged in series and one catalytic combustion gas sensor S1, a constant current circuit C3 arranged in series and the other catalytic combustion gas sensor S2 are arranged in parallel,
Detectors V1 and V2 for detecting the respective voltages of the two catalytic combustion gas sensors S1 and S2 are provided, and a mixed combustion gas is detected by comparing the two detected voltages. Detection circuit.   The detection circuit using the catalytic combustion type gas sensor according to claim 1, wherein the nickel-based conductor has a hollow coil shape. Combustible gas sensor, characterized in that it comprises a detection circuit using a been catalytic combustion type gas sensor according to claim 1 or 2.