JPH0157731B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in a gas detection method using a self-heating type gas detection element, and more particularly to a gas detection method capable of preventing deterioration of the element due to self-heating when used in a highly concentrated contaminated atmosphere. . The self-heating type gas detection element here refers to a gas detection element that uses changes in electrical conductivity due to gas adsorption of an N-type metal oxide semiconductor, and the self-heating caused by the detection current cannot be ignored. . The present inventors have studied gas detection in underground malls and the like. As a result, it was found that in such an environment, the resistance of the self-heating type element gradually decreases and the alarm concentration for gas decreases, and that the decrease in resistance value is particularly remarkable for gases such as hydrogen and alcohol. Such a phenomenon does not occur in an element in which self-heating is suppressed and the heating temperature of the element is kept constant. However, self-heating elements have many characteristics. This element is normally kept at a relatively low temperature, and when it comes into contact with gas, it automatically heats up due to self-heating. Therefore, gases that should be detected at high temperatures, such as methane and isobutane, can be detected without thermal deterioration. For example, in the case of the SnO _2- based #109 (#109 is the product name) sensor manufactured by Figaro Giken Co., Ltd., the temperature of the element in normal air is 300°C.
~350℃, while in gas (methane)
4000ppm) reaches 550℃. Next, self-heating type elements have particularly high output in high concentration gas. In other words, there is a characteristic that the dependence of the output on the gas concentration increases rapidly in high concentration gas. For example, in the #109 sensor mentioned above, isobutane
The resistance value of the element at 1000ppm is 5.9KΩ, 4000ppm
Inside it is 2.8KΩ, while in 10000ppm
It becomes 1.05KΩ. This characteristic is obtained by ensuring that the element reaches its maximum heating temperature in the gas at the alarm level, and that the temperature coefficient of resistance in the gas becomes positive around that temperature. When the gas concentration becomes higher than the alarm concentration, the resistance value of the element decreases and the amount of self-heating decreases. As the temperature of the device decreases, the resistance of the device decreases due to the positive temperature coefficient of resistance, creating a positive feedback cycle between decreasing temperature and decreasing resistance. For this reason, even if detection at the original alarm concentration fails, a warning will be reliably issued before any danger actually occurs. Further, a self-heating type element can be used even if the heating by the heater is uneven. The reason for this is unknown, but
This property has already been confirmed through long-term experiments. As described above, self-heating type gas detection elements have many advantages, and most of the gas detection elements used in Japan are self-heating type. The inventors of the present invention have confirmed the following facts as a result of studying the effects on devices in installation spaces such as underground malls. Lowering the resistance of the element occurs only in self-heating type elements. The atmosphere in which the resistance decreases contains a large amount of hydrogen and alcohol, especially hydrogen.
Moreover, its concentration is 500 ppm or less. The degree of resistance reduction depends on the hydrogen concentration in the atmosphere and
This phenomenon is determined by both the contact time with the gas and is irreversible. Temperature rise due to self-heating contributes to lowering the resistance, and this also occurs when the self-heating amount is artificially increased by increasing the applied voltage in a still atmosphere. In the end, these problems were caused by the unexpected progress of contamination in living spaces, and they first became apparent in the heavily contaminated underground malls. In other words, the basic design concept of a self-heating type gas detection element is that the atmosphere is normally clean, self-heating is small, and there is no risk of the element being overheated unnecessarily. Despite this, a situation arose in which the elements were constantly overheated due to unexpectedly significant environmental pollution. The present invention aims to solve such problems. The method of the present invention detects the gas concentration from the electrical conductivity of a self-heating type gas detection element, and issues an alarm when a gas exists at a predetermined concentration or higher, and when a gas at a lower concentration exists, the gas concentration is detected. After a certain delay time has elapsed, the temperature of the element is lowered to prevent deterioration of the element. FIG. 1 shows a flowchart for the embodiment. Numerical conditions for Examples are shown in Table 1. Table 1 Contents Self-heating type #109 Gas detection element made by Figaro Giken Sensor load resistance 3.5KΩ Heater voltage 1.1V Fixed Heater voltage 530m watts Applied to the series piece of circuit voltage element and load resistance,
100V or 50V self-heating value Approximately 100m watt in still air, maximum 715m watt in gas watt Alarm concentration methane 4000ppm, hydrogen
5000ppm, isobutane 2000ppm, alarm element temperature when element resistance is 4KΩ or less In still air 300 to 350℃ Maximum in gas 550℃ When the power is turned on and detection starts, the element first under normal operating conditions (circuit voltage (Vc) 100V, heater voltage (VH) and 1.1V element temperature in still air
300-350℃). Next, the gas concentration is detected from the electrical conductivity (σ _S ) of the element, and an alarm is issued when σ _S is 0.25 m or more. The gas concentration at this time is 4000ppm for methane, 2000ppm for isobutane, and 2000ppm for hydrogen.
At 5000ppm, the element is heated to 550℃.
When the gas concentration falls below the alarm concentration, the alarm will be stopped. When the element comes into contact with gas below the alarm concentration, that is, when σ _S is 0.031m (A level) or more, the temperature drops to 0.122m.
(B level) or below (this is hydrogen)
50ppm or more and 1000ppm or less. ), detection continues for a delay time _T1 of approximately 2 minutes, and if the gas concentration does not reach the alarm concentration during that time, it is determined that the atmosphere is contaminated, the circuit voltage is lowered to 50V, and detection is stopped. do. In this case, the maximum self-heating amount is only 180m watt, and the temperature of the element is 380℃.
That's all. Then, after maintaining this state for a time T ₂ of about 10 minutes, normal usage conditions (Vc =
100V). In this case, the B level is determined depending on the degree of environmental pollution and is set to be higher than the electrical conductivity in hydrogen of 500 ppm. Further, the delay time T ₁ shall be set to at least 10 seconds or more, taking into account the transient characteristics of the element with respect to temperature changes. Time to lower the temperature of the element T ₂
Detection will stop during this period, so the maximum period is 30 minutes. Make T ₁ as small as possible compared to T ₂ to avoid deterioration of the element. Furthermore, although the element temperature was lowered by reducing Vc here, _VH may be reduced instead of Vc. Furthermore, in order to simplify the circuit, the B level may be made to match the C level, and the A level may be abolished. An example circuit for the embodiment of FIG. 1 is shown in FIG. In the figure, 1 is an AC power supply (AC100V), which uses a transformer and a smoothing circuit to create an AC power supply V _H (1.1VAC) for the heater and a DC power supply (Vcc.+) for the control circuit.
5V). A DC constant voltage circuit 2 is connected to the AC power supply 1 via a half-wave rectifier circuit, and its output is connected to a self-heating type gas sensor 3 and a load resistor R1,
Apply to a 3.5KΩ series piece. Note that the circuit constants are determined so that the output of the constant voltage circuit 2 is 100V when the transistor Tr1 is conductive, and 50V when it is not conductive. The voltage applied to the load resistor R1 is detected by three comparators C1, C2, and C3. The comparator C1 detects whether or not a gas having a concentration below the alarm exists, and operates the buzzer 4 based on its output. Comparator C
2, C3 is for detecting the degree of pollution of the atmosphere. The outputs of the comparators C2 and C3 are each connected to an input terminal of an AND element 5, and the output thereof is connected to one input of a NOR element 6. The other input terminal of the NOR element 6 is connected to the NOR element 7 whose output is normally at a low level. Then, the output of the NOR element 6 is connected to the input of the OR element 8. The other input terminal of the OR element 8 is connected to a one-shot flip-flop circuit 9 whose output is normally 0. The output terminal S of the OR element 8 is connected to the gate of the FET transistor 10, and the source of the transistor 10 is connected to a CR timer T1 whose operating time is 2 minutes. CR
A comparator C4 is connected to the timer T1, and a CR timer T2 whose operating time is 10 minutes is connected to the output side of the comparator C4. A comparator C5 is connected to the CR timer T2. The outputs of comparator C4 and comparator C5 are each connected to NAND element 1.
Enter 1. Further, the output of the comparator C4 is connected to the set input of the reset flip-flop circuit 12, and the output of the NAND element 11 is connected to the reset input. Comparator C is connected to the input of Norer element 7.
5 and the output of the reset flip-flop circuit 12 are connected. Also, comparator C5
The output of the NOR element 7 is input to the one-shot flip-flop circuit 9, and the output of the NOR element 7 is input to the buffer
is connected to the base of the transistor Tr1 via the output terminal T of the transistor Tr1. This circuit operates as follows. When the power is turned on, the heater of element 3 has
An AC voltage of 1.1V is applied, and a DC voltage of 100V is applied to the series piece of element 3 and load resistor R1. When the gas concentration in the atmosphere exceeds the alarm concentration, the buzzer 4 is activated by the comparator C1. When the atmosphere is contaminated and the concentration is between the concentrations determined by the comparators C2 and C3, the AND element 5
The output of the NOR element 6 becomes high level, and the output of the NOR element 6 becomes low level. As a result, the output of the OR element 8 also becomes low level, the FET 10 becomes non-conductive, and the CR timer T1 starts operating.
When CR timer T1 charges, comparator C
The output of the reset flip-flop circuit 12 becomes 0, the output of the NOR element 7 is inverted to high level, and the transistor Tr1 becomes non-conductive via the buffer 13. This reduces the voltage applied to element 3 and load resistor R1 to 50V. Comparator C
When the output of 4 becomes high level, CR timer T
2 begins to operate, and after charging the CR timer T2, the output of the comparator C5 becomes high level. As a result, the output of the NAND element 11 becomes low level, and the reset set flip-flop circuit 12
is reset. With this, Noah element 7
and a transistor via buffer 13.
Tr1 becomes conductive again, and the voltage applied to element 3 and load resistor R1 becomes 100V again. On the other hand, when the output of the comparator C5 becomes high level, the one-shot flip-flop circuit 9 operates, and the OR element 8
The FET transistor 10 is made conductive through the .times.1.times.2.times.1.times.2.times.1.times.1.times.2.times..times..times..times..times..times..times..RTI. In this way, the operating state of the circuit returns to its original state. Test results according to this example are shown below. The test was conducted by examining the change in alarm concentration when the device was continuously energized for two weeks in 500 ppm hydrogen. As a comparative example, the same method as in the example was used except that the circuit voltage was set to 100 V regardless of the gas concentration. The results are shown in Table 2.

[Table] As explained above, according to the method of the present invention, deterioration of a self-heating type gas detection element due to use in a contaminated atmosphere can be prevented.

[Brief explanation of drawings]

FIG. 1 shows a flowchart of the method of the embodiment, and FIG. 2 shows an example of a circuit embodying the embodiment of FIG. 1...AC power supply, 2...DC constant current circuit, 3...self-heating type gas detection element, 4...load resistance, T1, T2...
Timer, Tr1...transistor.

Claims (1)

[Claims] 1. A gas detection element using an N-type metal oxide semiconductor is heated by self-heating generated by heater power and a detection current, and the temperature of the element is automatically raised when it comes into contact with gas, and in the method of detecting gas based on the electrical conductivity of the gas detection element and issuing an alarm, when the presence of gas at a concentration below the alarm concentration is detected, there is a delay time, and then the temperature of the gas detection element is lowered for a predetermined period of time. 1. A gas detection method characterized in that deterioration of a gas detection element due to the presence of a low concentration gas is prevented.