Patent Publication Number: US-2023146123-A1

Title: Gas sensing device including housing having connection passage

Description:
TECHNICAL FIELD 
     This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0056689, filed on May 12, 2020, and the entire contents of which are incorporated herein as a part of the specification. 
     The present disclosure relates to a gas sensing device. More particularly, the present disclosure relates to a gas sensing device having a housing in which a connection passage is formed. 
     BACKGROUND ART 
     There is a situation in which a method of measuring a concentration of a dissolved gas dissolved in a liquid is used so as to detect characteristics or a change in the characteristics of the liquid. For example, in oils used in various mechanical devices, such as an engine oil of a vehicle, an insulating oil of a transformer, and so on, as deterioration progresses, a change in a concentration of a dissolved gas, such as an increase in hydrogen, carbon monoxide, acetylene gas, and so on, occurs. Therefore, when the concentration of the dissolved gas is measured, whether the oil is deteriorated may be sensed. In actuality, regarding the insulating oil of the transformer, it is reported that there is a risk of explosion when 1000 ppm or more of dissolved hydrogen is generated. In addition, in the field of nuclear power generation, by measuring a concentration of oxygen or deuterium that is dissolved in water, information about corrosion of a pipe or information about power generation may be known. Further, in the metal industry, by measuring a concentration of a dissolved gas in a molten metal, quality of a manufactured metal may be constantly maintained and managed. 
     In order to measure a concentration of a dissolved gas, a method in which a dissolved gas is extracted from a liquid sample after the liquid sample is collected and then the dissolved gas is analyzed by a gas chromatography is generally used. However, there is a limitation that this method is not capable of measuring a concentration of a dissolved gas in real time in the industrial field. 
     In Korean Patent No. 10-1512189, a technology of measuring a concentration of a dissolved hydrogen gas by inserting a hydrogen sensor element, including a sensor unit that uses a solid electrolyte, into an oil has been proposed. In this technology, there is an advantage that a concentration of a dissolved gas may be easily measured in real time, but there is a problem that a sensing electrode of the sensor unit may be easily deteriorated since the sensing electrode is in direct contact with a liquid. 
     In Korean Patent Application Publication No. 2016-0011722, a technology of measuring a concentration of a dissolved hydrogen gas has been proposed. In this technology, a hydrogen sensor element in which a sensor unit is disposed in a sealed space formed by a housing and a gas separation membrane is inserted into a liquid, so that a sensing electrode of the sensor unit is not directly exposed to the liquid and the concentration of the dissolved hydrogen gas permeated in the sealed space through the gas separation membrane is measured. In this technology, there is an advantage that a concentration of a dissolved gas may be easily measured in real time while deterioration of the sensing electrode of the sensor unit is restrained. 
     However, when the sensing electrode of the sensor unit is disposed in the sealed space, evaporation of the dissolved gas in the sealed space and movement thereof to the sensing electrode of the sensor unit may be difficult due to a pressure in the sealed space. Particularly, a gas sensor is generally operated in a state in which the gas sensor is heated to a high temperature by using a heater, so that the pressure inside the sealed space in which the sensing electrode of the sensor unit is disposed is further increased. Therefore, evaporation of the dissolved gas and movement thereof from the liquid to a position of the sensing electrode of the sensor unit may be difficult, which may be an obstacle in rapidly and accurately measuring the concentration of the dissolved gas. 
     In order to solve this problem, a method of communicating external air to a space inside the housing where the sensing electrode of the sensor unit is disposed may be considered so that the pressure does not increase above atmospheric pressure even if the sensor unit is heated to a high temperature. However, in this situation, since the dissolved gas that is evaporated from the liquid is discharged to external air, there is a problem that it is difficult to be considered that the concentration of a gas in the space where the sensing electrode of the sensor unit is disposed accurately represents the concentration of the dissolved gas in the liquid. 
     DISCLOSURE 
     Technical Problem 
     Accordingly, the present disclosure has been made keeping in mind the above problems occurring in the related art, and an objective of the present disclosure is to provide a gas sensing device capable of measuring a concentration of a gas with a high response speed and a high accuracy even if a pressure of a space inside a housing where a sensor unit is disposed increases. 
     The objective of the present disclosure is not limited thereto, and other objectives and other advantages of the present disclosure will be understood from the following description. 
     Technical Solution 
     In order to achieve the above objective, according to an embodiment of the present disclosure, there is provided a gas sensing device including: a housing including an opening part through which a target gas to be sensed enters an inner space thereof; a sensor unit disposed in the inner space of the housing; and a connection passage connecting a first opening and a second opening that are formed in the housing such that the first opening and the second opening are open toward the inner space of the housing. Further, the gas sensing device may further include a heater unit for heating the sensor unit to a sensing temperature. 
     The inner space of the housing may be in communication with external air only through the opening part. 
     The inner space of the housing may include a first inner space between the sensor unit and the opening part and a second inner space that is a space except for the first inner space, the first opening may be open toward the first inner space, and the second opening may be open toward the second inner space. 
     The housing may be formed in a hollow tubular shape such that the opening part is formed in a first end portion of the housing in a longitudinal direction, the sensor unit may be disposed at the inner space of the housing while the sensor unit is in a state of being fixed to a first end portion of a frame in a longitudinal direction, the frame having a diameter smaller than an inner diameter of the housing, and a second end portion of the frame in the longitudinal direction may be gas-tightly fixed to the housing. 
     Therefore, a space between the frame and an inner wall of the housing may form the second inner space, and a circulation path circulating the first inner space, the first opening, the connection passage, the second opening, and the second inner space may be formed. 
     In addition, the housing may be formed in a hollow tubular shape such that the opening part is formed in a first end portion of the housing in a longitudinal direction, a second end portion of the housing in the longitudinal direction may be blocked by a cover part, and the sensor unit may be disposed at the inner space of the housing while the sensor unit is in a state of being coupled to an inner wall of the housing. Here, a gap may exist between the sensor unit and the inner wall of the housing, so that a circulation path circulating the first inner space, the first opening, the connection passage, the second opening, and the second inner space may be formed. 
     The sensor unit may include a hydrogen sensor element, and the hydrogen sensor element may include: a solid electrolyte; a sensing electrode formed on a first surface of the solid electrolyte in a direction toward the opening part; and a reference electrode formed on a second surface of the solid electrolyte, wherein the first opening may be positioned between the reference electrode and the opening part. 
     According to an embodiment of the present disclosure, there is provided a gas sensing device including: a housing which includes an inner space and which is formed in a hollow tubular shape, the housing being configured such that a lower end portion of the housing in a longitudinal direction is open toward the inner space, thereby forming an opening part such that a target gas to be sensed enters the inner space; a sensor unit disposed at a position of the inner space, the position being spaced apart from both an upper end portion and the lower end portion of the housing in the longitudinal direction by a predetermined distance, the sensor unit including a sensing electrode formed to face the opening part; and a heater unit provided to heat the sensor unit to a sensing temperature, wherein the inner space of the housing may include a first inner space and a second inner space that are respectively positioned below and above the sensing electrode of the sensor unit on the basis of the sensing electrode of the sensor unit, a first opening formed to be open toward the first inner space and a second opening formed to be open toward the second inner space may be formed in the housing, and a connection passage connecting the first opening and the second opening may be provided, thereby forming a circulation path in which the target gas to be sensed enters through the opening part and which is circulated through the first inner space, the first opening, the connection passage, the second opening, and the second inner space. 
     Here, the sensor unit may be disposed at the inner space while the sensor unit is in a state of being fixed to a lower end portion of a frame which is formed in a hollow tubular shape and which has a diameter smaller than an inner diameter of the housing, an inner portion of the frame may be exposed to external air while the inner portion of the frame is isolated from the inner space of the housing, and the sensor unit may further include a reference electrode that is exposed to the external air through the inner portion of the frame. 
     Otherwise, the sensor unit may be disposed at the inner space in a manner that a partial area of a border of the sensor unit is coupled to an inner wall of the housing through a coupling part, and a gap may be formed at a portion which is a position where the coupling part is not formed and which is positioned between the sensor unit and the inner wall of the housing, so that the first inner space and the second inner space may be in communication with each other through the gap. 
     Advantageous Effects 
     According to the present disclosure, since the connection passage that connects openings formed in the housing to each other so that the openings are respectively open toward an upper inner space and a lower inner space of the sensor unit, there is an effect that the gas sensing device capable of measuring a concentration of a gas with high response speed and high accuracy may be provided even if a pressure of the inner space of the housing where the sensor unit is disposed increases. 
     However, the effect of the present disclosure is not limited to those mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view schematically illustrating a gas sensing device according to a first embodiment of the present disclosure. 
         FIG.  2    is a cross-sectional view schematically illustrating the gas sensing device according to a second embodiment of the present disclosure. 
         FIG.  3    is a cross-sectional view taken along line A-A in  FIG.  2   . 
         FIGS.  4  to  6    are views illustrating a hydrogen sensor element capable of being used as a sensor unit in the present disclosure. 
         FIGS.  7  and  8    are views illustrating examples of use of the gas sensing device according to an embodiment of the present disclosure. 
         FIG.  9    is a graph illustrating a measuring result of a concentration of a gas by using the gas sensing device according to an embodiment of the present disclosure. 
         FIG.  10    is a graph illustrating a measuring result of a concentration of a gas by using the gas sensing device according to a comparative example of the present disclosure. 
     
    
    
     BEST MODE 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings, but the present disclosure is not limited or restricted to the embodiments. In describing the embodiments of the present disclosure, corresponding components will be described with the same name and the same reference numeral. In describing the present disclosure, a detailed description of known technologies will be omitted when it may obscure the subject matter of the present disclosure. Unless otherwise defined, terms used in this specification should be interpreted as content commonly understood by those of ordinary skill in the art. 
       FIG.  1    is a cross-sectional view schematically illustrating a gas sensing device according to a first embodiment of the present disclosure. Referring to  FIG.  1   , a gas sensing device  1 A according to a first embodiment of the present disclosure includes a housing  10  having a first side provided with an opening part  70 , a sensor unit  20  disposed in the housing  10 , a heater unit  50  for heating the sensor unit  20  to a sensing temperature, and a connection passage  60  connecting a first opening  61  and a second opening  62  that are formed in the housing  10 . 
     The housing  10  may be provided in a hollow tubular shape. The opening part  70  may be formed in the first side (a lower side of  FIG.  1   ) of the housing  10 , and may provide an entrance into which a target gas to be sensed is injected. When the gas sensing device  1 A is used for measuring a concentration of a gas in a measurement environment, the housing  10  may be mounted such that the opening part  70  is in communication with the measurement environment. For example, when the gas sensing device  1 A is used for measuring a concentration of a dissolved gas in a liquid, the housing  10  of the gas sensing device  1 A may be mounted such that the opening part  70  is in communication with a liquid storage container, or may be mounted such that a first end of the housing  10  where the opening part  70  is formed is submerged in the liquid. A dissolved gas evaporated from the liquid may be filled in an inner space  30  of the housing  10 , and the sensor unit  20  may sense the dissolved gas. 
     The sensor unit  20  may be disposed in the housing  10 , and may be disposed at a position spaced apart from opposite end portions of the housing  10  in a longitudinal direction (a vertical direction of  FIG.  1   ) of the housing  10 . For example, as illustrated in  FIG.  1   , the sensor unit  20  may be disposed at a position which is spaced apart from a first end portion of the housing  10  at the opening part  70  side by a d 1  distance in the longitudinal direction and which is spaced apart from a second end portion of the housing  10  by a d 2  distance in the longitudinal direction. Here, the position of the sensor unit  20  may be a position of a sensing electrode that is formed at the sensor unit  20 . 
     In order for disposing the sensor unit  20  at a position spaced apart from the end portions of the housing  10  in the longitudinal direction, the sensor unit  20  may be mounted in the housing  10  while being in a state in which the sensor unit  20  is fixed to a first end (the lower side of  FIG.  1   ) of a frame  22  that has a predetermined length. The frame  22  is formed in a tubular shape having a diameter smaller than a diameter of the housing  10 , so that a gap g may be formed between the frame  22  and an inner wall of the housing  10  in a width direction (a horizontal direction of  FIG.  1   ). The sensor unit  20  may be fixed to a first end portion of the frame  22 , and a second end portion of the frame  22  may be coupled to the housing  10 . When the sensor unit  20  is formed in a tubular shape, the frame  22  may be a component included in the sensor unit  20 . Various coupling methods such as adhesive coupling, screw coupling, brazing, and so on may be used to couple the frame  22  to the housing  10 , and a coupling method is not limited to a specific coupling method. Meanwhile, the frame  22  may be coupled to the housing  10  by a gas-tight coupling method. To this end, a sealing material  21  may be included at a coupling portion where the frame  22  and the housing  10  are coupled to each other. The sealing material  21  may be formed of an elastic polymer material such as an O-ring. Otherwise, when the frame  22  is coupled to the housing  10  by the adhesive coupling method, the sealing material  21  may be an adhesive material. 
     Since the housing  10  and the frame  22  are coupled to each other such that the housing  10  and the frame  22  are gas-tightly coupled to each other, the inner space  30  of the housing  10  except for the opening part  70  may form a sealed space blocked from external air. Otherwise, when the second side (an upper portion of  FIG.  1   ) of the housing  10  where the opening part  70  is not formed is formed in a confined structure, the sealed space is capable of being formed in the inner space  30  even if the frame  22  is not gas-tightly coupled to the housing  10 . 
     The inner space  30  of the housing  10  may be divided into a first inner space  31  and a second inner space  32  by a position where the sensor unit  20  is mounted. In the inner space  30  of the housing  10 , the first inner space  31  may be a space between the sensor unit  20  and the opening part  70 . Further, in the inner space  30  of the housing, the second inner space  32  may be a space except for the first inner space  31 . In an embodiment in  FIG.  1   , the first inner space  31  may be an inner space below the sensor unit  20 , and the second inner space  32  may be an inner space above the sensor unit  20 . That is, the second inner space  32  may be a space formed by the gap g between the frame  22  and the inner wall of the housing  10 . Here, an upper side and a lower side of the sensor unit  20  may be divided on the basis of the position of the sensing electrode that is formed at the sensor unit  20 . 
     The heater unit  50  is a configuration for heating the sensor unit  20  to the sensing temperature. The sensing temperature may vary according to a type of a sensor, and may be at least 300° C. As exemplarily illustrated in  FIG.  1   , the heater unit  50  may include a resistive heating type heating coil that is wound on an outer portion of the housing  10  where the sensor unit  20  is disposed, but may be provided in various types that are not limited thereto. For example, the heater unit  50  may be disposed inside the housing  10 , and may be provided in a heating pattern shape printed on a predetermined substrate, instead of the heating coil. Otherwise, the heater unit  50  may be provided as the heating coil wound on the frame  22 , or may be provided such that the heater unit  50  is mounted inside the frame  22  or the sensor unit  20 . The heater unit  50  is not limited to the resistive heating type, and may be provided as a light irradiation type heater unit such as a heating lamp, an LED, and so on. 
     When the sensor unit  20  does not required to be heated since the gas sensing device  1 A is used in a high temperature environment, the heater unit  50  may be omitted. For example, when the gas sensing device  1 A is used for measuring a concentration of a dissolved gas in a high temperature molten metal, the gas sensing device  1 A may not include the heater unit  50 . 
     When the gas sensing device  1 A is connected with a high pressure measurement environment or when the opening part  70  is connected to a liquid storage container so as to measure a dissolved gas in a liquid, the inner space  30  may become a high pressure space. Particularly, when the sensor unit  20  is heated to a high temperature by using the heater unit  50 , a pressure of the inner space  30  may further increase according to a temperature. 
     Due to such an increase in pressure, a target gas to be sensed may be difficult to be moved to the sensor unit  20 . 
     In order for solving this problem, the gas sensing device  1 A according to the first embodiment of the present disclosure forms the connection passage  60 . Specifically, the first opening  61  and the second opening  62  are formed in the housing  10 , and the first opening  61  and the second opening  62  are connected to each other by the connection passage  60 . The first opening  61  and the second opening  62  may be formed in the housing  10  such that the first opening  61  is open toward the first inner space  31  and the second opening  62  is open toward the second inner space  32 . Referring to  FIG.  1   , the first opening  61  may be formed below the sensor unit  20 , and the second opening  62  may be formed above the sensor unit  20 . Here, the upper side and the lower side of the sensor unit  20  may be divided on the basis of the position of the sensing electrode that is formed at the sensor unit  20 . 
     By this structure, a circulation path into which a gas at the first inner space  31  enters the first opening  61 , the connection passage  60 , the second opening  62 , the second inner space  32 , and the first inner space  31  may be formed again. By forming such a circulation path, even if a vicinity of the sensor unit  20  is in a high pressure state, the target gas which is to be sensed and which is injected into the inner space  30  through the opening part  70  may be more easily moved toward the sensor unit  20 . 
       FIG.  2    is a cross-sectional view schematically illustrating the gas sensing device according to a second embodiment of the present disclosure. Referring to  FIG.  2   , a gas sensing device  1 B according to the second embodiment of the present disclosure is different from the gas sensing device  1 A according to the first embodiment in that the sensor unit  20  is disposed in the housing  10  without using a frame. 
     The gas sensing device  1 B according to the second embodiment of the present disclosure includes the housing  10  having the first side provided with the opening part  70 , the sensor unit  20  disposed in the housing  10 , the heater unit  50  for heating the sensor unit  20  to the sensing temperature, and the connection passage  60  connecting the first opening  61  and the second opening  62  that are formed in the housing  10 . 
     The housing  10  may be provided in a hollow tubular shape having an open first side (a lower side of  FIG.  2   ). The open first side forms the opening part  70 , and the second side (an upper side of  FIG.  2   ) may have a structure confined by a cover part  12 . Therefore, except for the opening part  70 , a sealed space blocked from external air is formed in the inner space  30  of the housing  10 . 
     The sensor unit  20  may be disposed at the inner space  30  of the housing  10 , and may be disposed at a position spaced apart from the opposite end portions of the housing  10  in the longitudinal direction (a vertical direction of  FIG.  2   ). The sensor unit  20  may be coupled to the inner space  30  of the housing  10  by a coupling part  23 . The coupling part  23  may be an adhesive material, but is not limited thereto. Further, various coupling means capable of disposing the sensor unit  20  at the inner space  30  of the housing  10  may be used. 
     The inner space  30  of the housing  10  may be divided into the first inner space  31  and the second inner space  32  by a position where the sensor unit  20  is mounted. In the inner space  30  of the housing  10 , the first inner space  31  may be a space between the sensor unit  20  and the opening part  70 . Further, in the inner space  30  of the housing, the second inner space  32  may be a space except for the first inner space  31 . In an embodiment in  FIG.  2   , the first inner space  31  may be an inner space below the sensor unit  20 , and the second inner space  32  may be an inner space above the sensor unit  20 . That is, the second inner space  32  may be a space between the cover part  12  and the sensor unit  20 . Here, the upper side and the lower side of the sensor unit  20  may be divided on the basis of the position of the sensing electrode that is formed at the sensor unit  20 . 
     As exemplarily illustrated in  FIG.  2   , the heater unit  50  is a component for heating the sensor unit  20  to the sensing temperature, and the heater unit  50  may include a resistive heating type heating coil that is wound on an outer portion of the housing  10  where the sensor unit  20  is disposed, but may be provided in various types that are not limited thereto. For example, the heater unit  50  may be disposed inside the housing  10 , and may be provided in a heating pattern shape printed on a predetermined substrate, instead of the heating coil. Otherwise, the heater unit  50  may be provided such that the heater unit  50  is mounted inside the sensor unit  20 . The heater unit  50  is not limited to the resistive heating type, and may be provided as a light irradiation type heater unit such as a heating lamp, an LED, and so on. 
     When the sensor unit  20  does not required to be heated since the gas sensing device  1 B is used in a high temperature environment, the heater unit  50  may be omitted. For example, when the gas sensing device  1 B is used for measuring a concentration of a dissolved gas in a high temperature molten metal, the gas sensing device  1 B may not include the heater unit  50 . 
     The first opening  61  and the second opening  62  are formed in the housing  10 , and the first opening  61  and the second opening  62  are connected to each other by the connection passage  60 . The first opening  61  and the second opening  62  may be formed in the housing  10  such that the first opening  61  is open toward the first inner space  31  and the second opening  62  is open toward the second inner space  32 . Referring to  FIG.  2   , the first opening  61  may be formed below the sensor unit  20 , and the second opening  62  may be formed above the sensor unit  20 . Here, the upper side and the lower side of the sensor unit  20  may be divided on the basis of the position of the sensing electrode that is formed at the sensor unit  20 . 
     The sensor unit  20  is coupled to the housing  10  by the coupling part  23 , but the first inner space  31  and the second inner space  32  may be not completely blocked by the coupling part  23  and the sensor unit  20 . For example, the coupling part  23  may be provided such that the coupling part  23  couples only a partial area among a border of the sensor unit  20  to the inner wall of the housing  10 , so that the gap may be formed between the sensor unit  20  and the inner wall of the housing  10 .  FIG.  3    is a view illustrating the configuration described above, and is a cross-sectional view taken along line A-A in  FIG.  2   . Referring to  FIG.  3   , the coupling part  23  is formed only on four positions in the border of the sensor unit  20 . Further, in a position where the coupling part  23  is not formed, the predetermined gap g may be formed between the sensor unit  20  and the inner wall of the housing  10 . Accordingly, a gas may be freely moved between the first inner space  31  and the second inner space  32 . In addition to the coupling structure as described in  FIG.  3   , the first inner space  31  and the second inner space  32  may be in communication with each other by forming the coupling part  23  with an air-permeable material or by forming a through-hole (not illustrated) in the sensor unit  20 . 
     By this structure, a circulation path into which a gas at the first inner space  31  enters the first opening  61 , the connection passage  60 , the second opening  62 , the second inner space  32 , and the first inner space  31  again may be formed. By forming such a circulation path, even if a vicinity of the sensor unit  20  is in a high pressure state, the target gas which is to be sensed and which is injected into the inner space  30  through the opening part  70  may be more easily moved toward the sensor unit  20 . 
     In the sensor unit  20  used in the gas sensing device  1 A or  1 B according to embodiments of the present disclosure, various gas sensor elements may be used according to a target gas to be sensed, an application use, and so on. According to a target gas to be sensed, a hydrogen sensor element, a carbon monoxide sensor element, a hydrocarbon sensor element, and so on may be used as the sensor unit  20 . In addition, according to the type of the sensor unit  20 , various shapes of sensor elements, such as a pallet shape, a chip shape, a tube shape, and so on, may be used as the sensor unit  20 . In addition, according to a gas detection principle, an electrochemical type sensor element that measures a change in electromotive force (EMF) according to a gas concentration by using a solid electrolyte, a semiconductor type sensor element using a semiconductor material in which an electrical resistance is changed according to a gas concentration, and so on may be used as the sensor unit  20 . 
       FIGS.  4  to  6    are views illustrating a hydrogen sensor element capable of being used as the sensor unit  20  in the present disclosure. 
     A sensor unit  20 A in  FIG.  4    is a hydrogen sensor element specifically suitable for use in a hydrogen sensing device according to the first embodiment ( FIG.  1   ) of the present disclosure. Referring to  FIG.  4   , the sensor unit  20 A may include a solid electrolyte having a heterojunction structure in which an oxygen ion conductor  211  and a hydrogen ion conductor  212  are joined, a reference electrode  213  formed on a surface of the oxygen ion conductor  211 , and a sensing electrode  214  formed on a surface of the hydrogen ion conductor  212 . 
     As the oxygen ion conductor  211 , CeO 2  based compounds prepared by adding solid electrolyte or Gd 2 O 3  may be used, such as stabilized zirconia prepared by adding various substances to zirconia (ZrO 2 ), for example, Yttria stabilized zirconia (YSZ), calcium stabilized zirconia (CSZ), and magnesium stabilized zirconia (MSZ). As the hydrogen ion conductor  212 , substances acquired by substituting a B position of a substance having an ABO 3  type perovskite structure with various substances, for example, CaZrO 3  based compounds including CaZr 0.9 In 0.1 O 3−x , and the like, SrZrO 3  based compounds including SrZr 0.95 Y 0.05 O 3−x , and the like, BaCeO 3  based compounds including BaCe 0.9 Nd 0.1 O 3−x , and the like, and Ti based compounds including BaTiO 3 , SrTiO 3 , PbTiO 3 , and the like may be used. 
     In addition, the reference electrode  213  and the sensing electrode  214  may be formed of precious metal such as platinum (Pt), or the like. 
     The reference electrode  213  and the sensing electrode  214  are electrically connected to a measuring unit  90  through a lead wire, so that a concentration of a hydrogen gas may be measured by measuring an electromotive force. The electromotive force E measured between the reference electrode  213  and the sensing electrode  214  establishes the following relationship with oxygen partial pressure P O2  at the reference electrode  213  side and hydrogen partial pressure P H2  at the sensing electrode  214  side. 
         E=Eo+A ·log  P   H2 +( A/ 2)·log P   O2    (1)
 
     Since Eo and A are constants that depend on only a temperature in the equation, consequently, when the oxygen partial pressure P O2  at the reference electrode  213  side is known, it can be seen that the hydrogen partial pressure P H2  at the sensing electrode  214  side may be decided by measuring the electromotive force E. 
     The oxygen partial pressure P O2  at the reference electrode  213  side may be fixed by exposing the reference electrode  213  to the atmosphere. That is, referring to  FIG.  1    and  FIG.  4    together, by configuring the gas sensing device  1 A such that the frame  22  fixed to the sensor unit  20 A is gas-tightly coupled to the housing  10  by using the sealing material  21  and the reference electrode  213  is exposed to the atmosphere, the oxygen partial pressure P O2  at the reference electrode  213  side may be fixed to 0.21 atmospheric pressure which is the oxygen partial pressure in the air. Therefore, when the electromotive force E between the reference electrode  213  and the sensing electrode  214  is measured, the hydrogen partial pressure P H2  at the sensing electrode  214  side may be calculated by the Equation (1). 
     Instead of fixing the oxygen partial pressure P O2  by exposing the reference electrode  213  to the atmosphere, a sensor unit  20 B in  FIG.  5    has a structure in which the oxygen partial pressure P O2  is thermodynamically fixed by covering the reference electrode  213  with an oxygen partial pressure fixing reference substance  215 . Therefore, the sensor unit  20 B in  FIG.  5    is different from the sensor unit  20 A in  FIG.  4   . 
     As the oxygen partial pressure fixing reference substance  215 , mixtures of metal and metal oxides, such as Cu/CuO, Ni/NiO, Ti/TiO 2 , Fe/FeO, Cr/Cr 2 O 3 , Mo/MoO, and the like or mixtures of metal oxides having different oxidation degrees, such as Cu 2 O/CuO, FeO/Fe 2 O 3 , and the like may be used. Further, when the reference electrode  213  is covered with the oxygen partial pressure fixing reference substance  215 , the oxygen partial pressure at the reference electrode  213  side may be thermodynamically fixed. That is, the oxygen partial pressure at the reference electrode  213  side is decided by the oxygen partial pressure fixing reference substance  215  instead of the external air and similarly to the description referring to  FIG.  4   , the hydrogen partial pressure at the sensing electrode  214  side may be decided by the Equation (1) by measuring the electromotive force between the reference electrode  213  and the sensing electrode  214 . 
     The sensor unit  20 B in  FIG.  5    may be suitable for either the gas sensing device  1 A according to the first embodiment of the present disclosure or the gas sensing device  1 B according to the second embodiment of the present disclosure. In order to prevent the oxygen partial pressure fixing reference substance  215  from being affected by external air or a gas atmosphere in the inner space  30 , a sealing cover  218  for blocking the oxygen partial pressure fixing reference substance  215  from external air may be additionally provided. The sealing cover  218  may be formed of a dense ceramic material capable of preventing penetration of external air, and the like. Further, when the influence of external air is insignificant, the sealing cover  218  may be omitted. 
     A sensor unit  20 C in  FIG.  6    includes the hydrogen ion conductor  212 , the reference electrode  213  and the sensing electrode  214  that are respectively formed on opposite surfaces of the hydrogen ion conductor  212 , and a hydrogen partial pressure fixing reference substance  216  covering the reference electrode  213 . That is, comparing to the sensor unit  20 B in  FIG.  5   , the sensor unit  20 C in  FIG.  6    has a difference in that the sensor unit  20 C does not include the oxygen ion conductor and the reference electrode  213  is covered with the hydrogen partial pressure fixing reference substance  216  instead of the oxygen partial pressure fixing reference substance. 
     As the hydrogen partial pressure fixing reference substance  216 , mixed phases of metal and metal hydrides, such as Ti/TiH 2 , Zr/ZrH 2 , Ca/CaH 2 , Nd/NdH 2 , and the like may be used. Therefore, a hydrogen partial pressure P 2, H2  at the reference electrode  213  side may be thermodynamically fixed by the mixed phases. 
     In the sensor unit  20 C in  FIG.  6   , when the electromotive force E between the reference electrode  213  and the sensing electrode  214  is measured, a hydrogen partial pressure P 1, H2  at the sensing electrode  214  side may be decided by a Nernst equation given below. 
         E=− ( RT/ 2 F ) ln ( P   2, H2 )   (2)
 
     In Equation (2) given above, R represents a gas constant, F represents a Faraday constant, and T represents the sensing temperature and all of R, F, and T are constants. Further, since the hydrogen partial pressure P 2, H2  at the reference electrode  213  side is also a value decided by the hydrogen partial pressure fixing reference substance  216 , the hydrogen partial pressure P 1, H2  at the sensing electrode  214  side may be calculated from the measured electromotive force E value. 
     The sensor unit  20 C in  FIG.  6    may be suitable for either the gas sensing device  1 A according to the first embodiment of the present disclosure or the gas sensing device  1 B according to the second embodiment of the present disclosure. In order to prevent the hydrogen partial pressure fixing reference substance  216  from being affected by external air or a gas atmosphere in the inner space  30 , the sealing cover  218  for blocking the hydrogen partial pressure fixing reference substance  216  from external air may be additionally provided. 
     The gas sensing device according to embodiments of the present disclosure may be used for measuring a concentration of a dissolved gas in a liquid. 
       FIG.  7    is a view illustrating an example of use in which the gas sensing device of the present disclosure is used for measuring a concentration of a dissolved gas in a liquid stored in a liquid storage container  100 . In  FIG.  7   , although a structure of a gas sensing device  1  is briefly illustrated mainly in main components, all of the gas sensing device  1 A according to the first embodiment of the present disclosure or the gas sensing device  1 B according to the second embodiment of the present disclosure may be used. 
     Referring to  FIG.  7   , the gas sensing device  1  according to the present disclosure may be coupled to the liquid storage container  100  via a connector  110 . As illustrated in the drawing, the connector  110  may be formed on a side surface of the liquid storage container  100 , but is not limited thereto. Further, the connector  110  may be formed on various positions such as an upper surface of the liquid storage container  100 , a lower surface of the liquid storage container  100 , and so on. When the gas sensing device  1  is coupled to the liquid storage container  100  via the connector  110 , an inner portion of the liquid storage container  100  and the inner space  30  of the gas sensing device  1  are in communication with each other, and a dissolved gas in a liquid stored in the liquid storage container  100  may evaporate and be moved to the inner space  30 . 
     Generally, a solubility of a gas in a liquid is in accordance with Sievert&#39;s law in Equation (3). 
         C=k·P   gas    (3)
 
     C is a concentration of a dissolved gas in a liquid, k is a constant that depends on the type of gas, a temperature, and so on, and P gas  is a gas partial pressure in a space in contact with a liquid. That is, the concentration C of the dissolved gas and the gas partial pressure P gas  in the space in contact with the liquid are in proportion to each other, and reach an equilibrium state when an evaporation speed of the gas that is evaporated from the liquid to the space is equal to a speed at which the gas inside the space is dissolved in the liquid. 
     According to this principle, when the gas sensing device  1  of the present disclosure is connected to the liquid storage container  100  as illustrated in  FIG.  7   , the dissolved gas in the liquid that is stored in the liquid storage container  100  is evaporated in the inner space  30  of the gas sensing device  1 , and the equilibrium state is realized. Therefore, by measuring the concentration (gas partial pressure) of the gas in the inner space  30  by using the sensor unit  20 , the concentration of the dissolved gas in the liquid may be known. 
     Meanwhile, according to a pressure of the liquid in the liquid storage container  100 , a pressure inside the inner space  30  of the gas sensing device  1  that is connected to the liquid storage container  100  may increase. That is, the liquid stored in the liquid storage container  100  is moved to the inner space  30  of the gas sensing device  1  via the connector  110 , so that a liquid surface  120  may be formed in the inner space  30  of the gas sensing device  1 . Accordingly, a volume of the gas in the inner space  30  of the gas sensing device  1  may shrink until the pressure inside the inner space  30  is equal to the pressure of the liquid. 
     Due to such an increase in pressure inside the inner space  30 , the gas evaporated from the liquid may be difficult to be moved to the sensing electrode  214  of the sensor unit  20 . Particularly, when the sensor unit  20  is heated to the sensing temperature by using the heater unit  50 , the pressure inside the inner space  30  is further increased, so that the dissolved gas may be further difficult to be moved to the sensing electrode  214 . This situation may be an obstacle factor when the concentration of the dissolved gas is rapidly and accurately measured. 
     However, in the gas sensing device  1  according to the present disclosure, since the gas in the inner space  30  may be circulated along the connection passage  60  that connects the first opening  61  and the second opening  62  that are respectively open toward the first inner space  31  and the second inner space  32 , the gas injected through the opening part  70  may be smoothly moved to the sensing electrode  214  even if the inner space  30  is in the high pressure state. Accordingly, the concentration of the dissolved gas is capable of being rapidly and accurately measured. 
     In  FIG.  7   , preferably, the liquid surface  120  is formed below the first opening  61 , but the liquid surface  120  may be formed above the first opening  61 . That is, even if the first opening  61  is submerged in the liquid, the dissolved gas evaporated from the liquid may be moved toward the second opening  62  through the connection passage  60 . However, since the liquid surface  120  is required to be formed below the sensor unit  20 , it is preferable that a length of the housing  10  and/or a height of the connector  110  is designed in consideration of a use environment such as a liquid pressure and so on. Optionally, a gas permeating filter may be disposed at the connector  110  or the first inner space  31  so as to prevent the sensor unit  20  from being contaminated by the liquid. If the gas permeating filter has a configuration in which a liquid does not pass through the gas permeating filter and only a gas can pass through the gas permeating filter, a material or a shape of the gas permeating filter is not particularly limited. 
     For example, the gas permeating filter may be a gas permeating layer manufactured by including graphite, ceramic powder, a PTFE membrane, or the like. 
       FIG.  8    is a view illustrating another example of use in which the gas sensing device of the present disclosure is used for measuring a concentration of a dissolved gas in a liquid stored in a liquid storage container  300 . The liquid storage container  300  may be a transformer, and a liquid in the liquid storage container  300  may be an insulating oil of the transformer. 
     Referring to  FIG.  8   , a circulation pipe  310  for circulating a liquid is connected to the liquid storage container  300 . The circulation pipe  310  is provided with valves  313  and  314  and a circulation motor  320 . Further, in a state in which the valves  313  and  314  are open, when the circulation motor  320  is driven, a circulation path through which the liquid is circulated in a direction indicated by arrows in the drawing is formed. 
     A measurement tank  330  is provided on the circulation path, and the liquid circulated along the circulation pipe  310  is passing through the measurement tank  330 . That is, the circulating liquid may be temporarily stored in the measurement tank  330 . 
     The gas sensing device  1  according to the present disclosure may be coupled to the measurement tank  330  via the connector  340 . Accordingly, the dissolved gas evaporated from the liquid in the measurement tank  330  may be moved to the inner space  30  of the gas sensing device  1 . In order for preventing the liquid from moving to the gas sensing device  1 , a gas permeating filter  341  may be provided in the connector  340 . Either the gas sensing device  1 A according to the first embodiment of the present disclosure or the gas sensing device  1 B according to the second embodiment of the present disclosure may be used as the gas sensing device  1 . 
       FIG.  9    is a graph illustrating a measuring result of the electromotive force E of the sensor unit  20  after the gas sensing device  1 A according to the first embodiment of the present disclosure is connected with a measurement environment in which a hydrogen concentration is constantly maintained at 4%. At this time, the hydrogen sensor element in  FIG.  4    was used as the sensor unit  20 . That is, an electro-chemical hydrogen sensor in which the oxygen ion conductor  211  and the hydrogen ion conductor  212  are joined and the reference electrode  213  is formed on the oxygen ion conductor  211  and the sensing electrode  214  is formed on the hydrogen ion conductor  212  was used as the sensor unit  20 . As the oxygen ion conductor  211  and the hydrogen ion conductor  212 , Yttria stabilized zirconia (YSZ) and the CaZr 0.9 In 0.1 O 3−x  were respectively used, and the reference electrode  213  and the sensing electrode  214  were formed of platinum (Pt). 
     The sensor unit  20  was heated by using the heater unit  50  while measuring a temperature by connecting a thermocouple to the sensor unit  20 , and changes in temperature and electromotive force with time were measured. As can be seen in  FIG.  9   , the electromotive force continuously increases as the temperature increases. Further, a stable electromotive force of about 1.1 V was measured at a temperature of about at least 300° C. 
       FIG.  10    is a graph illustrating a measuring result of the electromotive force E under the same condition by using a gas sensing device in which the connection passage  60  is not formed for comparison. Except that the gas sensing device used in  FIG.  10    does not have the connection passage  60  and the first and second openings  61  and  62 , the gas sensing device used in  FIG.  10    was the same as the gas sensing device used in  FIG.  9   . Referring to  FIG.  10   , the electromotive force tends to increase as the temperature increases. However, comparing to the result in  FIG.  9   , it can be seen that the measured electromotive force value is very low and unstable. 
     From the result described above, it can be seen that the rapid and accurate measurement of the concentration of the gas may be realized when the gas sensing device of the present disclosure is used. 
     The present disclosure has been described with reference to the limited embodiments and drawings hereinabove, but it is apparent to those skilled in the art that various modifications can be made within the scope of the technical spirit of the present disclosure. For example, the connection passage included in the gas sensing device of the present disclosure may include a plurality of connection passages. Accordingly, a protection scope of the present disclosure should be determined according to disclosures of claims and a scope equivalent thereto.