Gas detector and method for evaluating deterioration of gas detector

Abstract

Provided is a gas detector in which it is easy to detect deterioration of a detecting element. A detecting element 10 includes a catalyst 12 for assisting combustion of a gas to be detected, and a resistor 11 for supporting the catalyst. A controller 80 supplies power to the resistor 11 to heat the resistor 11 to a predetermined temperature to burn the gas to be detected by means of the catalyst, and detects the concentration of the gas to be detected from a rise in the resistance value of the resistor 11 resulting from the fact that combustion of the gas to be detected causes the temperature of the resistor to rise by more than a rise in the temperature of the resistor resulting from the supply of electric power. The controller 80 detects deterioration of the detecting element 10 on the basis of a difference between the rate of change in the resistance value of the resistor 11 of the detecting element 10 from the start of the supply of power to the start of the detection of the concentration of the gas to be detected and a reference rate of change that serves as a detection standard.

Description

Gas detector and method for evaluating deterioration of gas detector

[0001] The present invention relates to a gas detector having a detection element that comes into contact with a gas to be detected, and a method for evaluating deterioration of a gas detector.

[0002] Conventionally, catalytic combustion gas detectors or thermal conduction gas detectors, such as those described in Patent Document 1 (JP 2014-235082 A), have been used to detect target gases such as hydrogen gas.

[0003] JP 2014-235082 A

[0004] In catalytic combustion gas detectors, molecules of poisonous substances (e.g., siloxane) in the air adhere to the catalyst surface of the detection element. In thermal conduction gas detectors, the detection element also comes into contact with the gas to be detected, so poisonous substances in the air also adhere to the detection element. When the detection element is poisoned, the sensitivity of the detection element decreases, making it difficult to detect the target concentration of the gas to be detected. Therefore, while it is important to evaluate the deterioration of the detection element, if a gas detector has already been installed at the detection location to detect the target gas, it is time-consuming to evaluate the deterioration of the detection element of the installed gas detector.

[0005] An object of the present invention is to provide a gas detector or a method for evaluating deterioration of a gas detector that can easily detect deterioration of a detection element.

[0006] Several aspects will be described below as means for solving the problems. These aspects can be combined as needed. A gas detector according to a first aspect includes a detection element having a resistor that is immersed in a gas containing a detectable gas, and a controller that heats the resistor to detect the concentration of the detectable gas. The controller detects deterioration of the detection element based on the difference between the rate of change in the resistance value of the resistor of the detection element from the start of power supply to the start of detection of the concentration of the detectable gas and a reference rate of change that serves as a detection reference. In the gas detector of the first aspect, the controller detects deterioration of the detection element based on the difference between the rate of change in the resistance value of the resistor of the detection element from the start of power supply to the start of detection of the concentration of the detectable gas and the reference rate of change, thereby easily detecting deterioration of the detection element.

[0007] In a gas detector according to a second aspect, in the gas detector of the first aspect, the controller detects deterioration of the detection element based on the difference between the rate of change of the resistance value of the resistor of the detection element from the start of power supply until the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change. In the gas detector of the second aspect, no catalytic combustion occurs during the period in which deterioration is detected, so deterioration of the detection element can be detected even when the detection target gas is contained. In a gas detector according to a third aspect, the controller uses, as the difference between the rate of change of the resistance value of the resistor of the detection element and the reference rate of change, a difference between a first resistance value of the resistor at a first time point close to the start of power supply and a second time point immediately before the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change, and a difference between a ratio of the first resistance value to the second resistance value and a ratio of the first reference resistance value to the second reference resistance value, where the first reference resistance value at the first time point and the second reference resistance value at the second time point are reference resistance values ​​that increase according to the reference rate of change. In the gas detector of the third aspect, the controller determines the first and second time points and recognizes the difference between the ratio of the first resistance value to the second resistance value and the ratio of the first reference resistance value to the second reference resistance value, thereby enabling accurate evaluation of deterioration of the detection element in a short time.

[0008] A gas detector according to a fourth aspect of the present invention is the gas detector of the third aspect, further comprising: a compensating element having a compensating resistor made of the same material as the resistor of the detection element; and a bridge circuit in which the detection element and the compensating element are connected in series. The controller compares a first output voltage generated across the resistor at a first time point with a second output voltage generated across the resistor at a second time point, thereby comparing the difference between the ratio of the first resistance value to the second resistance value and the ratio of the first reference resistance value to the second reference resistance value. In the gas detector of the fourth aspect, deterioration of the detection element can be detected using the bridge circuit, making it easy to implement in the gas detector a method for detecting deterioration of the detection element based on the difference between the rate of change of the resistance value of the resistor and the reference rate of change.

[0009] A fifth aspect of the gas detector is the gas detector of the fourth aspect, wherein the controller determines that the detection element has deteriorated and become unusable when the value obtained by subtracting the second output voltage from the first output voltage exceeds a predetermined threshold. In the gas detector of the fifth aspect, the controller can notify the user of the determination that the detection element has deteriorated and become unusable, for example, by issuing an alarm, thereby preventing malfunctions due to erroneous detection caused by deterioration of the detection element. A sixth aspect of the gas detector is the gas detector of any one of the first to third aspects, further comprising: a compensating element having a compensating resistor made of the same material as the resistor of the detection element; and a bridge circuit in which the detection element and the compensating element are connected in series. The controller compares the difference between the rate of change of the resistance value of the resistor of the detection element and the reference rate of change based on the slope of a graph of the voltage generated across the resistor at a third time point between the start of power supply and the time point at which the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change. In the gas detector of the sixth aspect, if deterioration of the detection element can be detected using a bridge circuit, it becomes easier to implement in the gas detector a method of detecting deterioration of the detection element based on the difference between the rate of change of the resistance value of the resistor and a reference rate of change.

[0010] A gas detector according to a seventh aspect is the gas detector according to any one of the first to fifth aspects, wherein the controller includes a memory that stores information related to the reference rate of change, and obtains the reference rate of change from the information related to the reference rate of change by reading the information related to the reference rate of change from the memory. In the gas detector of the seventh aspect, for example, if the rate of change of the resistance value of an unpoisoned detection element or information for obtaining the rate of change is stored in the memory, it is possible to improve the accuracy of detecting deterioration of the detection element compared to when the reference rate of change is obtained from an element other than the detection element.

[0011] A method for evaluating deterioration of a gas detector according to an eighth aspect is a method for evaluating deterioration of a gas detector including a detection element having a resistor that is immersed in a gas containing a detectable gas and that detects the concentration of the detectable gas by heating the resistor. In this method for evaluating deterioration of a gas detector, deterioration of the detection element is evaluated based on the difference between the rate of change in the resistance value of the resistor of the detection element from the start of power supply to the start of detection of the concentration of the detectable gas and a reference rate of change that serves as a detection reference. In the method for evaluating deterioration of a gas detector according to the eighth aspect, deterioration of the detection element can be easily evaluated by evaluating the rate of change in the resistance value of the resistor of the detection element from the start of power supply to the start of detection of the concentration of the detectable gas and the reference rate of change.

[0012] A gas detector according to a ninth aspect is the gas detector according to the first aspect, wherein the controller corrects the detected concentration of the detection target gas based on the amount of deterioration of the detection element. The gas detector according to the ninth aspect can obtain an accurate concentration of the detection target gas even when the detection element has deteriorated and the detection sensitivity has decreased.

[0013] A gas detector according to a tenth embodiment is the gas detector of the ninth embodiment, further comprising: a compensating element having a compensating resistor made of the same material as the resistor of the detection element; and a bridge circuit in which the detection element and the compensating element are connected in series. The controller obtains the amount of deterioration based on a value obtained by subtracting a second output voltage generated across the resistor at a second time point immediately before the rate of change in resistance value of the resistor of the detection element becomes equal to a reference rate of change from a first output voltage generated across the resistor at a first time point close to the start of power supply. The gas detector according to the tenth embodiment can obtain an accurate concentration of the detected gas even if the detection element deteriorates and its detection sensitivity decreases.

[0014] In the gas detector according to a tenth aspect, the gas to be detected is hydrogen gas.

[0015] According to the gas detector or the method for evaluating deterioration of a gas detector of the present invention, deterioration of the detection element can be easily detected.

[0016] FIG. 1 is a perspective view showing an example of the appearance of a gas detector according to the present invention. FIG. 2 is an explanatory diagram for explaining an example of the operation of a gas detector. FIG. 3 is an explanatory diagram for explaining an example of the operation of a gas detector according to a first embodiment. FIG. 4 is a circuit diagram showing an overview of an example configuration of a gas detector according to the first embodiment. FIG. 5 is an explanatory diagram for explaining the mechanism of deterioration of a detection element in a gas detector. FIG. 6 is a graph showing the change over time in voltage applied to detection elements with different degrees of deterioration in a bridge circuit. FIG. 7 is a graph showing the relationship between the difference in voltage across a resistor of a detection element at a first point in time and a second point in time and the detection sensitivity of hydrogen gas. FIG. 8 is a schematic graph showing the overview of the relationship between the usage time and sensitivity of a gas detector. FIG. 9 is a graph showing the change over time in voltage applied to an unpoisoned detection element in a bridge circuit in air with different hydrogen gas contents. FIG. 10 is a graph showing the change over time in voltage applied to detection elements with different degrees of deterioration in a bridge circuit. FIG. 11 is a schematic graph showing the change over time in voltage applied to a detection element for explaining a method of detecting deterioration of a detection element using integration. FIG. 12 is a circuit diagram showing an overview of an example configuration of a gas detector according to a second embodiment. FIG. 13 is a plan view showing an example configuration of a detection element according to the second embodiment. Fig. 14 is a cross-sectional view of the detection element taken along line II in Fig. 13. Fig. 15 is a block diagram of the gas detector 1 according to Modification 1G. Fig. 16 is a flowchart showing the control flow of gas concentration correction processing.

[0017] First Embodiment (1) Overall Configuration FIG. 1 shows the external appearance of a gas detector according to a first embodiment. The gas detector 1 includes a detection element 10 shown in FIG. 2. As shown in FIG. 2, the detection element 10 includes a catalyst 12 that aids in the combustion of a detection target gas and a resistor 11 that supports the catalyst 12. Examples of detection target gases include combustible gases. Examples of combustible gases to be detected include hydrogen and alcohol. As shown in FIG. 2, the gas detector 1 includes a controller 80. The controller 80 detects the concentration of the detection target gas from changes in the resistance value of the resistor 11. The controller 80 can be configured using, for example, a microcomputer. First, the operation of the controller 80 when detecting the concentration of the detection target gas will be described with reference to FIG. 2. When detecting a detection target gas, the controller 80 supplies power to the resistor 11 to heat the resistor 11 to a predetermined temperature, causing the detection target gas to be combusted by the catalyst 12. Power is supplied to the resistor 11 by, for example, applying a DC voltage VC via the bridge circuit 30. The predetermined temperature reached by the resistor 11 during detection varies depending on the type of gas detector 1, but is generally in the range of several tens to several hundred degrees Celsius. The controller 80 can detect the concentration of the detected gas from the increase in the resistance value of the resistor 11, since the temperature of the resistor 11 increases due to the combustion of the detected gas more than the temperature increase caused by the power supply. The operation of the gas detector 1 will be described below using the case where hydrogen gas is detected as the detected gas. Figure 2 shows a graph gh1 illustrating the relationship between the resistance value (Ω) of the resistor 11 of the detection element 10 and the concentration (%) of hydrogen, the detected gas. As the hydrogen concentration increases, the amount of heat imparted to the resistor 11 by the combustion of hydrogen by the catalyst 12 increases. Therefore, as the hydrogen concentration increases, the temperature of the resistor 11 increases and the resistance value of the resistor 11 also increases.

[0018] Next, the operation of the controller 80 when detecting deterioration of the detection element will be described with reference to FIG. 3 . The controller 80 detects deterioration of the detection element 10 based on the difference between the rate of change (Ω / sec) of the resistance of the resistor 11 of the detection element 10 from the start of power supply to the start of detection of the concentration of the target gas and a reference rate of change (Ω / sec) that serves as the detection reference. The rate of change of the resistance is given by (r2 - r1) / (t2 - t1), where r1 is the resistance at a first time point t1 and r2 is the resistance at a second time point t2. In FIG. 3 , the rate of change of the resistance of the compensation resistor 21 of the compensating element 20 is used as an example of the reference rate of change. In FIG. 3 , a graph gh2 showing the relationship between the resistance of the compensation resistor 21 of the compensating element 20 and time is shown. The slope of the graph gh2 at each time point corresponds to the reference rate of change of the resistance (Ω / sec). The resistor 11 of the detection element 10 and the compensating resistor 21 of the compensating element 20 are configured so that the resistance of the compensating resistor 21 and the resistance of the compensating resistor 21 are approximately the same at the temperature when the detectable gas is being detected. Furthermore, the compensating resistor 21 of the compensating element 20 is connected in series with the resistor 11 of the detection element 10. As a result, when the catalyst 12 of the detection element 10 is not combusting the detectable gas (when the detectable gas concentration is 0 ppm), the same current flows and generates heat in the same way when power is supplied, resulting in the resistance of the resistor 11 of the detection element 10 and the resistance of the compensating resistor 21 of the compensating element 20 increasing in the same way. In other words, graph gh3, which shows the relationship between the resistance of the detection element 10 and time, and graph gh2, which shows the relationship between the resistance of the compensating element 20 and time, shown in FIG. 3, are very similar, although there are differences depending on whether the catalyst 12 is present or not. Graph gh3 is a graph showing the relationship between the resistance of the resistor 11 of the detection element 10 and time. 3, the dashed line graph gh4 shows the relationship between the resistance value of the resistor 11 of the detection element 10 and time when the detection element 10 has deteriorated due to poisoning. When the detection element 10 deteriorates due to poisoning, it takes time for the temperature (resistance value) of the resistor 11 to rise. In other words, deterioration of the detection element 10 due to poisoning reduces the rate of temperature change of the resistor 11 in the early stage of the temperature rise of the resistor 11.Therefore, for a while after the power supply starts, the temperature change rate of the resistor 11 of the detection element 10, which is curved as shown in graph gh4 (proportional to the slope of graph gh4), is smaller than the temperature change rate of the resistor 11 of the detection element 10, which is curved as shown in graph gh3 (proportional to the slope of graph gh3). The temperature change rate (°C / sec) is given by (T2-T1) / (t2-t1), where T1 is the temperature at the first time point t1 and T2 is the temperature at the second time point t2. The reason why it takes time for the temperature of the resistor 11 to rise due to deterioration of the detection element 10 caused by poisoning is due to the adhesion of siloxane to the detection element 10 caused by the poisoning. One possible reason for this difference in the temperature change rate due to poisoning is the increase in heat capacity around the resistor 11 caused by the poisoning.

[0019] (2) Detailed Configuration (2-1) Appearance of the Gas Detector 1 As shown in FIG. 1 , the gas detector 1 includes, for example, a casing 2. It is possible to sell the gas detector 1 as a module without the casing 2 at the distribution stage. However, it is preferable to include the casing 2 from the viewpoint of protecting the detection element 10 when installing the gas detector at a measurement location. The casing 2 includes screw holes 2b for fixing the gas detector 1 to the measurement location with screws. The casing 2 includes an intake port 2a for taking in air that may contain the target gas from the space to be detected. The casing 2 also includes a connection portion 2c for connecting to an external device. After the gas detector 1 is fixed by passing a screw through the screw hole 2b and connected to an external device via the connection portion 2c, it is difficult to remove the gas detector 1 to detect deterioration of the detection element 10. Furthermore, when it is necessary to operate the gas detector 1 in air that does not contain the gas to be detected in order to evaluate the deterioration of the gas detector 1, if the gas detector 1 is fixed inside the body of a vehicle, for example, it is necessary to introduce air that does not contain the gas to be detected into the body of the vehicle, which is time-consuming.

[0020] (2-2) Detection Element 10 The resistor 11 of the detection element 10 is, for example, a metal wire or a metal pattern. The material of the metal wire or metal pattern is, for example, platinum, palladium, nickel, or an iron-palladium alloy. The catalyst 12 of the detection element 10 is, for example, a precious metal catalyst. The material of the catalyst 12 is, for example, palladium (Pd), rhodium (Rh), platinum (Pt), or an alloy thereof. The catalyst 12 is, for example, a dendritic or whisker-shaped electrodeposit. When the resistor 11 is a metal wire, the wire diameter of the metal wire is, for example, in the range of 10 μm to 50 μm, and the overall diameter of the detection element 10 including the electrodeposited catalyst 12 is, for example, in the range of 0.1 mm to 0.5 mm. The length of the metal wire is, for example, in the range of 0.3 mm to 1.2 mm, and the number of turns of the coiled portion is, for example, in the range of 4 turns to 15 turns.

[0021] (2-3) Compensating Element 20 The compensating resistor 21 of the compensating element 20 is made of the same material and has substantially the same shape as the resistor 11 of the detection element 10. For example, if the platinum wire constituting the resistor 11 has a wire diameter of 30 μm, a length of 1.0 mm, and five turns, the platinum wire constituting the compensating resistor 21 is set to a wire diameter of 30 μm, a length of 1.0 mm, and five turns. For example, a platinum wire whose surface has been deactivated before electrodeposition of the catalyst 12 is used as the compensating resistor 21. Figure 2 shows graph gh5, which shows the relationship between the resistance value (Ω) of the compensating resistor 21 of the compensating element 20 and the concentration (%) of hydrogen, the gas to be detected. As can be seen from graph gh5, the compensating resistor 21 exhibits a nearly constant resistance value regardless of the hydrogen concentration. Graph gh5 indicates that the resistance value of the compensating resistor 21 tends to decrease slightly as the hydrogen concentration increases. This tendency is thought to be due to the fact that hydrogen has a higher thermal conductivity than nitrogen and oxygen in the air.

[0022] (2-4) Bridge Circuit 30 The bridge circuit 30 incorporates a detection element 10 and a compensation element 20. In the bridge circuit 30, the resistor 11 of the detection element 10 and the compensation resistor 21 of the compensation element 20 are connected in series. A DC voltage VC is applied to the resistor 11 and the compensation resistor 21 connected in series. The application of the DC voltage VC supplies power to the resistor 11. The bridge circuit 30 incorporates known resistors 31 and 32 connected in series. The known resistors 31 and 32 have known resistance values. The known resistors 31 and 32 are connected in parallel to the resistor 11 and the compensation resistor 21 connected in series. One end of the known resistor 31 is connected to the detection element 10, one end of the known resistor 32 is connected to the other end of the known resistor 31, and the other end of the known resistor 32 is connected to the compensation resistor 21. The bridge circuit 30 is a Wheatstone bridge, and the connection point between the resistor 11 and the compensating resistor 21 and the connection point between the known resistors 31 and 32 serve as output terminals. Graph gh6 in FIG. 2 shows the relationship between the hydrogen concentration (%) and the output voltage (mV) of the bridge circuit 30. The bridge circuit 30 is adjusted so that its output voltage is 0 mV before detecting the hydrogen concentration (when the hydrogen concentration is 0%). When detecting the hydrogen concentration using the bridge circuit 30, the resistance of the resistor 11 of the detection element 10 increases as the hydrogen concentration increases, as shown in graph gh1. Meanwhile, as shown in graph gh5, the resistance of the compensating resistor 21 of the compensation element 20 barely changes even when the hydrogen concentration increases. When the hydrogen concentration increases, the balance of the bridge circuit 30 is disrupted, and the output voltage of the bridge circuit 30 increases. The output voltage Vout of the bridge circuit 30 is given by the following equation (1), where VC is the voltage value of the applied DC voltage, RS is the resistance value of the resistor 11, Rref is the resistance value of the compensating resistor 21, and R1 and R2 are the resistance values ​​of the known resistors 31 and 32. Vout=VC·(R2·RS−R1·Rref) / {(R1+R2)(RS+Rref)} (1) The bridge circuit 30 is adjusted so that it reaches an equilibrium state (a state in which Vout in equation (1) is 0) when the hydrogen concentration is 0% at a predetermined temperature at which detection of the target gas is performed.

[0023] An example of a detailed configuration of the bridge circuit 30 is the circuit configuration shown in FIG. 4 . The bridge circuit 30 includes a detection element 10, a compensation element 20, a first fixed resistor 41, a second fixed resistor 42, a first variable resistor 43, a second variable resistor 44, a DC power supply DC1, a first connection point Nd1, a second connection point Nd2, a third connection point Nd3, a fourth connection point Nd4, and wiring connecting these elements. The controller 80 includes a measuring device 81. In the bridge circuit 30 shown in FIG. 4 , the compensation element 20 is connected between the first connection point Nd1 and the fourth connection point Nd4, the detection element 10 is connected between the second connection point Nd2 and the fourth connection point Nd4, the first fixed resistor 41 is connected between the first connection point Nd1 and the third connection point Nd3, and the second fixed resistor 42 is connected between the second connection point Nd2 and the third connection point Nd3. A first variable resistor 43 for balancing is connected between the first connection point Nd1 and the second connection point Nd2. The center tap of this first variable resistor 43 is connected to the wiring between the first fixed resistor 41 and the second fixed resistor 42.

[0024] A second variable resistor 44 and a DC power supply DC1 are connected in series between the first connection point Nd1 and the second connection point Nd2. When detecting hydrogen gas, a DC voltage is applied to the detection element 10 from the DC power supply DC1, and the resistor 11 is heated to a predetermined temperature by the power consumed by the resistor 11. In this case, the temperature of the resistor 11 is, for example, 200 to 600°C. When the temperature of the resistor 11 is 250°C or higher, the resistance to poisoning is increased. This is thought to be because even if siloxane adheres to the catalyst 12, the volume of the siloxane decreases as the siloxane polymerizes on the catalyst 12, thereby preventing the catalyst 12 from being entirely covered with siloxane. Furthermore, when the temperature of the resistor 11 is 200°C or higher, the high catalytic activity of the catalyst 12 is thought to be sufficiently maintained even if the catalyst 12 is partially covered with siloxane. Furthermore, when the temperature of the resistor 11 is 600°C or lower, deterioration of the resistor 11 due to thermal stress is suppressed. The voltage value of the DC voltage VC applied between the first connection point Nd1 and the second connection point Nd2 is adjusted by adjusting the resistance value of the second variable resistor 44. The voltage applied to the resistor 11 is adjusted by the second variable resistor 44.

[0025] When hydrogen gas is detected using the gas detector 1, a DC voltage VC is applied from the DC power supply DC1 between the first connection point Nd1 and the second connection point Nd2. After a time has passed until the resistor 11 reaches a predetermined temperature, the first variable resistor 43 is adjusted in air containing no hydrogen gas to bring the bridge circuit 30 into a balanced state. A portion of the resistance of the first variable resistor 43 and the first fixed resistor 41 are connected in parallel between the first connection point Nd1 and the third connection point Nd3. The controller 80 has information about the adjustment of the balanced state of the bridge circuit 30. Therefore, the controller 80 obtains data on the resistance value between the first connection point Nd1 and the third connection point Nd3. The combined resistance formed by connecting the portion of the resistance of the first variable resistor 43 and the first fixed resistor 41 in parallel corresponds to the known resistance 31. Furthermore, the other portion of the resistance of the first variable resistor 43 and the second fixed resistor 42 are connected in parallel between the second connection point Nd2 and the third connection point Nd3. The controller 80 obtains data on the resistance value between the second connection point Nd2 and the third connection point Nd3. The combined resistance formed by connecting the other resistance of the first variable resistor 43 and the second fixed resistor 42 in parallel corresponds to the known resistor 32.

[0026] When hydrogen gas reaches the detection element 10 while the resistor 11 is maintained at a predetermined temperature, the hydrogen gas burns on the catalyst 12, raising the temperature of the resistor 11 and increasing its resistance. On the other hand, even when hydrogen gas reaches the compensating element 20, the hydrogen gas does not burn, and the resistance of the compensating element 20 remains unchanged. Therefore, a difference in resistance occurs between the detection element 10 and the compensating element 20, generating an output voltage from the bridge circuit 30 between the third node Nd3 and the fourth node Nd4. This output voltage is proportional to the concentration of hydrogen gas. The controller 80 measures the output voltage of the bridge circuit 30 with a measuring device 81 and, based on the measurement result, detects hydrogen gas and determines its concentration. The measuring device 81 can be configured, for example, with an AD converter that converts an analog voltage value into a digital value and a microcomputer that inputs the digital value from the AD converter as the output voltage of the bridge circuit 30.

[0027] (2-5) Detection of Deterioration of the Detector Element 10 Using the Bridge Circuit 30 Before the detector element 10 is poisoned, the resistance value of the resistor 11 of the detector element 10, as shown in graph gh3 of Figure 3, shows approximately the same tendency as the resistance value of the compensating resistor 21 of the compensating element 20, as shown in graph gh2. When power supply starts (when a DC voltage is applied), the voltage VS applied to the resistor 11 of the detector element 10 is a value obtained by dividing the voltage between the resistor 11 of the detector element 10 and the compensating resistor 21 of the compensating element 20. In other words, the voltage VS applied to the resistor 11 is given by the following equation (2) using the DC voltage VC and the voltage Vref applied to the compensating resistor 21 of the compensating element 20: VS = VC - Vref (2) When the detection element 10 is not poisoned (e.g., during adjustment at the manufacturing factory), the voltage VS generated across the resistor 11 when the DC voltage VC is applied (when power supply is initiated) behaves as shown in graph gh7 in FIG. 3. First, when the DC voltage VC is applied (when power supply is initiated), the voltage VS generated across the resistor 11 reaches its maximum. Because the rate of change of the resistance value of the resistor 11 (rate of change with temperature) is approximately the same as the rate of change of the resistance value of the compensating resistor 21 (rate of change with temperature), as shown in graph gh7, the voltage VS remains close to its maximum value, albeit with a slight decrease. In other words, both the resistance value of the resistor 11 in the detection element 10 and the resistance value of the compensating resistor 21 in the compensating element 20 increase in the same manner over time, and therefore the voltage VS generated by voltage division between the resistor 11 and the compensating resistor 21 also remains approximately constant. For example, if the resistor 11 and the compensating resistor 21 are exactly the same, VS will remain at VC / 2. However, graph gh7 schematically illustrates the output of the bridge circuit 30, ignoring slight changes in voltage VS. An example of the actual measured value of the output voltage Vout before poisoning is shown in graph gh10 of FIG. 6, which will be described later. When, for example, siloxane adheres to the resistor 11 due to poisoning, as can be seen by comparing graphs gh3 and gh4, in the initial stage, shortly after the DC voltage VC is applied, the rate of change in the resistance value of the resistor 11 of the detection element 10 (the slope of the graph) becomes smaller than before poisoning.Therefore, in the initial stage, the increase in the resistance value of the resistor 11 after the same time has elapsed is smaller than the increase in the resistance value of the compensating resistor 21 shown in graph gh2. However, in the final stage, when the temperature approaches the predetermined temperature for detecting the detection target gas, the increase in the resistance value of the resistor 11 after the same time has elapsed is larger than the increase in the resistance value of the resistor 11 before being poisoned shown in graph gh3 and the increase in the resistance value of the compensating resistor 21 shown in graph gh2. In other words, between the initial stage and the final stage, there is a time when the rate of change in the resistance value of the resistor 11 (the slope of the graph) and the rate of change in the resistance value of the compensating resistor 21 (the slope of the graph) are reversed. Note that graph gh2 of the compensating resistor 21 is a graph for obtaining information on graph gh3 of the resistor 11 before being poisoned, and the compensating resistor 21 is set so that graph gh2 approaches graph gh3 of the resistor 11 as closely as possible.

[0028] Graph gh8 shows the relationship between the output voltage of the bridge circuit 30 and time when the detection element 10 is poisoned. In the initial stage, the rate of change of the resistance of the resistor 11 (the slope of the graph) decreases due to poisoning, and the value of (VC - Vref) in equation (2) gradually decreases. At time tb, when the rate of change of the resistance of the resistor 11 (the slope of the graph) and the rate of change of the resistance of the compensation resistor 21 (the slope of the graph) become equal, graph gh8 reaches a minimum. By comparing the first output voltage v1 generated across the resistor 11 at time t1 with the second output voltage v2 generated across the resistor 11 at time t2, the controller 80 can compare the difference between the ratio of the first resistance value r1 to the second resistance value r2 and the ratio of the first reference resistance value q1 to the second reference resistance value q2. Over a certain period of time (t2-t1), the controller 80 can determine how much the increase in the resistance value of the resistor (r2-r1) has decreased compared to the increase in the resistance value of the compensating resistor 21 (q2-q1). Note that the time tb at which the rate of change in the resistance value of the resistor 11 and the rate of change in the resistance value of the compensating resistor 21 become equal is the time at which the rate of change in the resistance value of the resistor 11 becomes equal to the reference rate of change. The first time t1 is a time close to the start of power supply, in other words, the time at which the DC voltage VC is applied to the bridge circuit 30 (the time at which the DC voltage is applied to the resistor 11 and the compensating resistor 21). The second time t2 is a time immediately before the time tb at which the graph gh8 has a minimum value. The definition of the first time close to the start of power supply includes the case where the first time coincides with the start of power supply (the time at which the DC voltage VC is applied). Furthermore, the definition of the second time point immediately before the time point at which the rate of change of the resistance value of the resistor 11 becomes equal to the reference rate of change (here, the rate of change of the resistance value of the compensation resistor 21) includes the case where the second time point coincides with the time point (here, time tb) at which the rate of change of the resistance value of the resistor 11 becomes equal to the rate of change of the resistance value of the compensation resistor 21. The controller 80 evaluates that the sensitivity of the detection element 10 is within the allowable range if the difference between the first output voltage v1 and the second output voltage v2 of the bridge circuit 30 is smaller than a predetermined threshold. The fact that the difference between the first output voltage v1 and the second output voltage v2 of the bridge circuit 30 is smaller than the predetermined threshold means that the graph gh4 remains close to the graph gh2, and the sensitivity is within the allowable range.In other words, if the difference between the first output voltage v1 and the second output voltage v2 is smaller than the predetermined threshold, it means that the poisoning of the detection element 10 is within an allowable range.

[0029] (3) Deterioration and Sensitivity of the Detecting Element Figure 5 shows a schematic diagram of the concept of sensitivity reduction due to siloxane. Siloxane 91 adheres to the surface of the catalyst 12 of the detecting element 10. The siloxane adhered to the surface of the catalyst 12 of the detecting element 10 is oxidized by heating. Siloxane is oxidized and polymerized to form silica (SiO ２ During the formation of siloxane polymers 92, such as siloxane polymers 92, the surface of the catalyst 12 is coated with the siloxane polymers 92. The siloxane polymers 92 prevent contact between the active metal acting as the catalyst 12 and the target gas, preventing combustion of the target gas (e.g., hydrogen gas), resulting in a decrease in sensitivity. Therefore, as the amount of siloxane polymers 92 attached increases, the sensitivity decreases. As the amount of siloxane polymers 92 attached increases, the heat capacity of the detection element 10 increases, which is thought to change the behavior of the temperature rise when a DC voltage VC is applied. Figure 6 shows the bridge circuit 30 operating in air without hydrogen gas being applied to the gas detector 1, measuring the voltage across the resistor 11 of the detection element 10. Graph gh10 in Figure 6 shows the relationship between the output of the bridge circuit 30 and elapsed time before poisoning. The sensitivity of the detection element 10 in the state of graph gh10 is 100%. The graphs gh11, gh12, gh13, gh14, gh15, and gh16 are poisoned more and have lower sensitivity in this order.

[0030] The graph shown in Figure 7 shows the relationship between the change in the voltage VS across the resistor 11 of the detection element 10 shown in Figure 6 and the sensitivity of the detection element 10. The horizontal axis of the graph shown in Figure 7 shows the value obtained by subtracting the voltage (second output voltage v2) across the resistor 11 at 50 msec (second time t2) from the voltage (first output voltage v1) across the resistor 11 at 1.5 msec (first time t1) after the DC voltage VC is applied to the bridge circuit 30 for detection elements 10 at different stages of deterioration, which were degraded using multiple gas detectors 1. The vertical axis of the graph shown in Figure 7 shows the detection sensitivity of the detection element 10 for hydrogen gas. From the graph of FIG. 7, it can be seen that a detection element 10 in which the value obtained by subtracting the second output voltage v2 50 msec after 1.5 msec has elapsed from the application of the DC voltage VC to the bridge circuit 30 is less than the threshold voltage Vt (V) maintains a sensitivity of 80% or higher. In other words, a detection element 10 that satisfies the condition (first output voltage v1 - second output voltage v2) < Vt (V) has a sensitivity that decreases by no more than 20% from the initial state. The sensitivity of the detection element 10 decreases as the usage time increases, as shown in FIG. 8, for example. Therefore, it is preferable to issue a warning to the user of the gas detector 1 that the gas detector 1 has become unusable, for example by issuing an alarm at a time tc when the sensitivity does not fall below the allowable limit. When the relationship between the sensitivity and (first output voltage v1-second output voltage v2) shows the tendency as shown in FIG. 7, the controller 80 is configured to issue an alarm indicating that the device is no longer usable, for example, when (first output voltage v1-second output voltage v2)≧(Vt-α)(V), taking into consideration a safety margin α.

[0031] (4) Deterioration Evaluation of Gas Detector 1 In the above description, the controller 80 detects deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 of the detection element 10 and the reference rate of change (the rate of change of the resistance of the compensation resistor 21 of the compensating element 20). The controller 80 detects deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 of the detection element 10 from the application of the DC voltage VC to the bridge circuit 30 (start of power supply) until the rate of change of the resistance of the resistor 11 of the detection element 10 becomes equal to the reference rate of change and the reference rate of change that serves as the detection reference. Figure 9 shows a graph illustrating the change in the voltage VS across the resistor 11 of the detection element 10 over time after the DC voltage VC is applied to the bridge circuit 30. Graph gh20 is a graph obtained by measuring the voltage VS across the resistor 11 of the detection element 10 in air that does not contain hydrogen gas. Graphs gh21, gh22, and gh23 are graphs measuring the voltage VS across the resistor 11 when the air contains 1,000 ppm, 3,000 ppm, and 10,000 ppm of hydrogen, respectively. Comparing the four graphs gh20, gh21, gh22, and gh23, all of the graphs overlap until a short time after the time tb at which the minimum value is reached. This overlap between the graphs until a short time after the time tb at which the minimum value is reached is because the voltage VS across the resistor 11 is hardly affected by hydrogen gas until the time tb at which the minimum value is reached. Immediately after applying the DC voltage VC to the bridge circuit 30, the resistor 11 is at room temperature. It is believed that while the temperature of the resistor 11 is low, the catalyst 12 of the detection element 10 is inactive, and little hydrogen gas combustion occurs in the detection element 10. It is believed that the state in which almost no hydrogen gas combustion occurs continues until time tb, which is the time to wait for the minimum value, in each of the detection elements 10 shown in graphs gh20, gh21, gh22, and gh23. If there is no hydrogen gas combustion, the temperature of the resistor 11 depends on the power consumption of the resistor 11, regardless of the amount of hydrogen gas in the air.Therefore, if the elapsed time is the same for any of the detection elements 10 in graphs gh20, gh21, gh22, and gh23, the temperature of the resistor 11 will be the same, and so graphs gh20, gh21, gh22, and gh23 will overlap up to time tb. This indicates that if a decrease in sensitivity of the detection element 10 (in other words, deterioration of the detection element 10 or deterioration of the gas detector 1) is detected between the time when the DC voltage VC is applied to the bridge circuit 30 and time tb, it is possible to detect deterioration of the detection element 10 without the influence of hydrogen gas. In other words, if deterioration of the detection element 10 is evaluated between the time when the DC voltage VC is applied to the bridge circuit 30 and time tb, accurate deterioration evaluation is possible without removing the gas detector 1 or introducing air containing no hydrogen gas into the gas detector 1.

[0032] (5) Modifications (5-1) Modification 1A In the above description of the first embodiment, a case was described in which the value obtained by subtracting the voltage (second output voltage v2) generated across the resistor 11 when 50 msec has elapsed (second time point t2) from the voltage (first output voltage v1) generated across the resistor 11 when 1.5 msec has elapsed (first time point t1) since the DC voltage VC was applied to the bridge circuit 30 was used to detect deterioration of the detection element 10. However, deterioration of the detection element 10 may also be evaluated using, for example, the peak value Vmax of the voltage generated across the resistor 11 (the voltage VS when the DC voltage VC is applied to the bridge circuit 30) and the minimum value Vmin (bottom value) of the voltage generated across the resistor 11. In this case, for example, it is possible to determine whether deterioration of the detection element 10 has exceeded an allowable limit based on whether (Vmax - Vmin) exceeds a predetermined threshold. Alternatively, if fluctuations in the peak value can be ignored, the controller 80 can be configured to use only the minimum value Vmin to determine whether the deterioration of the detection element 10 has exceeded the tolerable limit based on whether the minimum value Vmin is equal to or less than a predetermined threshold. (5-2) Modification 1B To detect deterioration of the detection element 10, the first embodiment uses the difference between the voltage VS across the resistor 11 at the first time point t1 and the second time point t2. Modification 1A uses the difference between the peak value Vmax and the minimum value Vmin representing the bottom of the graph of the voltage VS over time. However, the method of detecting deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 of the detection element 10 and the reference rate of change is not limited to the method of using the difference between the voltage VS across the resistor 11 at different times, as described above. For example, as shown in FIG. 10, the slopes SL0 and SL1 of the graph at the third time point t3 may be compared. The third point in time t3 is, for example, a predetermined time until time tb at which many graphs showing changes in the voltage VS applied across the resistor 11 of the detection element 10 reach a minimum value. The slope SL0 is the slope of the graph gh10 at the third point in time t3, and the slope SL1 is the slope of the graph gh16 at the third point in time t3. As can be seen from a comparison of the slopes SL0 and SL1, the slopes increase as the deterioration of the detection element 10 progresses.Therefore, whether the degradation of the detection element 10 exceeds the allowable limit can be evaluated by determining whether the slope at the third time point t3 exceeds a predetermined threshold. (5-3) Modification 1C In the first embodiment and modifications 1A and 1B described above, the voltage VS generated across the resistor 11 of the detection element 10 is used to detect degradation of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and the reference rate of change. However, the voltage Vref generated across the compensation resistor 21 of the compensation element 20 incorporated in the bridge circuit 30 may also be used. The voltage Vref incorporated in the bridge circuit 30 is obtained by subtracting the voltage VS from a constant DC voltage VC. Therefore, using the voltage Vref indirectly uses the voltage VS. However, because an increase in the voltage VS appears as a decrease in the voltage Vref, it is necessary to use the voltage Vref with due consideration given to the fact that, for example, the peak value of the voltage VS appears as a minimum value of the voltage Vref and the bottom value of the voltage VS appears as a maximum value of the voltage Vref.

[0033] (5-4) Modification 1D In the first embodiment and modifications 1A to 1C described above, the difference in voltage VS generated across the resistor 11 at different times (e.g., t1 and t2) is used directly or indirectly to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and a reference rate of change. However, in addition to using the difference in voltage VS at different times, the voltage VS may also be used to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and a reference rate of change. For example, as shown in FIG. 11 , the time integral of the voltage VS of an unpoisoned detection element 10 from time t1 to time t2 may be compared with the time integral of the voltage VS of a poisoned detection element 10 from time t1 to time t2 to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and a reference rate of change. For example, when time integration is performed, the area of ​​graph gh7 for the unpoisoned detection element 10 from time t1 to time t2 decreases, while the area of ​​graph gh8 for the poisoned detection element 10 from time t1 to time t2 decreases. The greater the degree of poisoning, the smaller the value of the time integral over the same elapsed time period. Therefore, using the time integral of voltage VS, it is possible to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and the reference rate of change. Even if the time integral of voltage Vref is used instead of the time integral of voltage VS, it is possible to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and the reference rate of change. It is preferable to perform time integration over the period up to time tb, when the rate of change of the resistance value of the resistor 11 and the rate of change of the resistance value of the compensation resistor 21 become the same. By performing the time integration over the period up to time tb, it is possible to detect deterioration of the detection element 10 even in air containing hydrogen gas. If the deterioration of the detection element 10 is to be detected in air that does not contain hydrogen gas, the period in which the time integration is performed may include a period up to a time later than time tb.

[0034] (5-5) Modification 1E In the first embodiment and modifications 1A and 1B, the voltage VS generated across the resistor 11 is used to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 and a reference rate of change. However, the method of detecting deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 and a reference rate of change is not limited to using the voltage VS generated across the resistor 11. The controller 80 can measure the output voltage of the bridge circuit 30 between the third connection point Nd3 and the fourth connection point Nd4 using the measuring device 81. As shown in the above equation (1), the output voltage of the bridge circuit 30 changes depending on the resistance value RS of the resistor 11. Furthermore, the resistance value Rref of the compensating resistor 21 and the resistance values ​​R1 and R2 of the known resistors 31 and 32, other than the resistance value RS of the resistor 11, do not change with deterioration of the detection element 10. Furthermore, the bridge circuit 30 also incorporates the compensating resistor 21, and the part (R2 RS - R1 Rref) in which the difference in change between the resistance value RS of the resistor 11 and the resistance value Rref of the compensating resistor 21 affects the output voltage of the bridge circuit 30 is also included in equation (1). Therefore, for example, even if the output voltage of the bridge circuit 30 is used from the time point at which the DC voltage VC is applied (the time point at which power supply starts), deterioration of the detection element 10 can be detected based on the difference between the rate of change of the resistance value of the resistor 11 and the reference rate of change using the output voltage of the bridge circuit 30 up to time tb at which the rate of change of the resistance value of the resistor 11 and the rate of change of the resistance value of the compensating resistor 21 become equal. For example, even if the bridge circuit 30 is adjusted to be in equilibrium at a steady state where the temperature has reached a predetermined value at which the gas to be detected is to be detected, there will be intervals where the rate of change of the resistance value of the resistor 11 and the rate of change of the resistance value of the compensating resistor 21 differ, and in such intervals the equilibrium state of the bridge circuit 30 will be lost. Therefore, deterioration of the detection element 10 can be detected from the change in the output voltage of the bridge circuit 30 over time.

[0035] (5-6) Modification 1F In the first embodiment, the rate of change of the resistance of the compensating resistor 21 is used as the reference rate of change for comparison with the rate of change of the resistance of the resistor 11 of the detection element 10. However, the reference rate of change is not limited to the compensating resistor 21. The controller 80 may have a memory 82, store the resistance value of the resistor 11 of an unpoisoned detection element 10 in the memory 82, and calculate the reference rate of change from the stored resistance value. For example, for the resistor 11 of an unpoisoned detection element 10, the resistance values ​​q1 and q2 at times t1 and t2 shown in FIG. 3 can be read from the memory 82. The resistance values ​​r1 and r2 at times t1 and t2 can be calculated from the voltage VS across the resistor 11 during use. Then, (r2 - r1) can be compared with (q2 - q1) to detect deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 and the reference rate of change.

[0036] (5-7) Modification 1G As shown in FIG. 7 , it has been found that the greater the change in the voltage VS across the resistor 11, the lower the sensitivity of the detection element 10 (i.e., the more deteriorated the detection element 10 is). Using this property, the controller 80 may correct the measured concentration of the detected gas in accordance with the change in the voltage VS across the resistor 11. The controller 80 of the gas detector 1 according to Modification 1G executes a gas concentration correction process that corrects the gas concentration measurement results based on the change in the voltage VS across the resistor 11. FIG. 15 is a block diagram of the gas detector 1 according to Modification 1G. FIG. 16 is a flowchart showing the control flow of the gas concentration correction process. In the following description of the gas detector 1 according to Modification 1G, the same reference numerals are used to designate features already described, and detailed description thereof will be omitted.

[0037] 15, the gas detector 1 includes a bridge circuit 30, an output unit 50, a temperature sensor 60, and a controller 80. As shown in FIG. 15, the output unit 50 and the temperature sensor 60 are electrically connected to the controller 80 so as to be able to send and receive signals to and from the controller 80.

[0038] The output unit 50 outputs the corrected gas concentration calculated by the controller 80. Specifically, the output unit 50 acquires information on the corrected gas concentration calculated by the controller 80 and outputs it to a computer and / or a computer network external to the gas detector 1.

[0039] The temperature sensor 60 detects the air temperature at the installation position of the gas detector 1. The controller 80 acquires the detection result of the temperature sensor 60. The temperature sensor 60 is, for example, a thermistor.

[0040] Next, the gas concentration correction process will be described with reference to Fig. 16. When the controller 80 is powered on, it starts the gas concentration correction process (START), and the process proceeds to step S100.

[0041] In step S100, the controller 80 detects deterioration of the detection element 10 and proceeds to step S110 or step S160 based on the detection result. Specifically, if the detection element 10 has not deteriorated (No), the controller 80 proceeds to step S110, and if the detection element 10 has deteriorated (Yes), the controller 80 proceeds to steps S160 and S180. When detecting deterioration of the detection element 10, the controller 80 may use any of the deterioration detection methods disclosed in the first embodiment, Modification 1A, Modification 1B, Modification 1C, Modification 1D, Modification 1E, and Modification 1F, or may use a method other than these.

[0042] In step S110, the controller 80 acquires the output voltage of the bridge circuit 30 measured by the measuring device 81, and proceeds to step S120.

[0043] In step S120, the controller 80 acquires the air temperature at the installation position of the gas detector 1 detected by the temperature sensor 60, and proceeds to step S120.

[0044] In step S130, the controller 80 acquires the concentration of the detection target gas, and proceeds to step S140. Specifically, the controller 80 calculates the concentration of the detection target gas based on the output voltage of the bridge circuit 30, as described above.

[0045] In step S140, controller 80 acquires a corrected gas concentration obtained by correcting the concentration of the detection target gas, and proceeds to step S110. Specifically, controller 80 acquires the corrected gas concentration by performing a predetermined calculation to correct the concentration of the detection target gas based on the air temperature at the installation location.

[0046] In step S150, the controller 80 causes the output unit 50 to output the corrected gas concentration, and the process proceeds to step S110. In other words, the controller 80 repeats the processes from step S110 to step S150.

[0047] In step S160, the controller 80 acquires the deterioration amount of the detection element 10, and proceeds to step S170. The controller 80 acquires the change in the voltage VS across the resistor 11 as the deterioration amount of the detection element 10. More specifically, the controller 80 acquires the voltage obtained by subtracting the second output voltage v2 from the first output voltage v1 as the change in the voltage VS across the resistor 11.

[0048] In step S170, the controller 80 acquires a correction coefficient associated with the deterioration amount of the detection element 10, and proceeds to step S210. The controller 80 acquires the correction coefficient by performing a predetermined calculation to acquire the correction coefficient from the deterioration amount of the detection element 10. The controller 80 may acquire the correction coefficient using a predetermined table that specifies the relationship between the deterioration amount of the detection element 10 and the correction coefficient. The calculation and / or the table to acquire the correction coefficient from the deterioration amount of the detection element 10 may be stored in the memory 82.

[0049] In step S180, the controller 80 acquires the output voltage of the bridge circuit 30 measured by the measuring device 81, and proceeds to step S190.

[0050] In step S190, the controller 80 acquires the air temperature at the installation position of the gas detector 1 detected by the temperature sensor 60, and proceeds to step S200.

[0051] In step S200, the controller 80 acquires the concentration of the target gas, and proceeds to step S210. Specifically, the controller 80 calculates the concentration of the target gas based on the output voltage of the bridge circuit 30, as described above.

[0052] In step S210, controller 80 acquires a corrected gas concentration obtained by correcting the concentration of the detection target gas, and proceeds to step S220. Specifically, controller 80 acquires the corrected gas concentration by performing a predetermined calculation to correct the concentration of the detection target gas using a correction coefficient associated with the temperature at the installation location and the amount of deterioration of detection element 10.

[0053] In step S220, the controller 80 causes the output unit 50 to output the corrected gas concentration, and proceeds to steps S160 and S180. In other words, the controller 80 repeats the processes from step S160 and S180 to step S220.

[0054] The controller 80 repeatedly executes the gas concentration correction process while the power is on.

[0055] In the gas concentration correction process, the controller 80 may acquire the corrected gas concentration based on the amount of deterioration, regardless of whether the detection element 10 is deteriorated. In other words, in the control flow described above, after starting the gas concentration correction process, the controller 80 may always start the process from steps S160 and S180 without performing the process of step S100.

[0056] Second Embodiment The gas detector 1 according to the first embodiment is a catalytic combustion type. However, the present invention can also be applied to gas detectors other than catalytic combustion types, such as thermal conduction type detectors. As shown in FIG. 12 , the thermal conduction type gas detector 1 includes a bridge circuit 130 and a controller 80. The bridge circuit 130 includes a detection element 110, a compensation element 120, a first fixed resistor 41, a second fixed resistor 42, a first variable resistor 43, a second variable resistor 44, a DC power source DC1, a first connection point Nd1, a second connection point Nd2, a third connection point Nd3, a fourth connection point Nd4, and wiring connecting these elements. The bridge circuit 130 of the second embodiment differs from the bridge circuit 30 of the first embodiment in that the detection element 10 and the compensation element 20 are replaced with the detection element 110 and the compensation element 120. Since the bridge circuit 130 and the bridge circuit 30 have the same circuit configuration, a description of the connections between the circuit elements will be omitted. In the thermal conduction gas detector 1, differences in the thermal conductivity between the detection gas and the standard gas cause differences in the resistance values ​​of the detection element 110 and the compensation element 120 in the circuit. This causes a potential difference due to the difference in thermal conductivity between the detection gas and the standard gas. The controller 80 measures the potential difference due to the difference in thermal conductivity between the detection gas and the standard gas, output by the bridge circuit 130, using the measuring device 81, to detect the concentration of the detection gas.

[0057] As shown in FIGS. 13 and 14 , the detection element 110 includes a resistor 111 that contacts the gas to be detected. An insulating layer 115 and the resistor 111 are stacked in this order on a silicon substrate 114. A void 118 is formed within the silicon substrate 114, and the void 118 and the resistor 111 are preferably aligned in the stacking direction. The void 118 is open to the outside in a region surrounding the resistor 111. The formation of the void 118 suppresses thermal conduction between the resistor 111 and the silicon substrate 114, reducing the amount of current required to heat the resistor 111 and thereby saving power. Furthermore, the formation of the void 118 stabilizes the temperature of the resistor 111, improving detection sensitivity. The void 118 is formed, for example, by anisotropic etching using a KOH solution or the like, with the surface of the insulating layer 115 masked except for the region surrounding the resistor 111. The silicon substrate 114 is made of, for example, single crystal silicon. The thickness of the silicon substrate 114 is, for example, 0.4 mm. The insulating layer 115 is formed by sputtering SiO2. The thickness of the insulating layer 115 is, for example, 0.4 μm. The resistor 111 and the conductor wiring 116 are formed by sputtering platinum, followed by photolithography and etching. The thickness of both the resistor 111 and the conductor wiring 116 is, for example, 0.1 μm. The resistor 111 is patterned in a serpentine shape. The terminal electrode 117 is formed by vapor deposition of gold, for example.

[0058] The controller 80 detects the concentration of the target gas from the amount of heat absorbed by the target gas from the resistor 111. The controller 80 supplies power to the resistor 111 to maintain it at a specific temperature and detects the concentration of the target gas from the change in the resistance of the resistor 111 due to the heat absorbed by the target gas from the resistor 111. Because hydrogen gas has a higher thermal conductivity than other gases, the thermal conduction gas detector 1 is easy to detect hydrogen gas. The controller 80 detects deterioration of the detection element based on the difference between the rate of change of the resistance of the resistor 111 from the start of power supply until the resistor 111 reaches a specific temperature and a reference rate of change. Here, the rate of change of the resistance of the compensating element 112 can be used as the reference rate of change. As in the first embodiment, the rate of change of the resistance of an unpoisoned resistor 111 can be stored in the memory 82 as the reference rate of change, and the stored rate of change can be used as the reference rate of change. The voltage (power) of the resistor 111, which functions as a heater, increases in proportion to the hydrogen concentration.

[0059] (6) Features (6-1) The gas detector 1 described above includes a detection element 10, 110 having a resistor 11, 111 that can be immersed in a gas containing a detectable gas (e.g., hydrogen gas), and a controller 80 that heats the resistor 11, 111 to detect the concentration of the detectable gas. The controller 80 detects deterioration of the detection element 10, 110 based on the difference between the rate of change (Ω / sec) of the resistance of the resistor 11, 111 of the detection element 10, 110 from the start of power supply to the start of detection of the concentration of the detectable gas and a reference rate of change that serves as a detection reference. The deterioration evaluation method for the gas detector 1 includes a detection element 10, 110 having a resistor 11, 111 that can be immersed in a gas containing a detectable gas, and the detection element 10, 110 detects the concentration of the detectable gas by heating the resistor 11, 111. This degradation assessment method evaluates degradation of the detection element 10, 110 based on the difference between the rate of change in the resistance of the resistor 11, 111 of the detection element 10, 110 from the start of power supply to the start of detection of the concentration of the target gas and a reference rate of change serving as a detection reference. Degradation of the detection element 10, 110 can be easily detected. The reference rate of change (Ω / sec) is, for example, the rate of change (Ω / sec) in the resistance of the compensating resistor 21 of the compensating element 20, or the rate of change (Ω / sec) in the resistance of the resistor 11, 111 before poisoning, which is stored in the memory 82. In the catalytic combustion gas detector 1, the sensitivity degradation of the detection element 10 is closely related to the rate of change in the resistance of the resistor 11 of the detection element 10. Therefore, deterioration of the detection element 10 can be easily detected by the controller 80 detecting the deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 of the detection element 10 from the start of power supply until the start of detection of the concentration of the detection target gas and a reference rate of change that serves as the detection reference. Even in the thermal conduction gas detector 1, deterioration of the detection element 10 can be easily detected based on the difference between the rate of change of the resistance value of the resistor 111 from the start of power supply until the resistor 111 reaches a specific temperature and the reference rate of change. The start of power supply is, for example, the start of application of the DC voltage VC to the bridge circuit 30. The power source for the bridge circuit 30 can be either a current source or an AC power source.

[0060] (6-2) The controller 80 of the gas detector 1 detects deterioration of the detection element 10 based on the difference between the rate of change of the resistance of the resistor 11 of the detection element 10 from the start of power supply until the rate of change of the resistance of the resistor 11 of the detection element 10 becomes equal to the reference rate of change. Since no combustion occurs in the catalyst 12 during the deterioration detection period, deterioration of the detection element 10 can be detected even when the detection element 10 contains a detectable gas. This eliminates the need to remove the gas detector 1 from its installation location or introduce air containing no detectable gas into the installation location to prevent the gas detector 1 from containing the detectable gas. (6-3) The controller 80 can be configured to use the difference between the ratio of the first resistance value r1 to the second resistance value r2 and the ratio of the first reference resistance value q1 to the second reference resistance value q2 at a first time point t1, which is close to the start of power supply as shown in FIG. 3 , and a second time point t2, which is just before the rate of change of the resistance of the resistor 11 of the detection element 10 becomes equal to the reference rate of change. That is, the controller 80 can detect the following difference: the ratio between the first reference resistance value q1 and the second reference resistance value q2 is large because the rate of change of the resistance value of the compensating resistor 21 is large; whereas the ratio between the first resistance value r1 and the second resistance value r2 is small because the rate of change of the resistance value of the resistor 11 is small; and the controller 80 can detect the degree of difference. The first resistance value r1 is the resistance value of the resistor 11 at the first time point t1, and the second resistance value r2 is the resistance value of the resistor 11 at the second time point t2. The first reference resistance value q1 is the resistance value of the reference resistance that increases according to the reference rate of change at the first time point t1, and in the example of FIG. 3, it is the resistance value of the compensating resistor 21 at the first time point t1. The second reference resistance value q2 is the second reference resistance value q2 at the second time point t2, and in the example of FIG. 3, it is the resistance value of the compensating resistor 21 at the second time point t2. The controller 80 determines the first point in time t1 and the second point in time t2 and obtains data relating to the difference between the ratio of the first resistance value r1 to the second resistance value r2 and the ratio of the first reference resistance value q1 to the second reference resistance value q2, thereby enabling the deterioration of the detection element 10 to be evaluated accurately in a short period of time.

[0061] (6-4) In the gas detector 1 according to the first embodiment, in a bridge circuit 30 in which the detection element 10 and the compensating element 20 are connected in series as shown in Fig. 4, the controller 80 can obtain data relating to the difference between the ratio of the first resistance value r1 to the second resistance value r2 and the ratio of the first reference resistance value q1 to the second reference resistance value q2 shown in Fig. 3, for example, from the difference between the first output voltage v1 at the first time point t1 and the second output voltage v2 at the second time point t2 of the voltage VS across the resistor 11, which is generated by voltage division between the resistor 11 and the compensating resistor 21 connected in series in the bridge circuit 30 at the first time point t1. Being able to detect deterioration of the detection element 10 using the bridge circuit 30 in this way makes it easier to implement in the gas detector 1 a method of detecting deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and the reference rate of change. (6-5) For example, when the value obtained by subtracting the second output voltage v2 from the first output voltage v1 shown in Fig. 3 exceeds a predetermined threshold, the controller 80 determines that the detection element 10 has deteriorated and become unusable. The controller 80 can notify the user of the gas detector 1 of the determination result that the detection element 10 has deteriorated and become unusable, for example, by issuing an alarm, thereby preventing malfunctions caused by erroneous detection due to deterioration of the detection element 10.

[0062] (6-6) As shown in FIG. 4 , in a bridge circuit 30 in which a detection element 10 and a compensating element 20 are connected in series, the compensating element 20 can be configured using a compensating resistor 21 made of the same material as the resistor 11 of the detection element 10. The controller 80 of the gas detector 1 configured in this manner can be configured to compare the difference between the rate of change of the resistance value of the resistor 11 of the detection element 10 and the reference rate of change based on the slope of the graph of the voltage V S generated across the resistor 11 at a third time point r3 (see FIG. 10 ) between the start of power supply and the time when the rate of change of the resistance value of the resistor 11 of the detection element 10 becomes equal to the reference rate of change. In the example of FIG. 10 , the absolute value of the rate of change (V / sec) of the voltage V ref across the compensating resistor 21 at the third time point t3 is approximately equal to the absolute value of the reference rate of change (Ω / sec) at the third time point t3 because the current flowing through the compensating resistor 21 is approximately constant. If deterioration of the detection element 10 can be detected using the bridge circuit 30 in this way, it becomes easier to implement a method of detecting deterioration of the detection element 10 based on the difference between the rate of change of the resistance value of the resistor 11 and the reference rate of change in the gas detector 1. (6-7) As described in Modification 1F, if the resistance value of the resistor 11 of an unpoisoned detection element 10 is stored in the memory 82 and the controller 80 calculates the reference rate of change from the stored resistance value, there will be no difference between the detection element 10 and the compensating element 20, and the accuracy of detecting deterioration of the detection element 10 can be improved.

[0063] (6-8) As described in Modification 1G, the controller 80 may correct the detected concentration of the detection target gas based on the amount of deterioration of the detection element 10. This allows the gas detector 1 to obtain an accurate concentration of the detection target gas even if the detection element 10 has deteriorated and the detection sensitivity has decreased.

[0064] (6-9) The controller 80 may obtain the amount of deterioration of the detection element 10 based on the value obtained by subtracting the second output voltage v2 from the first output voltage v1. This allows the gas detector 1 to obtain an accurate concentration of the gas to be detected even if the detection element 10 has deteriorated and its detection sensitivity has decreased. While one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible without departing from the spirit and scope of the invention. In particular, the multiple embodiments and modifications described in this specification can be combined as needed.

[0065] REFERENCE SIGNS LIST 1 gas detector 10, 110 detection element 11, 111 resistor 12 catalyst 20, 120 compensation element 21 compensation resistor 30, 130 bridge circuit 80 controller 82 memory

Claims

1. A gas detector comprising: a detection element having a resistor that is immersed in a gas containing a detectable gas; and a controller that heats the resistor to detect the concentration of the detectable gas, wherein the controller detects deterioration of the detection element based on the difference between the rate of change in resistance value of the resistor of the detection element from the start of power supply to the start of detection of the concentration of the detectable gas and a reference rate of change that serves as the detection standard.

2. The gas detector according to claim 1, wherein the controller detects deterioration of the detection element based on the difference between the rate of change of the resistance value of the resistor of the detection element from the start of power supply until the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change and the reference rate of change.

3. The gas detector according to claim 2, wherein the controller uses, as the difference between the rate of change of the resistance value of the resistor of the detection element and the reference rate of change, a difference between a ratio of a first resistance value to a second resistance value and a ratio of a first reference resistance value to a second reference resistance value, for a first resistance value of the resistor at the first time point close to the start of power supply and a second time point immediately before the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change, and a difference between a ratio of a first resistance value to a second resistance value and a ratio of a first reference resistance value to a second reference resistance value, for a first reference resistance value at the first time point and a second reference resistance value at the second time point which increase according to the reference rate of change.

4. A gas detector as described in claim 3, comprising: a compensating element having a compensating resistor made of the same material as the resistor of the detection element; and a bridge circuit in which the detection element and the compensating element are connected in series, wherein the controller compares a first output voltage generated across the resistor at the first time point with a second output voltage generated across the resistor at the second time point, thereby comparing the difference between the ratio of the first resistance value to the second resistance value and the ratio of the first reference resistance value to the second reference resistance value.

5. The gas detector according to claim 4, wherein the controller determines that the detection element has become unusable due to deterioration of the detection element when a value obtained by subtracting the second output voltage from the first output voltage exceeds a predetermined threshold value.

6. A gas detector as claimed in any one of claims 1 to 3, comprising: a compensating element having a compensating resistor made of the same material as the resistor of the detection element; and a bridge circuit in which the detection element and the compensating element are connected in series, wherein the controller compares the difference between the rate of change of the resistance value of the resistor of the detection element and the reference rate of change based on the slope of a graph of the voltage generated across the resistor at a third point in time between the point in time when power supply begins and the point in time when the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change.

7. A gas detector as claimed in any one of claims 1 to 5, wherein the controller includes a memory that stores information relating to the reference rate of change, and the controller reads out the information relating to the reference rate of change from the memory to obtain the reference rate of change from the information relating to the reference rate of change.

8. A method for evaluating deterioration of a gas detector comprising a detection element having a resistor that is immersed in a gas containing a detectable gas, and which detects the concentration of the detectable gas by heating the resistor, said method evaluating deterioration of the detection element based on the difference between the rate of change in resistance value of the resistor of the detection element from the start of power supply to the start of detection of the concentration of the detectable gas and a reference rate of change that serves as the detection standard.

9. The gas detector according to claim 1, wherein the controller corrects the detected concentration of the target gas based on the amount of deterioration of the detection element.

10. A gas detector as described in claim 9, comprising: a compensating element having a compensating resistor made of the same material as the resistor of the detection element; and a bridge circuit in which the detection element and the compensating element are connected in series, wherein the controller obtains the amount of deterioration based on a value obtained by subtracting a second output voltage generated across the resistor at a second point in time immediately before the point in time at which the rate of change of the resistance value of the resistor of the detection element becomes equal to the reference rate of change from a first output voltage generated across the resistor at a first point in time close to the start of power supply.

11. The gas detector according to claim 1, wherein the gas to be detected is hydrogen gas.

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