JP2702272B2 - Gas detector - Google Patents

Description

The present invention relates to the detection of fuel gas leaks such as methane and propane, the detection of incomplete combustion gas such as carbon monoxide and hydrogen, and the detection of air pollution by tobacco and spray. The present invention relates to a gas detection device having a detection function.

[Prior Art] Conventionally used household gas leak alarms are methane,
It is provided for the purpose of detecting fuel gas leakage such as propane. In recent years, it has been required to detect the level of indoor air pollution caused by exhaust gas from petroleum fan heaters and smoke from cigarettes. However, Pt and Pd-added tin oxide semiconductor sensors used in conventional household gas leak alarms have less than 500 ppm of carbon monoxide and hydrogen in incomplete combustion exhaust gas from petroleum fan heaters and cigarette smoke. It was difficult to detect at low concentrations and output an alarm. Therefore, as an alarm that detects both fuel gas leakage and incomplete combustion exhaust gas, a tin oxide semiconductor sensor added with Pt or Pd used at about 400 ° C and a sensor for detecting carbon monoxide and hydrogen A gas detector using two sensors, ie, a tin oxide semiconductor sensor for detecting incomplete combustion exhaust gas used at about 100 ° C. is used.

[Problems to be Solved by the Invention] A gas detection device that can be commonly used for detecting fuel gas leakage and incomplete combustion exhaust gas has been configured using two types of semiconductor sensors as described above. Therefore, two types of accessory circuits are required for each semiconductor sensor, and there has been a problem that the device becomes complicated and large.

The present invention has been made to solve the above problems, and has as its object to provide a simple and miniaturized gas detection device.

[Means for Solving the Problems] A gas detection device according to the present invention includes a low-heat-capacity hot-wire semiconductor gas sensor mainly composed of tin oxide semiconductor, and a hot-wire semiconductor gas sensor in a highest concentration gas within the detection range. A power supply section for intermittently supplying power via a load resistor having a resistance value smaller than the resistance value; and a detection circuit section for measuring an output of the hot-wire semiconductor gas sensor in accordance with the intermittent cycle.

[Operation] In the gas detection device of the present invention, a load resistance smaller than the resistance value in the re-high concentration gas within the detection range of the gas sensor is connected to the hot-wire semiconductor gas sensor mainly composed of tin oxide semiconductor. When the gas sensor detects the gas to be detected, the temperature of the gas sensor increases. As the temperature rises, the relative sensitivity of the gas sensor to a plurality of detected gases changes.

The power supply unit of the gas sensor intermittently supplies power to the gas sensor, and the detection circuit unit measures the output of the gas sensor in accordance with the intermittent cycle, so that the substantial operation time of the gas sensor is short.

[Embodiment] FIG. 1 is a block diagram of a gas detection device showing an embodiment of the present invention. FIG. 2 is a perspective view of a hot-wire semiconductor gas sensor (5) used in the present gas detection device shown in FIG.

The semiconductor portion (1) is formed as a gas-sensitive layer containing tin oxide as a main component in a spherical shape of, for example, 0.6 mmφ around a coil (4) made of a noble metal, thereby forming a hot-wire semiconductor gas sensor (5). .

When antimony is added to tin oxide, which is a main component of the semiconductor portion (1), or one or more elements selected from Group IV elements and lanthanoids are added, the gas described below depends on the type and amount of the added element. The temperature dependence of the sensitivity, the durability of the sensor, and the like are adjusted to desired values.

Constant concentration (4000pp) of hot wire type semiconductor gas sensor (5)
FIG. 3 shows the temperature dependence of the sensitivity of m) to main gases. The numerical value shown as the relative sensitivity in FIG. 3 is a relative value of a sensor output described later. The measurement of the temperature dependency is performed by housing the hot-wire semiconductor gas sensor (5) in a high-temperature tank whose inside is kept at a constant temperature and measuring the sensor output. From FIG. 3, the hot-wire semiconductor gas sensor (5) is highly sensitive to miscellaneous gases such as carbon monoxide, hydrogen, and ethanol at low temperatures (about 300 ° C.), and is sensitive to fuel gases such as methane and propane. It shows that the sensitivity is relatively low. Also, it can be seen that the hot-wire semiconductor gas sensor (5) has a high sensitivity to fuel gas at a medium temperature (about 500 ° C.) and a relatively low sensitivity to miscellaneous gases. It can be seen that the sensitivity to the fuel gas is the best at a higher temperature (about 600 ° C.). Therefore, the hot wire semiconductor gas sensor (5) has high sensitivity to miscellaneous gases when used at low temperature (about 300 ° C.), and has high sensitivity to fuel gas when used at high temperature (600 ° C.).

However, if the present hot-wire type semiconductor gas sensor (5) (hereinafter abbreviated as a sensor) is always maintained at about 600 ° C., the sensor is formed within a very short period of one to two months due to crystal grain growth in the semiconductor tank (1). Changes with time. In order to use the sensor (5) stably even at a high temperature without causing such a temporal change, when the gas to be detected is not present in the atmosphere, the sensor temperature is kept at a low temperature, and the sensor is operated intermittently. Gas detector that can prevent sensor deterioration (1
2) is shown in FIG. Hereinafter, how the sensor (5) functions in the gas detection device (12) shown in FIG. 1 will be described with reference to the drawings. Here, the load resistance (7) of the sensor (5) is set to 1.2 Ω, which is about 1/8 of the operating resistance value of the sensor (5) in the atmosphere at about 350 ° C., for example, 9 Ω. The resistance value of the sensor (5) is measured by the detection circuit section (9) as a voltage between both points A and B of the bridge circuit. The measured voltage value Vs is input to the microcomputer circuit section (10). The gas sensor power supply circuit section (8) applies a constant voltage to a series circuit composed of the sensor (5) and the load resistor (7). For example, carbon monoxide,
When the sensor (5) detects miscellaneous gases such as hydrogen, the resistance of the sensor (5) decreases due to the adsorption of these gases. The decreased resistance value of the sensor (5) is determined according to the gas concentration at that time. At this time, as described above, a constant voltage is applied to the sensor (5) and the load resistor (7) in series, and the value of the load resistor (7) is set smaller than the resistance value of the sensor (5). Therefore, as the sensor current increases, the sensor power consumption increases, thereby further increasing the sensor temperature. That is, the sensor temperature rises to a constant temperature corresponding to the resistance value of the sensor (5) that has decreased according to the gas concentration. The actual sensor temperature at that time rises up to about 500 ° C. depending on the concentration of the miscellaneous gas. Such a decrease in the sensor resistance value and an increase in the sensor temperature also occur when a leak of fuel gas such as methane and propane is detected. In the measurement of the relative sensitivity shown in FIG. 3, the load resistance is not set small but is set large so as not to increase the sensor temperature in accordance with the gas concentration. ing.

An example of the change in the relative value of the voltage between the two points AB and the sensor surface temperature in the bridge circuit with respect to the gas concentration of the main gas is shown in FIG.
It is shown in the figure. As the resistance value of the sensor (5) decreases, the voltage between the two points A and B of the bridge circuit increases. The graph in FIG. 4 shows a monotonically increasing relationship and rises to the right.
Hereinafter, for simplicity, the voltage between both points A and B of the bridge circuit is referred to as a sensor output. For example, when this sensor (5) is placed in an atmosphere of 1000 ppm of carbon monoxide, it can be seen from FIG. 4 that the output of the sensor (5) has a relative value of 20 and the sensor surface temperature is about 400 ° C.

Further, as can be seen from FIG. 4, the miscellaneous gases such as carbon monoxide, hydrogen, and ethanol tend to saturate the sensor output at a high concentration of about 1000 ppm or more. This is because, as in the case of FIG. 3, the sensitivity to these miscellaneous gases decreases due to the increase in the sensor temperature accompanying the increase in the gas concentration. The cause of the decrease in sensitivity to miscellaneous gases on the high-temperature side is that most of the gases burn on the surface layer of the semiconductor unit (1) due to the flammability of the miscellaneous gas, and CO ₂ and H ₂ are insensitive to the sensor (5). Probably because it is converted to ₂ O.

On the other hand, fuel gases such as methane and propane have flame-retardant properties, so they are not burned and removed at the surface of the semiconductor, and the high-concentration gas exceeding 1000 ppm raises the sensor temperature, further increasing sensitivity. I do. As a result, almost 10,0
The sensor output does not saturate up to about 00 ppm, the relationship between gas concentration and sensor output increases monotonically, and the sensor temperature also increases.
Reach about 600 ° C. As the methane gas concentration increases, the resistance value of the sensor (5) decreases, the sensor power consumption increases until the sensor resistance becomes equal to the load resistance, and the sensor temperature increases. That is, the sensor (5)
When the resistance value becomes equal to the load resistance value, the sensor temperature also becomes maximum. In other words, the above-described relationship between the gas concentration and the monotonic increase in the sensor output is established up to the vicinity of the gas concentration at which the resistance value of the sensor (5) decreases and becomes equal to the value of the load resistance (7). In general, methane, propane, etc. in household kitchens using this sensor (5) are subject to the "Gas Leak Alarm Inspection Rules for City Gas" or "Liquefied Petroleum Gas Leak Alarm" issued by the Japan Gas Appliance Inspection Association or the High Pressure Gas Safety Association. Approximately 2000 ppm to about 1/4 LEL (Lower Explosi
ve Lebel (lower explosion limit) is set to emit a gas leak alarm when a gas with a gas concentration in the upper limit of the range is detected. Therefore, it is necessary to maintain the above-mentioned relationship between the gas concentration and the monotonic increase of the sensor output at least up to the gas concentration at the upper limit of the alarm concentration range. Therefore, it is necessary to select, as the resistance value of the load resistance (7), a value lower than the resistance value of the sensor (5) at the upper limit gas concentration at which this gas leakage alarm is transmitted. The resistance value of the sensor (5) used in the example at the gas concentration at the upper limit was about の of the operating resistance value of the sensor (5) in the atmosphere. Therefore, in this case, it is necessary that the value of the load resistance (7) is not more than 1/2 of the atmospheric resistance of the sensor (5).

As described above, the lower the value of the load resistance (7), the greater the sensor temperature rise due to the change in methane gas concentration, and the greater the rate of increase in the sensor output. Therefore, measurement of a gas having a higher concentration becomes possible. However, in that case, the sensor temperature may rise excessively, and the sensor (5) may be destroyed by heat. Therefore, the lower limit value of the load resistance is set so that the temperature of the sensor (5) when the resistance value of the sensor (5) decreases to the value of the load resistance (7) is within a temperature range where the sensor (5) is not damaged. Must be determined. This value slightly varies depending on the composition and structure of the sensor (5). According to the experiment, the preferable lower limit of the load resistance for the sensor (5) was about 1/20 of the operating resistance of the sensor (5) in the atmosphere.

Also, carbon monoxide, which is generated during incomplete combustion or contained in smoke of cigarettes, etc., as a miscellaneous gas, also triggers an air pollution alarm when a concentration of 50 to 500 ppm is detected based on the regulations of the Japan Gas Appliances Inspection Association. It is necessary to send. In FIG. 4, the gas sensor alarm generator (10) shown in FIG. 1 outputs an air pollution alarm when the sensor output reaches the region C, and issues a fuel gas leakage alarm when the sensor output reaches the region D.
The ranges of the alarm C 'and the alarm D' of b) are set in advance. More specifically, in a comparator (not shown) in the gas sensor alarm generation section (10b), the detection circuit section output Vs is set in advance in the lower limit c of the area C shown in FIG. Compare with a certain lower limit d. Comparison result c ≦
If Vs <d, an alarm C 'is generated, and if d≤Vs, an alarm D' is generated. If V <c, no alarm is issued. In this case, if the sensor (5) is placed in an atmosphere of methane gas of about 500 ppm, it enters the range of the alarm C 'and emits an air pollution alarm. This is about the concentration of unburned fuel gas that is emitted for a short time when the operation fails, and the operation of a ventilation fan or the like is promoted by transmitting an air pollution alarm, which is rather preferable in actual use. If there is a considerable amount of gas leakage that could lead to a gas leak, the indoor gas concentration will continue to rise while the air pollution signal is being transmitted,
When it reaches the area D, it changes to "fuel gas leak" warning transmission.

In addition, the output of the sensor (5) is C for various gases such as carbon monoxide, hydrogen, ethanol, etc.
Since the fuel gas stays within the area, there is no possibility of false alarm of "fuel gas leak" by detecting such miscellaneous gas.

As described above, in the sensor (5) of the present invention, the concentration of the sensor (5) changes according to the change in the gas concentration. As a result, the sensitivity to various gases changes, and the two types of gas detection functions are used to detect fuel gas leaks such as methane and propane and air pollution by miscellaneous gases such as carbon monoxide and hydrogen. Has both.

As described above, when two types of gases are detected by one sensor (5), quick response of the sensors is required. That is, it is necessary to reach an equilibrium state of about 600 ° C. as soon as possible when a sensor in a steady state of about 350 ° C. in the atmosphere is placed in an atmosphere of 6000 ppm methane gas. The response speed mainly depends on the voltage V applied to the sensor (5) -the load resistance (7) and the heat capacity of the sensor (5). Therefore, trials for reducing the heat capacity of the sensor (5) were repeated. As a result, the diameter of the semiconductor part (1) is 1 mm
Molded to less than φ, semiconductor part (1) compared to conventional sensor
It was found that when the volume was reduced to about 1/10 or less, a sensor having a sufficiently low heat capacity was obtained, and a rapid response to reach the above-mentioned equilibrium state in about 2 seconds was obtained. Since the responsiveness of the sensor is made very fast in this manner, there is no practical problem in having one type of gas detection function in one sensor (5) as described above.

Further, as described above, a sensor with a rapid response of 1 mmφ or less in diameter has a heating time of 350 ° C. from room temperature to a steady state of 2 seconds or less, and when the power is turned off from a steady state of 350 ° C. in the atmosphere. It was also confirmed that the temperature dropped to approximately room temperature within about 2 seconds. It was also confirmed that the temperature reached from room temperature to about 600 ° C. in about 2 seconds even in a 6000 ppm methane gas atmosphere.

By the way, generally, this kind of gas sensor needs to maintain stable sensitivity for 4 to 5 years. However, as described above, this sensor (5) is operated at about 350 ° C. in normal atmosphere, and 400 ° C. when detecting miscellaneous gas.
The temperature rises to about 600 ° C when fuel gas is detected. As described above, if continuous energizing operation is performed in a gas at a temperature of about 600 ° C., the sensor sensitivity deteriorates. Therefore, it is not a very preferable condition to maintain stable sensitivity for 4 to 5 years. On the other hand, since the sensor of the present invention has a very small heat capacity as described above, it reaches a normal use temperature of 350 ° C. in the atmosphere in about 2 seconds after energization. FIG. 5 shows a lapse of time after energization and a temperature change of the sensor in this state. As can be seen from FIG. 5, the temperature reached approximately 85% of the normal operating temperature in 1 second after the power was supplied and 99% in 2 seconds. Furthermore, when the sensor is placed in an atmosphere containing miscellaneous gas and fuel gas, about 2
It is confirmed that the sensor temperature (350 ° C. to 600 ° C.) corresponding to the gas concentration at that time is reached in about seconds as in FIG. Utilizing the above properties, the gas sensor power supply circuit (8) is controlled by the gas sensor power supply controller (10a) shown in FIG. Energizing the gas sensor intermittently with the frequency of
The output of the sensor (5) is measured only at the last moment of the energization time (for example, for 10 msec), and the gas sensor alarm generation unit (10b) is controlled so as to detect gas. In this way, by using the sensor with an energization time of substantially 2 seconds / 10 seconds = 1/5 or less due to intermittent energization, there is no deterioration in sensitivity due to high-temperature heating in the gas, and even when used for a long period of 4 to 5 years. It is possible to stably maintain the sensitivity. The measurement time of 2 seconds can be adjusted by the voltage applied to the sensor (5) and the load resistor (7) and the heat capacity of the sensor (5). For example, if the applied voltage is increased, the energization time for heating can be further reduced. In addition, when the sensor temperature with respect to the gas concentration is not in an equilibrium state,
That is, it has been confirmed that gas concentration measurement reproducibility can be obtained even in the measurement of the transient state. This type of gas detection device can issue an alarm or the like within approximately 30 seconds after detection of gas leakage according to the above-mentioned gas leakage alarm verification rule. There is no practical problem.

In addition, by intermittently energizing and measuring the sensor as described above, it is needless to say that deterioration of the sensor can be prevented and further power saving can be achieved. As a result, a portable or portable gas detection device that can be driven by a dry battery or a secondary battery is obtained.

The gas sensor power supply control unit (10a) and the gas sensor alarm generation unit (10b) described above are configured by programs in the microcomputer of the microcomputer circuit unit (10), but must be configured by general electronic circuits and the like. Is also possible.

In the embodiment described above, an example was described in which the ranges of two alarms C 'and D' were set in the gas sensor alarm generating section (10b). However, the present gas detection device is used as a general household gas leak alarm. When using (12), alarm C '
By omitting the alarm in the range of, and issuing only the alarm in the range of the alarm D ′ for methane, propane, etc., a household gas leak alarm with reduced cost can be obtained. A conventional household gas leak alarm detects alcohol used in cooking or mixed in a spray or the like, and generates an erroneous alarm several times more frequently than an actual gas leak. However, as described above, the gas detection device (12) issues an alarm in the range of the alarm D ',
Even if ethanol or the like exceeding the ambient temperature is present in the atmosphere, no false alarm is issued. That is, the present gas detection device can be used as a fuel gas-selective household gas leak alarm which is low in cost and is not affected by miscellaneous gases such as alcohol.

The sensor (5) described above is a precious metal coil (4)
This is an example in which a semiconductor portion (1) is formed three-dimensionally around a circle. On the other hand, the embodiment shown in FIG. 6 is an example of a substrate type hot-wire type semiconductor gas sensor (5b).
A platinum thin film is vapor-deposited on an alumina substrate (13) of about 1 mm × 1 mm in length and width, and is formed into a meandering pattern (4a) by photoetching. On top of that, a semiconductor (1) containing tin oxide as a main component is applied and molded. As described above, since the substrate type sensor is manufactured as a small sensor of about 1 × 1 × 0.4 mm, it has a sufficiently low heat capacity.

[Effect of the Invention] As described above, according to the gas detection device of the present invention, the load resistance at least smaller than the resistance value in the highest concentration gas to be measured by the gas sensor is applied to the hot-wire semiconductor gas sensor mainly made of tin oxide semiconductor. Is connected, the temperature of the gas sensor rises when the gas sensor detects the gas to be measured. Since the sensitivity of the gas to be detected by the gas sensor changes with the rise in temperature, one gas sensor can be used to mix 50 to 1000 ppm of carbon monoxide, hydrogen, ethanol, and other miscellaneous gases with 2000 ppm to 1/4 LEL of methane and propane. It is possible to sequentially detect two types of gas, such as fuel gas, etc., and it is possible to perform fuel gas leak detection, incomplete combustion exhaust gas detection, and air pollution detection without misinformation with a single gas sensor. The configuration is simplified and the size is reduced.

The power supply of the gas sensor intermittently supplies power to the gas sensor, and the detection circuit measures the output of the gas sensor according to the intermittent cycle. It is possible to maintain a stable gas sensitivity over a period of time.

[Brief description of the drawings]

FIG. 1 is a block diagram of a gas detection device according to one embodiment of the present invention, FIG. 2 is a perspective view of a hot-wire semiconductor gas sensor used in one embodiment of the present invention, and FIG. FIG. 4 is a graph showing the temperature dependence of the sensitivity of a hot-wire semiconductor gas sensor to a constant concentration of a main gas, and FIG. 4 is a graph showing the AB of the bridge circuit shown in FIG.
FIG. 5 is a graph showing a change in the relative value of the voltage between the two points and the sensor surface temperature. FIG. 5 shows a change in the temperature of the hot-wire semiconductor gas sensor used in one embodiment of the present invention with the passage of time after energization. FIG. 6 is a perspective view of a substrate type hot-wire semiconductor gas sensor. In the figure, 5 is a hot wire semiconductor gas sensor, 7 is a load resistance,
8 is a gas sensor power supply circuit unit, 9 is a detection circuit unit, 10a is a gas sensor power supply control unit, 10b is a gas sensor alarm generation unit, 1
2 is a gas detector.

Claims (3)

(57) [Claims] 1. A low-heat-capacity hot-wire semiconductor gas sensor mainly comprising a tin oxide semiconductor, wherein the hot-wire semiconductor gas sensor is connected to the hot-wire semiconductor gas sensor via a load resistor having a resistance smaller than the resistance in the highest concentration gas within its detection range. A power supply unit for intermittently supplying power; a detection circuit unit for measuring an output of the hot-wire semiconductor gas sensor according to the intermittent cycle. 2. The gas according to claim 1, further comprising an alarm generating unit which receives an output of said detection circuit unit and issues an alarm when the gas concentration is equal to or higher than a predetermined gas concentration corresponding to fuel gas leakage such as methane and propane. Detection device. 3. The alarm generating section receives the output of the detection circuit section and issues an alarm when the gas concentration exceeds a predetermined gas concentration corresponding to leakage of miscellaneous gases such as hydrogen, carbon monoxide, and ethanol. The gas detection device according to claim 1.