Gas detection device and gas detection method

Abstract

To provide a gas detection device and a gas detection method capable of detecting deterioration in sensitivity of gas detection irrespective of presence / absence of lowering of an air base.SOLUTION: A gas detection device 1 includes: a detection element 2 in which a resistance value increases by the combustion of an inflammable gas; a reference element 3 which is connected in series with the detection element 2 and in which the resistance value is constant with respect to the inflammable gas; a power source 41 for applying a pulse voltage to the detection element 2 and the reference element 3; a differential amplifier circuit 44 for outputting a potential difference between an intermediate potential Vin- and a reference potential Vin+ between the detection element 2 and the reference element 3; and an MCU 5 for executing gas detection processing for applying a pulse voltage of a first pulse width to the detection element 2 and the reference element 3 by controlling the power source 41 so as to detect gas based on the output Vout of the differential amplifier circuit 44, and gas sensitivity determination processing for applying the pulse voltage of a second pulse width equal to or greater than the first pulse width to the detection element 2 and the reference element 3 by controlling the power source 41 so as to determine the sensitivity of gas detection based on the output Vout of the differential amplifier circuit 44.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a gas detection device and a gas detection method. [Background technology]

[0002] A catalytic combustion type gas sensor that detects a target gas based on a neutral point potential difference of a bridge circuit equipped with a detection element and a reference element is known (see, for example, Patent Document 1). In the catalytic combustion type gas sensor described in Patent Document 1, deterioration of the catalytic combustion type gas sensor is determined based on an air base, which is a neutral point potential difference in a state in which the target gas is not present. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-235082 A Summary of the Invention [Problem to be solved by the invention]

[0004] It is known that the gas detection sensitivity of catalytic combustion type gas sensors deteriorates due to a decrease in the air base, but the inventors of the present application have discovered that the sensitivity may deteriorate even if the decrease in the air base is small. In such cases, it is difficult for conventional catalytic combustion type gas sensors to detect the deterioration in gas detection sensitivity.

[0005] In view of the above circumstances, it is an object of the present invention to provide a gas detection device and a gas detection method that are capable of detecting deterioration in gas detection sensitivity regardless of the presence or absence of a decrease in the air base. [Means for solving the problem]

[0006] a control unit that executes a gas detection process in which the power supply is controlled to apply the pulse voltage of a first pulse width to the detection element and the reference element to detect gas based on the output of the output unit, and a gas sensitivity determination process in which the power supply is controlled to apply the pulse voltage of a second pulse width equal to or greater than the first pulse width to the detection element and the reference element to determine the gas detection sensitivity based on the output of the output unit, and the control unit determines the gas detection sensitivity in the gas sensitivity determination process based on the ratio of outputs of the output unit between a point in time when the time from the rise of the pulse voltage is a first time less than the first pulse width and a point in time when the time from the rise of the pulse voltage is a second time equal to or greater than the second pulse width.

[0007] Further, a gas detection method of the present invention is a method of detecting gas using a gas detection device including a detection element whose resistance value increases due to combustion of a combustible gas, a reference element connected in series to the detection element and having a constant resistance value against the combustible gas, a power source which applies a pulse voltage to the detection element and the reference element, and an output unit which outputs a potential difference between a midpoint potential between the detection element and the reference element and a reference potential, the method comprising: a gas detection step of controlling the power source to apply the pulse voltage of a first pulse width to the detection element and the reference element and detect gas based on the output of the output unit; and a gas sensitivity determination step of controlling the power source to apply the pulse voltage of a second pulse width equal to or greater than the first pulse width to the detection element and the reference element and determine the gas detection sensitivity based on the output of the output unit, and in the gas sensitivity determination step, the gas detection sensitivity is determined based on a ratio of outputs from the output unit at a point in time when the time from the rise of the pulse voltage is a first time less than the first pulse width and a point in time when the time from the rise of the pulse voltage is a second time equal to or greater than the second pulse width. Effect of the Invention

[0008] According to the present invention, deterioration in sensitivity of gas detection can be detected regardless of the presence or absence of a decrease in the air base. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing the configuration of a gas detection device according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a cross-sectional view showing the sensing element and the reference element shown in FIG. [Diagram 3] FIG. 3 is a graph showing the sensor output characteristics of a catalytic combustion type gas sensor. [Figure 4] FIG. 4 is a waveform diagram showing a driving pattern of the bridge circuit by the MCU. [Diagram 5] FIG. 5 is a diagram for explaining the principle of the gas detection device of this embodiment. [Figure 6] FIG. 6 is a diagram showing the drive waveform and the response waveform of the catalytic combustion type gas sensor when the pulse width is set to 50 msec and 100 msec. [Figure 7] FIG. 7 is a graph showing the temperature characteristics of the gas detecting device of this embodiment. [Figure 8] FIG. 8 is a diagram for comparing the initial state and the poisoned state of the catalytic combustion type gas sensor. [Figure 9] FIG. 9 is a diagram for explaining the negative drift of the air base that occurs in a catalytic combustion type gas sensor. [Figure 10] FIG. 10 is a graph showing the relationship between the amount of change (mV) in the air base when the catalytic combustion type gas sensor is poisoned and the number of days that have passed (days). [Figure 11] FIG. 11 is a graph showing the relationship between the deterioration rate (%) of the detection sensitivity of a catalytic combustion type gas sensor to 3000 ppm methane when the sensor is poisoned and the number of days elapsed. [Figure 12] FIG. 12 is a graph showing the relationship between the response waveform of the output (mV) of the sensor L4 and the number of days that have passed. [Figure 13]FIG. 13 is a graph showing the relationship between the air base update value (mV), the 49 / 99 ratio, and the 30 / 99 ratio of the sensor L4 and the number of days elapsed. [Figure 14] FIG. 14 is a graph showing the relationship between the deterioration rate (%) of the detection sensitivity of sensor L4 to 3000 ppm methane and the number of elapsed days (days). [Figure 15] FIG. 15 is a graph showing the relationship between the output (mV) of the sensor L6 and the number of days elapsed. [Figure 16] FIG. 16 is a graph showing the relationship between the air base update value (mV), the 49 / 99 ratio, and the 30 / 99 ratio of the sensor L6 and the number of days elapsed. [Figure 17] FIG. 17 is a graph showing the relationship between the deterioration rate (%) of the detection sensitivity of the sensor L6 to 3000 ppm methane and the number of elapsed days (days). [Figure 18] FIG. 18 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 49 ratio, the 20 / 49 ratio, the 26 / 49 ratio, the 30 / 49 ratio, the 35 / 49 ratio, the 40 / 49 ratio, the 45 / 49 ratio, and the 49 / 49 ratio and the number of days elapsed. [Figure 19] FIG. 19 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 60 ratio, the 20 / 60 ratio, the 26 / 60 ratio, the 30 / 60 ratio, the 35 / 60 ratio, the 40 / 60 ratio, the 45 / 60 ratio, and the 49 / 60 ratio and the number of days elapsed. [Figure 20] FIG. 20 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 80 ratio, the 20 / 80 ratio, the 26 / 80 ratio, the 30 / 80 ratio, the 35 / 80 ratio, the 40 / 80 ratio, the 45 / 80 ratio, and the 49 / 80 ratio and the number of days elapsed. [Figure 21] FIG. 21 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 99 ratio, the 20 / 99 ratio, the 26 / 99 ratio, the 30 / 99 ratio, the 35 / 99 ratio, the 40 / 99 ratio, the 45 / 99 ratio, and the 49 / 99 ratio and the number of days elapsed. [Figure 22] FIG. 22 is a diagram showing the configuration of a gas detection device according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present invention will be described below along with preferred embodiments. Note that the present invention is not limited to the embodiments shown below, and can be modified as appropriate within the scope of the present invention. In addition, in the embodiments shown below, some configurations are omitted from illustration and description, but for the details of the omitted technologies, publicly known or well-known technologies are applied as appropriate within the scope of no contradiction with the contents described below.

[0011] 1 is a diagram showing the configuration of a gas detector 1 according to one embodiment of the present invention. As shown in the figure, gas detector 1 is a catalytic combustion type (or adsorption combustion type) gas sensor equipped with a bridge circuit 4 in which a detection element 2 and a reference element 3 are connected. Note that the adsorption combustion type is a gas detection method in which gas molecules to be detected are adsorbed to a catalyst in detection element 2 while the pulse voltage is OFF or at a LOW voltage, and when the pulse voltage becomes ON or at a HIGH voltage, the gas molecules are combusted on the catalyst in detection element 2, thereby obtaining a sensor output.

[0012] The detection element 2 includes a heater 21. The reference element 3 also includes a heater 31. Examples of the heaters 21 and 31 include micro-heaters of a diaphragm type, an air bridge type, a cantilever type, etc. Examples of the material of the heaters 21 and 31 include platinum (Pt) and the like.

[0013] The detection element 2 includes a detection unit 22 that covers the heater 21. The detection unit 22 is made of a material that has catalytic activity in an oxidation reaction. The detection unit 22 includes a catalyst and a catalyst carrier that supports the catalyst. Examples of the catalyst material of the detection unit 22 include platinum group metals such as platinum, palladium (Pd), iridium (Ir), ruthenium (Ru), and rhodium (Rh), and other metals such as gold (Au), silver (Ag), copper (Cu), nickel (Ni), and cobalt (Co). Examples of the catalyst carrier material of the detection unit 22 include alumina (Al2O3), titanium oxide (TiO2), cerium oxide (CeO2), and tin oxide (SnO2). Examples of the material of the detection unit 22 include a complex of a metal-organic framework (MOF) and a catalyst such as platinum.

[0014] The reference element 3 includes a reference part 32 that covers the heater 31. The reference part 32 is made of a substance that does not have catalytic activity in an oxidation reaction. The reference part 32 is made of the same material as the catalyst carrier of the detection part 22. An example of the material of the reference part 32 is MOF.

[0015] 2 is a cross-sectional view showing the detection element 2 and the reference element 3 shown in FIG. 1. As shown in this figure, when combustible gas molecules G to be detected come into contact with (or are adsorbed on) the detection section 22 heated to several hundred degrees by the heater 21, a catalytic combustion reaction (or an adsorption combustion reaction) occurs on the detection section 22, and the temperature of the heater 21 increases due to the reaction heat, increasing the resistance value. On the other hand, even if combustible gas molecules G come into contact with (or are adsorbed on) the reference section 32 of the reference element 3, no catalytic combustion reaction (or an adsorption combustion reaction) occurs. In such a gas detection device 1, the change in the resistance value of the heater 21 is obtained as a sensor output by the bridge circuit 4 to detect the combustible gas or identify the concentration of the combustible gas.

[0016] As shown in Fig. 1, the bridge circuit 4 includes paths 4A and 4B connected in parallel, and a power source 41. The pair of paths 4A and 4B branch off from a conductor connected to the positive electrode of the power source 41. The pair of branched paths 4A and 4B are reconnected to a conductor connected to the negative electrode of the power source 41. In one path 4A, the detection element 2 and the reference element 3 are connected in series. In the other path 4B, a pair of fixed resistors R A ,R B and a variable resistor VR are connected in series. In this embodiment, the detection element 2 is disposed on the positive electrode side of a power source 41, and the reference element 3 is disposed on the negative electrode side of the power source 41.

[0017] Examples of the power source 41 include a battery and a commercial power source. In this embodiment, the power source 41 can be a small battery by realizing low power consumption, which will be described later.

[0018] The bridge circuit 4 includes a terminal 42 connected between the detection element 2 and the reference element 3, and a fixed resistor R A and fixed resistor R B and a differential amplifier circuit 44. The intermediate potential between the detection element 2 and the reference element 3 is output from a terminal 42 to an output V in- The potential at the connection point between the variable resistor VR and the terminal 43 is output as the output V in+ The terminals 42 and 43 are connected to a differential amplifier circuit 44, and the output V in- and output V at terminal 43 in+ The difference (potential difference) between these is amplified by the differential amplifier circuit 44 and output as V out It becomes.

[0019] Output V of differential amplifier circuit 44 out is calculated using the following formula (1).

number

[0020] The differential amplifier circuit 44 outputs V outThe MCU 5 pulse-drives the bridge circuit 4 and outputs the output V out As will be described later, MCU 5 also performs temperature correction according to the ambient temperature (external temperature) around gas detection device 1.

[0021] Figure 3 is a graph showing the sensor output characteristics of a catalytic combustion type gas sensor. The output (V) of the catalytic combustion type gas sensor shown in this graph is a value after amplification. As shown in this graph, the output (V) of the catalytic combustion type gas sensor increases in proportion to the concentration of the combustible gas to be detected.

[0022] 4 is a waveform diagram showing a driving pattern of the bridge circuit 4 by the MCU 5. As shown in this figure, the MCU 5 pulse-drives the bridge circuit 4. In this embodiment, the period for pulse-driving the bridge circuit 4 is 30 sec, and the pulse width (ON time) is 50 msec. Note that it is not essential to perform ON / OFF control of the bridge circuit 4, and LOW / HIGH control of the bridge circuit 4 (switching control between LOW voltage and HIGH voltage) may be performed.

[0023] In this embodiment, in addition to the pulse drive with a pulse width of 50 msec for gas detection in a 30-second cycle, the sensor also executes pulse drive with a pulse width of 100 msec for checking the sensitivity of gas detection every predetermined time (for example, every 6 hours). This will be described in detail later.

[0024] FIG. 5 is a diagram for explaining the principle of gas detector 1 of this embodiment. As shown in the upper left part of this figure, in this embodiment, the pulse width of the pulse voltage is set to 50 msec. Here, as shown on the right side of the figure, it is assumed that there is a difference in film thickness between detection section 22 of detection element 2 and reference section 32 of reference element 3, and that there is no difference in film thickness. Specifically, as shown in the upper right part of the figure, it is assumed that the film thickness of detection section 22 is smaller than the film thickness of reference section 32 (hereinafter referred to as case 1), as shown in the middle right part of the figure, the film thickness of detection section 22 is equal to the film thickness of reference section 32 (hereinafter referred to as case 2), and as shown in the lower right part of the figure, the film thickness of detection section 22 is larger than the film thickness of reference section 32 (hereinafter referred to as case 3).

[0025] As shown in the lower left part of the figure, the output V of gas detection device 1 varies depending on the relationship between the film thickness of detection portion 22 and the film thickness of reference portion 32. out The response waveform varies. Specifically, the maximum peak output appears in case 1, and in case 2, the peak output is lower than in case 1. Furthermore, in cases 1 and 2, the peak output is shown between 10 and 30 msec from the start of the pulse, whereas in case 3, the peak output is shown 50 msec from the start of the pulse. The peak output in case 3 is lower than the peak output in cases 1 and 2.

[0026] Here, the output V in cases 1 to 3 at 50 msec from the start of the pulse (hereinafter, 50 msec. The end of the actual pulse width) out Specifically, the output V at 50 ms varies. out Case 1 is the highest, case 2 is the next highest, and case 3 is the lowest. That is, at the time of 50 msec, the output V of case 1, case 2, and case 3 are out does not converge.

[0027] 6 is a diagram showing the drive waveform and response waveform of gas detection device 1 when the pulse width is set to 50 msec and 100 msec. The vertical axis of the response waveform in this diagram is the output V out Output V at 50 ms out As shown in this figure, at 100 msec, the output V out In this way, the output V for cases 1 to 3 out The point where converges is called the saturation point.

[0028] 7 is a graph showing the temperature characteristics of gas detection device 1 of the present embodiment. The "air base fluctuation / mV" on the vertical axis in this graph represents the difference in air base from a reference value (hereinafter referred to as air base fluctuation) that is determined by using the air bases of cases 1 to 3 when the ambient temperature of gas detection device 1 is 20°C as the reference value. Therefore, when the ambient temperature of gas detection device 1 is 20°C, the air base fluctuation is always 0 mV.

[0029] In gas detection device 1 of this embodiment, as shown in Fig. 5, variation occurs in the response waveform due to the difference in film thickness between detection section 22 and reference section 32. In addition, in gas detection device 1 of this embodiment, as shown in Fig. 6, variation occurs in the relationship between the ambient temperature and the amount of air base fluctuation (hereinafter referred to as temperature characteristics) due to the difference in film thickness between detection section 22 and reference section 32.

[0030] The inventors of the present application have found that the temperature characteristics of gas detection device 1 show a correlation with the air base ratio, in other words, gas detection device 1 exhibits temperature characteristics that correspond to the air base ratio. Therefore, in this embodiment, the temperature characteristics that correspond to the air base ratio are confirmed in advance by experiment or the like. The air base ratio of gas detection device 1 is then obtained for each individual device, and a correction coefficient and correction formula that correspond to the obtained air base ratio are set for each individual device, thereby suppressing increases in the amount of air base fluctuation caused by temperature fluctuations.

[0031] However, if a poisonous substance adheres to detection portion 22 of detection element 2 or reference portion 32 of reference element 3, output V out (In an air environment considered to be fresh, the air base) fluctuates, and the sensitivity of gas detection decreases. In addition, if poisonous substances adhere to the detection part 22 or reference part 32 of the detection element 2, the sensitivity of gas detection may decrease even if there is no or only a small fluctuation in the air base. These points will be explained below.

[0032] 8 is a diagram for comparing the initial state and the poisoned state of a catalytic combustion type gas sensor. As shown in this figure, when a poisonous substance adheres to the detection part 22 of the detection element 2, if no measures are taken, the output V out Specifically, when a poisonous substance adheres to the detection section 22 of the detection element 2, the heat capacity of the detection section 22 becomes larger than that in the initial state. In contrast, when a constant voltage is applied to the detection element 2 and the reference element 3, the temperature of the detection section 22 becomes lower than that in the initial state, and the resistance value of the heater 21 decreases compared to that in the initial state. As a result, the voltage across the detection element 2 becomes lower than that in the initial state, and the midpoint potential (output V in- ) becomes higher than the initial state. Therefore, the output V calculated by the above formula (1) out decreases compared to the initial state.

[0033] 9 is a diagram for explaining the negative drift of the air base that occurs in a catalytic combustion type gas sensor. As shown in this figure, when a poisonous substance adheres to the detection part 22 of the detection element 2, a decrease in the air base (so-called negative drift) occurs.

[0034] In catalytic combustion gas sensors, the alarm threshold value ΔV A is set, and the output V out is the difference between the air base setting and ΔV A An alarm is issued when the air base rises by 5 mV (for example). The air base setting is initially set to an initial value V0, and is updated and reduced according to the negative drift of the air base. The alarm point V A(ΔV to the air base setting A The alarm determination threshold ΔV A is set and a warning is issued in the same manner.

[0035] On the other hand, in the gas detection device 1 of the present embodiment, the failure determination threshold V F is set, and the output V out is the failure judgment threshold V F If the sensitivity drops below this level, a sensor failure is detected. In addition, gas detection device 1 of the present embodiment executes a process for detecting deterioration in gas detection sensitivity that occurs regardless of the magnitude of the air-base negative drift. This process and the problems with catalytic combustion type gas sensors that are solved by this process are described below.

[0036] The inventors of the present application have confirmed through experiments that in catalytic combustion type gas sensors, deterioration of gas detection sensitivity may occur due to poisoning of the detection element 2 or reference element 3, regardless of the presence or absence of a change in the air base. Furthermore, the inventors of the present application have confirmed through experiments that in catalytic combustion type gas sensors, characteristics indicating deterioration of gas detection sensitivity appear in the response waveform of the sensor. Therefore, in gas detection device 1 of this embodiment, as shown in Figure 4, pulse driving with a pulse width of 100 msec is performed at predetermined time intervals to generate output V out The response waveform of the output V out Based on the response waveform, it is determined whether or not the gas detection sensitivity has deteriorated.

[0037] Fig. 10 is a graph showing the relationship between the amount of change in air base (mV) and the number of days elapsed when a catalytic combustion type gas sensor is poisoned. Fig. 11 is a graph showing the relationship between the deterioration rate (%) of the detection sensitivity to 3000 ppm methane and the number of days elapsed when a catalytic combustion type gas sensor is poisoned.

[0038] The inventors of the present application have confirmed through experiments that deterioration of gas detection sensitivity may occur regardless of the magnitude of the negative drift of the air base. The experiments were conducted using multiple types of catalytic combustion type gas sensors under poisoning conditions simulating the installation environment in a business environment. In the experiments, three types of catalytic combustion type gas sensors, two of each type, were used.

[0039] 10 and 11, it can be seen that even if the amount of change in the air base of the catalytic combustion gas sensor is extremely small (less than 1 / 3) compared to the failure judgment threshold, the gas detection sensitivity may deteriorate significantly due to the progression of poisoning. In addition, it can be seen that if the threshold for the deterioration rate of the gas detection sensitivity is set to 50%, the number of days that elapse until the deterioration rate of the gas detection sensitivity exceeds the threshold is 110 to 120 days for sensors L3 and L4, 120 to 130 days for sensors L5 and L6, and 150 to 170 days for sensors L1 and L2.

[0040] 12 is a graph showing the relationship between the response waveform of the output (mV) of sensor L4 and the number of days that have passed since installation in the environment of the above-mentioned poisoning conditions. This graph shows the relationship between the air base output (mV) of sensor L4 and the time from the start of the pulse voltage (hereinafter referred to as the pulse time) (msec) at 0.0, 20.9, 61.8, 96.3, and 151.5 days after installation in the environment of the above-mentioned poisoning conditions.

[0041] 13 is a graph showing the relationship between the air base update value (mV) of sensor L4, the ratio of the air base output when the pulse time is 49 msec to when it is 99 msec (hereinafter referred to as the 49 / 99 ratio), and the ratio of the air base output when the pulse time is 30 msec to when it is 99 msec (hereinafter referred to as the 30 / 99 ratio) and the number of days that have passed. The 49 / 99 ratio is the value obtained by dividing the air base output (mV) when the pulse time is 49 msec (hereinafter referred to as the 49 msec time) by the air base output (mV) when the pulse time is 99 msec (hereinafter referred to as the 99 msec time). The 30 / 99 ratio is the value obtained by dividing the air base output (mV) when the pulse time is 30 msec (hereinafter referred to as the 30 msec time) by the air base output (mV) at the 99 msec time.

[0042] Fig. 14 is a graph showing the relationship between the deterioration rate (%) of the detection sensitivity of sensor L4 to 3000 ppm methane and the number of days elapsed (days). The dashed line in Fig. 14 indicates that the threshold value of the deterioration rate of the gas detection sensitivity is set to -60%.

[0043] As shown in the graph of FIG. 12, the air base output at the point where the pulse time is 49 to 99 msec (hereinafter referred to as the 49 to 99 msec point) decreases as the number of days elapses increases. As shown in the graph of FIG. 13, a 3.5 mV decrease is observed when comparing the initial air base update value with that at the time 100 days have passed. However, since the deterioration rate of the gas detection sensitivity does not exceed the threshold value (60%) at the time 100 days have passed, it is difficult to determine the deterioration of the gas detection sensitivity based on the fluctuation of the air base output at the 49 to 99 msec point. In addition, since the 49 / 99 ratio does not change in accordance with the increase in the number of days elapsed, it is also difficult to determine the deterioration of the gas detection sensitivity based on the fluctuation of the 49 / 99 ratio.

[0044] On the other hand, the 30 / 99 ratio is 0.88 at the time when the deterioration rate of the gas detection sensitivity exceeds the threshold, compared to the initial value of 0.825. In other words, the 30 / 99 ratio at the time when the deterioration rate of the gas detection sensitivity exceeds the threshold is 1.067 times the initial value, and clearly rises from the initial value. The 30 / 99 ratio also rises before and after the deterioration rate of the gas detection sensitivity exceeds the threshold, and is 0.88 at the time when the deterioration rate of the gas detection sensitivity exceeds the threshold, compared to the minimum value of 0.815 at the start of the rise. In other words, the 30 / 99 ratio at the time when the deterioration rate of the gas detection sensitivity exceeds the threshold is 1.08 times the minimum value at the start of the rise, and clearly rises from the minimum value at the start of the rise. This phenomenon is considered to occur partly due to the increase in the air base output at 30 msec due to the combustion of VOCs adsorbed on the detection section 22 of the detection element 2.

[0045] Therefore, in an embodiment of gas detection device 1 having the same specifications as sensor L4, a pulse voltage with a pulse width of 100 msec is applied to bridge circuit 4 at a predetermined cycle to obtain a response waveform of the air base output, and deterioration of the gas detection sensitivity is detected according to the 30 / 99 ratio. Specifically, MCU 5 (see FIG. 1) determines whether the ratio between the current value and the initial value of the 30 / 99 ratio is equal to or greater than a first threshold value (e.g., 1.03 times) and / or the ratio between the current value of the 30 / 99 ratio and the minimum value at the start of the rise is equal to or greater than a second threshold value (e.g., 1.07 times), and detects deterioration of the gas detection sensitivity if the determination is affirmative.

[0046] In addition, in an embodiment of gas detection device 1 having the same specifications as sensor L4, MCU 5 performs a fault determination if the drop in air base output at 49 msec is 3 mV or more, the MCU 5 detects deterioration in gas sensitivity based on the ratio of the current value of the 30 / 99 ratio to its initial value and / or the ratio of the current value of the 30 / 99 ratio to the minimum value at the time the rise began, and the period of time that has elapsed since the start of use is 2 years or more.

[0047] 15 is a graph showing the relationship between the output (mV) of the sensor L6 and the number of days elapsed. This graph shows the relationship between the air base output (mV) of the sensor L6 and the pulse time (msec) when the number of days elapsed since the sensor was installed in the environment of the above-mentioned poisoning conditions is 0.0 days, 20.9 days, 61.8 days, 96.3 days, and 151.5 days.

[0048] Fig. 16 is a graph showing the relationship between the air base update value (mV), 49 / 99 ratio, and 30 / 99 ratio of sensor L6 and the number of days elapsed. Fig. 17 is a graph showing the relationship between the deterioration rate (%) of the detection sensitivity of sensor L6 to 3000 ppm methane and the number of days elapsed. The dashed line in Fig. 17 indicates that the threshold value of the gas sensitivity deterioration rate is set to about -70%.

[0049] As shown in the graph of FIG. 15, the air base output at 49-99 msec decreases as the number of days elapsed increases. As shown in the graph of FIG. 16, a comparison of the initial air base update value with that at 120 days elapsed shows a decrease of 3.5 mV or more. However, since the deterioration rate of the gas detection sensitivity does not exceed the threshold value (-70%) at the 120-day point, it is difficult to determine the deterioration of the gas detection sensitivity based on the fluctuation of the air base output at 49-99 msec. In addition, since the 49 / 99 ratio does not show any fluctuation in response to the increase in the number of days elapsed, it is also difficult to determine the deterioration of the gas detection sensitivity based on the fluctuation of the 49 / 99 ratio.

[0050] In contrast, the 30 / 99 ratio rises before and after the deterioration rate of the gas detection sensitivity exceeds the threshold, and is 1.04 at the point when the deterioration rate of the gas detection sensitivity exceeds the threshold, compared to the minimum value of 1.01 at the start of the rise. In other words, the 30 / 99 ratio at the point when the deterioration rate of the gas detection sensitivity exceeds the threshold is 1.03 times the minimum value at the start of the rise, and clearly rises from the minimum value at the start of the rise.

[0051] Therefore, in one embodiment of gas detection device 1 having the same specifications as sensor L6, a pulse voltage with a pulse width of 100 msec is applied to bridge circuit 4 at a predetermined cycle to obtain the response waveform of the air base output, and a deterioration in gas detection sensitivity is determined according to the 30 / 99 ratio. Specifically, MCU 5 determines whether the ratio between the current value of the 30 / 99 ratio and the minimum value at the start of the rise is equal to or greater than a threshold value (e.g., 1.03 times), and if the determination is affirmative, a deterioration in gas detection sensitivity is detected.

[0052] Furthermore, in one embodiment of gas detection device 1, MCU 5 performs a fault determination if the drop in air base output at 49 msec is 3 mV or more, if MCU 5 detects a deterioration in gas detection sensitivity based on the ratio of the current 30 / 99 ratio to the minimum value at the time the rise began, and if two years or more have elapsed since the start of use.

[0053] 18 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 49 ratio, the 20 / 49 ratio, the 26 / 49 ratio, the 30 / 49 ratio, the 35 / 49 ratio, the 40 / 49 ratio, the 45 / 49 ratio, and the 49 / 49 ratio and the number of days elapsed. The 16 / 49 ratio is the ratio of the air base output at a pulse time of 16 msec to that at a pulse time of 49 msec, and is the value obtained by dividing the air base output at the 16 msec time by the air base output at the 49 msec time. The 20 / 49 ratio is the ratio of the air base output at a pulse time of 20 msec to that at a pulse time of 49 msec, and is the value obtained by dividing the air base output at the 20 msec time by the air base output at the 49 msec time. The 26 / 49 ratio is the ratio of the air base output at a pulse time of 26 msec to that at a pulse time of 49 msec, calculated by dividing the air base output at 26 msec by the air base output at 49 msec. The 30 / 49 ratio is the ratio of the air base output at a pulse time of 30 msec to that at a pulse time of 49 msec, calculated by dividing the air base output at 30 msec by the air base output at 49 msec. The 35 / 49 ratio is the ratio of the air base output at a pulse time of 35 msec to that at a pulse time of 49 msec, calculated by dividing the air base output at 35 msec by the air base output at 49 msec. The 40 / 49 ratio is the ratio of the air base output at a pulse time of 40 msec to that at a pulse time of 49 msec, calculated by dividing the air base output at 40 msec by the air base output at 49 msec. The 45 / 49 ratio is the ratio of the air base output at a pulse time of 45 msec to that at a pulse time of 49 msec, calculated by dividing the air base output at 45 msec by the air base output at 49 msec. The 49 / 49 ratio is the ratio of the air base output at a pulse time of 49 msec to that at a pulse time of 49 msec, calculated by dividing the air base output at 49 msec by the air base output at 49 msec.

[0054] 19 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 60 ratio, the 20 / 60 ratio, the 26 / 60 ratio, the 30 / 60 ratio, the 35 / 60 ratio, the 40 / 60 ratio, the 45 / 60 ratio, and the 49 / 60 ratio and the number of days elapsed. The 16 / 60 ratio is the ratio of the air base output at a pulse time of 16 msec to that at a pulse time of 60 msec, and is the value obtained by dividing the air base output at the 16 msec time by the air base output at the 60 msec time. The 20 / 60 ratio is the ratio of the air base output at a pulse time of 20 msec to that at a pulse time of 60 msec, and is the value obtained by dividing the air base output at the 20 msec time by the air base output at the 60 msec time. The 26 / 60 ratio is the ratio of the air base output when the pulse time is 26 msec to when it is 60 msec, calculated by dividing the air base output at 26 msec by the air base output at 60 msec. The 30 / 60 ratio is the ratio of the air base output when the pulse time is 30 msec to when it is 60 msec, calculated by dividing the air base output at 30 msec by the air base output at 60 msec. The 35 / 60 ratio is the ratio of the air base output when the pulse time is 35 msec to when it is 60 msec, calculated by dividing the air base output at 35 msec by the air base output at 60 msec. The 40 / 60 ratio is the ratio of the air base output when the pulse time is 40 msec to when it is 60 msec, calculated by dividing the air base output at 40 msec by the air base output at 60 msec. The 45 / 60 ratio is the ratio of the air base output when the pulse time is 45 msec to when it is 60 msec, calculated by dividing the air base output at 45 msec by the air base output at 60 msec. The 49 / 60 ratio is the ratio of the air base output when the pulse time is 49 msec to when it is 60 msec, calculated by dividing the air base output at 49 msec by the air base output at 60 msec.

[0055] 20 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 80 ratio, the 20 / 80 ratio, the 26 / 80 ratio, the 30 / 80 ratio, the 35 / 80 ratio, the 40 / 80 ratio, the 45 / 80 ratio, and the 49 / 80 ratio and the number of days elapsed. The 16 / 80 ratio is the ratio of the air base output at a pulse time of 16 msec to that at a pulse time of 80 msec, and is the value obtained by dividing the air base output at the 16 msec time by the air base output at the 80 msec time. The 20 / 80 ratio is the ratio of the air base output at a pulse time of 20 msec to that at a pulse time of 80 msec, and is the value obtained by dividing the air base output at the 20 msec time by the air base output at the 80 msec time. The 26 / 80 ratio is the ratio of the air base output when the pulse time is 26 msec to when it is 80 msec, calculated by dividing the air base output at 26 msec by the air base output at 80 msec. The 30 / 80 ratio is the ratio of the air base output when the pulse time is 30 msec to when it is 80 msec, calculated by dividing the air base output at 30 msec by the air base output at 80 msec. The 35 / 80 ratio is the ratio of the air base output when the pulse time is 35 msec to when it is 80 msec, calculated by dividing the air base output at 35 msec by the air base output at 80 msec. The 40 / 80 ratio is the ratio of the air base output when the pulse time is 40 msec to when it is 80 msec, calculated by dividing the air base output at 40 msec by the air base output at 80 msec. The 45 / 80 ratio is the ratio of the air base output at pulse times 45 msec to 80 msec, calculated by dividing the air base output at 45 msec by the air base output at 80 msec. The 49 / 80 ratio is the ratio of the air base output at pulse times 49 msec to 80 msec, calculated by dividing the air base output at 49 msec by the air base output at 80 msec.

[0056] 21 is a graph showing the relationship between the air base update value (mV) of sensor L6, the 16 / 99 ratio, the 20 / 99 ratio, the 26 / 99 ratio, the 30 / 99 ratio, the 35 / 99 ratio, the 40 / 99 ratio, the 45 / 99 ratio, and the 49 / 99 ratio and the number of days elapsed. The 16 / 99 ratio is the ratio of the air base output when the pulse time is 16 msec to when it is 99 msec, and is the value obtained by dividing the air base output at the 16 msec time by the air base output at the 99 msec time. The 20 / 99 ratio is the ratio of the air base output when the pulse time is 20 msec to when it is 99 msec, and is the value obtained by dividing the air base output at the 20 msec time by the air base output at the 99 msec time. The 26 / 99 ratio is the ratio of the air base output when the pulse time is 26 msec to when it is 99 msec, calculated by dividing the air base output at 26 msec by the air base output at 99 msec. The 30 / 99 ratio is the ratio of the air base output when the pulse time is 30 msec to when it is 99 msec, calculated by dividing the air base output at 30 msec by the air base output at 99 msec. The 35 / 99 ratio is the ratio of the air base output when the pulse time is 35 msec to when it is 99 msec, calculated by dividing the air base output at 35 msec by the air base output at 99 msec. The 40 / 99 ratio is the ratio of the air base output when the pulse time is 40 msec to when it is 99 msec, calculated by dividing the air base output at 40 msec by the air base output at 99 msec. The 45 / 99 ratio is the ratio of the air base output at a pulse time of 45 msec to that at a pulse time of 99 msec, calculated by dividing the air base output at 45 msec by the air base output at 99 msec. The 49 / 99 ratio is the ratio of the air base output at a pulse time of 49 msec to that at 99 msec, calculated by dividing the air base output at 49 msec by the air base output at 99 msec.

[0057] 18 to 21, it can be seen that the 35 / 49 ratio, 35 / 60 ratio, 35 / 80 ratio, and 35 / 99 ratio increase before and after the deterioration rate of the gas detection sensitivity exceeds the threshold (after 120 days). It can also be seen that the 35 / 49 ratio, 35 / 60 ratio, 35 / 80 ratio, and 35 / 99 ratio increase from the minimum value of 1.00 to 1.01 at the start of the increase to 1.02 to 1.04 before and after the deterioration rate of the gas detection sensitivity exceeds the threshold.

[0058] 18 to 21, it can be seen that the 30 / 49 ratio, 30 / 60 ratio, 30 / 80 ratio, and 30 / 99 ratio increase before and after the deterioration rate of the gas detection sensitivity exceeds the threshold (after 120 days). It can also be seen that the 30 / 49 ratio, 30 / 60 ratio, 30 / 80 ratio, and 30 / 99 ratio increase from the minimum value of 1.01 to 1.02 at the start of the increase to 1.03 to 1.05 before and after the deterioration rate of the gas detection sensitivity exceeds the threshold.

[0059] 18 to 21, it can be seen that the 16 / 49 ratio, 16 / 60 ratio, 16 / 80 ratio, and 16 / 99 ratio increase before and after the deterioration rate of gas detection sensitivity exceeds the threshold. It can also be seen that the 16 / 49 ratio, 16 / 60 ratio, 16 / 80 ratio, and 16 / 99 ratio clearly increase from the minimum value at the start of the increase before and after the gas sensitivity deterioration rate exceeds the threshold.

[0060] From the above results, it can be confirmed that a deterioration in gas detection sensitivity can be detected according to the rate of change of the 16 / 49 ratio, 16 / 60 ratio, 16 / 80 ratio, 16 / 99 ratio, 20 / 49 ratio, 20 / 60 ratio, 20 / 80 ratio, 20 / 99 ratio, 26 / 49 ratio, 26 / 60 ratio, 26 / 80 ratio, 26 / 99 ratio, 30 / 49 ratio, 30 / 60 ratio, 30 / 80 ratio, 30 / 99 ratio, 35 / 49 ratio, 35 / 60 ratio, 35 / 80 ratio, and 35 / 99 ratio.

[0061] In other words, it was confirmed that the deterioration of gas detection sensitivity can be detected according to the rate of change in the ratio of the air base output when the pulse time is between 0.3 and 0.7 times the normal pulse width (50 msec) to the point when the pulse time is greater than or equal to the normal pulse width.

[0062] As described above, in gas detection device 1 of the present embodiment, MCU 5 applies a pulse voltage having a second pulse width (e.g., 100 msec) that is equal to or greater than the first pulse width (e.g., 50 msec) during normal gas detection to detection element 2 and reference element 3 at a predetermined cycle. Then, MCU 5 calculates the output V out The sensitivity of gas detection is determined based on this.

[0063] Specifically, the MCU 5 calculates the difference between the output V of the differential amplifier circuit 44 at a point in time when the pulse time is a first time (e.g., 30 msec) less than the first pulse width and a point in time when the pulse time is a second time (e.g., 99 msec) equal to or greater than the second pulse width. out The sensitivity of gas detection is determined based on the ratio of the output V of the differential amplifier circuit 44 at the time when the pulse time is the first time and the time when the pulse time is the second time. out It has been confirmed by the above experiment that the ratio of the output V of the differential amplifier circuit 44 at the time when the pulse time is the first time and the time when the pulse time is the second time fluctuates before and after the deterioration of the gas detection sensitivity is recognized. out By monitoring the variation in the ratio, the sensitivity of the gas detection can be determined, and thus, deterioration of the sensitivity of the gas detection can be detected regardless of the decrease in the air base.

[0064] In addition, the output V of the differential amplifier circuit 44 at the point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at the point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection is out It has been confirmed by the above experiment that the ratio of the output V of the differential amplifier circuit 44 at the point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at the point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection varies before and after the deterioration of the gas detection sensitivity is recognized. out By monitoring the variation in the ratio, degradation of the gas detection sensitivity can be detected despite a decrease in the air base.

[0065] In addition, the output V of the differential amplifier circuit 44 at the point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at the point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection is out It has been confirmed by the above experiment that the ratio of the output V of the differential amplifier circuit 44 at the point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at the point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection is continuously increased beyond a predetermined range. out When the ratio of the two increases continuously beyond a predetermined range, it can be determined that the sensitivity of the gas detection is deteriorating.

[0066] In this embodiment, the output V of the differential amplifier circuit 44 at a point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at a point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection is out The ratio of the former to the latter is the value obtained by dividing the former by the latter. Therefore, this value increases before and after deterioration of the gas detection sensitivity is recognized. In contrast, the output V of the differential amplifier circuit 44 at the point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at the point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection is out When the ratio is the latter divided by the former, the value decreases before and after deterioration of the gas detection sensitivity is recognized. In this case, deterioration of the gas detection sensitivity can be determined when the value continuously decreases beyond a predetermined range.

[0067] In addition, the output V of the differential amplifier circuit 44 at the point where the pulse time is 0.3 to 0.7 times the pulse width during normal gas detection and at the point where the pulse time is 1.0 to 2.0 times the pulse width during normal gas detection is outIt has been confirmed by the above experiment that the sensitivity of gas detection deteriorates when the ratio of the pulse width and the pulse width at the time of normal gas detection are varied beyond a predetermined range from the initial value. Therefore, the output V of the differential amplifier circuit 44 at the time when the pulse time is 0.3 to 0.7 times the pulse width at the time of normal gas detection and at the time when the pulse time is 1.0 to 2.0 times the pulse width at the time of normal gas detection is out When the ratio of the first and second voltages fluctuates beyond a predetermined range from the initial value, it is possible to determine that the sensitivity of the gas detection has deteriorated.

[0068] 22 is a diagram showing the configuration of gas detection device 100 according to another embodiment of the present invention. Note that the same components as those in the above-described embodiment are denoted by the same reference numerals, and the description of the above-described embodiment is incorporated herein.

[0069] 22, gas detector 100 includes series circuit 104, differential amplifier circuit 44, MCU 105, and D / A conversion circuit 106. Series circuit 104 includes power supply 41, detection element 2, and reference element 3. Power supply 41, detection element 2, and reference element 3 are connected in series. Detection element 2 is connected to the positive electrode of power supply 41, and reference element 3 is connected to the negative electrode of power supply 41.

[0070] In gas detection device 100, terminal 42 connected between detection element 2 and reference element 3 and terminal 43 of D / A conversion circuit 106 are connected to differential amplifier circuit 44. The midpoint potential between detection element 2 and reference element 3 is output from terminal 42 as output V in- The DA output of the D / A conversion circuit 106 is output as an output V in+ The output from terminal 42 is V in- and the output V from terminal 43 in+ The difference (potential difference) between these is amplified by the differential amplifier circuit 44 and output as V out to the MCU 105. It should be noted that it is not essential to connect the D / A conversion circuit 106 to the differential amplifier circuit 44 via the resistors R3 and R4, and the DA output of the D / A conversion circuit 106 may be directly input to the + side of the operational amplifier.

[0071] The MCU 105 outputs a digital signal indicating a reference potential to the D / A conversion circuit 106. The D / A conversion circuit 106 converts the digital signal output from the MCU 105 into an analog signal (output V in+ ) and outputs it from the terminal 43 to the differential amplifier circuit 44. The MCU 105 also applies a pulse voltage to the detection element 2 and the reference element 3 to drive the series circuit 104, and converts the output V out In addition, as in the above embodiment, the MCU 105 applies a pulse voltage having a pulse width equal to or greater than the pulse width during normal gas detection to the detection element 2 and the reference element 3 at a predetermined cycle, and detects the flammable gas to be detected based on the output V out The gas detection sensitivity is determined based on the response waveform.

[0072] The present invention has been described above based on the above-mentioned embodiment, but the present invention is not limited to the above-mentioned embodiment, and modifications may be made without departing from the spirit of the present invention, and publicly known or well-known technologies may be combined as appropriate.

[0073] For example, in the above embodiment, the pulse width of the pulse voltage during normal gas detection is set to 50 msec, but this pulse width may be set as appropriate. Furthermore, the first time period and the second time period may be set as appropriate according to the pulse width of the pulse voltage during normal gas detection. [Explanation of symbols]

[0074] 1: Gas detection device 2: Detector element 3: Reference element 5: MCU (control unit) 41:Power supply 44: Differential amplifier circuit (output section) 100: Gas detection device 105: MCU (control unit) V in+ : Output (reference potential) V in- : Output (midpoint potential) V out :output

Claims

1. A detection element whose resistance value increases due to the combustion of a combustible gas; a reference element connected in series with the detection element and having a constant resistance to the combustible gas; A power source that applies a pulse voltage to the detection element and the reference element; an output section that outputs a potential difference between a midpoint potential between the detection element and the reference element and a reference potential; a control unit that executes a gas detection process of controlling the power supply to apply the pulse voltage having a first pulse width to the detection element and the reference element and detect a gas based on an output from the output unit, and a gas sensitivity determination process of controlling the power supply to apply the pulse voltage having a second pulse width equal to or greater than the first pulse width to the detection element and the reference element and determine the gas detection sensitivity based on the output from the output unit; Equipped with The control unit, in the gas sensitivity determination process, determines the gas detection sensitivity based on the ratio of the output of the output unit at a point in time when the time from the rise of the pulse voltage is a first time that is less than the first pulse width, and a point in time when the time from the rise of the pulse voltage is a second time that is equal to or greater than the second pulse width.

2. 2. The gas detection device according to claim 1, wherein the first time period is equal to or greater than 0.3 times and equal to or less than 0.7 times the first pulse width.

3. 3. The gas detection device according to claim 2, wherein the second time period is equal to or greater than 1.0 times and equal to or less than 2.0 times the first pulse width.

4. 3. The gas detection device according to claim 1 or 2, wherein the control unit determines, in the gas sensitivity determination process, that the gas detection sensitivity has deteriorated when a ratio of an output of the output unit at a point in time when the time from the rise of the pulse voltage is the first time and a point in time when the time from the rise of the pulse voltage is the second time continuously increases or decreases beyond a predetermined range.

5. 3. The gas detection device according to claim 1 or 2, wherein the control unit determines that the gas detection sensitivity has deteriorated when, in the gas sensitivity determination process, a ratio of the output of the output unit at a point in time when the time since the rise of the pulsed voltage is the first time and a point in time when the time since the rise of the pulsed voltage is the second time increases or decreases beyond a predetermined range from an initial value.

6. A method for detecting a gas using a gas detection device comprising: a detection element whose resistance value increases due to combustion of a combustible gas; a reference element connected in series with the detection element and having a constant resistance value with respect to the combustible gas; a power source that applies a pulse voltage to the detection element and the reference element; and an output unit that outputs a potential difference between a midpoint potential between the detection element and the reference element and a reference potential, a gas detection step of controlling the power supply to apply the pulse voltage having a first pulse width to the detection element and the reference element, and detecting a gas based on an output of the output unit; a gas sensitivity determination step of controlling the power supply to apply the pulse voltage having a second pulse width equal to or greater than the first pulse width to the detection element and the reference element, and determining the gas detection sensitivity based on the output of the output section; Go, The gas detection method, in the gas sensitivity determination procedure, determines the sensitivity of gas detection based on a ratio of outputs from the output section at a point in time when the time from the rise of the pulse voltage is a first time that is less than the first pulse width, and a point in time when the time from the rise of the pulse voltage is a second time that is equal to or greater than the second pulse width.

Images