JPH0989835A - Cleaning method for carbon monoxide sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the aging of a solid electrolytic carbon monoxide sensor, i.e., the increase of H2 /CO ratio, deterioration of response and fluctuation of air base, by applying a voltage between the electrodes of solid electrolytic carbon monoxide sensor for a predetermined time and then performing heat-up processing. SOLUTION: The voltage being applied between the electrodes of a sensor must have same polarity as the electromotive force being generated between the electrodes of sensor. The applying voltage is 1-5V and when it is required to shorten the applying time, a voltage of 4-5V is applied thus obtaining sufficient effect through processing of 4-15sec. An applying voltage higher than 5V may cause damage on stabilization zirconia. A sensor deuterated through long term use is applied with voltages of 1, 3, 5V for 10sec (polarization) in order to lower the sensitivity of carbon monoxide thus enhancing the S/N ratio relatively. This effect is especially eminent when polarization is carried out with applying voltage of 5V.

Description

Detailed Description of the Invention

[0001]

[0001] The present invention relates to a method for cleaning a carbon monoxide sensor.

[0002]

2. Description of the Related Art Various carbon monoxide sensors have been put into practical use at present and are widely used in fields such as process control and safety control. Among these, semiconductor type sensors and catalytic combustion type sensors can be used as carbon monoxide sensors that detect incomplete combustion of combustion exhaust gas from internal combustion engines and the like. Among them, the contact sensor is usually used because the semiconductor sensor has a drawback that it cannot be accurately measured when the oxygen concentration or the water content in the sample gas changes. However, in general, in the case of combustion engine exhaust gas, the combustion exhaust gas temperature fluctuates in the range of several tens to several hundreds of degrees Celsius as the load changes. In order to cope with such a temperature change of the sample gas, it is necessary to perform extremely strict temperature correction even in the catalytic combustion type sensor.

Therefore, in order to solve the above-mentioned problems, development of an incomplete combustion detection sensor for exhaust gas using a solid electrolyte (oxygen ion conductor) is underway (Japanese Patent Publication No. 58-4985). FIG. 1 shows a cross-sectional view of such a solid electrolyte type carbon monoxide sensor. It should be noted that in FIG. 1, the center is a cross-sectional view of the sensor, and the drawings on both sides thereof are explanatory diagrams of the principle for explaining the reaction in the vicinity of the sensor electrode.

In the figure, reference numerals 1a and 1b are porous platinum electrodes,
2 is a combustible gas oxidation catalyst layer, 3 is a stabilized zirconia having oxygen ion conductivity (hereinafter also referred to as "YSZ"),
A heater 5 heats the alumina layer 4 to a temperature optimum for its conductivity.

Here, when such a sensor is placed in an atmosphere containing carbon monoxide gas, carbon monoxide in contact with the electrode 1a is combined with oxygen adsorbed on the electrode 1a to form carbon dioxide. On the other hand, since the combustible gas oxidation catalyst layer 2 exists around the electrode 1b, carbon monoxide in the atmosphere is oxidized by the catalyst layer 2 to carbon dioxide and does not reach the electrode 1b. Depending on the presence or absence of such an oxidation reaction on the electrode surface, an electromotive force corresponding to the carbon monoxide concentration in the atmosphere where the sensor is placed is generated between the electrodes 1a and 1b. By measuring this electromotive force, the concentration of carbon monoxide in the atmosphere can be detected.

In general, a small amount of combustible gas such as hydrogen gas often remains in combustion exhaust gas, but the above-mentioned solid electrolyte type carbon monoxide sensor has sensitivity to hydrogen gas as noise. If the sensor is used for a long time, the S / N ratio of the sensor decreases. This decrease in S / N ratio will be described with reference to FIG. FIG. 2 shows the ratio (H) of the sensor output value (noise) when the air containing 1000 ppm of hydrogen is used as the sample gas to the sensor output value (signal) when the air containing 1000 ppm of carbon monoxide is used as the sample gas. FIG. ₂ is a diagram showing a change over time of ( ₂ / CO ratio). Note that the data indicated by the two symbols in FIG. 2 are the results of two sensors of the same type of solid electrolyte type carbon monoxide sensor but of different production lots.

[0007] In addition, the long-term use causes a problem other than the decrease in the S / N ratio, that is, a decrease in responsiveness. Here, the deterioration of the response will be described with reference to FIG. FIG.
Are air containing 1000, 1500, 2000 and 3000 ppm of carbon monoxide as sample gases, respectively.
It is a figure which shows the sensor output at the time of making it contact with the solid electrolyte type carbon monoxide sensor alternately with the air which does not contain carbon monoxide every 10 minutes. It can be seen that the output gradually increases immediately after contact with the gas containing carbon monoxide, but is not stable even after 10 minutes. (In the case of a new sensor, this output value is stabilized in about 1 to 2 minutes.) Note that a similar change in output when air containing hydrogen gas is brought into contact instead of carbon monoxide is used. As shown in FIG. 4, it can be seen that in the case of hydrogen, the responsiveness is good even with a sensor that has been used for a long period of time and deteriorated, and it is almost stable within about one minute from the start of contact.

Alternatively, as a result of long-term use, the air base (output value when air containing neither carbon monoxide nor hydrogen is used as a sample gas) as shown in FIG. There is also a problem that the correction of is necessary. If the correction is to be performed automatically, the cost and the size of the apparatus will increase.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to prevent a change over time of a solid electrolyte type carbon monoxide sensor, that is, an increase in the H ₂ / CO ratio, deterioration of responsiveness, and fluctuation of an air base. I do.

[0010]

In order to solve the above-mentioned problems, the method for cleaning a solid electrolyte type carbon monoxide sensor according to the present invention is, as set forth in claim 1, a method for cleaning a solid electrolyte type carbon monoxide sensor. In the above, there is a configuration in which a voltage is applied between the electrodes of the sensor for a certain period of time.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the voltage applied between the electrodes of the sensor must be applied with the same polarity as the electromotive force generated between the sensor electrodes. The applied voltage at that time is 1
~ 5V, if it is necessary to shorten this application time,
If a voltage of 4 to 5 V is applied, a sufficient effect can be obtained by processing for about 4 to 15 seconds. At this time, applying a voltage of more than 5 V may damage the stabilized zirconia, so care must be taken.

[0012]

【Example】

[Effects of polarization treatment: Example 1] FIG. 6 shows that a sensor deteriorated after a long period of use is affected by a voltage of 1, 3 and 5 V.
Voltage application process (also called "polarization process") for 2 seconds,
10 before and after the solid electrolyte type carbon monoxide sensor
FIG. 5 shows the results of investigation of the difference in sensor output (signal value) with respect to air containing 00 ppm carbon monoxide gas, and the difference in sensor output (noise) with air containing 1000 ppm hydrogen before and after the polarization treatment. It is apparent from FIG. 6 that the polarization processing increases the sensor output with respect to carbon monoxide and decreases the sensitivity with respect to hydrogen, so that the S / N ratio is relatively improved. In particular, this effect is remarkable when polarization processing is performed with an applied voltage of 5V.

Further, the response to carbon monoxide is also improved by the polarization treatment. That is, the result of examining the response immediately after performing the above-mentioned polarization treatment (applied voltage: 5 V, application time: 10 seconds) on the sensor whose response to carbon monoxide has deteriorated as shown in FIG. As shown in FIG. Thus, the effect of improving the responsiveness by the polarization treatment according to the present invention is apparent. FIG. 8 shows the response to hydrogen gas at this time. (These measuring methods are the same as those in FIGS. 3 and 4, respectively.)

[Persistence of Effect of Polarization Treatment] Here, the persistence of the effect of the polarization treatment will be described. FIG. 9 shows 100
It is a figure which shows the time-dependent change of the output value when air containing 0 ppm carbon monoxide is used as a sample gas (polarization processing conditions, applied voltage: 5 V, applied time: 10 seconds). In addition, each code | symbol shows the result in the solid electrolyte type carbon monoxide sensor of a different manufacturing lot. FIG. 10 shows 1000 ppm
FIG. 5 is a diagram illustrating a change over time of an output value when hydrogen-containing air is used as a sample gas. Each symbol in FIG. 10 indicates a result obtained by the sensor having the same symbol in FIG.

As can be understood from FIGS. 9 and 10, the sensitivity and the S / N ratio are good for 3 hours after the polarization treatment, but the performance decreases thereafter. Therefore, it was found that the sensor can always be kept in a good condition by periodically performing the polarization treatment within 5 hours.

FIG. 11 shows the stability of the S / N ratio when the polarization treatment of 5 V for 10 seconds is performed every 30 minutes. This shows the ratio of the sensor output value of air containing 1000 ppm of hydrogen to the sensor output value of air containing 1000 ppm of carbon monoxide. In the figure, two types of symbols indicate results obtained by sensors of different lots. Thus, it has been found that the S / N ratio of the sensor can be stabilized over a long period of time by performing the polarization processing according to the present invention. This makes it clear that the present invention can increase the reliability of the measured values or extend the life of the sensor.

[Measures Against Fluctuation of Air Base Immediately after Polarization: Second Embodiment] The sensor is cleaned by the above-described polarization voltage application process, but the air base fluctuates at that time. In FIG. 12, as shown by the curve of "polarization only", it was found that after the polarization treatment for 10 seconds, the air base was significantly lowered and it took several minutes (here, 8 minutes) to return to the steady state. Was. Generally, a carbon monoxide detector is required to be able to measure at intervals of about one minute, so that the air base is required to be quickly stabilized.

As a result of the study, it was found that the air base can be immediately returned by increasing the heater voltage of the sensor and performing the heat-up process immediately after the completion of the polarization process. FIG. 12 also shows how the air base fluctuates when the sensor temperature is raised to 600 ° C. for 20 seconds after being polarized for 10 seconds at the sensor operating temperature of 400 ° C. and then returned to 400 ° C. again. Is shown. As is clear from this figure, the heat-up process returns the air base to the steady state in about 15 seconds.
When the heat-up temperature is higher than the temperature at the time of measurement, that is, the operating temperature, and 500 to 600 ° C., a sufficient effect can be obtained. Even if such heat-up treatment is performed, the effect of the polarization treatment is not impaired or reduced.

[Application to carbon monoxide detector: Example 3]
FIG. 13 shows a basic configuration diagram of the carbon monoxide detector according to the present invention. Power is supplied to the entire device by activation of the power supply circuit α, but at this time, the timer circuit β1 is activated first.
At the same time as the timer circuit β1 is activated, the constant voltage circuit γ3 for the heater is activated to keep the sensor at the measurement temperature, that is, the operating temperature (350 to 500 ° C), and the electrode application circuit γ1 is activated to polarize the sensor δ. Apply voltage. After a certain period of time, the operations of these voltage circuits γ1 and γ3 are terminated and the timer circuit β2 is activated, and at the same time, the operation of itself is terminated.

When the timer circuit β2 is activated, the constant voltage circuit γ2 for heat-up is activated to heat up the heater of the sensor δ, that is, a temperature higher than the operating temperature (5
(0 to 600 ° C). After a certain period of time, the operation of the power supply circuit γ2 is ended, the timer β3 is started, and at the same time, the operation of itself is ended.

Further, in the timer circuit β3, a time for keeping the heater constant voltage circuit γ3 sensor at the operating temperature is set. The sensor detection circuit ε is activated immediately after activation,
After the lapse of the set time, the timer β1 is started, and at the same time, the operation of the sensor detection circuit ε is ended and the operation of itself is ended. The above series of operations is repeated by the activation of the timer circuit β1 by the timer circuit β3. The sensor detection circuit ε measures the output of the sensor after a lapse of a certain time from the start.

In the carbon monoxide detector according to the present invention, the means for applying a voltage between the electrodes of the carbon monoxide sensor for a certain period of time is composed of the timer circuit β1 and the electrode application circuit γ1. Further, in the carbon monoxide detector according to the present invention, the means for keeping the temperature of the carbon monoxide sensor at a temperature higher than that at the time of measurement for a certain time is the timer circuit β2.
And a constant voltage circuit γ2 for heat-up.

Next, the carbon monoxide detector according to the present invention will be specifically described. As the sensor, the solid electrolyte type carbon monoxide sensor shown in FIG. 1 was used. A top view of the sensor is shown in Figure 1.
4 shows. The electrodes 1a and 1b of the sensor and the connecting terminal portions 6 provided at the corners of the sensor are connected by lead wires 7 made of platinum, respectively. This sensor is fixed to a pedestal 8 made of phenolic resin as shown in FIG. 15 for easy handling. Although four conductive pins 9 are shown in FIG. 15, two of them are connected to the sensor electrodes 1a and 1b, and the remaining two pins are used.
The book is connected to the heater inside the sensor.

Next, a circuit diagram of the carbon monoxide detector according to the present invention is shown in FIG. The output of the sensor section of the sensor 11 is connected to an input port 13i with an A / D converter of a microprocessing unit (hereinafter referred to as "MPU") 13 via an amplifier circuit 15. In addition, a power source 12 for voltage application for polarization processing is provided in the sensor section with a switch S.
Connection is possible when w is turned on. This switch Sw is connected from the MPU 13 to its output port O3 (reference numeral 13o).
It can be controlled by c). On the other hand, the sensor 11
An operating temperature power source 14l and a heat-up power source 14h are connected to the heater section 11b. These power sources 1
4l and 14h are selectively activated by the output ports O1 and O2 of the MPU (reference numerals 13oa and 13ob).

In addition to the above, the MPU 13 has a CPU.
(Reference numeral 13cp), ROM (reference numeral 13ro), RAM (reference numeral 13ra), and an output port O4 (reference numeral 13od) for outputting data to the LCD 16 for displaying the measurement result.

In the RAM, the timer I for controlling the polarization processing time and the timer I for controlling the heat-up processing by the constants a, b, c and d written in the ROM in advance.
I, a timer III that controls the time from the end of the heat-up process to the start of measurement, and a timer IV that controls the time from the start of measurement to the next polarization process. On the other hand, RO
In M, in addition to the above constants, a sensor correction constant or a control program is prepared in advance.

The detector operates according to the time chart of FIG. That is, when the power is turned on, the timer I starts operation after initialization, the switch Sw is turned on to start the polarization process, and the heater section 11b is heated to the operating temperature by the operating temperature power supply 14l. After the lapse of time a, the timer II is started after initialization and at the same time the switch S
w turns off and the polarization process is stopped. Further, the heater section 11b is heated by the heating power source 14h.

After the time measured by the timer II reaches the time b,
The timer III is started after initialization, and at the same time, the operation of the heat-up power source 14h is stopped, and the heater portion 11b is kept at the operating temperature by the operating temperature power source 14l. When the time measured by the timer III reaches the time c, the timer IV is initialized and then started. After that, until the time measured by the timer IV reaches the time d, the output of the sensor unit 11a of the sensor 11 is input to the MPU 13 from the input port 13I with the A / D converter via the amplifier circuit 15, and is calculated as necessary. The calculated carbon monoxide concentration value is output from the output port O4 of the MPU 13 to the LCD 16 and displayed.
When the measurement time of timer IV reaches time d, timer I is restarted.
Are initialized and the above operation is sequentially repeated. By using such a carbon monoxide detector, the user can always obtain stable and accurate measured values by simply turning on the power.

[0029]

According to the present invention, stable and accurate carbon monoxide measurement with a high S / N ratio can be carried out for a long period of time. Further, the responsiveness of the sensor to changes in the carbon monoxide concentration is significantly improved, which is very effective when applied to safety management and the like. At the same time, since the sensor life can be extended, it is possible to reduce the running cost and prevent the trouble of replacing the sensor.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a solid electrolyte type carbon monoxide sensor and an explanatory view of its principle.

FIG. 2 is a diagram showing a change over time in the ratio of the output value of hydrogen gas to the output value of carbon monoxide of the sensor.

FIG. 3 is a diagram showing the response of a deteriorated sensor to carbon monoxide.

FIG. 4 is a diagram showing the response of a deteriorated sensor to hydrogen.

FIG. 5 is a diagram showing a change over time of an air base of a sensor output.

FIG. 6 is a graph showing changes in sensitivity to carbon monoxide and sensitivity to hydrogen depending on the difference in voltage of the polarization treatment.

FIG. 7 is a diagram showing the response of a polarized sensor to carbon monoxide.

FIG. 8 is a diagram showing the response of a polarization-treated sensor to hydrogen.

FIG. 9 is a diagram showing a change over time of an output value when air containing 1000 ppm of carbon monoxide is used as a sample gas.

FIG. 10 is a diagram showing a change over time of an output value when air containing 1000 ppm of hydrogen is used as a sample gas.

FIG. 11 is a diagram showing a change over time of the S / N ratio when a polarization process is performed periodically.

FIG. 12 is a diagram showing the effect of air base fluctuation and heat-up processing due to polarization processing.

FIG. 13 is a basic configuration diagram of a carbon monoxide detector according to the present invention.

FIG. 14 is a top view of a solid electrolyte type carbon monoxide sensor.

FIG. 15 is a diagram (perspective view) showing a state where the sensor is fixed to the pedestal.

FIG. 16 is a circuit diagram of a carbon monoxide detector according to the present invention.

FIG. 17 is a time chart of the operation of the carbon monoxide detector of FIG. 16.

[Explanation of symbols]

 1a, 1b Porous platinum electrode 2 Combustible gas oxidation catalyst layer 3 Stabilized zirconia 4 Alumina layer 5 Heater 6 Connection terminal part 7 Lead wire 8 Pedestal 9 Pin 11 Sensor 11a Sensor part 11b Heater part 12 Voltage application power supply 13 MPU 14l Operating temperature power supply 14h Heat-up power supply 15 Amplifying circuit 16 LCD

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[Procedure amendment]

[Submission date] April 22, 1996

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mochizuki Total 23 Kashima, Minamata-cho, Futamachi, Shizuoka Prefecture Yazaki Keiki Co., Ltd. 72) Inventor Kazutoshi Agata 23 Yazaki Keiki Co., Ltd.

Claims (4)

[Claims] 1. A method for cleaning a solid electrolyte type carbon monoxide sensor, which comprises applying a voltage between electrodes of the sensor for a certain period of time. 2. The method for cleaning a solid electrolyte type carbon monoxide sensor according to claim 2, wherein the sensor temperature is maintained at a temperature higher than that during measurement for a certain period of time after the voltage application. 3. A carbon monoxide detector having a solid electrolyte type carbon monoxide sensor, comprising a means for applying a voltage for a predetermined time between electrodes of the sensor. 4. The carbon monoxide detector according to claim 3, further comprising means for keeping the carbon monoxide sensor at a temperature higher than that at the time of measurement for a certain period of time.