JP2013164401A - Gas detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas detector capable of simply and accurately detecting a gas to be detected for a long term.SOLUTION: The gas detector comprises: a semiconductor gas detection element; and correction means for correcting an output value of the semiconductor gas detection element on the basis of a formula (I) expressed by Vc=Vs+a*t-(Bt-B0)*b. In the formula (I), Vc represents a corrected output value, Vs represents an output value before correction, t represents lapsed time, B0 represents an initial base value, Bt represents a base value after t time unit, and a and b represent constants.

Description

  The present invention relates to a gas detection device including a semiconductor type gas detection element.

  In general, a semiconductor gas detection element is in a state in which oxygen is adsorbed on the surface in an atmosphere where there is no gas to be detected (hereinafter sometimes referred to as “detected gas”) such as in clean air. Since the space charge layer generated by the adsorbed oxygen spreads toward the inside of the particle, the free electron conduction path is narrowed and the electric resistance is increased. On the other hand, in an atmosphere in which a gas to be detected exists, the semiconductor gas detection element has a low electric resistance because adsorbed oxygen is desorbed from the surface by an oxidation-reduction reaction with the gas to be detected. Utilizing such properties, a gas detection device equipped with a semiconductor type gas detection element detects a gas to be detected by using a current value according to electric resistance as an output value and capturing a change in the output value. Yes. At that time, the magnitude of the output value changes according to the concentration of the gas to be detected.

  In addition, since the gas detection apparatus provided with the semiconductor type gas detection element used as the prior art in this invention is a general technique, prior art documents, such as a patent document, are not described.

  By the way, it has been found that the output characteristics of the semiconductor gas detection element provided in the gas detection device change over a long span. That is, even when the concentration of the gas to be detected is constant, a relatively early period after the start of use shows a high output value (hereinafter, sometimes referred to as “sensitivity enhancement characteristic change”). After a long period of time has elapsed since the start of use, the output value gradually decreases as time passes (hereinafter sometimes referred to as “sensitivity-changing characteristic change”). Therefore, even if the same output value is obtained in the gas detection device, the actual concentration of the gas to be detected may be different at each measurement time. Therefore, in the conventional gas detection device, there is a possibility that the concentration of the gas to be detected cannot be accurately detected over a long period of time. In addition, in order to eliminate the influence as described above as much as possible and to detect the concentration of the gas to be detected with high accuracy, it is necessary to frequently calibrate the output characteristics of the semiconductor gas detection element using, for example, a standard gas. There was a possibility of giving a complicated impression to the user.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a gas detection device that can detect a gas to be detected easily and accurately over a long period of time.

In order to achieve the above object, a first characteristic configuration of a gas detection device according to the present invention includes a semiconductor gas detection element and a correction unit that corrects an output value of the semiconductor gas detection element based on the following equation (I): It is in the point with and.
[Equation 1]
Vc = Vs + a.t- (Bt-B0) .b (I)
(In the formula, Vc is an output value after correction, Vs is an output value before correction, t is an elapsed time, B0 is an initial base value, Bt is a base value after t time units, and a and b are constants.)

  According to this configuration, an output change equivalent attributed to a change in sensitivity reduction characteristics is added in proportion to the elapsed time in the second term to the output value Vs of the first term obtained by actual measurement, and the high sensitivity By subtracting the amount corresponding to the change in output due to the change in conversion characteristics in proportion to the amount of change in the base value in the third term, the output value is corrected so as to cancel the influence of the change in the output characteristics of the semiconductor gas detection element over time can do. Therefore, the amount of change with time of the output value when the concentration of the gas to be detected is constant can be kept small over a long period of time. As a result, the frequency of calibration of the output characteristics can be reduced, and in some cases, such a calibration step can be eliminated. Therefore, it is possible to provide a gas detection device that can easily and accurately detect the gas to be detected over a long period of time.

  A second characteristic configuration of the gas detection device according to the present invention is that it further includes alarm means for informing an alarm when the corrected output value exceeds a predetermined threshold value.

  By appropriately adjusting the constants a and b for correcting the output value, it is possible to suppress the amount of change over time of the output value for the detected gas in the specific concentration range as small as possible. Therefore, the present invention can be suitably applied to a configuration further comprising alarm means for notifying an alarm when the corrected output value exceeds a predetermined threshold as described above, and the constants a and b are By appropriately setting, the operating state of the alarm means can be maintained well over a long period of time. That is, there is a false notification that the alarm means operates when the actual concentration of the detected gas is below the threshold, or a notification delay that the alarm means does not operate even if the actual concentration of the detected gas exceeds the threshold. Generation | occurrence | production can be suppressed favorably over a long period of time.

  A third characteristic configuration of the gas detection device according to the present invention is that the gas detection device further includes an abnormality notifying unit that notifies an abnormality when the absolute value of (Bt−B0) becomes larger than a predetermined value.

  In a state where the semiconductor gas detection element is operating normally, the amount of change in the base value based on the initial base value is expected to be within a predetermined range. In view of this point, according to the present configuration, it is predicted that an abnormality has occurred in the semiconductor gas detection element based on the absolute value of (Bt−B0), and when the abnormality is predicted, the abnormality is used. Can be promptly notified.

  The 4th characteristic structure of the gas detection apparatus which concerns on this invention exists in the point further provided with the memory | storage means to memorize | store Bt for every predetermined time unit t.

  According to this configuration, the base value Bt after t time units in the equation (I) can be easily obtained based on the history of the base value at each time point stored in the storage unit. In addition, based on the history of the base value, it is possible to obtain information regarding characteristic changes derived from the installation environment of the semiconductor gas detection element. Then, the information can be used for subsequent development and maintenance.

It is a graph which shows the relationship between the elapsed time and the sensor output before correction when the gas to be detected is ethanol. It is a graph which shows the relationship between ethanol concentration and the sensor output before correction | amendment. It is a schematic diagram which shows the mechanism of the output characteristic change of a semiconductor type gas detection element. It is a graph which shows the relationship between the elapsed time and the sensor output after correction | amendment when the to-be-detected gas is ethanol. It is a graph which shows the relationship between ethanol concentration and the sensor output after correction | amendment. It is a graph which shows the relationship between the elapsed time and the sensor output before correction when the gas to be detected is methyl ethyl ketone. It is a graph which shows the relationship between the elapsed time and the sensor output after correction | amendment when the to-be-detected gas is methyl ethyl ketone.

  A gas detection device according to the present invention includes a semiconductor gas detection element, and detects a gas to be detected by capturing a current value change accompanying a change in electrical resistance of the semiconductor gas detection element caused by the presence of the gas to be detected. It is. Such a gas detection device can be configured by incorporating a semiconductor type gas detection element into, for example, a known gas detection circuit.

  The form of the semiconductor gas detection element used in the present invention is not particularly limited. For example, a hot-wire semiconductor gas detection element, a substrate type semiconductor gas detection element, or the like can be used. Further, the type (material, composition, etc.) of the semiconductor type gas detection element is not particularly limited, and can be appropriately selected according to the type (gas type) of the gas to be detected. The gas species that can be detected by the gas detection device according to the present invention is not particularly limited, and examples thereof include flammable gas, toxic gas, inert gas, and VOC. The gas detection device according to the present invention can be used as, for example, a combustible gas sensor, a gas leak sensor, an odor sensor, and the like.

  In such a gas detection device including a semiconductor type gas detection element, the present invention is characterized by including correction means for correcting the output value of the semiconductor type gas detection element based on the following equation (I).

[Equation 2]
Vc = Vs + a.t- (Bt-B0) .b (I)
(In the formula, Vc is an output value after correction, Vs is an output value before correction, t is an elapsed time, B0 is an initial base value, Bt is a base value after t time units, and a and b are constants.)

  As shown in Examples described later, the present inventors experimentally examined the influence of a change in output characteristics (including a change in sensitivity characteristics and a change in sensitivity characteristics) over a long span in a semiconductor gas detection element. confirmed. The present inventors pay attention to the point that the influence due to the change in the output characteristic can be canceled by quantitatively evaluating each of the change in the sensitivity improvement characteristic and the change in the sensitivity reduction characteristic. The formula was derived.

  In the above formula (I), a and b are constants, and can take different values depending on the material constituting the semiconductor gas detection element and the type of gas to be detected. Further, B0 is a base value in the initial state (can be handled as a constant), and can take a different value depending on the material constituting the semiconductor gas detection element and the type of gas to be detected. These constants a and b and the initial base value B0 can be appropriately set as experience values based on various experiments.

  In the above formula (I), t is the elapsed time. As the unit of elapsed time in this case, [hour], [day], [week], [month] and the like can be adopted, and there is no particular limitation. Bt is a base value after t time units according to the elapsed time t. For example, when the unit of the elapsed time t is [week], Bt represents the base value after t weeks. Bt in each time unit can be set as the minimum value of the base value (actually measured value) in the time unit immediately before the time unit.

  In order to easily obtain such a base value Bt after t time units, as an example, a method of using information that is acquired at any time and stored in the storage means can be employed. More specifically, a storage unit such as a RAM is further provided, and a base value Bt for each predetermined time is sequentially stored and stored in the storage unit, and the base in the nearest (t-1) time unit is configured. The value can be a base value after t time units. In the case where such storage means is provided, information on characteristic changes derived from the installation environment of the semiconductor gas detection element is acquired based on the history of the base value, and the acquired information is used for subsequent development, maintenance, etc. There is also an advantage that it can be utilized.

  In the gas detection device according to the present invention, the correction means corrects the output value of the semiconductor type gas detection element based on the formula (I), so that the amount of change in the output value with time when the concentration of the gas to be detected is constant is calculated. It can be kept small over a long period of time. In consideration of this point, the gas detection device according to the present invention can be preferably applied as a gas alarm device that notifies a user of the fact when the presence of a gas to be detected exceeding a predetermined concentration is detected. In this case, the gas detection device according to the present invention may be configured to further include alarm means for notifying an alarm when the corrected output value exceeds a predetermined (predetermined) threshold value.

  It is preferable that the gas detection device according to the present invention further includes abnormality notification means for notifying abnormality when the absolute value of (Bt−B0) becomes larger than a predetermined value. In a state in which the semiconductor gas detection element is operating normally, the amount of change in the base value based on the initial base value is expected to be within a predetermined range (for example, 5 to 10 mA). Therefore, as described above, by adopting the configuration including the abnormality notifying unit that notifies the abnormality when the absolute value of (Bt−B0) is larger than the predetermined value, abnormality has occurred in the semiconductor gas detection element. Can be promptly notified to the user.

  The gas detection device according to the present invention preferably further comprises notification means for notifying the arrival of the inspection time when the elapsed time from the start of use exceeds a predetermined time. The predetermined time in this case is not particularly limited, but it is preferably a long time enough for the change in sensitivity-reducing characteristics to sufficiently proceed. As an example, a time corresponding to a unit of 50 to 100 hours is given. Can do. Also, the notification method is not particularly limited, and may be based on lighting or blinking display using an LED or the like, voice guidance, image display on a monitor, or the like. By adopting a configuration including such a notification means, it is possible to appropriately notify the user of the arrival of the inspection time of the gas detection device.

  Examples of the gas detection device according to the present invention will be described below, and the present invention will be described in more detail. However, the present invention is not limited to the following examples.

  As the semiconductor gas detection element, a hot-wire semiconductor sensor using tin oxide as a sensitive material is used. (1) Laboratory air, (2) Laboratory air containing 20 ppm of ethanol as a gas to be detected, (3) Changes in the output characteristics of the semiconductor gas detection element were investigated under the respective atmospheres of laboratory air containing 50 ppm of ethanol as the gas to be detected and (4) air in the laboratory containing 100 ppm of ethanol as the gas to be detected. The results are shown in FIGS.

  The experimental results shown in FIG. 1 and FIG. 2 show that the gas detector is left in an environment where 5 ppm of hexamethyldisiloxane, 20 ppm of isopropanol, and 3 ppm of diethyl phthalate coexist, and the laboratory for the elapsed time. It is the result of tracking the output value (base value) in air and the sensitivity to ethanol. In this test condition, a gas that can coexist in the environment where the gas detector is actually installed is selected, and its concentration is set higher than the concentration in the actual environment. This experiment can be regarded as an acceleration test when the gas detection device is installed in an actual environment. Moreover, in FIG. 1, the current value (output value) as a sensor output by the semiconductor gas detection element is plotted against the elapsed time after the start of use. In FIG. 2, the output value of the semiconductor gas detection element is plotted against the concentration of the logarithm of ethanol.

  As can be clearly understood from FIGS. 1 and 2, even if the ethanol concentration is constant, a relatively early period after the start of use of the gas detection device (in the illustrated example, a period of at least 0 to 25 hours). It can be seen that the output value rises with a larger change width as the base value rises. According to the study by the present inventors, such a change in output characteristics at an early stage is mainly caused by the progress of reaction (catalyst poisoning) with miscellaneous gas components in the environment as the surface of the semiconductor gas detection element. This is considered to be due to the fact that the gas to be detected deteriorates moderately within a short period of time, and as a result, the gas to be detected easily diffuses into the semiconductor gas detection element. Examples of miscellaneous gas components in the environment include siloxanes, sulfur compounds, and hardly oxidizable organic compound gases. Such an output characteristic change can be referred to as a “sensitivity-enhancing characteristic change”.

  On the other hand, after a certain amount of time has elapsed after the start of use (in the example shown, at least a period of 25 to 60 hours), the concentration of each gas to be detected remains constant while the base value remains substantially constant. However, it can be seen that the output value gradually decreases with time. According to the study by the present inventors, such a change in output characteristics after a lapse of a predetermined time is mainly caused by the progress of sintering (sintering) accompanying the accumulation of thermal stress of the sensitive material particles and the environmental noise. This is considered to be due to the fact that the semiconductor gas detection element is naturally deteriorated with the reaction with the gas component and the like, and sufficient responsiveness cannot be obtained. Such a change in output characteristics can be referred to as a “sensitivity reduction characteristic change”.

  Considering the above phenomenon comprehensively, in the semiconductor gas detection element, the sensitivity change and the sensitivity change occur simultaneously with the passage of time, and so-called dynamic equilibrium is formed. It turns out that it can be interpreted as being. The increase in the output value in the early stage is because the high sensitivity change is superior to the low sensitivity change, and the low sensitivity change is dominant over the high sensitivity change. After that, the output value gradually decreases with time. FIG. 3 schematically shows a change in the sensitivity enhancement characteristic and a change in the sensitivity reduction characteristic, which are two factors causing the change in the output characteristic, conceptually divided.

  In addition, the sensitivity with respect to ethanol was tracked similarly using the gas detector installed in the normal indoor environment separately from said experiment. From this performance tracking result, it was confirmed that the phenomenon shown in FIG. 1 appears over a long period of time. These facts mean that the difference in the influence of the semiconductor gas detection element from the installation environment can be expressed by a function having time as a variable.

  The inventors of the present invention have tried to correct the output value in order to cancel the influence of each of the sensitivity enhancement characteristic change and the sensitivity reduction characteristic change. Then, it has been found that the effect due to the change in the sensitivity reduction characteristic can be canceled by adding the output change equivalent due to the change in the sensitivity reduction characteristic in proportion to the elapsed time using the proportional coefficient a for sensitivity reduction. It was. Similarly, high sensitivity can be obtained by subtracting the output change equivalent attributed to the change in sensitivity enhancement characteristic in proportion to the amount of change in the base value relative to the initial base value using the proportional coefficient b for sensitivity enhancement. It has been found that the influence of change in chemical properties can be counteracted. Here, the low-sensitivity proportional coefficient a can be calculated based on sensitivity tracking data when the semiconductor gas detection element is installed in clean air, and the high-sensitivity proportional coefficient b is a severe environment. It can be calculated based on the result of the acceleration test in the inside.

Based on this concept, it was found that the output value after correction so as to cancel the influence of the change in the output characteristics of the semiconductor gas detection element can be expressed by the following relational expression.
[Equation 3]
Vc = Vs + a.t- (Bt-B0) .b (I)
(In the formula, Vc is an output value after correction, Vs is an output value before correction, t is an elapsed time, B0 is an initial base value, Bt is a base value after t time units, and a and b are constants.)

  Next, the proportionality factors a and b in the above formula (I) are experimentally calculated by an acceleration test or the like, and the low-sensitivity proportionality factor a = 0.15 and the high-sensitivity proportionality factor b = 20. . Then, using these and the initial base value B0 obtained by actual measurement, the output value obtained by actual measurement shown in FIGS. 1 and 2 was corrected based on the above formula (I). The results are shown in FIGS. Like FIG.1 and FIG.2, the electric current value (output value) as a sensor output after correction | amendment is plotted with respect to the elapsed time after a use start in FIG. 4, and the logarithm display of ethanol in FIG. The corrected output value is plotted against the density.

  As can be understood from FIGS. 4 and 5, it can be seen that the amount of change in the output value after correction at each density is kept small over a long period of time. Referring to FIG. 5 in particular, when the ethanol concentration range is 10 ppm to 100 ppm, the corrected output value itself varies depending on the ethanol concentration, but the fluctuation amount is a period of 60 hours or more. It can be seen that the current is suppressed within the range of 2.5 mA to 3 mA. Referring to FIG. 2, if it is considered that the output value increases in a period of 10 to 24 hours in the state before correction and the fluctuation amount expands to the range of 6 mA to 6.5 mA, this It can be understood that the output value correction according to the invention exhibits an excellent effect in suppressing the change with time.

  Thus, by setting the proportionality factor a for desensitization and the proportionality factor b for sensitivity enhancement to appropriate values, the amount of change with time of the output value after correction is long within a concentration range of at least 10 ppm to 100 ppm. It is kept small over the period. Therefore, the gas detection device according to the present embodiment has exceptional utility for the use of the gas alarm device in the case where ethanol is used as the gas to be detected. That is, in a gas alarm device that notifies a user when ethanol gas having a concentration exceeding a threshold value is detected as a predetermined value within a range of 10 ppm to 100 ppm, for a long period of time. The alarm concentration can be stabilized.

  4 and 5 show the case where the detected gas is ethanol, the gas type of the detected gas is not limited to ethanol. Even if a gas other than ethanol is used as the gas to be detected, each gas type can be set by setting the proportionality factor a for desensitization and the proportionality factor b for sensitivity enhancement to appropriate values according to the gas type. The amount of change over time in the output value can be kept small over a long period of time.

  However, when the above correction is performed by the correction unit according to the present embodiment, the sensor output value (base value) in the air is changed from the initial value as time passes as shown in FIG. It will rise gradually. Therefore, the base value (Bt; in this example, the lowest value of the sensor output value in the previous fixed period) is monitored, and an abnormality is detected when the base value exceeds a predetermined value (for example, 10 mA). It can be set as the structure which alert | reports. That is, if the sensor abnormality or the sensor life is determined based on the base value exceeding the predetermined value and the sensor replacement time is notified to the user, the soundness of the gas detection device can be maintained. It becomes easy to plan.

In another example, a hot wire type semiconductor sensor using zinc oxide as a sensitive material was used as a semiconductor type gas detection element, and an acceleration test was performed under the same conditions as in the above example using methyl ethyl ketone as the gas to be detected. The result of confirming the effect of the correction by the formula (I) in this case is shown in FIGS. Here, FIG. 6 shows the result of actual measurement of the output value, and FIG. 7 shows the result when the output value is corrected based on the formula (I). In the formula (I), the coefficients a and b are set to a = 0.17 and b = 20, respectively. As is clear from this result, according to the formula (I), it is caused by the change in sensitivity characteristics of the semiconductor gas sensing element due to disturbance substances in the environment and the deterioration of the sensitive material itself due to long-term use. It can be seen that the change in sensitivity-reducing characteristics can be corrected simultaneously. Thereby, in this example, the special utility was confirmed by the use of the gas alarm device in the case of using methyl ethyl ketone as the gas to be detected. That is, in a gas alarm device that notifies a user when methyl ethyl ketone having a predetermined value near 20 ppm (for example, 15 ppm to 25 ppm) is detected as a threshold and a concentration exceeding the threshold is detected over a long period of time. It was confirmed that the alarm concentration could be stabilized over the entire period.

Claims (4)

A gas detection apparatus comprising: a semiconductor gas detection element; and correction means for correcting an output value of the semiconductor gas detection element based on the following equation (I).
[Equation 1]
Vc = Vs + a.t- (Bt-B0) .b (I)
(In the formula, Vc is an output value after correction, Vs is an output value before correction, t is an elapsed time, B0 is an initial base value, Bt is a base value after t time units, and a and b are constants.)   The gas detection device according to claim 1, further comprising alarm means for notifying an alarm when the corrected output value exceeds a predetermined threshold value.   The gas detection device according to claim 1, further comprising abnormality notifying means for notifying abnormality when the absolute value of (Bt−B0) becomes larger than a predetermined value.   The gas detection device according to any one of claims 1 to 3, further comprising storage means for storing Bt for each predetermined time unit t.

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