JPS62223662A - Gas detecting device - Google Patents

Abstract

PURPOSE:To detect inflammable gas and carbon monoxide at the same time and to shorten the detection time by providing a detecting device with an inflammable gas detecting means, an auxiliary detecting means, a temperature varying means, and a carbon monoxide detecting means. CONSTITUTION:The heating temperature of a gas sensor 2 which uses a metal oxide semiconductor which varies in resistance value according to gas is varied periodically between a high-temperature range and a low-temperature range. Then, the inflammable gas detecting means composed of a comparing circuit 24, a D-FF 26, and an LED 28 detects the inflammable gas from the output of the sensor in the high-temperature range. The auxiliary detecting means composed of a converter 30 and a D-FF 32, on the other hand, serves as the temperature varying means, and detects carbon monoxide with the output of the sensor 2 in the low-frequency range preliminarily to vary the heating temperature into the intermediate temperature between the high-temperature range and low-temperature range. The carbon monoxide detecting means consisting of a converter 34, amplifiers 36 and 38, a D-FF 40, and an LED 42 compensates the output of the sensor 2 in the low-temperature range with the intermediate temperature to detect carbon monoxide selectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a gas detection device using a gas-sensitive metal oxide semiconductor, and particularly to a gas detection device that periodically changes the temperature of the semiconductor between a high temperature range and a low temperature range. .

More specifically, the present invention relates to a gas detection device that simultaneously detects flammable gases such as methane and isobutane and carbon monoxide using one gas sensor.

[Prior art] Japanese Patent Publication No. 53-43.320 discloses that a gas sensor using a metal oxide semiconductor such as Snow is heated alternately to a high temperature range and a low temperature range, and carbon monoxide is selectively extracted from the output in the low temperature range. discloses a device for detecting The inventor considered using the output of this device in a high temperature range to detect flammable gases such as methane at the same time as carbon monoxide.

However, when it comes to combustible gases, there is a limit to the dead time of detection. For example, the self-inspection regulations for gas leak alarms for city gas stipulate that the alarm must be emitted for 1000 to 12,500 ppm of methane, and 12,50 hpm. On the other hand, the effect of CO on the human body is slow, e.g.
It is sufficient to detect approximately 400 ppm of CO within 5 minutes.

However, if the temperature of the sensor is changed periodically,
The heating period determines the detection speed, and in order to perform detection quickly, it is necessary to shorten the period. If the heating period is shortened here, the relative sensitivity between carbon monoxide and hydrogen will decrease, causing false alarms due to hydrogen. The concentration of hydrogen that can normally be produced is 11,000 pp or less. In other words, the requirement for detecting carbon monoxide is to detect approximately 1100pp of carbon monoxide.
The purpose is to detect hydrogen separately from hydrogen of about 00 pp.

[Problems to be solved by the invention] The problems to be solved by this invention are as follows: (1) Detecting flammable gases such as methane and isobutane and carbon monoxide with one gas sensor; (2) shortening the detection period; (3) ) It consists in selectively detecting carbon monoxide from hydrogen.

[Structure of the Invention] In the gas detection device of the present invention, the heating temperature of a gas sensor using a metal oxide semiconductor whose resistance value changes depending on the gas is alternately and periodically changed between a high temperature range and a low temperature range. Detect combustible gas from the output of the gas sensor.

Next, carbon monoxide is preliminarily detected from the output of the sensor in the low temperature range, and based on the result, the sensor temperature is changed to a temperature intermediate between the high temperature range and the low temperature range. Then, the output in the low temperature range is compensated by the output at intermediate temperatures, and carbon monoxide is selectively detected.

[Example 1] Hereinafter, an example will be explained using methane and CO as detection targets, but various types of gas sensors to be used are already known, and each part of the device is within the range of known technology. You can freely change it within. For example, there is a gas sensor in which a single heater electrode is embedded in a metal oxide semiconductor, and gas is detected from a change in resistance between both ends of this electrode 71i. In this case, a change in the resistance of the semiconductor due to adsorption of gas acts as a change in the parallel resistance of the heater electrode. Further, in this case, a change in the thermal conductivity of the semiconductor due to adsorption of gas causes a change in the temperature of the sensor, which changes the resistance value of the electrode that also serves as a heater.

Furthermore, a thermistor can be used to compensate for the dependence of the gas sensor on the ambient temperature. Furthermore, a temperature compensation element having almost the same temperature dependence as the gas sensor may be provided and incorporated in the bridge circuit in series with the sensor.

"Configuration of Example" In the figure, (2) is a gas sensor, and (4) and (6) are electrodes that also serve as heaters.
nOt with a small amount of Pd catalyst added is used. The temperature of the sensor is 400°C as a steady value in the high temperature range, 80°C as a steady value in the low temperature range, and 250°C as a steady value in the intermediate temperature range. Note that the temperature in the low temperature range may be room temperature. Furthermore, if false alarms caused by organic solvents such as ethanol are a problem, a filter such as activated carbon may be provided. Of course, the semiconductor may be replaced with other semiconductors such as ZnO or IntOs.

(8) is a power supply whose output (+Vcc) is used as the power supply for the entire device, and (10) is a timer that operates at a cycle of, for example, 90 seconds. emits a low signal (L), and for the last 30 seconds it emits an M signal (
M) is emitted. The timer (10) outputs the sampling signal (Sh) in the high temperature range immediately before the end of the high signal (11), and the sampling signal (S1) immediately before the end of the low signal (I7).
), and issue a sampling signal (Sm) within the period of the M signal (M), preferably near its end. Note that when the sampling signals (Sh), (Sl), and (Sm) are generated, the signals (1-1), (I7), and (M) are turned off. Immediately after the signal (Sl) is generated, a reset signal (Sl') is issued, and unless there are special conditions, the timer (■
0) returns to signal (II) without passing through the period of signal (M).

([2) is a heater power supply consisting of a three-value power supply, and the sensor ([2]
) to three temperatures: a high temperature range, a low temperature range, and an intermediate temperature. Note that the output of the heater power source (12) in the low temperature range may be 0.

(14) and (16) are diodes for heating the two heaters (4) and (6) together, and (18) is a detection voltage (V
cc) to the sensor (2), which connects the signals (Sh) and (Sl) via the OR circuit (20).
, (Sm). (22) is an analog switch, and when there is no temperature change signal (F) described later, the signal (
SL') to reset the timer (10).

(R1) is a load resistance (IOKΩ here), and its applied voltage (Vrl) is the sensor output. In addition to the sensor output, the electrical conductivity of the sensor or the voltage (V r
l) or electrical conductivity raised to the power of 0.5 to 1.4, more preferably 0.6 to 1.2, or the like can also be used. However, it is easiest to use the voltage applied to the load resistance.

(24) is a comparison circuit, (26) is operated by the signal (Sh), F F, (28) is for methane notification, and E
In D, these constitute a combustible gas detection means.

(30) is a comparison circuit, (32) is operated by the signal (S1), and F and F are used as preliminary detection means;
It also serves as a temperature change means. Also, the outputs of F and F(32) are used as the temperature change signal (F).

(34) is a Δ/D・I)/Δ converter, and the signal (Sl
) Record the sensor output at time D4. Note that the converter (34
) can be changed to the recording element of Ren and Ha. (36) is the resistance (
R2) is an amplifier with a gain (here 0.6), (38) is an amplifier for obtaining the difference in output between converter (34) and amplifier (36), (40) beams, F, F.
It is operated by the signal (Sm).

Further, (50) is an LED for CO notification, which constitutes carbon dioxide detection means.

(44) is an OR circuit, (46) is a buzzer, and D and F.

Methane detection signal of F (2G), D, F', F (40)
It operates based on the carbon monoxide detection signal.

"Sensor Characteristics" Figure 3 shows the characteristics of the sensor (2) at 400°C. The figure shows the response to carbon monoxide, methane, hydrogen, and air when a sensor in steady state at 80°C is heated to 400°C, at an ambient temperature of 20°C and humidity of 60%. The sensor (2) is equipped with an activated carbon filter to remove the influence of ethanol.

The response to methane is complete within 20 seconds and has high relative sensitivity.

Figure 4 shows the sensor (2
) shows the characteristics when cooled. Note that the measurement conditions are the same.

Response to carbon monoxide and hydrogen at low temperatures (slower, 3
In a time of about 0 seconds, the output reaches a steady value tt-1', and carbon monoxide and hydrogen cannot be distinguished. Also, the output (Ma, (CO
+0. trxt) to the 0.9 to 0.7 power.

FIG. 5 shows the characteristics when the heating temperature was changed from 80°C to 250°C. The output at this temperature (it is highly selective to hydrogen, the response is fast, and the output is approximately proportional to the hydrogen concentration to the 8th power).

“Example operation” The operation of the device will be explained with reference to FIG. Senzu.

(2) is heated to a high temperature for 30 seconds by a high signal (1-H, and held at a low temperature for 30 seconds by a low signal (I,).
, if no gas is present, reset signal (Sl')l
This resets the temperature and returns to high-temperature heating.

, Nolto FL/eyes) 7+'-1 release 712! Soup; 1 loaf. −
Take out the high-temperature side output by L top Ai (8h),
Performs methane detection.

Next, near the end of the low signal (L), the low temperature side output is taken out by the signal (Sl) and compared with the base potential of the comparator circuit (30) to perform preliminary detection of carbon monoxide. If the sensor output is large, the timer (10
), the sensor (2) is heated to an intermediate temperature and the hydrogen concentration is detected.

And the output in the low temperature range recorded in the converter (34),
The difference between the output of the amplifier (36) at the intermediate temperature is Vl-0
, 6Vm, and is determined by the amplifier (38). Here, Vl is the output in the low temperature range, and Vm is the output in the medium temperature range. Further, 0 and 6 are hli compensation coefficients for hydrogen, and although the theoretical value from FIGS. 4 and 5 is 0.7, it is set to 06 here. In this way, the effects of hydrogen are compensated for and carbon monoxide is selectively detected. In general, the compensation coefficient can be changed freely according to the characteristics of the sensor.

By the way, various types of compensation are known when two outputs are used, and the compensation is not limited to the difference. The difference was used here because the circuit configuration is the simplest and because using the ratio would cause an error when hydrogen and carbon monoxide coexist. In other words, the ratio of the output in the low-temperature range to the output in the medium-temperature range is higher than that in the case of only CO, even at the same CO concentration.
It is smaller when and II coexist. Further, in the embodiment, the voltage applied to the load resistance is used as the sensor output, but this may be replaced by the electrical conductivity of the sensor or the like.

Further, the sensor output may be a power of 0.5 to 1.4, more preferably 046 to 1.2. As described with reference to FIGS. 4 and 5, the sensor output is proportional to the gas concentration to the 0.8th power, and when a power is used, compensation can be performed using the output proportional to the gas concentration. However, it is preferable not to perform the exponentiation because a simple circuit can be used, and the compensation for highly sensitive hydrogen can be limited and the detection can be shifted to the safe side.

In this embodiment, a timer (10) is used to change the temperature to the medium temperature range.
Although this was done using Noricent, other means such as using an auxiliary timer may also be used. Moreover, the conditions for changing the temperature to the intermediate temperature range can be changed in various ways. Furthermore, the output in the medium temperature range can be recorded in a converter (34), etc., and the output in the low/speech range can be used as analog. Regarding the comparator circuits (24) and (30), these may be made into a single circuit and the potentials may be switched. Further, the temperatures in the low temperature range, medium temperature range, and high temperature range, the detection cycle, etc. can be changed as appropriate depending on the characteristics of the sensor.

[Effects of the invention] In this invention, flammable gases such as methane and isobutane,
It is possible to simultaneously detect carbon monoxide and shorten the detection time without impairing selectivity to carbon monoxide.

[Brief explanation 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 the gas sensor used in the embodiment. In the figure, (2) gas sensor, (4), (6) heater electrode, (8) power supply, (10) timer, (I2) heater power supply, (24), (30) comparison circuit, (26) , (32), (40) D, F, F.

Claims (1)

[Claims] (1) In a gas detection device in which the heating temperature of a gas sensor using a metal oxide semiconductor whose resistance value changes depending on the gas is alternately and periodically changed between a high temperature range and a low temperature range, the heating temperature of the gas sensor in the high temperature range is A combustible gas detection means for detecting combustible gas from the output, a preliminary detection means for preliminary detection of carbon monoxide from the output of the gas sensor in a low temperature range, and a preliminary detection means for detecting flammable gas from the output of the gas sensor. A temperature changing means for changing the heating temperature to an intermediate temperature between the high temperature range and the low temperature range, a means for compensating the output of the gas sensor in the low temperature range with an output at the intermediate temperature, and a selective output for carbon monoxide. A gas detection device comprising: carbon monoxide detection means for detecting carbon monoxide.