WO2018110520A1

WO2018110520A1 - Gas concentration measurement device

Info

Publication number: WO2018110520A1
Application number: PCT/JP2017/044460
Authority: WO
Inventors: 寛北川; 鈴木　洋介; 平松　秀彦
Original assignee: 株式会社デンソー
Priority date: 2016-12-15
Filing date: 2017-12-12
Publication date: 2018-06-21
Also published as: JP2018096900A

Abstract

This gas concentration measurement device is provided with: an anode (12) which is capable of storing and releasing alkali metal ions; a cathode (11) at which the alkali metal ions and a specific gas contained in a measurement gas are reacted with each other, thereby producing a reaction product; an electrolyte (13) which is interposed between the anode and the cathode and is capable of conducting the alkali metal ions; and a gas concentration measurement unit (16) which determines the concentration of the specific gas in accordance with the current value at the time when a voltage is applied to the cathode.

Description

Gas concentration measuring device

Cross-reference of related applications

This application is based on Japanese Patent Application No. 2016-243130 filed on December 15, 2016, the contents of which are incorporated herein by reference.

The present disclosure relates to a gas concentration measurement device that detects the concentration of a specific gas contained in a measurement gas.

Conventionally, a gas sensor that uses a lithium ion conductive electrolyte is known as a gas sensor for measuring a specific gas concentration in a measurement gas (see, for example, Patent Document 1). This gas sensor is configured to measure the carbon dioxide gas concentration by measuring the electromotive force utilizing the fact that an electromotive force is generated between a pair of electrodes provided in the electrolyte in accordance with the carbon dioxide gas concentration.

JP 2000-34134 A

However, the gas sensor described in Patent Document 1 has low measurement accuracy in the high concentration region, and the electromotive force generated between the electrodes is likely to shift in the presence of the impurity gas.

This disclosure aims to improve measurement accuracy in a gas concentration measurement apparatus using an electrolyte having alkali metal ion conductivity.

According to an aspect of the present disclosure, a gas concentration measurement device includes an anode capable of storing and releasing alkali metal ions, and a cathode that generates a reaction product by reacting alkali metal ions with a specific gas included in a measurement gas. A gas that measures the concentration of a specific gas based on the value of the current that flows between the anode and the cathode when a voltage is applied to the cathode, and an electrolyte that is interposed between the anode and the cathode and can conduct alkali metal ions A concentration measuring unit.

For example, when a gas concentration measurement device using an electrolyte having alkali ion conductivity is configured as a current sensor, the oxygen concentration is higher than that of an electromotive force sensor that measures the gas concentration based on the electromotive force. Even if it exists, it becomes possible to detect with high precision.

The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings.
It is a figure which shows the structure of the gas concentration measuring apparatus of 1st Embodiment. It is a figure which shows the relationship between the electric current value of a sensor part, and the oxygen concentration in measurement gas. It is a figure which shows the saturated vapor pressure of water and lithium hydroxide saturated aqueous solution. It is a figure which shows the structure of the gas concentration measuring apparatus of 2nd Embodiment. It is a figure which shows the change of the electric current value of the 1st, 2nd cathode of 2nd Embodiment. It is a figure which shows the change of the electric current value of the 1st, 2nd cathode of other embodiment. In the gas concentration measuring apparatus of 1st Embodiment, it is a block diagram which shows an example in case it has a humidity control apparatus. FIG. 2 is a configuration diagram showing an example of a case where a temperature adjustment device is included in the gas concentration measurement device of the first embodiment.

(First embodiment)
Hereinafter, a gas concentration measuring apparatus 1 according to a first embodiment to which the present disclosure is applied will be described with reference to the drawings. The gas concentration measuring apparatus 1 is configured to measure a specific gas concentration contained in a measurement gas. The gas concentration measuring apparatus 1 of this embodiment measures the concentration of oxygen as a specific gas.

As shown in FIG. 1, the gas concentration measuring apparatus 1 of this embodiment includes a sensor unit 10 and a gas concentration measuring unit 16.

The sensor unit 10 includes a cathode 11, an anode 12, and a solid electrolyte layer 13. The gas concentration measuring apparatus 1 of the present embodiment is configured as a laminated body in which a cathode 11, an anode 12, and a solid electrolyte layer 13 are laminated. The cathode 11 is disposed on one surface side of the solid electrolyte layer 13, and the anode 12 is disposed on the other surface side of the solid electrolyte layer 13.

The cathode 11 has a positive electrode material made of metal or metal oxide. The positive electrode material has a catalytic function for promoting a reaction (that is, a reduction reaction) of oxygen that is a positive electrode active material. As the oxygen of the positive electrode active material, oxygen (oxygen contained in the atmosphere) existing around the gas concentration measuring device 1 (particularly the cathode 11) is used.

The cathode 11 of this embodiment includes a Ti electrode 11a and a Pt electrode 11b. The Ti electrode 11a is configured as a porous body, and the Pt electrode 11b is configured as a comb-shaped electrode. The porous Ti body constituting the Ti electrode 11a has a porosity of 80%. The Ti electrode 11a and the Pt electrode 11b are stacked. The Pt electrode 11b is disposed so as to be in contact with the solid electrolyte layer 13, and the Ti electrode 11a is disposed so as to be in contact with the Pt electrode 11b.

The anode 12 has a negative electrode material including a negative electrode active material capable of occluding and releasing alkali metal ions. Examples of alkali metal ions include lithium ions and sodium ions. In the present embodiment, lithium ions are used as alkali metal ions.

The negative electrode active material is selected from the group consisting of metallic lithium, a lithium alloy, a metal material capable of occluding and releasing lithium, an alloy material capable of occluding and releasing lithium, and a compound capable of occluding and releasing lithium. One or more materials. In this embodiment, metallic lithium is used as the anode 12.

The solid electrolyte layer 13 is interposed between the cathode 11 and the anode 12 and is made of a solid electrolyte capable of conducting lithium ions. The solid electrolyte constituting the solid electrolyte layer 13 is preferably made of a material that has no electron conductivity and high lithium ion conductivity. The solid electrolyte layer 13 functions as a transmission path through which lithium ions move between the cathode 11 and the anode 12. The solid electrolyte layer 13 may be a single layer or a plurality of layers.

The solid electrolyte layer 13 of the present embodiment is a laminate of two layers, a first electrolyte layer 13a and a second electrolyte layer 13b. The first electrolyte layer 13a is provided on the cathode 11 side, and the second electrolyte layer 13b is provided on the anode 12 side.

As the first electrolyte layer 13a, LATP (Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ ), which is a Li—Al—Ti—P—O-based NASICON type oxide, is used. As the second electrolyte layer 13b, an electrolyte made of PEO (polyethylene oxide) and LiTFSI (lithium bistrifluoromethanesulfonimide: Li (CF ₃ SO ₂ ) ₂ N) is used. The second electrolyte layer 13b is provided to prevent LATP constituting the first electrolyte layer 13a from reacting with the metal Li constituting the anode 12.

As shown in FIG. 1, the cathode 11 is covered with a seal portion 14. The seal part 14 may be made of resin or alumina as long as it can block the measurement gas. The seal portion 14 is formed with an opening portion 14a through which the measurement gas can pass.

A diffusion layer 15 for limiting the diffusion of the measurement gas to the surface of the cathode 11 is provided in the opening 14a of the seal portion 14. As the diffusion layer 15, for example, a porous body of alumina can be used.

The gas concentration measuring unit 16 is configured to detect the oxygen concentration based on a current value when a constant voltage is applied to the sensor unit 10. The gas concentration measurement unit 16 includes a power supply unit, a current measurement unit, and a calculation unit. The power supply unit applies a voltage to the cathode 11 and the anode 12. The current measuring unit measures a current value flowing between the cathode 11 and the anode 12 when a voltage is applied by the power supply unit. The computing unit computes the oxygen concentration based on the current value measured by the current measuring unit.

In the gas concentration measuring apparatus 1 of the present embodiment, the following reaction proceeds at the cathode 11 and the anode 12. In the present embodiment, the constant voltage applied to the sensor unit 10 is 2.9 V or less.

[When measuring gas concentration]
Anode side: Li → Li ⁺ + e ⁻
Cathode side: 2Li ⁺ + O ₂ + 2e ⁻ → Li ₂ O ₂
[When refreshing]
Anode side: Li ⁺ + e ⁻ → Li
Cathode side: Li ₂ O ₂ → 2Li ⁺ + O ₂ + 2e ⁻
When measuring the gas concentration, lithium ions are generated at the anode 12, and the lithium ions move to the cathode 11 side through the electrolyte layer 13. At the cathode 11, lithium ions react with oxygen in the air, and Li ₂ O ₂ is mainly deposited as a reaction product.

When the gas concentration measurement is continued, lithium atoms are unevenly distributed on the cathode 11 side as reaction products, and the gas concentration measurement cannot be performed. For this reason, it is necessary to periodically refresh the direction in which the current flows.

At the time of refresh, the reaction product is decomposed at the cathode 11 to generate lithium ions and oxygen. Lithium ions pass through the electrolyte layer 13 and move to the anode 12 side to become lithium metal.

By performing the refresh for a predetermined time, the gas concentration measurement device 1 can perform the gas concentration measurement again. In the gas concentration measuring apparatus 1 of the present embodiment, the gas concentration measurement and the refresh are repeatedly performed every predetermined time.

When measuring the gas concentration, since the amount of oxygen supplied to the anode 11 is limited by the diffusion layer 15, a saturation phenomenon appears in which the current becomes a constant value (that is, the limit current) even if the voltage is increased. Since the limit current is proportional to the oxygen concentration in the measurement gas, the oxygen concentration can be detected from the current value when a constant voltage is applied to the sensor unit 10. That is, the gas concentration measuring apparatus 1 of the present embodiment is configured as a limiting current type sensor.

FIG. 2 shows the relationship between the current value when a constant voltage is applied to the sensor unit 10 and the oxygen concentration in the measurement gas. FIG. 2 shows the relationship between the current value of the sensor unit 10 and the oxygen concentration when the temperature of the sensor unit 10 is 70 ° C. and the dew point temperature is 60 ° C.

As shown in FIG. 2, the current value of the sensor unit 10 at the time of measuring the gas concentration is proportional to the oxygen concentration in the air, and the current value increases as the oxygen concentration increases. Therefore, the oxygen concentration in the air can be measured by measuring the current value with the gas concentration measuring unit 16 applying a constant voltage to the sensor unit 10. As shown in FIG. 2, the gas concentration measuring apparatus 1 according to the present embodiment can measure a high concentration oxygen concentration on the order of%.

The relationship between the current value of the sensor unit 10 and the oxygen concentration shown in FIG. 2 differs depending on the temperature of the sensor unit 10 and the humidity of the measurement gas. For this reason, the gas concentration measurement part 16 should just be provided with the relationship between the electric current value of the sensor part 10, and oxygen concentration as a map for every temperature of the sensor part 10, and humidity of measurement gas. In addition, in this indication, humidity shall mean relative humidity.

Here, the surface ion conductive layer formed on the surface of the reaction product (mainly Li ₂ O ₂ ) generated at the cathode 11 by gas concentration measurement will be described. A gas containing water in the vapor phase is in contact with the surface of the reaction product generated at the cathode 11. For this reason, on the surface of the reaction product, a part of lithium contained in the reaction product is bonded to the hydroxy group of the water molecule, and LiOH is generated.

LiOH is a water-soluble substance, and absorbs moisture in the presence of vapor phase water to form a lithium hydroxide layer. As a result, a surface ion conductive layer composed of a lithium hydroxide layer is formed on the surface of the reaction product.

The surface ion conductive layer forms a lithium ion conductive layer in the reaction field of the cathode 11. As a result, the conductivity in the sensor unit 10 can be improved. Further, it is possible to suppress the occurrence of an overvoltage in which the voltage greatly increases when the sensor unit 10 performs refresh after the gas concentration measurement. For this reason, it is possible to stably measure and refresh the gas concentration.

The magnitude of the ionic conductivity of the surface ion conductive layer varies based on the amount of moisture in the gas phase (that is, humidity) contained in the measurement gas. Specifically, the surface ion conductive layer expands when the humidity of the measurement gas is high, and the surface ion conductive layer contracts when the humidity of the measurement gas is low.

If the surface ion conductive layer is enlarged too much, the reaction product may be reduced and the surface ion conductive layer may flow out. In addition, if the surface ion conductive layer is excessively reduced, appropriate ion conductivity cannot be secured, and gas concentration measurement by the sensor unit 10 may not be performed.

For this reason, when the gas concentration measurement and refresh of the sensor unit 10 are performed, it is necessary to appropriately adjust the humidity of the measurement gas and properly maintain the surface ion conductive layer. For example, as shown in FIG. 7A, the humidity of the measurement gas can be adjusted by humidifying or dehumidifying the measurement gas by the humidity adjusting device 21. Alternatively, as shown in FIG. 7B, the humidity of the measurement gas can be adjusted by adjusting the temperature of the sensor unit 10 with the temperature adjustment device 22. As an example of the temperature adjustment device 22, a heating device such as a heater may be provided. In these cases, the humidity adjusting device 21 and the temperature adjusting device 22 correspond to a humidity adjusting unit that adjusts the humidity of the measurement gas. Note that only one or both of the humidity adjusting device 21 and the temperature adjusting device 22 may be provided as the humidity adjusting unit.

When vapor phase water contained in the measurement gas is condensed on the surface of the cathode 11, the surface ion conductive layer becomes excessive, and the reaction product may be completely dissolved and dissolved. For this reason, in this embodiment, the amount of water vapor contained in the measurement gas is made smaller than the amount of saturated water vapor at the temperature of the sensor unit 10.

When the water vapor partial pressure of the measurement gas is equal to the saturated vapor pressure (hereinafter referred to as “LiOH saturated vapor pressure”) when the lithium hydroxide layer constituting the surface ion conductive layer is made into a saturated aqueous solution, the surface ion conductive layer Is in an equilibrium state. That is, if the water vapor partial pressure of the measurement gas is higher than the LiOH saturated vapor pressure, the surface ion conductive layer expands, and if the water vapor partial pressure of the measurement gas is lower than the LiOH saturated vapor pressure, the surface ion conductive layer shrinks.

FIG. 3 shows changes in saturated water vapor pressure (P1) and LiOH saturated vapor pressure (P2) accompanying temperature change, and humidity (relative humidity RH) corresponding to LiOH saturated vapor pressure. As shown in FIG. 3, the saturated water vapor pressure and the LiOH saturated vapor pressure change according to the temperature change, and the saturated water vapor pressure and the LiOH saturated vapor pressure increase as the temperature rises.

The LiOH saturated vapor pressure is lower than the saturated water vapor pressure due to the vapor pressure drop caused by the aqueous solution. Moreover, since the solubility of lithium hydroxide increases at a high temperature, the difference between the saturated water vapor pressure and the LiOH saturated vapor pressure increases.

Since the saturated water vapor pressure is 100% humidity, the ratio of the LiOH saturated vapor pressure to the saturated water vapor pressure is the humidity (%) corresponding to the LiOH saturated vapor pressure. As shown in FIG. 3, the humidity corresponding to the LiOH saturated vapor pressure is around 84%, although it varies depending on the temperature. For this reason, the surface ion conductive layer can be brought into an equilibrium state by controlling the humidity of the measurement gas at the apparatus temperature to be around 84%.

If the humidity of the measurement gas is too low, the surface ion conductive layer will shrink too much, so it is desirable that the water vapor pressure of the measurement gas be 40% or more of the LiOH saturated vapor pressure. In addition, if the humidity of the measurement gas is too high, the surface ion conductive layer becomes too large. Therefore, the water vapor pressure of the measurement gas is preferably 100% or less of the LiOH saturated vapor pressure. That is, the relationship of 1 ≧ (water vapor pressure of measurement gas) / (LiOH saturated vapor pressure) ≧ 0.4 may be satisfied.

In the gas concentration measuring apparatus 1 according to the present embodiment described above, an electrolyte having lithium ion conductivity is used as the electrolyte membrane 13 sandwiched between the cathode 11 and the anode 12, and measurement is performed based on a current value when a constant voltage is applied. The oxygen concentration in the gas is measured. By configuring the gas concentration measuring device 1 using an electrolyte having lithium ion conductivity as a current type sensor in this way, the oxygen concentration is lower than that of an electromotive force type sensor that measures the gas concentration based on the electromotive force. It becomes possible to detect with high accuracy even at a high concentration.

In the present embodiment, since the Ti porous body having a large specific surface area is used as the cathode 11, the gas concentration detection accuracy based on the current value can be improved.

In this embodiment, the water vapor pressure of the measurement gas is 40 to 100% of the LiOH saturated vapor pressure. Thereby, a lithium ion conductive layer can be formed on the surface of the reaction product produced on the cathode 11, and the detection accuracy of the gas concentration based on the current value can be improved.

Further, in this embodiment, a lithium ion conductive solid electrolyte is used as the electrolyte membrane 13 sandwiched between the cathode 11 and the anode 12, and the gas concentration measuring device 1 can be operated at a relatively low temperature.

(Second Embodiment)
Next, a second embodiment of the present disclosure will be described. In the second embodiment, description of the same parts as in the first embodiment will be omitted, and only different parts will be described.

As shown in FIG. 4, the gas concentration measuring apparatus 2 of the second embodiment is configured as a bipolar gas sensor having a plurality of cathodes. In the second embodiment, two

cathodes

11 and 17 are provided. One anode 12 and one electrolyte layer 13 are provided, as in the first embodiment.

The two

cathodes

11 and 17 are provided on the surface of the electrolyte layer 13, respectively. These

cathodes

11 and 17 have the same structure, and include Ti electrodes 11a and 17a and

Pt electrodes

11b and 17b, respectively.

The first cathode 11 and the second cathode 17 are each covered with a seal portion 14. In the seal portion 14, a first opening 14 a is formed corresponding to the first cathode 11, and a second opening 14 b is formed corresponding to the second cathode 17. A first diffusion layer 15 is provided in the first opening 14a, and a second diffusion layer 18 is provided in the second opening 14b.

The gas concentration measuring unit 16 reverses the current flow in the first cathode 11 and the second cathode 17. For this reason, refreshing can be performed on the

other cathodes

11 and 17 while the gas concentration measurement is performed on one of the

cathodes

11 and 17.

In the second embodiment, the gas concentration measurement unit 16 switches the gas concentration measurement and the refresh at different timings using the two

cathodes

11 and 17. That is, the gas concentration measurement unit 16 switches the current flow between the two

cathodes

11 and 17 at different timings.

As shown in FIG. 5, in the second embodiment, the gas concentration measurement unit 16 alternately performs gas concentration measurement and refresh switching at the

cathodes

11 and 17. Thereby, the gas concentration measurement by the 1st cathode 11 and the gas concentration measurement by the 2nd cathode 17 can be performed alternately, and a gas concentration measurement can be performed continuously.

In the limit current type sensor, it takes some time until the current value becomes constant after the start of voltage application during gas concentration measurement, that is, until the gas concentration can be measured. For this reason, the timing at which the current flow is switched from the refresh to the gas concentration measurement at the

other cathode

11, 17 is set earlier than the timing at which the current flow is switched from the gas concentration measurement to the refresh at the one

cathode

11, 17.

According to the second embodiment described above, the gas concentration measurement can be continuously performed by alternately measuring the gas concentration with the plurality of

cathodes

11 and 17. In the second embodiment, the gas concentration is measured alternately by the two

cathodes

11 and 17, but three or more cathodes may be provided and the gas concentration may be measured in order.

(Other embodiments)
Although the present disclosure has been described based on the embodiments, it is understood that the present disclosure is not limited to the embodiments and structures. The present disclosure includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms including only one element, more or less, are within the scope and spirit of the present disclosure. For example, various modifications can be made as follows without departing from the spirit of the present disclosure.

(1) In the gas

concentration measuring apparatuses

1 and 2 of each of the above embodiments, the oxygen concentration is measured. However, as described below, oxygen is included by controlling the voltage applied to the sensor unit 10. A plurality of types of gas concentrations may be measured.

In the gas

concentration measuring apparatuses

1 and 2, lithium ions can react with various gases when measuring the gas concentration. For example, (a) 4Li ⁺ + O ₂ + 2CO ₂ + 4e ⁻ → 2Li ₂ CO ₃ , (a) 4Li ⁺ + O ₂ + 2H ₂ O + 4e ⁻ → 4LiOH, (c) 2Li ⁺ + O ₂ + 2e ⁻ → There is Li ₂ O ₂ . In reaction (a), lithium ions react mainly with carbon dioxide, and in reactions (a) and (c), lithium ions react mainly with oxygen.

Depending on the voltage applied to the sensor unit 10, at least one of the reactions (a) to (c) proceeds. Specifically, the reaction by voltage of 3.65V reaction by a (vs.Li ⁺ / Li) or less (A) proceeds, and less 3.2V (vs.Li ⁺ / Li) voltage (I) proceeds, and the reaction (c) proceeds by setting the voltage to 2.9 V (vs. Li ⁺ / Li) or less.

For this reason, by setting the voltage applied to the sensor unit 10 to 3.65 to 3.2 V, the reaction between lithium ions and carbon dioxide is the main component, and the carbon dioxide concentration can be measured. In addition, by setting the voltage applied to the sensor unit 10 to 3.2 V or less, the reaction between lithium ions and oxygen is mainly performed, and the oxygen concentration can be measured. In this way, by controlling the voltage applied to the sensor unit 10, it is possible to measure a plurality of types of gas concentrations.

(2) Alternatively, the concentration of a plurality of gases including oxygen may be measured by differentiating a plurality of cathode electrode materials. For example, the Pt electrode is inert to carbon dioxide and the Ru electrode is active to carbon dioxide. For this reason, in a configuration in which two cathodes are provided as in the gas concentration measuring apparatus 2 of the second embodiment, a Pt electrode is provided on one cathode and a Ru electrode is provided on the other cathode. When oxygen and carbon dioxide are contained in the measurement gas, the oxygen concentration can be measured using the Pt electrode, and the oxygen concentration and carbon dioxide concentration can be measured using the Ru electrode. Further, the carbon dioxide concentration can be obtained by subtracting the measured gas concentration value (that is, oxygen concentration) of the Pt electrode from the measured gas concentration value of the Ru electrode (that is, the total value of oxygen concentration and carbon dioxide concentration).

(3) In the above-described embodiment, the gas

concentration measuring devices

1 and 2 are configured as limit current type sensors. However, the present invention is not limited to this, and a transient that measures the gas concentration based on a change in the current value when a voltage is applied. You may comprise as an electric current type sensor. In the transient current type sensor, the gas concentration can be measured immediately after the voltage application is started when the gas concentration is measured.

Therefore, when the gas concentration measuring device 2 of the second embodiment is a transient current type sensor, as shown in FIG. 6, the current flows from the gas concentration measurement to the refresh at one of the

cathodes

11 and 17. The timing for switching and the timing for switching the current flow from refresh to gas concentration measurement at the

other cathodes

11 and 17 may be made simultaneously.

Claims

An anode (12) capable of storing and releasing alkali metal ions;
A cathode (11) in which a reaction product is produced by a reaction between the alkali metal ion and a specific gas contained in the measurement gas;
An electrolyte (13) interposed between the anode and the cathode and capable of conducting the alkali metal ions;
A gas concentration measuring device comprising: a gas concentration measuring unit (16) that measures the concentration of the specific gas based on a current value flowing between the cathode and the anode when a voltage is applied to the cathode.
The gas concentration measuring device according to claim 1, wherein the alkali metal ion is lithium ion.
The gas concentration measuring device according to claim 1 or 2, wherein the specific gas is oxygen.
A plurality of the cathodes are provided,
The gas concentration measurement unit may flow a current so that the reaction product is decomposed at another cathode when the reaction product is generated at any one of the plurality of cathodes. The gas concentration measuring device according to any one of claims 1 to 3, wherein the gas concentration measuring device is configured to be switchable.
The specific gas includes a plurality of types of gas,
5. The gas concentration measurement unit according to claim 1, wherein the gas concentration measurement unit measures the concentrations of the plurality of types of specific gases based on the current values when different voltages are applied to the cathode. Gas concentration measuring device.
The specific gas includes a plurality of types of gas,
A plurality of the cathodes are provided, and the plurality of cathodes have different electrode materials,
In the plurality of cathodes, the different types of the specific gas and the alkali metal ions react,
5. The gas concentration measuring unit measures a concentration of a different kind of the specific gas at each cathode based on each current value when a voltage is applied to the plurality of cathodes. 6. The gas concentration measuring device according to claim 1.
A water-soluble substance based on the reaction product can be generated on the surface of the cathode,
Furthermore, it has a humidity adjustment part (21, 22) which adjusts the humidity of measurement gas so that the water vapor pressure of said measurement gas may be 40% or more of the saturated vapor pressure of said water-soluble substance. The gas concentration measuring apparatus according to one.