JPH0129257B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a gas detection method, and in particular to a method in which harmful gases such as alcohol and ketones are removed by adsorption to a filter, and gas leaks and generation of toxic gases are detected. It is about improvement.

In order to selectively detect gases such as CH ₄ and H ₂ produced by gas leaks, and toxic gases such as CO produced by incomplete combustion, a method of adsorbing and removing harmful gases using filters such as activated carbon has been used for a long time. Are known. For example, when ethanol is removed using activated carbon as a filter, the concentration of ethanol can be reduced to several tenths or less using a simple filter. In this way, conventional
The CH ₄ detection method gave a false alarm when ethanol was around 300ppm (CH ₄ 3000ppm was the detection concentration), and the CO detection method gave a false alarm when 20ppm ethanol was detected (CO _100ppm was the detection concentration). ), false alarms can be almost eliminated.

However, the activity of such a filter decreases over time because it does not re-release the adsorbed gas. Heat cleaning can be performed to regenerate the filter, but it is difficult to detect a decrease in the adsorption capacity of the filter. It is also possible to set up a timer that runs on a monthly basis and perform heat cleaning periodically.
Such timers are extremely expensive. Tokukai Akira
No. 54-38190 states that heat cleaning activated carbon filters is not practical.

An object of the present invention is to provide a gas detection method that performs heat cleaning in response to a decrease in filter activity.

The gas detection method of this invention is a method in which a filter such as activated carbon or silica gel adsorbs and removes a harmful gas, and only the gas to be detected is detected by a gas sensor. When the output of the gas sensor becomes equal to or less than the predetermined value, the filter is heat-cleaned after a delay time has elapsed since the output of the gas sensor became equal to or less than the predetermined value.

In this invention, the output of the gas sensor is used as an index of the decrease in filter activity.

This is to avoid deterioration of the filter due to an increase in the number of times of heat cleaning. The following three cases can be considered as cases in which the output of the gas sensor increases.

When the harmful gas cannot be removed due to a decrease in filter activity, when there is more harmful gas than the filter's capacity, or when the target gas is generated.

Among these cases, it is thought that the number of cases is small, considering that the filter can reduce the concentration of the harmful gas to less than a few tenths. In this case, the frequency is also low, and is thought to occur once a month at most.
Therefore, there is no problem in using the output of the gas sensor as an indicator of a decrease in filter activity. Further, even if heat cleaning is performed in the above case, the number of times of heat cleaning is extremely small, and it is not considered to accelerate the deterioration of the filter. Useless heat cleaning can be avoided by using the output of the gas sensor as an indicator of a decrease in filter activity.

Various values can be used as the output of the gas sensor as a reference for heat cleaning, and for example, an output corresponding to the alarm concentration of the gas to be detected may be used. Alternatively, the time average of the output of the gas sensor may be set to a predetermined value to avoid heat cleaning caused by a transient high-concentration harmful gas.

The reason why we decided to perform heat cleaning after a delay when the output of the gas sensor exceeds a predetermined value and then falls below this predetermined value again is to take into consideration the case where the gas to be detected is actually generated. It is something. In this case, if heat cleaning is performed without a delay time, it will not be possible to distinguish between the gas to be detected and the gas released from the filter. By providing a delay time, heat cleaning can be performed after gas detection is prioritized.

The delay time can be determined in various ways. For example, the delay time can be set as a certain period of time after the output of the gas sensor has fallen below the predetermined value, or the delay time can be set as a certain period of time after the output of the gas sensor has decreased below the predetermined value. It may be determined by a method such as taking the time until the output decreases. Note that if the interfering gas concentration is kept at a high concentration for a long period of time, false alarms will continue during that time, and heat cleaning of the filter will not be performed until the interfering gas is removed. However, this kind of thing is rare. The generation of ethanol, which is the main interfering gas, is usually temporary, and it is rare for ethanol to be generated at high concentrations for a long period of time.

As the gas sensor, one that utilizes a change in the resistance value of a metal oxide semiconductor, one that detects combustion heat due to catalytic oxidation as a temperature change of a temperature measuring resistor, etc. may be used.

The material of the filter is not limited to activated carbon, and a wide variety of materials such as silica gel, molecular sieve, activated alumina, etc. can be used. Such an adsorbent,
Filters are made of heat-resistant fibers or insulated stainless steel wire covered with a mesh. The filter may be attached directly to the gas sensor case or placed around the gas sensor.
This is because if the filter and gas sensor are placed around the filter, they can absorb a considerable amount of the harmful gas even if the filter and the gas sensor are far apart. An example of a filter is one in which 20 to 50 meshes of activated carbon are wrapped in heat-resistant fiber.

There are various types of heaters used for heat cleaning, and positive temperature coefficient thermistors and the like can be used. The heater is installed inside or around the side of the filter.

The heat cleaning temperature and time may be appropriately selected depending on the type of adsorbent. When activated carbon is used, heating at 120° C. for 1 minute may be used as heat cleaning, for example.

An example of a circuit used in this gas detection method will be explained below based on the drawings.

The circuit shown in FIG. 1 includes a gas detection circuit 1, a buzzer drive circuit 11, and a heat cleaning circuit 21.

The gas detection circuit 1 extracts the output of the gas sensor 2. The gas sensor 2 includes a metal oxide semiconductor such as SnO ₂ and a pair of electrodes 3 and 4 connected to the metal oxide semiconductor. The electrode 3 also serves as a heater, and a voltage reduced to about 1V is applied by a transformer 5 to heat the gas sensor 2. Two load resistors 6 and 7 are connected in series to the gas sensor 2, and a DC voltage stabilized by a capacitor 8, a Zener diode 9, etc. is applied. In this way, a change in the resistance value of the gas sensor 2 is detected as a change in the voltage applied to the load resistor 7.

The potential at point _P1 of the gas detection circuit 1 is input to the positive terminal of the comparator 12 of the buzzer drive circuit 11. The comparator 12 compares this potential with a reference potential, operates the transistor 13, and operates the buzzer 14. The buzzer 14 sounds to notify of gas leakage or generation of toxic gas.

The potential at P ₂ points of gas detection circuit 1 is connected to diode 2.
2 to the heat cleaning circuit 21.

In the heat cleaning circuit 21, a capacitor 23 and resistors 24 and 25 cancel out the effects of the transient gas. In this case, change the resistance value of resistor 24 to resistor 2
Keep it smaller than 5.

The potential of the _three points of the positive side electrode P of the capacitor 23 is
Input to the positive terminal of comparator 26, DC power supply and resistor 2
The potential at the _P4 point of the series piece such as No. 5 is input to the negative terminal of the comparator 26.

The output terminal P ₅ of the comparator 26 is inputted via the diode 27 to the positive terminal of the comparator 28 . A DC power supply 29, a capacitor 30, and a resistor 31 are connected in series, and the intermediate junction between the capacitor 30 and the resistor 31 is connected to the positive terminal of the comparator 28. Comparator 28
Connect the negative terminal of to the _P4 point.

The output terminal P ₆ of the comparator 28 is inputted to the negative terminal of the comparator 33 via the diode 32 . A DC power supply 34, a capacitor 35, and a resistor are connected in series, and the negative terminal of the capacitor 35 is connected to the negative terminal of the comparator 33. Connect the positive terminal of comparator 33 to point _P4 .

36 is a NAND circuit, which is the output terminal of the comparator 28.
P ₆ and the output terminal P ₇ of the comparator 33 are input.
The output of the NAND circuit 36 operates the SCR 37 and causes the heater 38 for heat cleaning to generate heat.

Note that in this circuit, the potential of the _P1 point is lower than the _P2 point, but the _P1 point and the _P2 point may be made to match, or the _P1 point may be made to be on the high potential side. Also, the heater 38 for heat cleaning
An issuing diode or the like may be connected in parallel with the above to notify the execution of heat cleaning.

Next, the operation of this circuit will be explained based on FIG.

When the output of the gas sensor 2 fluctuates as indicated by the broken line in FIG. 2A, the buzzer drive circuit 11 operates the buzzer for _t1 seconds to detect gas.

Out of the output of the gas sensor 2, the output due to the influence of transient gases is removed by the capacitor 23, and even if a gas with a high concentration exceeding the filter's capacity temporarily exists, no heat cleaning is performed. The capacitor 23 transforms the output of the gas sensor 2 as shown by the solid line in FIG. 2A.

This output is compared with the reference potential at point _P4 , and an output as shown in Figure _2B is obtained at point P5. Next, this output is stretched by t ₂ seconds by capacitor 30, and the second
Obtain the output shown in Figure C at point _P6 . t ₂ corresponds to the delay time. Using capacitor 35,
Obtain the output shown in Figure 2D at point _P7 . Then
The NAND circuit 38 obtains an output that exists for only t ₃ seconds, and heat cleaning is performed during this time.

Figure 3 shows a modification of the heat cleaning circuit.
The operation is shown in FIG. The gas detection circuit and the buzzer drive circuit can be constructed in the same manner as shown in FIG. 1, so their explanation will be omitted. Furthermore, the third
In the illustrated circuit, a circuit configuration for eliminating the influence of transient gas is omitted, but the circuit configuration shown in FIG. 1 or the like may be adopted as necessary.

The output of the gas detection circuit is input to the heat cleaning circuit 41 from point _P10 . P The potential at ₁₀ points is V ₁
shall be. This potential is input to the positive terminal side of the comparator 42, the negative terminal side of the comparator 43, and the positive terminal side of the comparator 44. Four series resistors 4 to DC power supply 45
6, 47, 48, 49 are connected and the reference potential V ₂ ,
Obtain V ₃ and V ₄ . V ₂ to the negative terminal side of the comparator 42,
V ₃ to the positive terminal side of comparator 43, V ₄ to comparator 44
Connect to the negative terminal side of. The output terminal of the comparator 42 is connected via a diode 50 between a capacitor 52 and a resistor 53 in a series series consisting of a DC power supply 51, a capacitor 52, and a resistor 53. The potential at this point is input to the positive terminal of the comparator 54 and compared with an appropriate reference potential.

Each output terminal of comparator 54, comparator 43, and comparator 44 is input to AND element 55. The output of the AND element 55 operates the heater 56 for heat cleaning.

Assume that a voltage as shown by the solid line in FIG. 4A is input to the heat cleaning circuit 41. Each of the reference potentials V ₂ , V ₃ , and V ₄ is indicated by a dashed-dotted line. An output shown in FIG. 4B is obtained at the output terminal of the comparator 54. Further, the output terminal of the comparator 43 receives the voltage shown in FIG. 4C, and the output terminal of the comparator 44 receives the voltage shown in FIG.
The voltage shown is obtained. As a result, on the output side of the AND element 55, V ₁ decreases to below V ₃ and V ₄
Output is obtained only until . Note that if V ₁ does not fall below V ₄ for some reason, the heat cleaning is discontinued after the delay time caused by the capacitor 52 and resistor 53 has elapsed.

As explained above, in this invention, the output of the gas sensor is used as an index of the decrease in filter activity, so that heat cleaning can be performed in accordance with the decrease in filter activity. Furthermore, since a delay time is provided before heat cleaning, gas detection and heat cleaning can be performed separately.

[Brief explanation of drawings]

1 and 3 show circuit examples embodying the gas detection method of the present invention, and FIGS. 2 and 4 are explanatory diagrams showing the operation of each circuit example. 1...Gas detection circuit, 2...Gas sensor, 11
... Buzzer drive circuit, 21, 41 ... Heat cleaning circuit, 38, 56 ... Heater for heat cleaning.

Claims (1)

[Claims] 1. In a method where a filter such as activated carbon or silica gel adsorbs interfering gases and removes them, and only the gas to be detected is detected by a gas sensor, the output of the gas sensor once exceeds a predetermined value and then falls below the predetermined value again. In this case, the filter is heat-cleaned after a delay time has elapsed since the output of the gas sensor became equal to or less than the predetermined value.