Patent Publication Number: US-2021180237-A1

Title: Steam generator and clothes care apparatus having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2019-0166131, filed on Dec. 12, 2019 in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     The disclosure relates to a steam generator including a water level sensor with an improved structure, and a clothes care apparatus having the steam generator. 
     2. Description of the Related Art 
     A clothes care apparatus is equipment for clothes care, such as drying wet clothes, removing dust gathered on clothes or smell permeated in clothes, and smoothing out the wrinkles of clothes. 
     The clothes care apparatus includes a heat exchanger for supplying hot air to a care room in which clothes are accommodated to dry the clothes, and a steam generator for performing a refresh function, such as wrinkle removal, odor removal, and static electricity removal of clothes, etc. 
     The heat exchanger and the steam generator are installed in a machine room, and the machine room is positioned in the lower space of the care room. 
     The steam generator includes a water level sensor for sensing a level of water stored therein, and a heater for heating the water. 
     However, the water stored in the steam generator may be hard water containing a large amount of minerals. Heat applied to the hard water by the heater may form scale. Such scale formed on the water level sensor may cause wrong detections of the water level sensor. In this case, the water level sensor may wrongly recognize a low water level as a high water level so that the heater operates, or wrongly recognize a high water level as a low water level, so that water continues to be supplied to the steam generator causing it to overflow. 
     SUMMARY 
     Therefore, it is an aspect of the disclosure to provide a steam generator having a water level sensor for preventing electric conduction between electrodes by separating the electrodes from each other, and a clothes care apparatus including the steam generator. 
     It is another aspect of the disclosure to provide a steam generator having a water level sensor for preventing formation of scale through an air cap structure, and a clothes care apparatus including the steam generator. 
     It is another aspect of the disclosure to provide a steam generator for preventing loss of components by increasing exposed portions of electrodes, and a clothes care apparatus including the steam generator. 
     Additional aspects of the disclosure will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the disclosure. 
     In accordance with an aspect of the disclosure, a clothes care apparatus includes: a main body including a care room accommodating clothes and configured to care the clothes; and a steam generator configured to generate a steam to be supplied to the care room, and including a water level sensor configured to sense an inside water level, wherein the steam generator includes a case, and the water level sensor includes: a first housing coupled to the case; a second housing coupled to the case at the same height as the first housing; a first electrode supported by the first housing; and a second electrode supported by the second housing and separated from the first electrode. 
     The first electrode and the second electrode may be provided without any coating to prevent a wrong detection caused by scales. 
     The first electrode may be a low water level electrode configured to sense a low water level inside the case, and the second electrode may be a high water level electrode configured to sense a high water level inside the case, a length of the high water level electrode being shorter than a length of the low water level electrode. 
     The clothes care apparatus may further include a common electrode supported by the first housing together with the first electrode. 
     The second housing may be symmetrical to the first housing on the same line as the first housing with respect to the case. 
     The water level sensor may further include: a first air cap coupled to the first housing and accommodating a portion of the first electrode; and a second air cap coupled to the second housing and accommodating a portion of the second electrode. 
     The case may include a plurality of partition walls respectively being adjacent to the first air cap and the second air cap and extending upward from a bottom of the case. 
     An air pocket may be formed in a space of the first air cap in which the portion of the first electrode is accommodated and a space of the second air cap in which the portion of the second electrode is accommodated, to prevent water from entering the spaces. 
     The water level sensor may further include a plurality of sealing members respectively positioned between the first housing and the first air cap and between the second housing and the second air cap to form the air pocket. 
     The first air cap may include: a first flange being in a shape corresponding to a lower surface of the first housing; and a plurality of partition walls extending toward a bottom of the case from the first flange to form an air pocket. 
     The second air cap may include: a second flange being in a shape corresponding to a lower surface of the second housing; and a plurality of partition walls extending toward a bottom of the case from the second flange to form an air pocket. 
     A length of the first electrode exposed to outside of the first air cap may be longer than 3 mm and shorter than 15 mm. 
     The case may include a first side surface, and a second side surface being opposite to the first side surface with respect to a center line, the first housing may be coupled to the case between the first side surface and the center line, and the second housing may be coupled to the case between the second side surface and the center line. 
     The case may include: a third side surface connected to the first side surface and the second side surface; and a fourth side surface being opposite to the third side surface with respect to a reference line being perpendicular to the center line, wherein the first housing may be coupled to the case between the first side surface, the third side surface, the center line, and the reference line, and the second housing may be coupled to the case between the second side surface and the center line such that the second housing is on the reference line and diagonal to the first housing. 
     The water level sensor may further include a third housing supporting the third electrode, aligned with the first housing, and coupled to the case between the first side surface and the center line, the first electrode may be a low water level electrode configured to sense a low water level inside the case, the second electrode may be a high water level electrode configured to sense a high water level inside the case, and the third electrode may be a common electrode. 
     In accordance with another aspect of the disclosure, a steam generator includes: a case; and a water level sensor configured to sense a water level inside the case, wherein the water level sensor includes: a first housing coupled to one side of the case; a second housing coupled to the other side of the case, the other side being opposite to the one side of the case with respect to a center line of the case; a first electrode supported by the first housing; a second electrode supported by the second housing; a first air cap coupled to the first housing and accommodating a portion of the first electrode; and a second air cap coupled to the second housing and accommodating a portion of the second electrode. 
     The first air cap may include a first air pocket configured to prevent water from entering a space in which the portion of the first electrode is accommodated, and the second air cap may include a second air pocket configured to prevent water from entering a space in which the portion of the second electrode is accommodated. 
     The case may include a plurality of partition walls respectively positioned below the first air cap and the second air cap and extending upward from a bottom of the case. 
     The first electrode may be a low water level electrode configured to sense a low water level inside the case, and the second electrode may be a high water level electrode configured to sense a high water level inside the case, a length of the high water level electrode being shorter than a length of the low water level electrode. The steam generator may further include a common electrode supported by the first housing together with the low water level electrode. 
     The first housing and the second housing may be coupled at the same height with respect to the case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the disclosure will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a perspective view of a clothes care apparatus according to an embodiment of the disclosure; 
         FIG. 2  shows a state in which a door opens in a clothes care apparatus according to an embodiment of the disclosure; 
         FIG. 3  is a side cross-sectional view of a clothes care apparatus according to an embodiment of the disclosure; 
         FIG. 4  is an exploded perspective view of a clothes care apparatus according to an embodiment of the disclosure; 
         FIG. 5  is a perspective view of a steam generator according to a first embodiment of the disclosure; 
         FIG. 6  shows a cross section taken along line A-A′ of  FIG. 5 ; 
         FIG. 7  shows a cross section taken along line B-B′ of  FIG. 5 ; 
         FIG. 8  is an exploded perspective view of a first water level sensor according to a first embodiment of the disclosure in the steam generator according to the first embodiment of the disclosure; 
         FIG. 9  is an exploded perspective view of a second water level sensor according to a first embodiment of the disclosure, in the steam generator according to the first embodiment of the disclosure; 
         FIG. 10  is a front view showing a coupled state of the first water level sensor according to the first embodiment of the disclosure, in the steam generator according to the first embodiment of the disclosure; 
         FIG. 11  is a top view of the steam generator according to the first embodiment of the disclosure; 
         FIG. 12  is a front view of a first water level sensor according to a second embodiment of the disclosure, in the steam generator according to the first embodiment of the disclosure; 
         FIG. 13  is a front view of a first water level sensor according to a third embodiment of the disclosure, in the steam generator according to the first embodiment of the disclosure; 
         FIG. 14  is a front view of a first water level sensor according to a fourth embodiment of the disclosure, in the steam generator according to the first embodiment of the disclosure; 
         FIG. 15  is a top view of a steam generator according to a second embodiment of the disclosure; 
         FIG. 16  is a top view of a steam generator according to a third embodiment of the disclosure; 
         FIG. 17  is a top view of a steam generator according to a fourth embodiment of the disclosure; 
         FIG. 18  is a front view showing a coupled state of a first water level sensor according to a fifth embodiment of the disclosure with a third water level sensor according to a first embodiment of the disclosure, in the steam generator according to the fourth embodiment of the disclosure; 
         FIG. 19  is a front view showing a first water level sensor according to a sixth embodiment of the disclosure and a third water level sensor according to a second embodiment of the disclosure, in the steam generator according to the fourth embodiment of the disclosure; 
         FIG. 20  is a front view showing a first water level sensor according to a seventh embodiment of the disclosure and a third water level sensor according to a third embodiment of the disclosure, in the steam generator according to the fourth embodiment of the disclosure; 
         FIG. 21  is a front view showing a first water level sensor according to an eighth embodiment of the disclosure and a third water level sensor according to a fourth embodiment of the disclosure, in the steam generator according to the fourth embodiment of the disclosure; 
         FIG. 22  is a top view of a steam generator according to a fifth embodiment of the disclosure; and 
         FIG. 23  is a front view of a water level sensor according to a ninth embodiment of the disclosure, in a steam generator according to a sixth embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Configurations illustrated in the embodiments and the drawings described in the present specification are only the preferred embodiments of the disclosure, and thus it is to be understood that various modified examples, which may replace the embodiments and the drawings described in the present specification, are possible when filing the present application. 
     Also, like reference numerals or symbols denoted in the drawings of the present specification represent members or components that perform the substantially same functions. 
     The terms used in the present specification are merely used to describe the embodiments, and are not intended to limit the disclosure. An expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. 
     In the present specification, it is to be understood that the terms such as “including” or “having,” etc., are intended to indicate the existence of the features, numbers, operations, components, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, operations, components, parts, or combinations thereof may exist or may be added. 
     It will be understood that, although the terms “first”, “second”, etc., may be used herein to describe various components, these components should not be limited by these terms. The above terms are used only to distinguish one component from another. 
     For example, a first component discussed below could be termed a second component, and similarly, a second component may be termed a first component without departing from the teachings of this disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     In the following description, the terms “front direction”, “rear direction”, “upper portion”, “lower portion”, etc. are defined based on the drawings, and the shapes and positions of the corresponding components are not limited by the terms. 
     Throughout the disclosure, the expression “at least one of a, b or c” indicates only a, only b, only c, both a and b, both a and c, both b and c, all of a, b, and c, or variations thereof. 
     Hereinafter, embodiments of the disclosure will be described in detail with reference to the appended drawings. 
       FIG. 1  is a perspective view of a clothes care apparatus according to an embodiment of the disclosure, and  FIG. 2  shows a state in which a door opens in a clothes care apparatus according to an embodiment of the disclosure. 
     As shown in  FIGS. 1 and 2 , a clothes care apparatus  1  may include a main body  10  forming an outer appearance, and a door  20  rotatably coupled to the main body  10 . 
     The main body  10  may be substantially in a shape of a hexahedron of which one side opens. In a front side of the main body  10 , an opening  10   a  may be formed. On the main body  10 , the door  20  rotatably coupled to the main body  10  to open and close a care room  30  may be mounted. 
     The door  20  may be mounted on the main body  10  through a hinge or link, although not shown in the drawings. 
     The main body  10  may include the care room  30  formed inside the main body  10  and accommodating clothes to care the clothes. The care room  30  may open at the front side. The care room  30  may be opened or closed by the door  20  for opening and closing the opening  10   a.    
       FIG. 3  is a side cross-sectional view of the clothes care apparatus  1  according to an embodiment of the disclosure, and  FIG. 4  is an exploded perspective view of the clothes care apparatus  1  according to an embodiment of the disclosure. 
     As shown in  FIGS. 3 and 4 , the main body  10  may include an external cabinet  11  and an internal cabinet  12  positioned inside the external cabinet  11 . The main body  10  may include a supporting member  50  installed inside the care room  30  to hang clothes. 
     The main body  10  may include a machine room  40  accommodating a heat exchanger  60 , etc. for dehumidifying and heating inside air of the care room  30 . 
     The care room  30  may form a space for accommodating clothes. The care room  30  may be defined by a top plate  12   a,  a bottom plate  12   b,  a left plate  12   c,  a right plate  12   d,  and a rear plate  12   e  that constitute the internal cabinet  12 . 
     The internal cabinet  12  may include a frame  13  for supporting the top plate  12   a,  the bottom plate  12   b,  the left plate  12   c,  the right plate  12   d,  and the rear plate  12   e.    
     The frame  13  may define the care room  30  and the machine room  40  positioned below the care room  30 , although not limited thereto. 
     The supporting member  50  may be installed in the top plate  12   a  of the care room  30 . The supporting member  50  may be separable from the care room  30 . At least one supporting member  50  may be provided. The supporting member  50  may be in a shape of a hanger to hang clothes, although not limited thereto. 
     The supporting member  50  may enable air to flow therein. Dust or foreign materials collected in clothes may be removed by air supplied to the inside of the supporting member  50 . 
     In the supporting member  50 , an air hole  51  for supplying air to the clothes may be formed. The air hole  51  may be formed at an upper end of the supporting member  50 , and air may be supplied to the clothes through the air hole  51 , although not limited thereto. 
     The air hole  51  may be formed with various sizes at various locations to spray supplied air over a wide area of the clothes. 
     The care room  30  may include a first inlet  31   a,  a second inlet  32   a,  a first outlet  31   b,  a second outlet  32   b,  and a steam inlet  33 . 
     The first inlet  31   a  and the first outlet  31   b  may be formed in the bottom plate  12   b  of the care room  30 . The first inlet  31   a  may be positioned at a rear portion of the bottom plate  12   b  of the care room  30 . The first outlet  31   b  may be positioned at a front portion of the bottom plate  12   b  of the care room  30 . The first inlet  31   a  may be adjacent to the first outlet  31   b.    
     The steam inlet  33  may be positioned at a lower portion of the rear plate  12   e  of the care room  30 . The steam inlet  33  may be positioned above the first inlet  31 . 
     The second inlet  32   a  may be formed above the top plate  111   e  or  12   a  of the care room  30 . The second outlet  32   b  may be formed at a center of the rear plate  12   e  of the care room  30 . The second inlet  32   a  may be adjacent to the second inlet  32   b.    
     The second inlet  32   a  of the care room  30  may be connected to the supporting member  50 . Air entered through the second inlet  32   a  may be transferred to the supporting member  50  through the air hole  51  and transferred to clothes hung on the supporting member  50 . 
     In a lower portion of the main body  10 , a drain container  61  and a water supply container  101  that are separable from the main body  10  may be installed. The drain container  61  and the water supply container  101  may be positioned below the care room  30 . 
     The drain container  61  may be used to easily process condensed water generated by the heat exchanger  60 . The water supply container  101  may store water required for a steam generator  100  to generate steam. 
     The water stored in the water supply container  101  may be supplied to the steam generator  100  to be used to generate a steam. The water supply container  101  may be separable from the main body  10  to easily add water. 
     The drain container  61  and the water supply container  101  may be installed in a front area of the machine room  40 . The machine room  40  may be positioned in the lower portion of the main body  10 . The machine room  40  may be positioned below the care room  30 . 
     A plurality of hoses  68  may be provided to be respectively used in the heat exchanger  60  or the steam generator  100 . 
     The heat exchanger  60  may dehumidify and heat inside air of the care room  30  as necessary. 
     The heat exchanger  60  may supply hot air to the inside of the care room  30 . The heat exchanger  60  may include an evaporator  63 , a condenser  64 , and a compressor  65 , through which a refrigerant circulates. The heat exchanger  60  may dehumidify and heat air. 
     In the evaporator  63  of the heat exchanger  60 , the refrigerant may evaporate to absorb latent heat from surrounding air to condense and remove moisture in the air. 
     The refrigerant passed through the compressor  65  and condensed in the condenser  64  may emit latent heat toward surrounding air to thereby heat the surrounding air. 
     The evaporator  63  and the condenser  64  may perform a heat exchange function so that air entered the machine room  40  by a first fan  62  may pass through the evaporator  63  and the condenser  64 , sequentially, to be dehumidified and heated. 
     The heat exchanger  60  installed in the machine room  40  may include a first duct  66  connecting the evaporator  63 , the condenser  64 , and the first fan  62  to each other. The first duct  66  may be connected to the care room  30  to form a first flow path  67  to circulate air between the care room  30  and the first duct  66 . 
     The first duct  66  may be connected to the first inlet  31   a  and the first outlet  31   b  of the care room  30 . One end of the first duct  66  may be connected to the first inlet  31   a,  and the other end of the first duct  66  may be connected to the first outlet  31   b.    
     Air of the care room  30  may be discharged to the first duct  66  via the first outlet  31   b,  and the discharged air may be dehumidified and again enter the care room  30  via the first inlet  31   a.    
     The first inlet  31   a  may be positioned in a rear area of the care room  30 , and the first outlet  31   b  may be positioned in a front area of the care room  30 , although not limited thereto. The first inlet  31   a  and the first outlet  31   b  may be positioned at different locations as necessary. 
     The first duct  66  may dehumidify air discharged from the care room  30  through the first outlet  31   b,  and cause the dehumidified air to enter the care room  30  through the first inlet  31   a.  The first fan  62  may be positioned on the first duct  66  to inhale inside air of the care room  30  to the inside of the first duct  66 . 
     In the machine room  40 , the steam generator  100  may be provided to receive water from the water supply container  101  and generate steam. 
     The steam generator  100  may be connected to the water supply container  101  to receive water and generate a stream, and include a steam supply pipe  104  for guiding the generated steam to a steam spray portion  103 . The steam spray portion  103  may be positioned at the lower portion of the rear plate  12   e  of the care room  30 . 
     An end of the steam spray portion  103  may be in a shape of a nozzle to smoothly spray a steam into an inside space of the care room  30 . The steam spray portion  103  may be exposed to the inside of the care room  30 . 
     The care room  30  may include a blower  80  for causing inside air of the care room  30  to flow. The blower  80  may include a second duct  81 , and a second fan  82  may be installed inside the second duct  81 . 
     The second duct  81  may communicate with the care room  30  and form a second flow path  83  to circulate air between the care room  30  and the second duct  81 . The second fan  82  may be positioned on the second flow path  83 . 
     The second duct  81  may be positioned behind the second outlet  32   b  of the care room  30 . The second duct  81  may be positioned at an upper portion of the rear plate  12   e  of the care room  30 , and include a filter  90  therein. 
     The filter  90  may be a High Efficiency Particulate Air (HEPA) filter, although not limited thereto. 
     The second duct  81  may be coupled to a top cover  91  positioned above the care room  30 . The blower  80  may be positioned at a rear area above the care room  30 , and include a motor  84  for generating a rotating force, and at least one second fan  82  rotating by the motor  84 . 
     The motor  84  may include a shaft extending at both sides, and a pair of second fans  82  may be respectively coupled to both ends of the shaft. Through this structure, the pair of second fans  82  may rotate by the motor  84 . 
     The pair of second fans  82  may be centrifugal fans that inhale air in their shaft directions and discharge the inhaled air outward in their radial directions, although not limited thereto. 
     The second fan  82  may be accommodated in a fan case  85 . The fan case  85  may be coupled to a duct bracket  86  formed on the top plate  12   a  of the care room  30 . 
     In the duct bracket  86 , at least one duct hole  86   a  may be formed. The second fans  82  may be coupled to the at least one duct hole  86   a  to move inside air of the second duct  81  to the second inlet  32   a.    
     The second duct  81  may be connected to the second inlet  32   a  and the second outlet  32   b  of the care room  30 . One end of the second duct  81  may be connected to the second inlet  32   a,  and the other end may be connected to the second outlet  32   b  of the care room  30 . 
     The second inlet  32   a  may be connected to the supporting member  50  to transfer inside air of the second duct  81  to the supporting member  50 . 
     The second fans  82  positioned inside the second duct  81  may inhale inside air of the care room  30  through the second outlet  32   b  and again discharge the air to the care room  30  through the second inlet  32   a.    
     On the rear plate  12   e  of the care room  30 , a filter installing portion may be provided to install the filter  90  thereon. The second outlet  32   b  may be positioned to correspond to the filter installing portion. 
     Inside air of the care room  30 , which is discharged to the second duct  81 , may be filtered by the filter  90  of the second outlet  32   b.  Dust and smell included in the air discharged to the second duct  81  may be removed by the filter  90 . 
     The air filtered by the filter  90  may be discharged to the supporting member  50  through the blower  80 . The filter  90  may include a dust filter (not shown) for removing dust or a device for deodorization. 
     For clothes caring, after clothes are hung on the supporting member  50  and the door  20  is closed, the care room  30  may operate. Inside air of the care room  30  may circulate along the first flow path  67  and the second flow path  83 . 
       FIG. 5  is a perspective view of a steam generator according to a first embodiment of the disclosure.  FIG. 6  shows a cross section taken along line A-A′ of  FIG. 5 .  FIG. 7  shows a cross section taken along line B-B′ of  FIG. 5 . 
     Referring to  FIGS. 5 to 7 , the steam generator  100  may include a case  110  for storing water therein. The case  110  may form an outer appearance of the steam generator  100 . 
     The case  110  may receive water from the water supply container  101  (see  FIG. 3 ) through the hose  68 . 
     The case  110  may include an upper case  111  and a lower case  112 . The upper case  111  may be positioned above the upper case  112 . The upper case  111  may be separable from the lower case  112 . Water may be stored in the lower case  112 . 
     The case  110  may have a capacity capable of accommodating a predefined amount of water therein. The case  110  may be substantially in a shape of a rectangular parallelepiped, although not limited thereto. 
     The upper case  111  may include a water supply portion  113  and a discharge portion  114 . The water supply portion  113  may be connected to the hose  68  to enable water to enter the upper case  111 . The discharge portion  114  may be connected to the steam supply pipe  104  to supply a steam generated by heating water entered the inside of the case  110  by the heater  120  to the care room  30 . 
     Referring to  FIGS. 5 and 11 , the upper case  111  may include a first side surface  111   a,  a second side surface  111   b,  a third side surface  111   c,  a fourth side surface  111   d,  and a top plate  111   e  connected to the plurality of side surfaces  111   a,    111   b,    111   c,  and  111   d.  The discharge portion  114  may be positioned on the top plate  111   e  of the upper case  111 , although not limited thereto. 
     The first side surface  111   a  may be opposite to the second side surface  111   b  with respect to a center line L 1 . The third side surface  111   c  may be connected to the first side surface  111   a  and the second side surface  111   b.  The fourth side surface  111   d  may be connected to the first side surface  111   a  and the second side surface  111   b,  and may be opposite to the third side surface  111   c  with respect to a reference line L 2 . 
     The center line L 1  and the reference line L 2  may pass an intersecting point C of the steam generator  100  and may be perpendicular to each other. 
     The center line L 1  may be parallel to the first side surface  111   a  and the second side surface  111   b  and pass a geometric center of the steam generator  100 . The reference line L 2  may be parallel to the third side surface  111   c  and the fourth side surface  111   d  and pass centers of water level sensors of the steam generator  100 . 
     The top plate  111   e  of the upper case  111  may include a first coupling portion  117  and a second coupling portion  118  to which the water level sensors are coupled. The water level sensors may include a first water level sensor  200  and a second water level sensor  300 . 
     The first water level sensor  200  may be coupled to the first coupling portion  117 , and the second water level sensor  300  may be coupled to the second coupling portion  118 . The first water level sensor  200  and the second water level sensor  300  may be coupled at the same height with respect to the case  110 . 
     Referring to  FIGS. 5 to 7 , the first water level sensor  200  may include a first electrode  210  and a third electrode  220  (see  FIG. 8 ). The second water level sensor  300  may include a second electrode  310  (see  FIG. 9 ). The first electrode  210  may be a low water level electrode for sensing a low water level in the case  110 . The second electrode  310  may be a high water level electrode for sensing a high water level in the case  110 . The third electrode  220  may be a common electrode. 
     The common electrode may extend to near a bottom  115  of the case  110  to contact water stored in the inside of the case  110  until the water is consumed up. The low water level electrode may be electrically connected to the common electrode until the water stored in the inside of the case  110  is consumed up. 
     The low water level electrode may sense a lowest water level to prevent overheating of the heater  120 . 
     The low water level electrode may be positioned at a location contacting water stored in the inside of the case  110  to prevent a fire that may occur as a result of exposure of the heater  120  to above the surface of water stored in the inside of the case  110  due to a slope of a place where the steam generator  100  is installed. 
     The low water level electrode may be positioned above the heater  120 . One end of the low water level electrode may be supported by a first housing  240 , and the other end of the low water level electrode may be positioned above the heater  120  in such a way to be spaced from the heater  120 . 
     The high water level electrode may be positioned at a location contacting water stored in the inside of the case  110  to prevent water supplied to the inside of the case  110  from exceeding a high water level and thus overflowing out of the case  110 . The high water level electrode may have a shorter length than the low water level electrode. That is, the second electrode  310  may have a shorter length than the first electrode  210 . 
     The first water level sensor  200  may be provided as a low water level sensor for sensing a low water level in the case  110 , and the second water level sensor  300  may be provided as a high water level sensor for sensing a high water level in the case  110 . 
     The low water level sensor for sensing a low water level may be provided together with a common electrode, and the high water level sensor for sensing a high water level may be provided separately, thereby preventing electric conduction between electrodes, which may be caused by scale formed by hard water. Accordingly, wrong operations of the water level sensor may be prevented. 
     The heater  120  may be installed in the lower case  112 . The heater  120  may be positioned adjacent to the bottom  115  of the case  110  to heat water stored in the case  110  regardless of a height in water level of the water. The heater  120  may be installed on a bottom of the lower case  112  to directly heat water filled in the lower case  112 , in the state of completely sinking under the water. 
     The heater  120  may be coupled to a heater fixing portion  122  of the lower case  112  by a heater coupling member  121 . 
     The heater  120  may be a sheath heater  120  having high thermal efficiency and capable of heating water in a relatively short time, although not limited thereto. For example, the heater  120  may be a coil heater for heating water stored in the case  110  from outside of the case  110 . 
     The case  110  may include a temperature sensor (not shown) for measuring temperature of water stored in the case  110 , and a heater temperature sensor (not shown) such as a thermo-fuse may be installed in the heater  120  to prevent the heater  120  from being overheated and damaged, although not limited thereto. 
     As shown in  FIGS. 6 and 7 , the first water level sensor  200  (see  FIG. 7 ) may include a first housing  240  and a first air cap  230 . 
     The first air cap  230  may accommodate portions of the first electrode  210  and the third electrode  220  (see  FIG. 8 ). 
     The second water level sensor  300  may include a second housing  340 , a second air cap  330 , and the second electrode  310 . The second air cap  330  may accommodate a portion of the second electrode  310 . In spaces in which the first and third electrodes  210  and  220  and the second electrode  310  are accommodated inside the first air cap  230  and the second air cap  330 , air pockets  233  and  333  may be formed to prevent water from entering the spaces. Details about the air pockets  233  and  333  will be described later. 
     The lower case  112  may include a plurality of partition walls  116   a  and  116   b.  The partition wall  116   b  may be formed below the first air cap  230 , and the partition wall  116   a  may be formed below the second air cap  330 . The plurality of partition walls  116   a  and  116   b  may be positioned adjacent to the first air cap  230  and the second air cap  330 . The plurality of partition walls  116   a  and  116   b  may extend upward from the bottom  115  of the lower case  112 . The plurality of partition walls  116   a  and  116   b  may have a substantially ‘⊏’ shape, although not limited thereto. 
     The first and third electrodes  210  and  220  and the second electrode  310  may be at appropriate heights from the bottom  115  of the lower case  112  to sense a water level of water to be stored in the case  110 . 
     Due to the plurality of partition walls  116   a  and  116   b,  substantially independent spaces may be formed around the first and third electrodes  210  and  220  and the second electrode  310 . Accordingly, the partition walls  116   a  and  116   b  may reduce sloshing of water even though water is supplied to the inside of the case  110  or vibrations are transferred from outside so that water filled in the case  110  sloshes. Also, the partition walls  116  and  116   b  may prevent water from being splashed to the first and third electrodes  210  and  220  and the second electrode  310  of the first and second water level sensors  200  and  300  while being supplied. Thereby, the first and second water level sensors  200  and  300  may sense a water level with high accuracy. 
     The first water level sensor  200  and the second water level sensor  300  may be coupled at the same height with respect to the case  110 . Thereby, spatial utilization of the machine room  40  provided in a limited lower space of the clothes care apparatus  1  may increase. 
       FIG. 8  is an exploded perspective view of the first water level sensor  200  according to a first embodiment of the disclosure, in the steam generator  100  according to the first embodiment of the disclosure.  FIG. 9  is an exploded perspective view of the second water level sensor  300  according to a first embodiment of the disclosure, in the steam generator  100  according to the first embodiment of the disclosure. 
     Referring to  FIGS. 5 and 8 , the first water level sensor  200  may be installed in the first coupling portion  117  of the upper case  111 . The first water level sensor  200  may be coupled to the first coupling portion  117  of the upper case  111  by a plurality of sensor coupling members  250 . The sensor coupling members  250  may be bolts, etc., although not limited thereto. 
     Referring to  FIG. 8 , the first water level sensor  200  may include the first housing  240  and the first electrode  210 . The first water level sensor  200  may further include the third electrode  220 . The first housing  240  may be detachably coupled to the upper case  111 . On a top of the first housing  240 , a first socket  241  for an electrical connection to outside may be mounted. A connector (not shown) configured to connect the first and third electrodes  210  and  220  to a controller (not shown) of the clothes care apparatus  1  may be inserted in the first socket  241 . 
     The first socket  241  may be mounted on the top of the first housing  240 . The first socket  241  may be exposed to the outside of the case  110 , although not limited thereto. 
     The first housing  240  may support the first electrode  210  and the third electrode  220 . The first electrode  210  and the third electrode  220  may be provided without any coating. An electrode with a coating may no longer sense a water level upon formation of scale at one end of the electrode exposed out of the coating. Accordingly, the first electrode  210  and the third electrode  230  may be formed without any coating to maintain a function of sensing a water level even though scale is formed at one ends of the first and third electrodes  210  and  230 . Thereby, it may be possible to prevent loss of components and lengthen the life span of a product. Also, because no coating is formed, material costs may be reduced through a simplified structure. 
     The first water level sensor  200  may include the first air cap  230  coupled to the first housing  240 . The first air cap  230  may be coupled to a lower side of the first housing  240 . 
     The first air cap  230  may include a first flange  231  being in a shape corresponding to a lower surface of the first housing  240 . The first air cap  230  may include a plurality of partition walls  232  extending toward the bottom  115  of the case  110  from the first flange  231 . The plurality of partition walls  232  may be substantially in a shape of a box having openings at both sides, although not limited thereto. 
     The first air cap  230  may accommodate a portion of the first electrode  210 . Also, the first air cap  230  may accommodate a portion of the third electrode  220 . The portion of the first electrode  210  and the portion of the third electrode  220  may be accommodated in an inside space defined by the plurality of partition walls  232 . 
     The first air cap  230  may form the air pocket  233  in the space in which the portion of the first electrode  210  is accommodated. That is, the air pocket  233  may be formed in the inside space defined by the plurality of partition walls  232 . The air pocket  233  may prevent the portion of the first electrode  210  from contacting water. Also, the air pocket  233  may prevent the portion of the third electrode  220  from contacting water. 
     By preventing the electrodes from contacting water, formation of scale may be prevented. Also, because no scale is formed on the electrodes, electric conduction between the electrodes may be prevented, thereby avoiding wrong operations of the water level sensors. 
     The first water level sensor  200  may include a sealing member  260 . The sealing member  260  may be positioned between the first housing  240  and the first air cap  230 . The sealing member  260  may be accommodated in a space in which a step is formed downward toward inside of the first flange  231  of the first air cap  230 . The sealing member  260  may prevent air from flowing between inside and outside of the first air cap  230 , thereby forming the air pocket  233  in the inside of the first air cap  230 . The sealing member  260  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     Referring to  FIGS. 5 and 9 , the second water level sensor  300  may be coupled to the second coupling portion  118  of the upper case  111 . The second water level sensor  300  may be coupled to the second coupling portion  118  of the upper case  111  by a plurality of sensor coupling members  350 . The sensor coupling members  350  may be bolts, etc., although not limited thereto. 
     As shown in  FIG. 9 , the second water level sensor  300  may include the second housing  340  and the second electrode  310 . The second housing  340  may be detachably coupled to the upper case  111 . On a top of the second housing  340 , a second socket  341  for an electrical connection to outside may be mounted. A connector (not shown) configured to connect the second electrode  310  to the controller (not shown) of the clothes care apparatus  1  may be inserted in the second socket  341 . 
     The second socket  341  may be mounted on the top of the second housing  340 . The second socket  341  may be exposed to the outside of the case  110 , although not limited thereto. 
     The second housing  340  may support the second electrode  310 . The second electrode  310  may be provided without any coating. An electrode with a coating may no longer sense a water level due to formation of scale at one end of the electrode exposed out of the coating. Accordingly, the second electrode  310  may be formed without any coating to maintain a function of sensing a water level even though scale is formed at one end of the second electrode  310 . Thereby, it may be possible to prevent loss of components and lengthen the life span of a product. Also, because no coating is formed, material costs may be reduced through a simplified structure. 
     The second water level sensor  300  may include the second air cap  330  coupled to the second housing  340 . The second air cap  330  may be coupled to a lower side of the second housing  340 . 
     The second air cap  330  may include a second flange  331  being in a shape corresponding to a lower surface  12   b  of the second housing  340 . The second air cap  330  may include a plurality of partition walls  332  extending toward the bottom  115  of the case  110  from the second flange  331 . The plurality of partition walls  332  may be substantially in a shape of a box having openings at both sides, although not limited thereto. 
     The second air cap  330  may accommodate a portion of the second electrode  310 . Also, the portion of the second electrode  310  may be accommodated in an inside space defined by the plurality of partition walls  332 . 
     The second air cap  330  may form the air pocket  333  in the space in which the portion of the second electrode  310  is accommodated. That is, the air pocket  333  may be formed in the inner space defined by the plurality of partition walls  332 . The air pocket  333  may prevent the portion of the second electrode  310  from contacting water. 
     By preventing the electrodes from contacting water, formation of scale may be prevented. Also, because no scale is formed on the electrodes, electric conduction between the electrodes may be prevented, thereby avoiding wrong operations of the water level sensors. 
     The second water level sensor  300  may include a sealing member  360 . The sealing member  360  may be positioned between the second housing  340  and the second air cap  330 . The sealing member  360  may be accommodated in a space in which a step is formed downward toward inside of the second flange  331  of the second air cap  330 . The sealing member  360  may prevent air from flowing between inside and outside of the second air cap  330 , thereby forming the air pocket  333  in the inside of the second air cap  330 . The sealing member  360  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     Hereinafter, an operation principle of the first and second water level sensors  200  and  300  of the steam generator  100  will be described in detail with reference to  FIGS. 5 to 9 . 
     First, water may enter the inside of the case  110  through the water supply portion  113 , and the water inside of the case  110  may be heated by the heater  120  to be converted into steam. 
     The steam may enter the inside of the care room  30  through the discharge portion  114  of the case  110 . 
     Water may be fully filled inside the case  110  to reach a full water level. In this case, the common electrode, the low water level electrode, and the high water level electrode may all sink under the water to convey electricity, so that the controller (not shown) may determine that this state is a high water level. 
     The water filled inside the case  110  may be consumed to reach a half-full water level. In this case, the high water level electrode may be exposed to above the surface of water to thus convey no electricity to the common electrode, while the low water level electrode and the common electrode may remain under the water to convey electricity, so that the controller (not shown) may determine that this state is a low water level. 
     The water filled inside the case  110  may be almost consumed to become lower than the low water level, so that both the common electrode and the low water level electrode may be exposed to the air. In this case, no electricity may be conveyed. 
       FIG. 10  is a front view showing a coupled state of the first water level sensor  200  according to the first embodiment of the disclosure, in the steam generator  100  according to the first embodiment of the disclosure. 
     Referring to  FIGS. 8 and 10 , the first water level sensor  200  may include the first housing  240 , the first air cap  230 , and the plurality of sensor coupling members  250 . Although not shown in the drawings, the sealing member  260  may be accommodated in the inside of the first air cap  230 . 
     The first housing  240  may support the first electrode  210  and the third electrode  220 . A portion of the first electrode  210  and a portion of the third electrode  220 , which are supported by the first housing  240 , may be accommodated inside the first air cap  230 . One end of the first electrode  210  and one end of the third electrode  220 , which are not accommodated inside the first air cap  230 , may be exposed to the outside of the first air cap  230 . The exposed end of the first electrode  210  may be a first sensor  211 . The exposed end of the third electrode  220  may be a third sensor  221 . In this case, a length d of the first electrode  210  and the third electrode  220 , which is exposed to the outside of the first air cap  230 , that is, a length d of the first sensor  211  and the third sensor  221  may be longer than 3 mm and shorter than 15 mm. 
     The portions of the first electrode  210  and the third electrode  220 , which are accommodated inside the first air cap  230 , may not contact water due to the air pocket  233 . 
     Accordingly, the first sensor  211  and the third sensor  221 , which are exposed to the outside of the first air cap  230 , may sense a water level. 
     In the case in which the length d of the first electrode  210  and the third electrode  220 , which is exposed to the outside of the first air cap  230 , is too short, water level sensing areas may be reduced. Accordingly, upon formation of scale at the ends of the first and third electrodes  210  and  220 , the first and third electrodes  210  and  220  may fail to properly perform a water level sensing function. Accordingly, the length d of the first sensor  211  and the third sensor  221  according to an embodiment of the disclosure may be longer than 3 mm. 
     Meanwhile, in the case in which the length d of the first electrode  210  and the third electrode  220 , which is exposed to the outside of the first air cap  230 , is too long, there may be a high probability of wrongly sensing a water level upon splashing of water inside the case  110 . Accordingly, the length d of the first sensor  211  and the third sensor  221  may be shorter than 15 mm. That is, the length d of the first electrode  210  and the third electrode  220 , which is exposed to the outside of the first air cap  230 , may be appropriately within a range that is longer than 3 mm and shorter than 15 mm. Also, the length d may be appropriately 5 mm. 
     In  FIG. 10 , the first water level sensor  200  is shown, however, the same technical feature may also be applied to the second water level sensor  300 . A length of the second electrode  310  of the second water level sensor  300 , which is exposed to the outside of the second air cap  330 , that is, a length of the second sensor  311  (see  FIG. 6 ) may also be longer than 3 mm or shorter than 15 mm. 
       FIG. 11  is a top view of the steam generator  100  according to the first embodiment of the disclosure. 
     Hereinafter, coupling positions of the first water level sensor  200  and the second water level sensor  300  will be described with reference to  FIGS. 5, 8, 9, and 11 . 
     The first water level sensor  200  may be coupled to the first coupling portion  117  of the upper case  111 . The first coupling portion  117  may be formed between the first side surface  111   a  and the center line L 1  of the upper case  111 . That is, the first water level sensor  200  may be coupled to the top plate  111   e  of the case  110  between the first side surface  111   a  and the center line L 1  of the upper case  111 . More particularly, the first housing  240  and the first air cap  230  may be coupled to the top plate  111   e  of the case  110  between the first side surface  111  and the center line L 1  by the plurality of sensor coupling members  250 . 
     The second water level sensor  300  may be coupled to the second coupling portion  118  of the upper case  111 . The second coupling portion  118  may be formed between the second side surface  111   b  and the center line L 1  of the upper case  111 . 
     That is, the second water level sensor  300  may be coupled to the top plate  111   e  of the case  110  between the second side surface  111   b  and the center line L 1  of the upper case  111 . More particularly, the second housing  340  and the second air cap  330  may be coupled to the top plate  111   e  of the case  110  between the second side surface  111   b  and the center line L 1  by the plurality of sensor coupling members  350 . 
     In the steam generator  100  seen from above, the first water level sensor  200  and the second water level sensor  300  may be collinear. In other words, the first housing  240  and the second housing  340  may be collinear. That the first water level sensor  200  and the second water level sensor  300  are collinear may represent that the first water level sensor  200  and the second water level sensor  300  are positioned on the reference line L 2 . That is, the first water level sensor  200  may be opposite to the second water level sensor  300  on the center line L 1  with respect to the case  110 . 
     The first water level sensor  200  may be a low water level sensor, and the second water level sensor  300  may be a high water level sensor. Accordingly, through the collinear arrangement, the clothes care apparatus  1  or the steam generator  100  may sense, even though being installed on an inclined floor, an inside water level based on the same criterion. Also, by separating the high water level electrode from the low water level electrode, a probability of electric conduction caused by formation of scales may be reduced. Thereby, wrong detections of the first and second water level sensors  200  and  300  may be prevented. 
       FIG. 12  is a front view of a first water level sensor according to a second embodiment of the disclosure, in the steam generator  100  according to the first embodiment of the disclosure. 
     Referring to  FIG. 12 , a first water level sensor  200   a  according to the second embodiment of the disclosure may include a first housing  240   a,  and a first electrode  210   a  and a third electrode  220   a  supported by the first housing  240   a,  unlike the first water level sensor  200 . In other words, the first water level sensor  200   a  may not include a configuration such as an air cap for forming a layer of air around portions of the first electrode  210   a  and the third electrode  220   a.    
     Although not shown in  FIG. 12 , a second water level sensor (not shown) may also omit a configuration such as an air cap, like the first water level sensor  200   a.    
       FIG. 13  is a front view of a first water level sensor according to a third embodiment of the disclosure, in the steam generator  100  according to the first embodiment of the disclosure. 
     Referring to  FIG. 13 , a first water level sensor  200   b  according to the third embodiment of the disclosure may further include an electrode coating  270   b,  unlike the first water level sensor  200  according to the first embodiment of the disclosure. The electrode coating  270   b  may surround a first electrode  210   b  and a third electrode  220   b.    
     The electrode coating  270   b  may be integrated and injection-molded into the first housing  240   b,  although not limited thereto. Through the configuration of the electrode coating  270   b,  the first electrode  210   b  and the third electrode  220   b  may be prevented from being corroded. 
     Also, the first air cap  230   b  may accommodate the electrode coating  270   b  and portions of the first electrode  210   b  and the third electrode  220   b  accommodated inside the electrode coating  270   b.    
     By providing the first air cap  230   b,  the first water level sensor  200   b  and the second water level sensor (not shown), which are separated from each other, may be prevented from conveying electricity due to scale formed on an inner surface of the upper case  111 . 
     The first water level sensor  200   b  may include a plurality of sealing members  260   b.  The sealing members  260   b  may be positioned between the housing  240   b  and the first air cap  230   b.  Also, the sealing members  260   b  may be installed at a connected portion between structures of the first air cap  230   b.  The sealing members  260   b  may prevent air from flowing between inside and outside of the first air cap  230   b,  thereby forming an air pocket  233   b  inside the first air cap  230   b.  The sealing members  260   b  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     Although not shown in  FIG. 13 , the second water level sensor (not shown) may also further include an electrode coating surrounding electrodes, like the first water level sensor  200   b.    
       FIG. 14  is a front view of a first water level sensor according to a fourth embodiment of the disclosure, in the steam generator  100  according to the first embodiment of the disclosure. 
     As shown in  FIG. 14 , a first water level sensor  200   c  according to the fourth embodiment of the disclosure may include a first housing  240   c,  a first electrode  210   c  and a third electrode  220   c  supported by the first housing  240   c,  and an electrode coating  270   c,  unlike the first water level sensor  200   b  according to the third embodiment of the disclosure. In other words, the first water level sensor  200   c  may not include a configuration such as an air cap for forming a layer of air around portions of the first electrode  210   c  and the third electrode  220   c.    
     Although not shown in  FIG. 14 , a second water level sensor (not shown) may also omit a configuration such as an air cap, like the first water level sensor  200   c.    
     Although not shown in  FIG. 14 , the steam generator  100  may include the water level sensors  200   a  and  300   a  according to the second embodiment of the disclosure, the water level sensors  200   b  and  300   b  according to the third embodiment of the disclosure, or the water level sensors  200   c  and  300   c  according to the fourth embodiment of the disclosure, instead of the water level sensors  200  and  300  according to the first embodiment of the disclosure. 
       FIG. 15  is a top view of a steam generator according to a second embodiment of the disclosure. 
     As shown in  FIG. 15 , a steam generator  100   a  may include the first water level sensor  200  and the second water level sensor  300 . 
     The upper case  111  may include the first side surface  111   a,  the second side surface  111   b,  the third side surface  111   c,  the fourth side surface  111   d,  and the top plate  111   e  connected to the plurality of side surfaces  111   a,    111   b,    111   c,  and  111   d.    
     The first side surface  111   a  may be opposite to the second side surface  111   b  with respect to the center line L 1 . The third side surface  111   c  may be connected to the first side surface  111   a  and the second side surface  111   b.  The fourth side surface  111   d  may be connected to the first side surface  111   a  and the second side surface  111   b,  and may be opposite to the third side surface  111   c  with respect to the reference line L 2 . 
     The center line L 1  and the reference line L 2  may pass an intersecting point C of the steam generator  100   a  and may be perpendicular to each other. 
     The center line L 1  may be parallel to the first side surface  111   a  and the second side surface  111   b  to pass a geometric center of the steam generator  100   a.  The reference line L 2  may be parallel to the third side surface  111   c  and the fourth side surface  111   d  to pass a center of the second water level sensor  300  of the steam generator  100   a.    
     The first water level sensor  200  may be coupled to a first coupling portion  117   a  of the upper case  111 . The first coupling portion  117   a  may be formed between the first side surface  111   a,  the third plate  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . That is, the first water level sensor  200  may be coupled to the case  110  between the first side surface  111   a,  the third side surface  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . 
       FIG. 15  is a top view of the steam generator  100   a,  and, as seen from above, the first housing  240  may be coupled between the first side surface  111   a,  the third side surface  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . 
     The second water level sensor  300  may be coupled to a second coupling portion  118   a  of the upper case  111 . The second coupling portion  118   a  may be formed between the second side surface  111   b  and the center line L 1  of the upper case  111 . That is, the second water level sensor  300  may be coupled to the case  110  between the second side surface  111   b  and the center line L 1  of the upper case  111 . Also, the second coupling portion  118   a  may be positioned on the reference line L 2 . 
       FIG. 15  is a top view of the steam generator  100   a,  and, as seen from above, the second housing  340  may be positioned between the second side surface  111   b  and the center line L 1  and coupled to the case  110  on the reference line L 2 . 
     Accordingly, the first water level sensor  200  may be diagonal to the second water level sensor  300 . That is, the first water level sensor  200  may be farthest from the second water level sensor  300  with respect to the case  110 . 
     Thereby, a probability of electric conduction between the first and second water level sensors  200  and  300 , caused by formation of scale, may be reduced. Other detailed configurations except for the arrangement of the first water level sensor  200  and the second water level sensor  300  may be the same as those of the steam generator  100  according to the first embodiment of the disclosure. 
     However, the positions of the first water level sensor  200  and the second water level sensor  300  may be reversed. 
     Also, although not shown in  FIG. 15 , the steam generator  100   a  may include the water level sensors  200   a  and  300   a  according to the second embodiment of the disclosure, the water level sensors  200   b  and  300   b  according to the third embodiment of the disclosure, or the water level sensors  200   c  and  300   c  according to the fourth embodiment of the disclosure, instead of the water level sensors  200  and  300  according to the first embodiment of the disclosure. 
       FIG. 16  is a top view of a steam generator according to a third embodiment of the disclosure. 
     As shown in  FIG. 16 , the first water level sensor  200  and the second water level sensor  300  may be coupled to the top plate  111   e  of the upper case  111  between the second side surface  111   b  and the center line L 1 . 
     The first water level sensor  200  may be coupled to a first coupling portion  117   b  of the upper case  111 . The first coupling portion  117   b  may be formed between the second side surface  111   b  and the center line L 1  of the upper case  111 . That is, the first water level sensor  200  may be coupled to the case  110  between the second side surface  111   b  and the center line L 1  of the upper case  111 . 
       FIG. 16  is a top view of the steam generator  100   b,  and, as seen from above, the first housing  240  may be coupled between the second side surface  111   b  and the center line L 1  of the upper case  111 . 
     The second water level sensor  300  may be coupled to a second coupling portion  118   b  of the upper case  111 . The second coupling portion  118   b  may be formed between the second side surface  111   b  and the center line L 1  of the upper case  111 , like the first coupling portion  117   b.  That is, the second water level sensor  300  may be coupled to the case  110  between the second side surface  111   b  and the center line L 1  of the upper case  111 . 
       FIG. 16  is a top view of the steam generator  100   b,  and, as seen from above, the second housing  340  may be coupled between the second side surface  111   b  and the center line L 1  of the upper case  111 . 
     Accordingly, the first housing  240  and the second housing  340  may be aligned along one side of the case  110 . In  FIG. 16 , the first housing  240  and the second housing  340  are shown to be coupled to the top plate  111   e  of the upper case  111  between the second side surface  111   b  and the center line L 1 , however, the first housing  240  and the second housing  340  may be coupled side by side to the top plate  111   e  of the upper case  111  between the first side surface  111   a  and the center line L 1 . 
     Also, although not shown in  FIG. 16 , the steam generator  100   b  may include the water level sensors  200   a  and  300   a  according to the second embodiment of the disclosure, the water level sensors  200   b  and  300   b  according to the third embodiment of the disclosure, or the water level sensors  200   c  and  300   c  according to the fourth embodiment of the disclosure, instead of the water level sensors  200  and  300  according to the first embodiment of the disclosure. 
       FIG. 17  is a top view of a steam generator according to a fourth embodiment of the disclosure.  FIG. 18  is a front view showing a coupled state of a first water level sensor according to a fifth embodiment of the disclosure with a third water level sensor according to a first embodiment of the disclosure, in the steam generator according to the fourth embodiment of the disclosure. 
     A third water level sensor  400  may be a configuration having the same shape as a first water level sensor  500 . Therefore, a configuration of the third water level sensor  400  will be described together with that of the first water level sensor  500  with reference to  FIG. 18 . 
     As shown in  FIGS. 17 and 18 , a steam generator  100   c  may include three water level sensors. The steam generator  100   c  may include the first water level sensor  500 , a second water level sensor  600 , and the third water level sensor  400 . The first water level sensor  500  may include a first electrode  510 , a first housing  540  supporting the first electrode  510 , and a first air cap  530  accommodating a portion of the first electrode  510 . 
     The second water level sensor  600  may include a second housing  640 . The second water level sensor  600  may include a second electrode  610 , the second housing  640  supporting the second electrode  610 , and a second air cap  630  accommodating a portion of the second electrode  610  (see  FIG. 9 ). 
     The third water level sensor  400  may include a third electrode  410 , a third housing  440  supporting the third electrode  410 , and a third air cap  430  accommodating a portion of the third electrode  410 . 
     The first water level sensor  500  according to the fifth embodiment of the disclosure may include the first electrode  510 , and the third water level sensor  400  according to the first embodiment of the disclosure may include the third electrode  410 . 
     The first electrode  510  may be a low water level electrode for sensing a low water level inside the case  110 . The second electrode  610  (see  FIG. 9 ) may be a high water level sensor for sensing a high water level inside the case  110 . The third electrode  410  may be a common electrode. 
     Accordingly, in the steam generator  100   c  according to the fourth embodiment of the disclosure, the first electrode  510 , the second electrode  610 , and the third electrode  410  may be separated from each other. 
     The first water level sensor  500  may include the first housing  540  and the first electrode  510 . The first housing  540  may be detachably coupled to the upper case  111 . On a top of the first housing  540 , a first socket  541  for an electrical connection to outside may be mounted. 
     The first socket  541  may be mounted on the top of the first housing  540 . The first socket  541  may be exposed to the outside of the case  110 , although not limited thereto. 
     The first housing  540  may support the first electrode  510 . The first electrode  510  may be provided without any coating. An electrode with a coating may no longer sense a water level upon formation of scales at one end of the electrode exposed out of the coating. Accordingly, the first electrode  510  may be formed without any coating to maintain a function of sensing a water level even though scale is formed at one end of the first electrode  510 . Thereby, it may be possible to prevent loss of components and lengthen the life span of a product. Also, because no coating is formed, material costs may be reduced through a simplified structure. 
     The first water level sensor  500  may include the first air cap  530  coupled to the first housing  540 . The first air cap  530  may be coupled to a lower side of the first housing  540 . 
     The first air cap  530  may include a first flange  531  being in a shape corresponding to a lower surface of the first housing  540 . The first air cap  530  may include a plurality of partition walls  532  extending toward the bottom  115  of the case  110  from the first flange  531 . The plurality of partition walls  532  may be substantially in a shape of a box having openings at both sides, although not limited thereto. 
     The first air cap  530  may accommodate a portion of the first electrode  510 . The portion of the first electrode  510  may be accommodated in an inside space defined by the plurality of partition walls  532 . 
     The first air cap  530  may form an air pocket  533  in the space in which the portion of the first electrode  510  is accommodated. That is, the air pocket  533  may be formed in the inner space defined by the plurality of partition walls  532 . The air pocket  533  may prevent the portion of the first electrode  510  from contacting water. 
     By preventing the electrodes from contacting water, formation of scale may be prevented. Also, because no scale is formed on the electrodes, electric conduction between the electrodes may be prevented, thereby avoiding wrong operations of the water level sensors. 
     The first water level sensor  500  may include a sealing member  560 . The sealing member  560  may be positioned between the first housing  540  and the first air cap  530 . The sealing member  560  may be accommodated in a space in which a step is formed downward toward inside of the first flange  531  of the first air cap  530 . The sealing member  560  may prevent air from flowing between inside and outside of the first air cap  530 , thereby forming the air pocket  533  inside the first air cap  530 . The sealing member  560  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     The first water level sensor  400  may include the third housing  440  and the third electrode  410 . The third housing  440  may be detachably coupled to the upper case  111 . On a top of the third housing  440 , a third socket  441  for an electrical connection to outside may be mounted. 
     The third socket  441  may be mounted on the top of the first housing  440 . The third socket  441  may be exposed to the outside of the case  110 , although not limited thereto. 
     The third housing  440  may support the third electrode  410 . The third electrode  410  may be provided without any coating. An electrode with a coating may no longer sense a water level upon formation of scales at one end of the electrode exposed out of the coating. Accordingly, the third electrode  410  may be formed without any coating to maintain a function of sensing a water level even though scale is formed at one end of the third electrode  410 . Thereby, it may be possible to prevent loss of components and lengthen the life span of a product. Also, because no coating is formed, material costs may be reduced through a simplified structure. 
     The third water level sensor  400  may include the third air cap  430  coupled to the third housing  440 . The third air cap  430  may be coupled to a lower side of the third housing  440 . 
     The third air cap  430  may include a third flange  431  being in a shape corresponding to a lower surface of the first housing  440 . The third air cap  430  may include a plurality of partition walls  432  extending toward the bottom  115  of the case  110  from the first flange  431 . The plurality of partition walls  432  may be substantially in a shape of a box having openings at both sides, although not limited thereto. 
     The third air cap  430  may accommodate a portion of the third electrode  410 . The portion of the third electrode  410  may be accommodated in an inside space defined by the plurality of partition walls  432 . 
     The third air cap  430  may form an air pocket  433  in the space in which the portion of the third electrode  410  is accommodated. That is, the air pocket  433  may be formed in the inner space defined by the plurality of partition walls  432 . The air pocket  433  may prevent the portion of the third electrode  410  from contacting water. 
     By preventing the electrodes from contacting water, formation of scale may be prevented. Also, because no scale is formed on the electrodes, electric conduction between the electrodes may be prevented, thereby avoiding wrong operations of the water level sensors. 
     The third water level sensor  400  may include a sealing member  460 . The sealing member  460  may be positioned between the third housing  440  and the third air cap  430 . The sealing member  460  may be accommodated in a space in which a step is formed downward toward inside of the third flange  431  of the third air cap  430 . The sealing member  460  may prevent air from flowing between inside and outside of the third air cap  430 , thereby forming the air pocket  433  inside the third air cap  430 . The sealing member  460  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     Referring to  FIG. 9 , a structure of the second water level sensor  600  will be described. 
     The second water level sensor  600  may include the second housing  640  and the second electrode  610 . The second housing  640  may be detachably coupled to the upper case  111 . On a top of the second housing  640 , a second socket  641  for an electrical connection to outside may be mounted. 
     The second socket  641  may be mounted on the top of the second housing  640 . The second socket  641  may be exposed to the outside of the case  110 , although not limited thereto. 
     The second housing  640  may support the second electrode  610 . The second electrode  610  may be provided without any coating. An electrode with a coating may no longer sense a water level upon formation of scale at one end of the electrode exposed out of the coating. Accordingly, the second electrode  610  may be formed without any coating to maintain a function of sensing a water level even though scale is formed at one end of the second electrode  610 . Thereby, it may be possible to prevent loss of components and lengthen the life span of a product. Also, because no coating is formed, material costs may be reduced through a simplified structure. 
     The second water level sensor  600  may include the second air cap  630  coupled to the second housing  640 . The second air cap  630  may be coupled to a lower side of the second housing  640 . 
     The second air cap  630  may include a second flange  631  being in a shape corresponding to a lower surface of the second housing  640 . The second air cap  630  may include a plurality of partition walls  632  extending toward the bottom  115  of the case  110  from the second flange  631 . The plurality of partition walls  632  may be substantially in a shape of a box having openings at both sides, although not limited thereto. 
     The second air cap  630  may accommodate a portion of the second electrode  610 . The portion of the second electrode  610  may be accommodated in an inside space defined by the plurality of partition walls  632 . 
     The second air cap  630  may form an air pocket  633  in the space in which the portion of the second electrode  610  is accommodated. That is, the air pocket  633  may be formed in the inner space defined by the plurality of partition walls  632 . The air pocket  633  may prevent the portion of the second electrode  610  from contacting water. 
     By preventing the electrodes from contacting water, formation of scale may be prevented. Also, because no scale is formed on the electrodes, electric conduction between the electrodes may be prevented, thereby avoiding wrong operations of the water level sensors. 
     The second water level sensor  600  may include a sealing member  660 . The sealing member  660  may be positioned between the second housing  640  and the second air cap  630 . The sealing member  660  may be accommodated in a space in which a step is formed downward toward inside of the second flange  631  of the second air cap  630 . The sealing member  660  may prevent air from flowing between inside and outside of the second air cap  630 , thereby forming the air pocket  633  inside the second air cap  630 . The sealing member  660  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     The second water level sensor  600  may be coupled to the second coupling portion  118   b  of the upper case  111  by a plurality of sensor coupling members  650 . The sensor coupling members  650  may be bolts, etc., although not limited thereto. 
     The first housing  540  may support the first electrode  510 . A portion of the first electrode  510 , which is supported by the first housing  540 , may be accommodated inside the first air cap  530 . One end of the first electrode  510 , which is not accommodated in the inside of the first air cap  530 , may be exposed to the outside of the first air cap  530 . The exposed end of the first electrode  510  may be a first sensor  511 . In this case, a length d of the first electrode  510 , which is exposed to the outside of the first air cap  530 , that is, a length d of the first sensor  511  may be longer than 3 mm and shorter than 15 mm. 
     The portion of the first electrode  510 , which is accommodated inside the first air cap  530 , may not contact water due to the air pocket  533 . 
     Accordingly, the first sensor  511  and the third sensor  521 , which are exposed to the outside of the first air cap  530 , may sense a water level. 
     In the case in which the length d of the first electrode  510 , which is exposed to the outside of the first air cap  530 , is too short, a water level sensing area may be reduced. Accordingly, upon formation of scale at the end of the first electrode  510 , the first electrode  510  may fail to properly perform a water level sensing function. Accordingly, the length d of the first sensor  511  according to an embodiment of the disclosure may be longer than 3 mm. 
     Meanwhile, in the case in which the length d of the first electrode  510 , which is exposed to the outside of the first air cap  530 , is too long, there may be a high probability of wrongly sensing a water level upon splashing of water inside the case  110 . Accordingly, the length d of the first sensor  511  according to an embodiment of the disclosure may be shorter than 15 mm. 
     In the third water level sensor  400 , likewise, a length d of a third sensor  411  may be longer than 3 mm and shorter than 15 mm. 
     Referring to  FIG. 17 , the steam generator  100   c  may include the first water level sensor  500 , the second water level sensor  600 , and the third water level sensor  400 . 
       FIG. 17  is a top view of the steam generator  100   c,  and, as seen from above, the first housing  540 , the second housing  640 , and the third housing  440  may be coupled to the case  110 . 
     The upper case  111  may include the first side surface  111   a,  the second side surface  111   b,  the third side surface  111   c,  the fourth side surface  111   d,  and the top plate  111   e  connected to the plurality of side surfaces  111   a,    111   b,    111   c,  and  111   d.    
     The first side surface  111   a  may be opposite to the second side surface  111   b  with respect to the center line L 1 . The third side surface  111   c  may be connected to the first side surface  111   a  and the second side surface  111   b.  The fourth side surface  111   d  may be connected to the first side surface  111   a  and the second side surface  111   b,  and may be opposite to the third side surface  111   c  with respect to the reference line L 2 . 
     The center line L 1  and the reference line L 2  may pass an intersecting point C of the steam generator  100   c  and may be perpendicular to each other. 
     The center line L 1  may be parallel to the first side surface  111   a  and the second side surface  111   b  to pass a geometric center of the steam generator  100   c.  The reference line L 2  may be parallel to the third side surface  111   c  and the fourth side surface  111   d  to pass centers of the first and second water level sensors  500  and  600  of the steam generator  100   c.    
     The first water level sensor  500  may be coupled to a first coupling portion  117   c  of the upper case  111 . The first coupling portion  117   c  may be formed on the upper plate  111   e  between the first side surface  111   a  and the center line L 1  of the upper case  111 . That is, the first water level sensor  500  may be coupled to the case  110  on the top plate  111   e  between the first side surface  111   a  and the center line L 1 . Also, the first water level sensor  500  may be coupled to the case  110  such that a center of the first housing  540  is on the reference line L 2 . Accordingly, in the steam generator  100   c  seen from above, the first housing  540  may be coupled to the case  110  between the first side surface  111   a  and the center line L 1  such that the first housing  540  is on the reference line L 2 . 
     The second water level sensor  600  may be coupled to a second coupling portion  118   c  of the upper case  111 . The second coupling portion  118   c  may be formed on the top plate  111   e  between the second side surface  111   b  and the center line L 1  of the upper case  111 . That is, the second water level sensor  600  may be coupled to the case  110  on the top plate  111   e  between the second side surface  111   b  and the center line L 1  of the upper case  111 . Also, the second water level sensor  600  may be coupled to the case  110  such that a center of the second housing  640  is on the reference line L 2 . Accordingly, in the steam generator  100   c  seen from above, the second housing  640  may be coupled to the case  110  between the second side surface  111   b  and the center line L 1 , such that the center of the second housing  640  is on the reference line L 2 . Also, the second water level sensor  600  may be coupled to the case  110  such that the center of the second housing  640  is on the reference line L 2 . Accordingly, in the steam generator  100   c  seen from above, the second housing  640  may be coupled to the case  110  between the second side surface  111   b  and the center line L 1  such that the center of the second housing  640  is on the reference line L 2 . 
     The second water level sensor  600  may be opposite to the first water level sensor  500  with respect to the center line L 1 . 
     The third water level sensor  400  may be coupled to a third coupling portion  119   c  of the upper case  111 . The third coupling portion  119   c  may be formed on the top plate  111   e  of the upper case  111  between the first side surface  111   a,  the third side surface  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . That is, the third water level sensor  400  may be coupled to the case  110  on the top plate  111   e  between the first side surface  111   a,  the third side surface  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . In the steam generator  100   c  seen from above, the third housing  440  may be coupled to the case  110  between the first side surface  111 , the third side surface  111   c,  the center line L 1 , and the reference line L 2 . 
     Accordingly, the third housing  440  may be aligned with the first housing  540  to be coupled to the case  110 . 
     As shown in  FIGS. 17 and 18 , a low water level electrode, a high water level electrode, and a common electrode may be separated from each other, thereby preventing electric conduction between the electrodes, which may be caused by formation of scale. Also, independency between the plurality of electrodes may be secured to reduce a generation probability of wrong detections of the water level sensors. 
       FIG. 19  is a front view showing a first water level sensor according to a sixth embodiment of the disclosure and a third water level sensor according to a second embodiment of the disclosure, in the steam generator  100   c  according to the fourth embodiment of the disclosure. 
     A third water level sensor  400   a  may be a configuration having the same shape as a first water level sensor  500   a.  Therefore, the configuration of the third water level sensor  400   a  will be described together with that of the first water level sensor  500   a  with reference to  FIG. 19 . 
     Referring to  FIG. 19 , the first water level sensor  500   a  according to the sixth embodiment of the disclosure may include a first housing  540   a,  and a first electrode  510   a  supported by the first housing  540   a,  unlike the first water level sensor  500  according to the fifth embodiment of the disclosure. In other words, the first water level sensor  500   a  may not include a configuration such as an air cap for forming a layer of air around a portion of the first electrode  510   a.    
     Also, the third water level sensor  400   a  according to the second embodiment of the disclosure may include a third housing  440   a,  and a third electrode  410   a  supported by the third housing  440   a,  unlike the third water level sensor  400  according to the first embodiment of the disclosure. In other words, the third water level sensor  400   a  may not include a configuration such as an air cap for forming a layer of air around a portion of the third electrode  410   a.    
     Although not shown in  FIG. 19 , a second water level sensor (not shown) may also not include a configuration such as an air cap, like the first water level sensor  500   a  and the third water level sensor  400   a.    
       FIG. 20  is a front view showing a first water level sensor according to a seventh embodiment of the disclosure and a third water level sensor according to a third embodiment of the disclosure, in the steam generator  100   c  according to the fourth embodiment of the disclosure. 
     Referring to  FIG. 20 , a first water level sensor  500   b  according to the seventh embodiment of the disclosure may further include an electrode coating  570   b,  unlike the first water level sensor  500  according to the fifth embodiment of the disclosure. The electrode coating  570   b  may surround a first electrode  510   b.    
     The electrode coating  570   b  may be integrated and injection-molded into a first housing  540   b,  although not limited thereto. Through the configuration of the electrode coating  570   b,  the first electrode  510   b  may be prevented from being corroded. 
     Also, a first air cap  530   b  may accommodate the electrode coating  570   b  and a portion of the first electrode  510   b  accommodated inside the electrode coating  570   b.    
     By providing the first air cap  530   b,  the first water level sensor  500   b,  a second water level sensor (not shown), and a third water level sensor  400   b,  which are separated from each other, may be prevented from conveying electricity due to scale formed on the inner surface of the upper case  111 . 
     The first water level sensor  500   b  may include a plurality of sealing members  560   b.  The sealing members  560   b  may be positioned between the housing  540   b  and the air cap  530   b.  Also, the sealing members  560   b  may be installed at a connected portion between structures of the first air cap  530   b.  The sealing members  560   b  may prevent air from flowing between inside and outside of the first air cap  530   b,  thereby forming an air pocket  533   b  inside the air cap  530   b.  The sealing members  560   b  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     The third water level sensor  400   b  according to the third embodiment of the disclosure may further include an electrode coating  470   b,  unlike the third water level sensor  400  according to the first embodiment of the disclosure. The electrode coating  470   b  may surround the third electrode  410   b.    
     The electrode coating  470   b  may be integrated and injection-molded into a third housing  440   b,  although not limited thereto. Through the configuration of the electrode coating  470   b,  the first electrode  410   b  may be prevented from being corroded. 
     Also, a third air cap  430   b  may accommodate the electrode coating  470   b  and a portion of the third electrode  410   b  accommodated inside the electrode coating  470   b.    
     By providing the third air cap  430   b,  the first water level sensor  500   b,  the second water level sensor (not shown), and the third water level sensor  400   b,  which are separated from each other, may be prevented from conveying electricity due to scale formed on the inner surface of the upper case  111 . 
     The third water level sensor  400   b  may include a plurality of sealing members  460   b.  The sealing members  460   b  may be positioned between the third housing  440   b  and the third air cap  430   b.  Also, the sealing members  460   b  may be installed at a connected portion between structures of the first air cap  430   b.  The sealing member  460   b  may prevent air from flowing between inside and outside of the first air cap  430   b,  thereby forming an air pocket  433   b  inside the first air cap  430   b.  The sealing members  460   b  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     Although not shown in  FIG. 20 , the second water level sensor (not shown) may also further include an electrode coating surrounding an electrode, like the first water level sensor  500   b  and the third water level sensor  400   b.    
       FIG. 21  is a front view showing a first water level sensor according to an eighth embodiment of the disclosure and a third water level sensor according to a fourth embodiment of the disclosure, in the steam generator  100   c  according to the fourth embodiment of the disclosure. 
     As shown in  FIG. 21 , a first water level sensor  500   c  according to the eighth embodiment of the disclosure may include a first housing  540   c,  a first electrode  510   c  supported by the first housing  540   c,  and an electrode coating  570   c,  unlike the first water level sensor  500   b  according to the seventh embodiment of the disclosure. In other words, the first water level sensor  500   c  may not include a configuration such as an air cap for forming a layer of air around a portion of the first electrode  510   c.    
     A third water level sensor  400   c  according to the fourth embodiment of the disclosure may include a third housing  440   c,  a third electrode  410   c  supported by the third housing  440   c,  and an electrode coating  470   c,  unlike the third water level sensor  400   b  according to the third embodiment of the disclosure. In other words, the third water level sensor  400   c  may not include a configuration such as an air cap for forming a layer of air around a portion of the third electrode  410   c.    
     Although not shown in  FIG. 21 , a second water level sensor (not shown) may also not include a configuration such as an air cap, like the first water level sensor  500   c  and the third water level sensor  400   c.    
     Although not shown in  FIG. 21 , the steam generator  100   c  may include the water level sensors  500   a  and  600   a  according to the sixth embodiment of the disclosure and the water level sensor  400   a  according to the second embodiment of the disclosure, the water level sensors  500   b  and  600   b  according to the seventh embodiment of the disclosure and the water level sensor  400   b  according to the third embodiment of the disclosure, or the water level sensors  500   c  and  600   c  according to the eighth embodiment of the disclosure and the water level sensor  400   c  according to the fourth embodiment of the disclosure, instead of the water level sensors  500  and  600  according to the fifth embodiment of the disclosure and the water level sensor  400  according to the first embodiment of the disclosure. 
       FIG. 22  is a top view of a steam generator according to a fifth embodiment of the disclosure. 
     Referring to  FIG. 22 , a steam generator  100   d  may include the first water level sensor  500 , the second water level sensor  600 , and the third water level sensor  400 . 
       FIG. 22  is a top view of the steam generator  100   d,  and as seen from above, the first housing  540 , the second housing  640 , and the third housing  440  may be coupled to the case  110 . 
     The upper case  111  may include the first side surface  111   a,  the second side surface  111   b,  the third side surface  111   c,  the fourth side surface  111   d,  and the top plate  111   e  connected to the plurality of side surfaces  111   a,    111   b,    111   c,  and  111   d.    
     The first side surface  111   a  may be opposite to the second side surface  111   b  with respect to the center line L 1 . The third side surface  111   c  may be connected to the first side surface  111   a  and the second side surface  111   b.  The fourth side surface  111   d  may be connected to the first side surface  111   a  and the second side surface  111   b,  and may be opposite to the third side surface  111   c  with respect to the reference line L 2 . 
     The center line L 1  and the reference line L 2  may pass an intersecting point C of the steam generator  100   d and may be perpendicular to each other.    
     The center line L 1  may be parallel to the first side surface  111   a  and the second side surface  111   b  to pass a geometric center of the steam generator  100   d.  The reference line L 2  may be parallel the third side surface  111   c  and the fourth side surface  111   d  to pass a center of a first water level sensor  500  of the steam generator  100   d.    
     The first water level sensor  500  may be coupled to a first coupling portion  117   d  of the upper case  111 . The first coupling portion  117   d  may be formed on the top plate  111   e  between the first side surface  111   a  and the center line L 1  of the upper case  111 . 
     That is, the first water level sensor  500  may be coupled to the case  110  on the top plate  111   e  between the first side surface  111   a  and the center line L 1 . Also, the first water level sensor  500  may be coupled to the case  110  such that the center of the first housing  540  is on the reference line L 2 . Accordingly, in the steam generator  100   d  seen from above, the first housing  540  may be coupled to the case  110  between the first side surface  111   a  and the center line L 1  such that the center of the first housing  540  is on the reference line L 2 . 
     The second water level sensor  600  may be coupled to a second coupling portion  118   d  of the upper case  111 . The second coupling portion  118   d  may be formed on the top surface  111   e  between the second side surface  111   b  and the center line L 1  of the upper case  111 . That is, the second water level sensor  600  may be coupled to the case  110  on the top plate  111   e  between the second side surface  111   b  and the center line L 1  of the upper case  111 . Also, the second water level sensor  600  may be coupled to the case  110  such that the second housing  640  is on the reference line L 2 . Accordingly, in the steam generator  100   d  seen from above, the second housing  640  may be coupled to the case  110  between the second side surface  111   b  and the center line L 1  such that the center of the second housing  640  is on the reference line L 2 . 
     Accordingly, the second water level sensor  600  may be opposite to the first water level sensor  500  with respect to the center line L 1 . 
     The third water level sensor  400  may be coupled to a third coupling portion  119   d  of the upper case  111 . The third coupling portion  119   d  may be formed on the top plate  111   e  of the upper case  111  between the second side surface  111   b,  the third side surface  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . That is, the third water level sensor  400  may be coupled to the case  110  on the top plate  111   e  between the second side surface  111   b,  the third side surface  111   c,  the center line L 1 , and the reference line L 2  of the upper case  111 . In the steam generator  100   d  seen from above, the third housing  440  may be coupled to the case  110  between the second side surface  111   b,  the third side surface  111   c,  the center line L 1 , and the reference line L 2 . 
     Accordingly, the third housing  440  may be aligned with the second housing  640  to be coupled to the case  110 . 
     Although not shown in  FIG. 22 , the steam generator  100   d  may include the water level sensors  500   a  and  600   a  according to the sixth embodiment of the disclosure and the water level sensor  400   a  according to the second embodiment of the disclosure, the water level sensors  500   b  and  600   b  according to the seventh embodiment of the disclosure and the water level sensor  400   b  according to the third embodiment of the disclosure, or the water level sensors  500   c  and  600   c  according to the eighth embodiment of the disclosure and the water level sensor  400   c  according to the fourth embodiment of the disclosure, instead of the water level sensors  500  and  600  according to the fifth embodiment of the disclosure and the water level sensor  400  according to the first embodiment of the disclosure. 
       FIG. 23  is a front view of a water level sensor according to a ninth embodiment of the disclosure in a steam generator according to a sixth embodiment of the disclosure. 
     As shown in  FIG. 23 , a water level sensor  700  according to the ninth embodiment of the disclosure may include a housing  740  and an air cap  750 . 
     The water level sensor  700  may include a first electrode  710 , a second electrode  720 , and a third electrode  730 . 
     The first electrode  710  may be a low water level electrode for sensing an inside low water level. The second electrode  720  may be a high water level electrode for sensing an inside high water level. The third electrode  730  may be a common electrode. 
     The housing  740  may be fixed to an outer surface of the upper case  111  by bolts, etc. On a top of the housing  740 , a socket  741  for an electrical connection to outside may be mounted. 
     The housing  740  may support the first electrode  710 , the second electrode  720 , and the third electrode  730 . 
     The water level sensor  700  may include electrode coatings  771 ,  772 , and  773  surrounding outer surfaces of the first, second, and third electrodes  710 ,  720 , and  730 . 
     The electrode coatings  771 ,  772 , and  773  may be integrated and injection-molded into the housing  740 , although not limited thereto. Through the configuration of the electrode coatings  771 ,  772 , and  773 , the first, second, and third electrodes  710 ,  720 , and  730  may be prevented from being corroded. 
     The water level sensor  700  may include the air cap  750  coupled to the housing  740 . The air cap  750  may be coupled to a lower side of the housing  740 . 
     The air cap  750  may include a first flange  731  being in a shape corresponding to a lower surface of the first housing  240 . The air cap  750  may include a plurality of partition walls  752  extending toward the bottom  115  of the case  110  from the first flange  731 . The plurality of partition walls  752  may be substantially in a shape of a box having openings at both sides, although not limited thereto. 
     The air cap  750  may accommodate portions of the first electrode  710  and the first electrode coating  771 . Also, the air cap  750  may accommodate portions of the second electrode  720 , the second electrode coating  772 , the third electrode  730 , and the third electrode coating  773 . The portions of the first electrode  710  and the first electrode coating  771 , the portions of the second electrode  720  and the second electrode coating  772 , and the portions of the third electrode  730  and the third electrode coating  773  may be accommodated in the plurality of partition walls  752 . 
     More particularly, the plurality of partition walls  752  may include a middle partition wall  752   a  for partitioning a space. The middle partition wall  752   a  may partition a space in which the first electrode  710  and the third electrode  730  are accommodated from a space in which the second electrode  720  is accommodated. 
     The plurality of partition walls  752  may include a first partition wall  752   a  accommodating the first electrode  710  and the third electrode  730 , and a second partition wall  752   c  accommodating the second electrode  720 . The second electrode  720  being a high water level electrode may be shorter than the first electrode  710  and the third electrode  730  being a low water level electrode and a common electrode. Therefore, the second partition wall  752   c  accommodating the second electrode  720  may also be shorter than the first partition wall  752   b.  That is, a structure of the plurality of partition walls  752  of the air cap  750  may be a step structure. 
     Accordingly, a space may be partitioned by the middle partition wall  752   a  so that the high water level electrode, the low water level electrode, and the common electrode may be separated from each other. 
     The air cap  750  may form an air pocket  753  in the space accommodating the portions of the first, second, and third electrodes  710 ,  720 , and  730  and the first, second, and third electrode coatings  771 ,  772 , and  773 . That is, the air pocket  753  may be formed in an inside space defined by the plurality of partition walls  752 . The air pocket  753  may prevent the portions of the first electrode  710  and the first electrode coating  771  and the third electrode  730  and the third electrode coating  773  from contacting water. Also, the air pocket  753  may prevent the portions of the second electrode  720  and the second electrode coating  772  from contacting water. 
     By preventing the electrodes and the electrode coatings from contacting water, formation of scale may be prevented. Also, because no scale is formed, electric conduction between the electrodes may be prevented, thereby avoiding wrong operations of the water level sensors. 
     The water level sensor  700  may include a plurality of sealing members  760 . The sealing members  760  may be positioned between the housing  740  and the air cap  750 . Also, the sealing members  760  may be installed at a connected portion between a flange  751  of the air cap  750  and the plurality of partition walls  752 . The sealing members  760  may prevent air from flowing between inside and outside of the air cap  750 , thereby forming the air pocket  753  inside the air cap  750 . The sealing members  760  may be made of a resin material such as rubber having elasticity, although not limited thereto. 
     By separating the plurality of electrodes for sensing a water level from each other, the water level sensor may be prevented from wrongly sensing a water level due to scales formed between the electrodes. 
     By preventing portions of the electrodes from contacting water through an air cap structure, formation of scale may be prevented. 
     By increasing exposed portions of the electrodes to widen sensing areas, a water level may be sensed even though scale is formed at ends of the electrodes. 
     So far, although the technical concept of the disclosure has been described based on specific embodiments, the scope of rights of the disclosure is not limited to these embodiments. 
     It should be interpreted that various embodiments modified or changed by a person skilled in the art within a scope not deviating from the gist of the disclosure as the technical concept of the disclosure, which is defined in the claims, also belong to the scope of rights of the disclosure.