JPH0220682Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an improvement of a CO gas detection device using a SnO ₂ gas sensor.

Japanese Patent Publication No. 53-43320 periodically changes the heating temperature of a gas sensor using changes in the resistance value of a metal oxide semiconductor, heat-cleans the sensor during high-temperature heating, and cleans the sensor from the resistance value during low-temperature heating.
Discloses technology to detect CO gas. This technology dramatically improves the relative sensitivity between CO gas and other gases, and is a pioneer in the detection of CO gas.

The inventor attempted to put this technology to practical use based on a gas sensor made by adding a small amount of noble metal catalyst to SnO ₂ . During this process, the following five problems became clear.

(1) The relative sensitivity between CO gas and ethanol vapor is insufficient, causing false alarms due to ethanol.

(2) Combustion equipment, which is the main source of CO gas,
It also generates _NOx gas. The sensitivity to _NO
Sensitivity to NO _X cannot be excluded.

(3) The heating cycle and heating temperature of the sensor have a significant effect on the characteristics of the sensor. Therefore, the heating temperature and period must be precisely determined.

(4) In a normal living space, even under normal conditions, CO may temporarily exist at levels of 50 to 100 ppm. Even at such low concentrations, if CO exists for a long period of time, it has a negative effect on the human body. Therefore, low concentration
Regarding CO, it is necessary to distinguish between temporary and long-lasting CO.

(5) Gas sensors are periodically heat-cleaned and CO detection can only be performed intermittently. C.O.
When this occurs, the dead time between detections must be shortened.

The purpose of this invention is to suppress the effects of ethanol vapor and _NOx , and to determine the heating cycle and heating temperature suitable for detecting CO. In this invention, SnO ₂ containing 1 to 15 mg of a noble metal catalyst (at least one member of Pt, Pd, Rh, Ir, Os, Ru, Au, Ag; the amount added is shown in terms of metal) per 1 g of SnO ₂ is used. A gas sensor is made by using a pellet as a gas sensing part and integrating a heater for heating and an activated carbon filter for removing ethanol and _NOx .
Activated carbon is used in an amount of 0.5 to 5 g, preferably 0.8 to 3 g, per SnO ₂ pellet. In this way, CO gas is detected in the absence of ethanol and _NOx . The condition of 0.5 to 5 g for activated carbon was determined based on the size of gas sensors currently on the market, and it is assumed that the gas sensor of this invention is the same size as the gas sensor currently on the market. There is. The lower limit of 0.5 g was determined empirically to prevent ethanol breakthrough even in high-concentration ethanol.
That is, if 0.5 g or more of activated carbon is used, the output for ethanol will be 100 ppm or less for ethanol concentrations in the range that can occur on a daily basis, and there is little risk of false alarms. On the other hand, increasing the amount of activated carbon decreases the gas sensor's permeability and response speed to CO. In the case of a normal-sized gas sensor, the range in which the response speed does not decrease significantly due to activated carbon is:
The amount of activated carbon should be 5g or less. Therefore, the amount of activated carbon should be set at 0.5 to 0.5 to 100,000 to prevent the ethanol from breaking through and not cause a large drop in response speed.
It was set to 5g.

Next, a timer circuit is used to control the heating cycle of the gas sensor and the sampling timing of the resistance value of the gas sensor. Connect the heater power supply circuit to the timer circuit, heat the gas sensor to 250 to 400℃ for 20 to 120 seconds to perform heat cleaning, and then heat the gas sensor to 250 to 400℃ for 20 to 120 seconds.
Hold the gas sensor at 60-100 °C for 180 seconds. This process is repeated to periodically change the temperature of the gas sensor. If the heat cleaning temperature is set below 250℃ or the heat cleaning time is set below 20 seconds, the heat cleaning may be insufficient.
The response speed to CO gas decreases, making it impossible to detect CO. If the heat cleaning temperature is 400°C or higher, the gas sensor will deteriorate over time. If the heat cleaning time is set to 120 seconds or more, the heating cycle of the gas sensor becomes longer and the dead time between detections increases. If the temperature on the low-temperature side of the gas sensor is set to 60°C or lower, the response to CO gas will be delayed, making detection difficult. At temperatures above 100℃,
The relative sensitivity between H ₂ and CO decreases, and the detection accuracy decreases. If the gas sensor is kept at a low temperature for less than 30 seconds, the response to CO will be insufficient. If it is 180 seconds or more, the heating cycle of the gas sensor increases, which is also not preferable.

Based on the sampling signal of the timer circuit,
The output of the gas sensor is sampled during a period in which the gas sensor is held at a low temperature. Then, CO is detected from this sampling value. For the detection of CO,
Immediate detection of high concentrations of CO and low concentrations
Two things are important: detecting the presence of CO over a long period of time. For these purposes, it is preferable to do the following: Two comparator means are used, one in the presence of high concentrations of CO and one in the presence of low concentrations of CO.
Detects two cases where . high concentration of CO
is present, or several times in a row at low concentrations.
If CO is present, a flip-flop element is set to activate a second alarm means, such as a buzzer. At the same time, the heating temperature of the sensor is held on the low temperature side, and the sampling signal interval is shortened to shorten the dead time. Furthermore, if a low concentration of CO is present, a first alarm means such as an LED is activated.

Examples of this invention will be described below based on the drawings. The structure of the gas sensor 2 is shown in FIG. This gas sensor 2 has a pair of coil-shaped heater electrodes (one of which is shown as 6) embedded in a SnO ₂ pellet 4, and outside air is introduced through an activated carbon filter 10 surrounded by two wire meshes 8a and 8b. It was designed to do so. SnO ₂ pellet 4 is SnO ₂
This is a sintered body in which 3 mg of Pd (added amount in terms of metal, the same applies hereinafter) is added per gram of Pd. but,
Instead of Pd, one or more of noble metals such as Pt, Rh, Ru, Au, etc. may be used. Further, the amount added may be varied within the range of 1 mg to 15 mg per 1 g of SnO ₂ . The activated carbon filter 10 is for removing ethanol, _NOx such as NO, _NO2, etc., and is constructed using 1 g of activated carbon granules here. Note that the particle size of the activated carbon has little effect on the characteristics of the filter 10, and the amount of activated carbon used is 0.5 to 5 g, preferably 0.3 to 3 g.

This device consists of a sensor circuit centered around the gas sensor 2, a heater power supply circuit for applying power to the heater 6 of the gas sensor 2, a timer circuit for generating a clock signal, and a sensor circuit for evaluating the danger of CO generation. It consists of an alarm circuit, etc.

Figure 2 shows the heater power supply circuit 12 and the sensor circuit 2.
0. In the heater power supply circuit 12, the battery 14
The output of the circuit is stabilized by a Zener diode 16, and its output Vcc is used as a power source for each circuit. Next, a step variable stabilized power supply is constructed using the stabilized power supply IC, Ic ₁ , etc. The output capacitance of the stabilized power supply Ic, Ic ₁ is changed to two boost transistors Tr ₁ ,
Increased by Tr ₂ . The outputs of the stabilized power supplies Ic and Ic ₁ are divided by resistors and fed back to the error amplification input N of the power supplies Ic and Ic ₁ . Here, the switching transistor Tr3 is turned on and off by the current from the HC terminal connected to the timer circuit 30 in FIG. ₃ , and the feedback voltage to the error amplification input N is changed. In this way, when the switching transistor _Tr3 is turned on, the outputs of the power supplies Ic and _Ic1 are increased.

The outputs of the power supplies Ic and Ic ₁ are converted into alternating current and supplied to the two heaters 6 and 6' of the gas sensor 2 via the transformer 18, thereby uniformly heating the gas sensor 2.

In the sensor circuit 20, a load resistance is connected to the gas sensor 2.
Connect R _L and power supply Vcc in series. Load resistance R _L
A negative characteristic thermistor 22 is connected in parallel with the gas sensor 2 to compensate for the dependence of the gas sensor 2 on the ambient temperature. Load resistance
The voltage V _RL applied to R _L is input as is or after being divided appropriately to the alarm circuit 40 in FIG. 3.

In the timer circuit 30, two NAND elements
An oscillation circuit that oscillates with a time constant of 1.5 seconds is provided using Ic ₃ and Ic ₄ . The output of the oscillation circuit is input to two decimal counters Ic ₅ and Ic ₆ , and a 100-decimal counter (operating at a cycle of 150 seconds) is provided. Counter Ic ₆ Q ₀
~The output of Q ₃ is connected to an AND element via a diode.
Connect to IC ₇ . Note that the other input terminal (HCF) of the AND element Ic ₇ is connected to the alarm circuit 40.
A high potential is applied to the input terminal (HCF) unless there is a hold signal. Adding the output (high signal and low signal) of the AND element Ic ₇ to the switching transistor Tr ₃ of the heater power supply circuit 12,
Gas sensor 2 is heated to 300℃ for the first 60 seconds, then 90℃ for the next 60 seconds.
Heat to 80 °C for seconds. The output of a differentiating circuit consisting of a capacitor ₃₂ and a resistor ₃₄ is connected to the clear inputs of the counters Ic 5 and Ic 6, and the counters Ic ₅ and Ic ₆ are cleared when the power is turned on.

Q ₉ output of counter Ic ₆ and Q ₈ of counter Ic ₅
and Q ₉ output to NAND element Ic ₈ , and take out the sampling signal for the last 3 seconds of the low signal. Conducting the sampling signal at high input 2
It is applied to the alarm circuit 40 via two analog switches Ic ₉ and Ic ₁₀ . In addition, the Q ₉ output of counter Ic ₆ and the Q ₉ input of counter Ic ₅ are connected to NAND element Ic _11.
In addition, clock pulse CP2 is taken out during the last 1.5 seconds of the low signal.

In the alarm circuit 40, a buffer amplifier 42 is connected to the load resistance R _L of the sensor circuit 20 to increase the input capacitance. Next, analog switch IC ₁₀
The switch piece S1 of is used as a sampling means,
Sampling is performed based on the sampling signal of NAND element _Ic8 .

The input from the switch piece S1 is applied to a low concentration comparator C1, and a signal 1 is generated when the CO concentration is 100 ppm or more, and a signal 0 is generated in other cases. Similarly, high concentration comparator C2 determines the CO concentration.
Generates signal 1 when it is 500ppm or more, and generates signal 0 in other cases.

The signal from the low concentration comparator C1 is added to the edge trigger type shift register _Ic13 operated by the clock pulse CP2, and the results of four times are stored in the shift register _Ic13 . The four outputs of the shift register Ic ₁₃ are applied to the NAND element Ic ₁₄ , and the flip-flop element FF is set when the CO concentration is 100 ppm or more four times in a row. Furthermore, the monostable multivibrator _Ic12 is operated by the signal 1 of the high concentration comparator C2, and the flip-flop element FF is set. 0 signal of low concentration comparator C1 is shifted to A of shift register _Ic13
Take out through the output, flip-flop element
Reset FF. Furthermore, an integrating circuit consisting of a resistor 44 and a capacitor 46 resets the flip-flop element FF even when the power is turned on.

The set output of flip-flop element FF is taken out via a diode and used as a sampling control signal. This signal is added to the NAND elements Ic ₈ and Ic ₁₁ , and sampling is performed at 15 second intervals when set. In addition, the reset output (HCF) of the flip-flop element FF (low when set) is used as a hold signal, and the heater power supply circuit 1 is
Hold the output of 2 to the low voltage side.

The A output of the shift register Ic ₁₃ operates the first alarm means 48, such as a light emitting diode, to indicate the presence of low concentration CO gas. The set output of the flip-flop element FF activates the second alarm means such as the buzzer 50, and alerts the user to the presence of a high concentration of CO.
Or indicate the presence of low concentrations of CO over a long period of time. Instead of the buzzer 50, combustion stopping means of a combustion device or the like may be used.

Next, the operation of this device will be explained together with the characteristics of the gas sensor 2. In the example, SnO ₂ pellets 4
As a sample, SnO ₂ containing 3 mg of Pd per 1 g was used. Then, for pellet 4, at 300℃ for 60 seconds and 80 seconds.
A 90 second thermal cycle was applied at °C. Here, the catalyst
Even when noble metals other than Pd were used, and the amount added was changed in the range of 1 to 15 mg/g, the properties of Pellet 4 remained almost unchanged. Set the heating temperature on the high temperature side to 250
When the temperature was below 0.degree. C. or the heating time was below 20 seconds, the response speed of the pellets 4 to CO gas decreased. When the heating temperature on the high temperature side was increased to 400°C or higher, the sensitivity to CO on the low temperature side gradually decreased. Further, if the time for holding the pellets at a high temperature is unnecessarily prolonged, not only the dead time during sampling becomes longer but also causes thermal deterioration of the pellets 4. Therefore, the heating conditions on the high temperature side are 250 to 400℃ and a time of 20 to 400℃.
It was set as 120 seconds.

When the heating temperature on the low-temperature side was set to 60° C. or lower, the response speed to CO decreased significantly, and the dependence of the resistance value of the pellet 4 on the ambient temperature also became extremely large. If the heating temperature on the low temperature side is 100℃ or higher, H ₂ and CO
The relative sensitivity was poor, resulting in false alarms due to _H2 . Next, the time required to respond to CO at a temperature of 60 to 100°C was about 30 to 120 seconds. Therefore, the holding time on the low temperature side was set to 30 to 180 seconds, which is approximately the same as the time required for response.

Even if the heating conditions for pellet 4 were changed, the decrease in detection accuracy caused by ethanol and _NOx could not be resolved. Therefore, when a filter 10 made of 1 g of activated carbon was used, the ethanol concentration inside the filter 10 was reduced to less than 1/100 of that outside, _NO
The concentration was less than 1/20 of that outside. The activated carbon filter 10's NO _X adsorption capacity is lower than that for ethanol, but the NO _X generation concentration is generally low (usually 30 ppm or less, at most 50 ppm or less).
Therefore, no problem occurred.

When ethanol adsorption is saturated, filter 1
0 has no effect. Therefore, we installed gas sensor 2 at 10
They were exposed to 3000 ppm ethanol for 4 hours each day during the day. Under these conditions, the gas sensor 2 was heated in the thermal cycle described in this example, and the output of the sensor 2 was measured. The concentration of ethanol generated in homes is at most 1000 ppm, usually less than 200-300 ppm, and its duration is less than 2-3 hours. Therefore, this test is more severe than the actual ethanol generation situation. Activated carbon absorbs ethanol when it comes into contact with it, and when left alone releases ethanol and regenerates it. After 20 hours of regeneration, the release of ethanol is insufficient and the capacity of the filter 10 gradually decreases.

In the case where 0.5 g of activated carbon was used, the output of the gas sensor 2 in ethanol was approximately 80 ppm in terms of CO after 10 days. For those using 0.8g or more of activated carbon, the output after 10 days was less than 50ppm in terms of CO. Detection of CO at low concentrations is performed in the range of 50 to 200 ppm, centered around 100 ppm. The results of the ethanol durability test revealed that using 0.5g or more of activated carbon would cause false alarms only under severe conditions, while using 0.8g or more of activated carbon would almost eliminate false alarms. If the amount of activated carbon is increased unnecessarily, the gas permeability of the gas sensor 2 will be reduced, so the amount of activated carbon is set to at most 5 g, preferably 3 g or less.

The operation of this device will be explained with reference to the flowchart shown in FIG. When the power is turned on, flip-flop element FF and counters Ic ₆ and Ic ₅ are reset. Next, the counter Ic ₆ generates a high signal for 60 seconds, and the gas sensor 2 is kept on the high temperature side. Then
Hold gas sensor 2 on the low temperature side for 90 seconds, and then
Samples are taken every second. CO concentration
Above 500ppm, the flip-flop element FF is set immediately, and between 100ppm and 500ppm, the flip-flop element FF is set for 10 minutes.
Flip-flop element FF is set only when 100 ppm or more of CO is present.

Here, the operation centered on the low concentration comparator C1 is shown in FIG. The effect of CO on the human body is CO
It depends not only on concentration, but also on both CO concentration and time. Exposure to several thousand ppm of CO, even for less than a minute, can have fatal effects on the human body. As the concentration considering the safety factor for this
Select 500ppm, and when it detects that the CO concentration exceeds this value, it immediately sets the flip-flop to issue an alarm. The detection cycle in the example is 2 minutes and 30 seconds,
There is a detection delay of up to 2 minutes and 30 seconds, but this delay is acceptable because it detects at 500 ppm CO with a safety factor in mind. On the other hand, the effects of CO at levels around 100 ppm on the human body do not occur immediately, but are caused by continued exposure to low concentrations of CO over a long period of time. As an environmental standard for low-concentration CO, it has been proposed that the average CO concentration in workplaces and factories be 50 ppm or less. Therefore, in the example, a slightly higher CO concentration of 100 ppm or more was used.
The detection condition for low concentration CO was that it lasted for 10 minutes or more. For this purpose, a shift register Ic ₁₃ is used, and by using the fact that all four outputs A to D of the shift register are set to high, a low concentration of CO2 is detected.
Detect. In order for the four outputs A to D to be set, the CO concentration must exceed 100 ppm for 10 minutes (2 minutes 30 seconds x 4). The reliability from the comparator C1 at the time of the sampling signal (indicated here as CD1) is determined by the shift register at the moment the clock pulse CP2 changes from high to low.
Read into A output of IC ₁₃ , clock pulse CP2
Each time, the data is sent from A to B, C, and D, and then removed from register _Ic13 . and register
Flip-flop element FF is set only when the outputs of A to D of IC ₁₃ are all high.

Returning to FIG. 4, when the flip-flop element FF is set, the buzzer 5 as the second alarm means is activated.
0 sounds, and at the same time sensor 2 is activated by a hold signal.
is held on the low temperature side, and sampling is performed at 15 second intervals according to the sampling control signal. Then, when the CO concentration becomes 100 ppm or less, the flip-flop element FF is reset and the device returns to its normal state.

This devised CO gas detection device has the following features: (1) An activated carbon filter removes ethanol and _NO
(2) Select heating conditions suitable for CO detection to improve the detection accuracy of the gas sensor.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the gas sensor used in the example.
2 and 3 are partial circuit diagrams of the device of the embodiment, FIG. 4 is a flow chart showing the operation of the device of the embodiment, and FIG. 5 is a waveform diagram of the device of the embodiment. 2...Gas sensor, 4...SnO ₂ pellet, 6
... Heater, 10 ... Activated carbon filter, 12 ...
...Heater power supply circuit, 20...Sensor circuit, 30...
...Timer circuit, 40...Alarm circuit, S1...
...Sampling means, C1...Comparator for low concentration, C2...Comparator for high concentration, Ic ₁₃ ...
Shift register, FF...flip-flop element.

Claims (1)

[Scope of utility model registration claim] SnO ₂ contains 1 to 15 mg of metal equivalent per 1 g of SnO ₂
SnO ₂ pellets added with a noble metal catalyst,
A gas sensor equipped with a heater to heat _the SnO ₂ pellets and an activated carbon filter consisting of 0.5 to 5 g of activated carbon to remove _ethanol vapor and NO A sensor circuit that is connected in series, a timer circuit that alternately emits a high signal that lasts for 20 to 120 seconds and a low signal that lasts for 30 to 180 seconds, and that emits a sampling signal within the period of emitting the low signal; Two types of high and low power are applied to the heater of the gas sensor based on the high signal and low signal of
Set the temperature to 400℃, and set the temperature of the gas sensor at low signal to 60℃.
A heater power supply circuit to maintain the temperature at ~100°C and a sampling means for extracting the voltage applied to the load resistance of the sensor circuit as an input signal based on the sampling signal of the timer circuit are provided, and CO gas is detected from the output of the sampling means. A CO gas detection device configured as follows.