JPH06100560B2 - Gas detector - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an improvement in a gas detection device using a gas-sensitive metal oxide semiconductor, and in particular, the semiconductor temperature is periodically changed to a high temperature region and a low temperature region. The present invention relates to a gas detection device that changes.
More specifically, the present invention relates to a gas detection device that simultaneously detects combustible gases such as methane and isobutane and carbon monoxide with one gas sensor.

[Prior Art] JP-B-53-43,320 discloses that a gas sensor using a metal oxide semiconductor such as SnO ₂ is heated alternately in a high temperature region and a low temperature region,
An apparatus for selectively detecting carbon monoxide from an output in a low temperature range is disclosed. The inventor examined using the output of this device in a high temperature range to detect a combustible gas such as methane at the same time as carbon monoxide.

By the way, regarding combustible gas, there is a limit to the dead time of detection. For example, the voluntary verification rules for gas leak alarms for city gas stipulate that an alarm is issued for methane of 1000 to 12,500 ppm and an alarm is issued for methane of 12,500 ppm within 60 seconds. On the other hand, the effect of CO on the human body is slow, and for example, CO of about 100 to 400 ppm may be detected within 5 minutes. However, when the temperature of the sensor is changed cyclically, the heating cycle is the rate-determining rate of the detection speed, and it is necessary to shorten the cycle in order to perform detection quickly. Here, if the heating cycle is shortened, the relative sensitivity between carbon monoxide and hydrogen is lowered, and a false alarm due to hydrogen occurs. And the concentration of hydrogen that can usually be generated is 1000 ppm or less. That is, the request for the detection of carbon monoxide is about 100 ppm of carbon monoxide.
It is to be distinguished from hydrogen of about ppm.

[Problems to be Solved by the Invention] An object of the present invention is to (1) detect combustible gases such as methane and isobutane and carbon monoxide with one gas sensor; (2) shorten the detection cycle; ) To selectively detect carbon monoxide from hydrogen.

[Configuration of the Invention] In the gas detection device of the present invention, the heating temperature of the gas sensor using a metal oxide semiconductor whose resistance value changes depending on the gas is periodically changed alternately to a high temperature region and a low temperature region, and the high temperature region is changed. Combustible gas is detected from the output of the gas sensor in. Next, carbon monoxide is preliminarily detected from the output of the sensor in the low temperature region, and the sensor temperature is changed to an intermediate temperature between the high temperature region and the low temperature region based on the result. Then, carbon monoxide is selectively detected from the difference between the output of the gas sensor in the low temperature range and the output of the intermediate temperature.

The output in the high temperature region mainly represents the concentration of combustible gases such as methane, isobutane, hydrogen, etc., the output at the intermediate temperature mainly represents the hydrogen concentration, and the output in the low temperature region mainly represents the concentrations of hydrogen and carbon monoxide. When detecting methane or isobutane from the output in the high temperature range, there are few false alarms due to hydrogen originally, and the problem is the distinction between carbon monoxide and hydrogen. The output of the intermediate temperature gas sensor mainly represents the hydrogen concentration, and carbon monoxide can be selectively detected by compensating so as to subtract the intermediate temperature gas sensor output from the low temperature region gas sensor output. In this specification, carbon monoxide is not included in combustible gas, and hydrogen is included in combustible gas. This is because carbon monoxide is a highly toxic gas, and it is necessary to detect carbon monoxide at a low concentration in distinction from other combustible gases.

[Examples] Examples will be described below with methane and CO as detection targets. However, various kinds of gas sensors are already known, and each part of the apparatus is within the range of known technology. You can change it freely in. For example, there is a gas sensor in which a single electrode also serving as a heater is embedded in a metal oxide semiconductor, and gas is detected from a change in resistance between both ends of this electrode. In this case, the resistance change of the semiconductor due to the adsorption of gas acts as a change in the parallel resistance of the heater / electrode. Further, in this case, the change in the thermal conductivity of the semiconductor due to the adsorption of the gas brings about a change in the temperature of the sensor, which changes the resistance value of the heater / electrode. Further, the thermistor may compensate the ambient temperature dependency of the gas sensor. Further, a temperature compensating element having a temperature dependency almost equal to that of the gas sensor may be provided and incorporated in the bridge circuit in series with the sensor.

[Structure of Example] In the figure, (2) is a gas sensor, (4) and (6) are electrodes that also serve as heaters, and here, Sn is used as a metal oxide semiconductor.
O ₂ with a small amount of Pd catalyst added is used. The temperature of the sensor is 400 ℃ in the high temperature range and 80 ℃ in the low temperature range.
The steady-state value at the intermediate temperature is 250 ° C. The temperature in the low temperature region may be room temperature. Further, when false alarm due to an organic solvent such as ethanol becomes a problem, a filter such as activated carbon may be provided. Of course, the semiconductor may be replaced with another semiconductor such as ZnO or In ₂ O ₃ .

(8) is a power supply, and its output (+ Vcc) is the power supply for the entire device. (10) is, for example, a timer that operates in a 90-second cycle. It outputs a high signal (H) for the first 30 seconds and a second 30 seconds. A low signal (L) is issued, and an M signal (M) is issued for the last 30 seconds.
The timer (10) issues a sampling signal (Sh) in the high temperature range immediately before the end of the high signal (H) and a sampling signal (Sl) immediately before the end of the low signal (L), and the period of the M signal (M) Of these, a sampling signal (Sm) is emitted, preferably near the end of the sampling. Each sampling signal (Sh), (S
l) and (Sm) are generated, signals (H), (L),
(M) is turned off. The reset signal (Sl ') is issued immediately after the signal (Sl) is generated, and the timer (10) returns to the signal (H) without passing through the period of the signal (M) unless there is a special condition.

(12) is a heater power supply consisting of a three-value power supply, and the sensor (2)
Is for heating to three temperatures of a high temperature range, a low temperature range, and an intermediate temperature thereof. The output of the heater power supply (12) in the low temperature range may be zero.

(14) and (16) are diodes for heating the two heaters (4) and (6) together, and (18) and (19) are analog switches for applying the detection voltage (Vcc) to the sensor (2). Then, the switch (18) is normally open and the switch (19) is normally closed, and it operates by the signals (Sh), (Sl), (Sm) through the OR circuit (20). (2
2) is an analog switch, which is a temperature change signal (F) described later.
When there is no signal, it is for resetting the timer (10) by the signal (Sl ').

(R ₁ ) is a load resistance (10KΩ in this case), and its applied voltage (Vrl) is the sensor output. Besides, as the sensor output, the electric conductivity of the sensor, or the voltage (Vrl) or the electric conductivity to the power of 0.5 to 1.4, more preferably to the power of about 0.6 to 1.2 can be used. However, it is easiest to use the applied voltage to the load resistance.

(24) operates by comparison circuit, (26) operates by signal (Sh)
DFF and (28) are methane notification LEDs, which constitute combustible gas detection means.

(30) operates by comparison circuit, (32) operates by signal (Sl)
The DFF serves as both the preliminary detecting means and the temperature changing means. The output of DFF (32) is used as the temperature change signal (F).

(34) is an A / D / D / A converter, which records the sensor output during signal (Sl). The converter (34) can be changed to any recording element. (36) is an amplifier having a gain (here, 0.6) determined by the resistance (R ₂ ), (38) is an amplifier for obtaining the output difference between the converter (34) and the amplifier (36), and (40) is DFF operates with signal (Sm). Further, (50) is a CO notification LED, which constitutes a carbon monoxide detection means.

(44) is an OR circuit, and (46) is a buzzer, which operates by the methane detection signal of DFF (26) and the carbon monoxide detection signal of DFF (40).

"Characteristics of the sensor" Fig. 3 shows the characteristics of the sensor (2) at 400 ° C. The illustration shows 8
It shows the response to carbon monoxide, methane, hydrogen, and air when a sensor in a steady state at 0 ℃ is heated to 400 ℃. The ambient temperature is 20 ℃ and the humidity is 60%. The sensor (2) was equipped with an activated carbon filter to eliminate the influence of ethanol. The response to methane is completed within 20 seconds and the relative sensitivity is high.

Fig. 4 shows the characteristics when the sensor (2) was cooled from a steady state of 400 ° C to 80 ° C. The measurement conditions are the same.

The response to carbon monoxide and hydrogen at low temperature is slow, and the output does not reach a steady value in about 30 seconds, and carbon monoxide and hydrogen cannot be distinguished. The output is 0.9 of (CO + 0.1H ₂ ).
~ Proportional to 0.7.

Fig. 5 shows the characteristics when the heating temperature is changed from 80 ° C to 250 ° C. The output at this temperature is selective to hydrogen and has a fast response, and the output is proportional to the hydrogen power of about 0.8.

"Operation of Embodiment" The operation of the apparatus will be described with reference to FIG. The sensor (2) is heated to a high temperature for 30 seconds by the high signal (H) and kept at a low temperature for 30 seconds by the low signal (L). When the gas is not present, it is reset by the reset signal (Sl ′) and heated to a high temperature. Return.

Near the end of the high signal (H), the high temperature side output is taken out by the signal (Sh) and methane is detected.

Next, near the end of the row signal (L), the low temperature side output is taken out by the signal (Sl) and compared with the reference potential of the comparison circuit (30) to perform preliminary detection of carbon monoxide. If the sensor output is large, the resetting of the timer (10) is blocked by the signal (F), the sensor (2) is heated to an intermediate temperature, and the hydrogen concentration is detected. Then, the difference between the output in the low temperature range recorded in the converter (34) and the output at the intermediate temperature of the amplifier (36) is determined by the amplifier (38) as V1−0.6Vm. Here, Vl is the output in the low temperature range and Vm is the output in the medium temperature range. Further, 0.6 is a compensation coefficient for hydrogen, and the theoretical value from FIGS. 4 and 5 is 0.7, but here it is set to 0.6. In this way, the effect of hydrogen is compensated and carbon monoxide is selectively detected. The theory and the compensation coefficient can be freely changed according to the characteristics of the sensor.

By the way, various compensation methods using two outputs are known, and the compensation method is not limited to the difference. The difference is used here because the circuit configuration is the simplest, and when the ratio is used, an error occurs when hydrogen and carbon monoxide coexist. That is, the ratio between the output in the low temperature region and the output in the medium temperature region is smaller when CO and H ₂ coexist than when only CO is used, even at the same CO concentration. Further, in the embodiment, the voltage applied to the load resistance is used as the sensor output, but this may be replaced by the electric conductivity of the sensor. In addition, the sensor output is a power such as 0.5 to 1.4, and more preferably 0.6 to 1.2.
You may use a square etc. As described with reference to FIGS. 4 and 5, the sensor output is proportional to the 0.8th power of the gas concentration,
When power is used, compensation can be performed using an output proportional to the gas concentration. However, if you do not use exponentiation,
A simple circuit can be used and the compensation for high concentrations of hydrogen can be limited and the detection can be shifted to the safe side, which is preferable.

In this embodiment, the temperature change to the medium temperature range is performed by using the reset of the timer (10), but it may be performed by other means such as using an auxiliary timer. The conditions for changing the temperature to the medium temperature range can be changed variously. Further, the output in the medium temperature range may be recorded in the converter (34) or the like, and the output in the low temperature range may be used as it is as an analog. Even if it is attached to the comparison circuits (24) and (30),
These may be single and the reference potential may be switched. In addition, each temperature of low temperature range, medium temperature range, high temperature range,
The detection cycle and the like can be appropriately changed according to the characteristics of the sensor.

[Advantages of the Invention] In the present invention, a combustible gas such as methane or isobutane,
Carbon monoxide can be detected simultaneously, and the detection time can be shortened without impairing the selectivity to carbon monoxide.

[Brief description of drawings]

FIG. 1 is a circuit diagram of the embodiment, FIG. 2 is a waveform diagram thereof, and FIGS. 3 to 5 are characteristic diagrams of a gas sensor used in the embodiment. In the figure, (2) gas sensor, (4), (6) electrode also serving as a heater,
(8) Power supply, (10) Timer, (12) Heater power supply, (2
4), (30) comparison circuit, (26), (32), (40) DFF,
(34) A / D / D / A converter,

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-55-103453 (JP, A) JP-A-61-110046 (JP, A) Actual opening Sho-56-7056 (JP, U) Actual opening Sho-53- 138893 (JP, U)

Claims (1)

[Claims] 1. A gas detection device in which the heating temperature of a gas sensor using a metal oxide semiconductor, the resistance value of which changes depending on gas, is cyclically changed alternately between a high temperature region and a low temperature region. By detecting the flammable gas from the output of the gas sensor, the flammable gas detecting means, the preliminary detecting means for preliminarily detecting carbon monoxide from the output of the gas sensor in the low temperature range, and the output of the preliminary detecting means, Selective for carbon monoxide from the difference between the output of the gas sensor in the low temperature range and the output at the intermediate temperature, as well as the temperature changing means for changing the heating temperature of the gas sensor to an intermediate temperature between the high temperature range and the low temperature range. And a carbon monoxide detecting means for obtaining various outputs, and a gas detecting device.