Thermal conductivity gas analyzer

Abstract

A thermal conductivity gas analyzer according to the present invention comprises: a plurality of cells respectively housing resistors constituting a Wheatstone bridge circuit; a first flow path section through which a sample gas or a reference gas flows; a second flow path section provided so as to branch off of the first flow path section; and a supply gas switching unit connected to the first flow path section. Each resistor is constituted by a thermal conductivity thin-film sensor or a thermal conductivity semiconductor sensor. The plurality of cells include measurement cells that respectively house resistors located on one pair of opposite sides of the Wheatstone bridge circuit. The second flow path section connects to the measurement cells. The supply gas switching unit alternately supplies the sample gas and the reference gas to the first flow path section.

Description

Thermal Conductivity Gas Analyzer

[0001] The present invention relates to a thermal conductivity gas analyzer.

[0002] Conventionally, a thermal conductivity gas analyzer has been proposed that measures the concentration of a predetermined component contained in a sample gas by utilizing the difference in thermal conductivity between the predetermined component and a reference gas (see, for example, Patent Document 1).

[0003] Japanese Unexamined Patent Publication No. 7-248304

[0004] Thermal conductivity gas analyzers experience drift due to contamination inside the cell into which the sample or reference gas is introduced. Drift refers to the phenomenon in which the output value (indicated value) fluctuates over time. Drift that occurs over a long period of time (e.g., one week or more) is also called long-term drift. Long-term drift can be improved by performing periodic calibrations (e.g., zero calibration, span calibration). However, in industrial plants that operate 24 hours a day, 365 days a year, operation must be stopped to perform calibration, which reduces operational efficiency. To avoid this decrease in operational efficiency, it is necessary to reduce the effects of long-term drift without requiring frequent calibration. However, such a thermal conductivity gas analyzer has not yet been proposed.

[0005] The present invention has been made to solve the above problems, and its object is to provide a thermal conduction gas analyzer that can reduce the effects of long-term drift without requiring frequent calibration.

[0006] A thermal conduction gas analyzer according to one aspect of the present invention calculates the concentration of a predetermined component contained in a sample gas by detecting a change in resistance value due to a temperature change of a resistor corresponding to the thermal conductivity of the predetermined component as an unbalanced voltage in a Wheatstone bridge circuit including the resistors. The thermal conduction gas analyzer includes a plurality of cells each accommodating each of the resistors constituting the Wheatstone bridge circuit, a first flow path section through which the sample gas or the reference gas flows, a second flow path section branching from the first flow path section, and a supply gas switching section connected to the first flow path section, wherein each of the resistors is constituted by a thermal conduction thin-film sensor or a thermal conduction semiconductor sensor, and the plurality of cells include measurement cells each accommodating the resistors located on opposite sides of the Wheatstone bridge circuit, the second flow path section being connected to the measurement cells, and the supply gas switching section alternately supplies the sample gas and the reference gas to the first flow path section.

[0007] According to the thermal conduction gas analyzer of the present invention, the influence of long-term drift can be reduced without frequent calibration.

[0008] FIG. 1 is an explanatory diagram schematically showing the configuration of a thermal conduction gas analyzer according to one embodiment of the present invention. FIG. 2 is a cross-sectional view of a block body included in the thermal conduction gas analyzer. FIG. 3 is a cross-sectional view showing the detailed configuration of a resistor included in the thermal conduction gas analyzer. FIG. 4 is a graph showing the difference in response speed between a thermal conduction thin-film sensor and a platinum resistance temperature sensor. FIG. 5 is a graph showing the waveform of an output signal when a sample gas and a reference gas are alternately supplied to a measurement cell in an initial state. FIG. 6 is a graph showing the waveform of an output signal when a sample gas and a reference gas are alternately supplied to a measurement cell after a long period of time has elapsed. FIG. 7 is a cross-sectional view showing the general configuration of a thermal conduction semiconductor sensor applicable to the resistor. FIG. 8 is an explanatory diagram schematically showing the configuration of a thermal conduction gas analyzer according to another embodiment of the present invention.

[0009] Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings.

[0010] [Embodiment 1] Fig. 1 is an explanatory diagram schematically illustrating the configuration of a thermal conduction gas analyzer 1 according to one embodiment of the present invention. The thermal conduction gas analyzer 1 is an analyzer that measures the concentration of a gas by utilizing the thermal conductivity inherent to the gas. The thermal conduction gas analyzer 1 is installed in various industrial plants, such as hydrogen refining plants, and is used to measure the concentration of a predetermined gas. Such a thermal conduction gas analyzer 1 includes a Wheatstone bridge circuit WB, a power supply unit PS, and a processing unit PU.

[0011] The Wheatstone bridge circuit WB is configured by connecting a plurality of resistors 21. The plurality of resistors 21 includes a first resistor R1, a second resistor R2, a third resistor R3, and a fourth resistor R4. The first resistor R1 and the second resistor R2 are connected in series via a first connection point P1. The first resistor R1 and the second resistor R2 constitute a first resistor group G1. The third resistor R3 and the fourth resistor R4 are connected in series via a second connection point P2. The third resistor R3 and the fourth resistor R4 constitute a second resistor group G2.

[0012] The first resistor group G1 and the second resistor group G2 are connected in parallel. More specifically, the first resistor R1 of the first resistor group G1 and the third resistor R3 of the second resistor group G2 are connected via a third connection point P3. A variable resistor R5 for offset adjustment is provided at the third connection point P3. The second resistor R2 of the first resistor group G1 and the fourth resistor R4 of the second resistor group G2 are connected via a fourth connection point P4.

[0013] Due to the above connection relationship, the first resistor R1 and the fourth resistor R4 are located on one pair of opposing sides of the Wheatstone bridge circuit WB, and the second resistor R2 and the third resistor R3 are located on the other pair of opposing sides of the Wheatstone bridge circuit WB.

[0014] The power supply unit PS is configured, for example, by a constant current source. The power supply unit PS is connected to the third connection point P3 and the fourth connection point P4 of the Wheatstone bridge circuit WB. This allows a constant current to flow between the third connection point P3 and the fourth connection point P4.

[0015] The processing unit PU includes a voltage amplifier VA and a calculation unit CA. The voltage amplifier VA is connected to the first connection point P1 and the second connection point P2 of the Wheatstone bridge circuit WB, and detects and amplifies the potential difference between the first connection point P1 and the second connection point P2. The calculation unit CA calculates the concentration of a predetermined component in the sample gas SG flowing through the cell 2, which will be described later, based on the potential difference amplified by the voltage amplifier VA. The calculation unit CA is configured, for example, by a central processing unit called a CPU (Central Processing Unit).

[0016] The thermal conduction gas analyzer 1 further includes a plurality of cells 2, a first flow path section 3, a second flow path section 4, and a supply gas switching section 5. The plurality of cells 2, the first flow path section 3, and the second flow path section 4 are integrated into a single, substantially cylindrical block body 6 (see FIG. 2 ). Details of the block body 6 will be described later.

[0017] The plurality of cells 2 accommodate and hold each of the resistors 21 that make up the Wheatstone bridge circuit WB. The plurality of cells 2 include a measurement cell 2A and a comparison cell 2B. The measurement cell 2A accommodates and holds each of the resistors 21 located on a pair of opposite sides of the Wheatstone bridge circuit WB. In this embodiment, the cell 2 accommodating the second resistor R2 and the cell 2 accommodating the third resistor R3 each constitute a measurement cell 2A. In other words, two measurement cells 2A are provided.

[0018] The comparison cells 2B accommodate and hold the resistors 21 located on the opposite sides of the other pairs of resistors in the Wheatstone bridge circuit WB. In this embodiment, the cell 2 accommodating the first resistor R1 and the cell 2 accommodating the fourth resistor R4 constitute the comparison cells 2B. In other words, two comparison cells 2B are provided, just like the measurement cells 2A.

[0019] The first flow path section 3 constitutes a flow path through which the sample gas SG or the comparison gas CG flows. Here, the comparison gas CG has the same components as the gas sealed in the comparison cell 2B. In this embodiment, the gas sealed in the comparison cell 2B is referred to as the "comparison sealed gas" to distinguish it from the comparison gas CG flowing through the first flow path section 3.

[0020] The sample gas SG can be a mixture of a gas (base gas) with the same components as the comparison fill gas and comparison gas CG and a gas (predetermined component) with a different thermal conductivity from the comparison fill gas and comparison gas CG. For example, if the comparison fill gas and comparison gas CG are nitrogen, the sample gas SG can be a gas containing two components: nitrogen and hydrogen. The thermal conductivity of nitrogen is 1.000, and the thermal conductivity of hydrogen is 6.968. The thermal conductivities shown here are values ​​at 0°C and 1 atmosphere.

[0021] The comparative gas and comparative gas CG are not limited to the above nitrogen, but may be, for example, hydrogen, argon, carbon monoxide, carbon dioxide, methane gas (CH ４ ), air, etc. Furthermore, the predetermined component contained in the sample gas SG is not limited to the above-mentioned hydrogen, but may be other gases such as oxygen, carbon monoxide, carbon dioxide, water vapor, argon, etc. Furthermore, the sample gas SG may be a gas containing one component. In this case, it is possible to detect the type of sample gas SG.

[0022] The second flow path section 4 is provided by branching off from the first flow path section 3 and connected to the measurement cell 2A. In the configuration of Fig. 1, two measurement cells 2A are provided, and therefore two second flow path sections 4 are also provided corresponding to the respective measurement cells 2A.

[0023] The supply gas switching unit 5 is connected to the first flow path section 3. The supply gas switching unit 5 alternately supplies the sample gas SG and the comparison gas CG to the first flow path section 3 (cross-flow system). The supply gas switching unit 5 includes a sample gas flow path section 51, a comparison gas flow path section 52, a supply flow path section 53, and a switching valve SV.

[0024] The sample gas flow path 51 is a pipe through which the sample gas SG supplied from the sample gas supply unit (not shown) flows. The sample gas flow path 51 is connected to a first connection port of the switching valve SV. The comparison gas flow path 52 is a pipe through which the comparison gas CG supplied from the comparison gas supply unit (not shown) flows. The comparison gas flow path 52 is connected to a second connection port of the switching valve SV. The supply flow path 53 is a pipe connected to the first flow path 3. The side of the supply flow path 53 opposite to the side connected to the first flow path 3 is connected to a third connection port of the switching valve SV.

[0025] The switching valve SV is, for example, a three-way solenoid valve. The switching valve SV alternates between supplying the sample gas SG from the sample gas flow path 51 to the supply flow path 53 and supplying the reference gas from the reference gas flow path 52 to the supply flow path 53. For example, by controlling the on / off of a current flowing through a solenoid (particularly a coil) of the three-way solenoid valve, communication between the first connection port and the third connection port and communication between the second connection port and the third connection port within the switching valve SV can be alternately switched. This allows the switching valve SV to alternately switch between supplying the sample gas SG and supplying the reference gas to the supply flow path 53.

[0026] Next, the above-mentioned block body 6 will be described. Fig. 2 is a cross-sectional view of the block body 6. For convenience, Fig. 2 shows a cross-sectional view of the measurement cell 2A on the right side and a cross-sectional view of the comparison cell 2B on the left side with respect to the central axis AX along the up-down direction of the approximately cylindrical block body 6. The block body 6 has the above-mentioned multiple cells 2, a first flow path section 3, and a second flow path section 4.

[0027] The first flow path section 3 is formed by an inner wall that penetrates the block body 6 in the vertical direction. The supply flow path section 53 of the supply gas switching unit 5 is connected to the upstream side of the first flow path section 3. The gas outlet section 7 is connected to the downstream side of the first flow path section 3. Therefore, in FIG. 2 , the sample gas SG or the reference gas CG supplied from the supply flow path section 53 flows from the bottom to the top through the first flow path section 3 formed in the block body 6 and is discharged from the first flow path section 3 via the gas outlet section 7.

[0028] The second flow path section 4 described above is formed by an inner wall extending within the block body 6 in a direction away from the first flow path section 3 (e.g., a direction perpendicular to the central axis AX). The measurement cell 2A has a measurement space 2As. The above-mentioned resistor 21 (e.g., the second resistor R2 or the third resistor R3) is disposed in the measurement space 2As. The second flow path section 4 communicates with the measurement space 2As of the measurement cell 2A. Therefore, a portion of the sample gas SG or the reference gas CG flowing through the first flow path section 3 diffuses from the first flow path section 3 to the second flow path section 4, flows into the measurement space 2As of the measurement cell 2A, and comes into contact with the resistor 21 disposed in the measurement space 2As.

[0029] On the other hand, the comparison cell 2B has a comparison space 2Bs. The above-mentioned resistor 21 (e.g., the first resistor R1 or the fourth resistor R4) is also disposed in the comparison space 2Bs. The comparison space 2Bs is a closed space, and is filled with a comparison fill gas having the same components as the comparison gas CG. The comparison fill gas filled in the comparison space 2Bs comes into contact with the resistor 21 disposed in the comparison space 2Bs.

[0030] In each cell 2, the resistor 21 generates heat when current is supplied from the power supply PS (see FIG. 1 ). When the sample gas SG, the reference gas CG, or the reference fill gas contacts the resistor 21, heat is removed from the resistor 21, causing a temperature change in the resistor 21 and a change in the resistance of the resistor 21. The temperature change and the resistance change of the resistor 21 depend on the thermal conductivity of the gas in contact with the resistor 21. For example, if the reference fill gas is nitrogen and the sample gas SG contains nitrogen and hydrogen, the resistance of the resistor 21 changes differently in the measurement cell 2A and the comparison cell 2B because nitrogen and hydrogen have different thermal conductivities. If a difference occurs in the resistance of the resistor 21 between the measurement cell 2A and the comparison cell 2B, this difference results in an unbalanced voltage (potential difference between the first connection point P1 and the second connection point P2) in the Wheatstone bridge circuit WB. The calculation unit CA of the processing unit PU (see FIG. 1) can calculate the concentration of a predetermined component (for example, hydrogen) contained in the sample gas SG based on the unbalanced voltage.

[0031] In this way, the thermal conduction gas analyzer 1 of this embodiment calculates the concentration of a specific component by detecting the change in resistance value of the resistor 21 due to the temperature change in the resistor 21, which corresponds to the thermal conductivity of the specific component contained in the sample gas SG, as an unbalanced voltage of the Wheatstone bridge circuit WB including the resistor 21.

[0032] Next, the resistor 21 of each cell 2 will be described in detail. Fig. 3 is a cross-sectional view showing the detailed configuration of the resistor 21. The resistor 21 is adhered to the base body 22 with an adhesive 24, with an intermediate body 23 interposed between the resistor 21 and the base body 22. The resistor 21 includes a diaphragm holding film 212, a thin-film resistor 213, a pad portion 214, and a surface protection film 215 on a substrate 211.

[0033] The substrate 211 is made of, for example, a silicon substrate. The diaphragm holding film 212 is made of, for example, a silicon oxide film (SiO ２ film) and silicon nitride film (Si ３ N ４ The thin-film resistor 213 is made of, for example, platinum (Pt), and is patterned on the diaphragm holding film 212. The pad portions 214 are wiring connection portions (electrode pads) made of, for example, gold (Au), and are formed in contact with each end of the thin-film resistor 213. The surface protection film 215 is made of, for example, TEOS (tetraethoxysilane)-SiO ２ It is made up of a film or the like, and partially covers the diaphragm holding film 212 and the thin film resistor 213 .

[0034] The pad portion 214 is connected (wire-bonded) via a gold wire 27 to a coating layer 26 that covers one end of the lead pin 25. The lead pin 25 is provided to penetrate the base body 22 in the thickness direction. The through hole of the base body 22 through which the lead pin 25 passes is airtightly sealed with a sealant 22a. The other end of the lead pin 25 is covered with a coating layer (not shown), and a wiring 28 (see FIG. 2) is connected to the coating layer. The wiring 28 is connected to one of the first connection point P1 to the fourth connection point P4 shown in FIG. 1.

[0035] In the resistor 21 shown in FIG. 3 , the thickness Td from the bottom surface of the substrate 211 to the surface of the surface protection film 215 is, for example, about 1 μm. In contrast, the thickness of a typical platinum resistance thermometer, which is a glass-coated platinum resistor, is about 1.2 mm. In other words, the resistor 21 of this embodiment is significantly thinner than a typical platinum resistance thermometer. The resistor 21 of this embodiment is called a thermal conduction thin-film sensor. Because the thermal conduction thin-film sensor is a thin film, it is prone to losing heat when it comes into contact with external gas. For this reason, in a thermal conduction thin-film sensor, changes in resistance value due to temperature change occur in a shorter time than in a platinum resistance thermometer.

[0036] Figure 4 is a graph showing the difference in response speed between a thermal conduction thin-film sensor and a platinum resistance thermometer. Here, the graph shows the change in resistor output (current value when a constant current is passed) over time when hydrogen with a concentration of 1 vol% is passed at a flow rate (flow rate) of 0.5 L / min, with full scale being 100%. The response time until 90% output is obtained is defined as T ９０ (sec), as shown in the figure, in a typical platinum resistance thermometer, T ９０ On the other hand, in the thermal conduction type thin film sensor, T ９０ = 5 (sec). That is, the response speed of the thermal conduction type thin film sensor is much faster than that of the platinum resistance thermometer.

[0037] By using such a thermal conduction thin-film sensor with a fast response speed as the resistor 21, even when the sample gas SG and the reference gas CG are alternately supplied to the measurement cell 2A at a short switching cycle (for example, every 10 seconds), the change in resistance value of the resistor 21 in the measurement cell 2A due to a temperature change can be detected in accordance with the switching cycle. In other words, before the gas supplied to the measurement cell 2A is switched between the sample gas SG and the reference gas CG, the change in resistance value of the resistor 21 due to contact with the gas supplied to the measurement cell 2A can be detected. This allows the switching cycle of the gas supplied to the measurement cell 2A to be shortened and the gas concentration to be detected.

[0038] 5A and 5B are graphs showing the waveforms of signals output from the voltage amplifier VA of the processing unit PU when the supply gas switching unit 5 (see FIG. 1) alternately supplies the sample gas SG and the comparison gas CG to the measurement cell 2A. Note that FIG. 5A shows the signal waveform in the initial state of the thermal conduction gas analyzer 1 (immediately after zero calibration), while FIG. 5B shows the signal waveform after a long period of time (e.g., one week) has elapsed since the initial state of FIG. 5A. Note that the types and concentrations of the sample gas SG, comparison gas CG, and comparison filler gas used in the initial state and after the long period of time are assumed to be the same. For convenience, FIGS. 5A and 5B ignore the distortion of the signal waveforms due to response delays of the resistors 21. The signal waveforms of FIGS. 5A and 5B are determined according to the unbalanced voltage of the Wheatstone bridge circuit WB.

[0039] In a configuration in which the sample gas SG and the comparison gas CG are alternately supplied to the measurement cell 2A as in this embodiment, the difference ΔV between the detection value of the sample gas SG (e.g., voltage value V1) output from the voltage amplifier VA when the sample gas SG is supplied to the measurement cell 2A and the detection value of the comparison gas CG (e.g., voltage value V2) output from the voltage amplifier VA when the comparison gas CG is supplied to the measurement cell 2A can be output from the calculation unit CA as an indication of the concentration of a predetermined component contained in the sample gas SG. Even if the detection value of the sample gas SG fluctuates from V0 to V1 between the initial state and after a long period of time due to contamination in the cell 2 (e.g., contamination on the surface of the resistor 21), the detection value of the comparison gas CG also fluctuates due to the influence of the same contamination in the cell 2. Therefore, the value of the difference ΔV itself does not change between the initial state and after a long period of time. Therefore, by using the difference ΔV as the indicated value of the concentration of the predetermined component, it is possible to stably obtain the concentration value of the predetermined component without frequent calibration (zero calibration, span calibration, etc.) and without being affected by long-term drift, which causes the detected value (indicated value) of the sample gas SG to fluctuate over a long period of time. In other words, it is possible to reduce the influence of long-term drift even if calibration is not performed frequently.

[0040] In particular, by having the switching valve SV of the supply gas switching unit 5 alternately switch between supplying the sample gas SG from the sample gas flow path 51 to the supply flow path 53 and supplying the reference gas CG from the comparison gas flow path 52 to the supply flow path 53, the configuration of this embodiment can be reliably realized, in which the sample gas SG and the reference gas CG are alternately supplied to the measurement cell 2A at short intervals, and the above-mentioned effects can be reliably obtained.

[0041] Furthermore, by adopting a cross-flow method in which the sample gas SG and the reference gas CG are alternately supplied in the thermal conduction gas analyzer 1, it is possible to expect the effect of being able to perform measurements with high sensitivity without forming a long cell 2. In other words, it is expected that the S / N ratio can be improved and high sensitivity can be achieved. As a result, while conventional thermal conduction gas analyzers could only detect concentrations down to a lower limit of 1%, by adopting the cross-flow method, it may be possible to manufacture a thermal conduction gas analyzer 1 that can detect concentrations down to a lower limit of 0.1%.

[0042] In the processing unit PU, the voltage amplifier VA detects a difference ΔV between a first detection value (e.g., V1 in FIG. 5B ) obtained based on the output from the Wheatstone bridge circuit WB when the sample gas SG is supplied to the measurement cell 2A and a second detection value (e.g., V2 in FIG. 5B ) obtained based on the output from the Wheatstone bridge circuit WB when the reference gas CG is supplied to the measurement cell 2A. The calculation unit CA then outputs this difference ΔV as an indication value of the concentration of a predetermined component contained in the sample gas SG. This makes it possible to stably obtain the concentration value (indication value) of the predetermined component even after a long period of time has passed, without being affected by long-term drift.

[0043] In this embodiment, an example has been described in which the sample gas SG contains hydrogen as the predetermined component. Hydrogen has a much higher thermal conductivity than other gases (e.g., nitrogen). Therefore, compared to when a gas other than hydrogen is used as the predetermined component, the resistance value of the resistor 21 is more likely to change due to a change in temperature in response to a change in the concentration of the predetermined component. This makes it easier to detect the concentration of the predetermined component.

[0044] Although the above description has been given of an example in which a thermal conduction type thin film sensor is used as each resistor 21 of the Wheatstone bridge circuit WB, the resistors 21 are not limited to the above-described thermal conduction type thin film sensors. Each resistor 21 may be formed of a thermal conduction type semiconductor sensor.

[0045] Fig. 6 is a cross-sectional view showing the schematic configuration of a thermal conduction semiconductor sensor applicable to the resistor 21. The resistor 21 (thermal conduction semiconductor sensor) shown in Fig. 6 is configured by laminating an insulating layer 222 and a palladium-nickel alloy layer 223 in this order on a semiconductor substrate 221. The palladium-nickel alloy layer 223 is configured by laminating an interface layer 223a, a bulk layer 223b, and a surface layer 223c in this order from the insulating layer 222 side.

[0046] Palladium (Pd) is a hydrogen molecule (H ２ ) into hydrogen atoms (H). When hydrogen is adsorbed onto the palladium-nickel alloy layer 223, the total resistance of the palladium-nickel alloy layer 223 changes. Therefore, even when such a thermal conduction semiconductor sensor is used as the resistor 21, the change in resistance value of the resistor 21 due to a temperature change can be detected as an unbalanced voltage in the Wheatstone bridge circuit WB, and the concentration of a predetermined component (e.g., hydrogen) contained in the sample gas SG can be calculated. As such a thermal conduction semiconductor sensor, for example, a hydrogen analyzer H2scan (manufactured by H2scan) can be used. The thermal conduction semiconductor sensor is known to have a faster response speed than a platinum resistance temperature sensor.

[0047] That is, in this embodiment, a thermal conduction thin-film sensor or a thermal conduction semiconductor sensor can be used as each resistor 21 constituting the Wheatstone bridge circuit WB. Because these sensors have a fast response speed, even if the switching valve SV is switched at a short switching cycle, they can detect ΔV in response to the switching and detect the concentration of a predetermined component. In other words, by using a thermal conduction thin-film sensor or a thermal conduction semiconductor sensor as each resistor 21, a cross-flow system can be adopted in the thermal conduction gas analyzer 1, allowing the thermal conduction gas analyzer 1 to function as an analyzer.

[0048] In this embodiment, the switching period is assumed to be about 10 seconds, but it is desirable that the switching period be short in order for the thermal conduction gas analyzer 1 to function as an analyzer. Specifically, it is desirable that the switching period be within one minute (at most several tens of seconds).

[0049] 7 is an explanatory diagram schematically illustrating the configuration of a thermal conduction gas analyzer 1 according to another embodiment of the present invention. The thermal conduction gas analyzer 1 of this embodiment has the same configuration as that of embodiment 1, except that all of the cells 2 in the configuration of embodiment 1 are used as measurement cells through which the sample gas SG and the reference gas CG flow alternately, and the configuration of the supply gas switching unit 5 is changed. Differences from embodiment 1 will be described below.

[0050] For the sake of convenience in the following description, the measurement cell 2A described in the first embodiment will be referred to as the first measurement cell 2X, and the comparison cell 2B will be referred to as the second measurement cell 2Y. That is, when the measurement cell 2A is the first measurement cell 2X, the plurality of cells 2 includes the second measurement cell 2Y. As described above, the measurement cell 2A corresponding to the first measurement cell 2X accommodates the resistors 21 (the second resistor R2 and the third resistor R3) located on one pair of opposite sides of the Wheatstone bridge circuit WB. Therefore, it can be said that the second measurement cell 2Y corresponding to the comparison cell 2B accommodates the resistors 21 (the first resistor R1 and the fourth resistor R4) located on the remaining pair of opposite sides of the Wheatstone bridge circuit WB.

[0051] As in the first embodiment, the supply gas switching unit 5 includes a sample gas flow path 51 through which the sample gas SG flows and a comparison gas flow path 52 through which the comparison gas CG flows. The supply gas switching unit 5 further includes a first supply flow path 53P connected to the first flow path 3, a second supply flow path 54P connected to the second measurement cell 2Y, a first switching valve SV1, and a second switching valve SV2.

[0052] The sample gas flow path 51 is connected to a first connection port of the first switching valve SV1, and the reference gas flow path 52 is connected to a first connection port of the second switching valve SV2.

[0053] The first supply flow path section 53P has a first pipe 53a, a second pipe 53b, and a third pipe 53c. The first pipe 53a is connected to the first flow path section 3. One end of the second pipe 53b is connected to the second connection port of the first switching valve SV1. The other end of the second pipe 53b is connected to the first pipe 53a. One end of the third pipe 53c is connected to the second connection port of the second switching valve SV2. The other end of the third pipe 53c is connected to the first pipe 53a.

[0054] The second supply flow path section 54P has a fourth pipe 54a, a fifth pipe 54b, a sixth pipe 54c, and a seventh pipe 54d. The fourth pipe 54a is connected to the seventh pipe 54d. The seventh pipe 54d is connected to the second measurement cell 2Y. One end of the fifth pipe 54b is connected to the third connection port of the first switching valve SV1. The other end of the fifth pipe 54b is connected to the fourth pipe 54a. One end of the sixth pipe 54c is connected to the third connection port of the second switching valve SV2. The other end of the sixth pipe 54c is connected to the fourth pipe 54a.

[0055] The first selector valve SV1 is, for example, a three-way solenoid valve. The first selector valve SV1 alternates between supplying the sample gas SG from the sample gas flow path 51 to the first supply flow path 53P and supplying the sample gas SG from the sample gas flow path 51 to the second supply flow path 54P. For example, by controlling the on / off state of a current flowing through the solenoid of the first selector valve SV1, communication between the first connection port and the second connection port and communication between the first connection port and the third connection port within the first selector valve SV1 can be alternately switched. This allows the first selector valve SV1 to alternately supply the sample gas SG to the first supply flow path 53P and supply the sample gas SG to the second supply flow path 54P.

[0056] The second selector valve SV2 is, for example, a three-way solenoid valve. The second selector valve SV2 alternately switches between supplying the reference gas CG from the reference gas flow path 52 to the second supply flow path 54P and supplying the reference gas CG from the reference gas flow path 52 to the first supply flow path 53P in synchronization with the switching of the first selector valve SV1. For example, by controlling the on / off state of a current flowing through the solenoid of the second selector valve SV2, communication between the first and third connection ports and communication between the first and second connection ports within the second selector valve SV2 can be alternately switched. This allows the second selector valve SV2 to alternately switch between supplying the reference gas CG to the second supply flow path 54P and supplying the reference gas CG to the first supply flow path 53P. In addition, by synchronizing the switching operation of the second switching valve SV2 with the switching operation of the first switching valve SV1, the supply of the comparison gas CG from the second switching valve SV2 to the second supply flow path section 54P can be synchronized with the supply of the sample gas SG from the first switching valve SV1 to the first supply flow path section 53P, and the supply of the sample gas SG from the first switching valve SV1 to the second supply flow path section 54P can be synchronized with the supply of the comparison gas CG from the second switching valve SV2 to the first supply flow path section 53P.

[0057] In the configuration of this embodiment, the switching operation of the first selector valve SV1 and the second selector valve SV2 described above alternates between the sample gas SG and the comparison gas CG flowing through the first measurement cell 2X and the second measurement cell 2Y. That is, while the sample gas SG flows through the first measurement cell 2X, the comparison gas CG flows through the second measurement cell 2Y. Furthermore, while the comparison gas CG flows through the first measurement cell 2X, the sample gas SG flows through the second measurement cell 2Y. This allows the processing unit PU to obtain a signal amount twice the signal amount (ΔV) shown in FIGS. 5A and 5B compared to the configuration of the first embodiment, further improving the concentration detection sensitivity. Furthermore, since this embodiment does not have a comparison cell in which the comparison gas CG is sealed or enclosed, there is no need to worry about fluctuations in the reading due to gas leakage (gas loss) from the comparison cell.

[0058] In this embodiment, the supply of the sample gas SG to the first supply passage portion 53P by the first selector valve SV1 and the supply of the reference gas CG to the second supply passage portion 54P by the second selector valve SV2 are synchronized. The supply of the sample gas SG to the second supply passage portion 54P by the first selector valve SV1 and the supply of the reference gas CG to the first supply passage portion 53P by the second selector valve SV2 are synchronized. This allows the sample gas SG and the reference gas CG to flow alternately through the first measurement cell 2X and the second measurement cell 2Y, thereby ensuring the advantages of this embodiment, such as improved concentration detection sensitivity.

[0059] [Supplementary Note] In the analyzer described above, it is desirable that both the comparison filler gas and the comparison gas CG are gases with the same components as the base gas of the sample gas SG.

[0060] The comparison fill gas and the comparison gas CG are preferably selected taking the following points into consideration. That is, as described above, a thermal conduction analyzer uses a comparison cell and a measurement cell to form a Wheatstone bridge circuit, and outputs the change in current in the circuit as a change in concentration. In the case of the cross-flow method described in this specification, the output is the difference in thermal conductivity between the sample gas and the comparison gas, so the key to measurement is that the concentration (gas composition) of the comparison gas does not fluctuate. Taking this into consideration, the comparison fill gas and the comparison gas CG are preferably, for example, 100% nitrogen or 100% hydrogen.

[0061] As mentioned above, air can also be used as the comparison filler gas and the comparison gas CG because it does not exhibit large concentration changes. However, if the gas to be measured is a flammable gas, it is desirable to use another gas (e.g., nitrogen) from a safety standpoint. Furthermore, it is known that it is difficult to accurately measure minute concentration changes when air is used as the comparison filler gas and the comparison gas CG. Therefore, when measurement of minute concentration changes is required, it is desirable to use a gas other than air as the comparison filler gas and the comparison gas CG.

[0062] Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to these, and the invention can be expanded or modified without departing from the spirit of the invention.

[0063] The present invention can be used in a thermal conduction gas analyzer that measures the concentration of a specific component contained in a sample gas by utilizing a change in resistance value due to a change in temperature of a resistor that corresponds to the thermal conductivity of the specific component.

[0064] REFERENCE SIGNS LIST 1 Thermal conduction gas analyzer 2 Cell 2A Measurement cell 2X First measurement cell 2Y Second measurement cell 3 First flow path section 4 Second flow path section 5 Supply gas switching section 21 Resistor 51 Sample gas flow path section 52 Reference gas flow path section 53 Supply flow path section 53P First supply flow path section 54P Second supply flow path section 55 Third flow path section CA Calculation section (processing section) CG Reference gas PU Processing section R1 First resistor R2 Second resistor R3 Third resistor R4 Fourth resistor SG Sample gas SV Switching valve SV1 First switching valve SV2 Second switching valve V1 Voltage value (first detected value) V2 Voltage value (second detected value) VA Voltage amplifier (processing section) WB Wheatstone bridge circuit ΔV Difference

Claims

1. A thermal conduction gas analyzer that calculates the concentration of a predetermined component contained in a sample gas by detecting a change in resistance value due to a temperature change of a resistor corresponding to the thermal conductivity of the component as an unbalanced voltage in a Wheatstone bridge circuit including the resistors, the thermal conduction gas analyzer comprising: a plurality of cells each accommodating each of the resistors that make up the Wheatstone bridge circuit; a first flow path section through which the sample gas or the reference gas flows; a second flow path section branching off from the first flow path section; and a supply gas switching section connected to the first flow path section, wherein each of the resistors is formed by a thermal conduction thin-film sensor or a thermal conduction semiconductor sensor, the plurality of cells including measurement cells each accommodating the resistors located on one pair of opposite sides of the Wheatstone bridge circuit, the second flow path section being connected to the measurement cells, and the supply gas switching section alternately supplies the sample gas and the reference gas to the first flow path section.

2. A thermal conduction gas analyzer as described in claim 1, wherein the supply gas switching section includes: a sample gas flow path section through which the sample gas flows; a comparison gas flow path section through which the comparison gas flows; a supply flow path section connected to the first flow path section; and a switching valve, wherein the switching valve alternately switches between supplying the sample gas from the sample gas flow path section to the supply flow path section and supplying the comparison gas from the comparison gas flow path section to the supply flow path section.

3. The thermal conduction gas analyzer according to claim 1, wherein, when the measurement cell is a first measurement cell, the plurality of cells include second measurement cells each accommodating a resistor located on the remaining pair of opposite sides of the Wheatstone bridge circuit, and the supply gas switching unit includes: a sample gas flow path section through which the sample gas flows; a comparison gas flow path section through which the reference gas flows; a first supply flow path section connected to the first flow path section; a second supply flow path section connected to the second measurement cell; a first selector valve and a second selector valve, wherein the first selector valve alternates between supplying the sample gas from the sample gas flow path section to the first supply flow path section and supplying the sample gas from the sample gas flow path section to the second supply flow path section, and the second selector valve alternates between supplying the reference gas from the reference gas flow path section to the second supply flow path section and supplying the reference gas from the reference gas flow path section to the first supply flow path section in synchronization with switching of the first selector valve.

4. A thermal conduction gas analyzer as described in claim 3, wherein the supply of the sample gas to the first supply flow path section by the first switching valve and the supply of the comparison gas to the second supply flow path section by the second switching valve are performed synchronously, and the supply of the sample gas to the second supply flow path section by the first switching valve and the supply of the comparison gas to the first supply flow path section by the second switching valve are performed synchronously.

5. A thermal conduction gas analyzer as described in any one of claims 1 to 4, further comprising a processing unit that outputs the difference between a first detection value obtained based on the output from the Wheatstone bridge circuit when the sample gas is supplied to the measurement cell and a second detection value obtained based on the output from the Wheatstone bridge circuit when the comparison gas is supplied to the measurement cell as an indication value of the concentration of the predetermined component contained in the sample gas.

6. A thermal conduction gas analyzer according to any one of claims 1 to 5, wherein the sample gas contains hydrogen as the predetermined component.

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