Patent Publication Number: US-2022233977-A1

Title: Measurement unit and filter device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Patent Application No. PCT/JP2020/041180 filed on Nov. 4, 2020, which claims priority to Japanese Patent Application No. 2019-201240 filed on Nov. 6, 2019, the entire contents of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a measurement unit and a filter device. 
     BACKGROUND ART 
     Patent Document 1 discloses a filter device in which an IC tag is provided to inside an upper plate provided so as to cover an upper end face of a filtration member in a filter element. An antenna is provided on a lid body that covers a case into which the filter element is inserted. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: JP 2019-115879 A 
       
    
     In recent years, to reduce mounting of an imitation and management of an operating time and the like, attaching an IC tag to a filter element has been demanded. However, in the invention described in Patent Document 1, an antenna assembly needs to be mounted on the lid body alone. Therefore, to provide a measurement portion, such as a differential pressure sensor, to the filter device, the two members, the measurement portion and the antenna assembly, need to be mounted on the lid body, and this caused an issue of a mistake in mounting and an increase in cost. 
     SUMMARY OF INVENTION 
     One or more embodiments of the present invention provide a measurement unit and a filter device with which a measurement portion and an antenna can be provided in the filter device by simply mounting one member. 
     A measurement unit according to one or more embodiments of the present invention includes, for example, a case, an antenna unit, and a sensor. The case has a pillar shape. The antenna unit includes an antenna. The antenna is provided on the case and is configured to communicate with an IC tag. The sensor is provided in the case. The antenna unit is provided in a vicinity of a first end face as one end of the case. The sensor is provided inside the case and a back side of the case with respect to the antenna unit. 
     According to the measurement unit according to one or more embodiments of the present invention, the antenna unit that includes the antenna configured to communicate with the IC tag is provided in the vicinity of the first end face as one end of the case. The sensor is provided inside the case and a back side of the case with respect to the antenna unit. As a result, a measurement portion and the antenna can be provided in a filter device by simply mounting one member. 
     Here, when viewed along a center axis of the case, the antenna unit may overlap with the sensor. Accordingly, the measurement unit can be downsized. 
     Here, the case may have a first hole, a second hole, a third hole, and a fourth hole. The first hole may open at the first end face. The second hole may open at a second end face as an end face on a side opposite to the first end face. The third hole is a through-hole having an end opening to the first hole and another end opening to the second hole. The fourth hole may open to the second hole. When viewed along a center axis of the case, the third hole may be non-overlapping with the fourth hole. The antenna unit may have a plate shape. A wiring pattern as the antenna may be formed on one surface of the antenna unit. The antenna unit may be provided in the first hole such that the antenna is exposed from an opening of the first hole. An antenna wire having one end connected to the antenna unit may be provided in the third hole. At least the sensor or a first substrate to which the sensor is connected may be disposed in the fourth hole. As a result, the antenna unit, the antenna wire, and the measurement portion (the sensor or the first substrate) can be provided at different positions, and both functions of the antenna and the measurement portion can be achieved. 
     Here, a cover covering an opening of the second hole may be provided. A second substrate may be provided in the second hole. The antenna wire may have another end provided on the second substrate. The first substrate may be provided in the fourth hole. A power cable that penetrates the cover and supplies the first substrate and the second substrate with power may be provided. This allows supplying the power to the two substrates with one set of the power cables and saving a space. 
     Here, the case may have a fifth hole and a sixth hole. The fifth hole may open to the first hole. The sixth hole is a through-hole having an end opening to a side surface of the case and another end opening to a vicinity of a bottom surface of the fifth hole. The antenna unit may have a diameter a same as an inner diameter of the first hole. The antenna unit may have a seventh hole that is a through-hole, which penetrates in a direction crossing an axial direction of the antenna. A spool and an elastic member may be provided in the fifth hole. The spool may be slidable inside the fifth hole and divide the fifth hole into a first space and a second space and divide the fifth hole into a first space and a second space. The elastic member may urge a force in a direction heading from the first space to the second space on the spool. A magnet may be provided on a surface facing the bottom surface of the fifth hole of the spool. The first substrate may be provided in the fourth hole. A magnetic field detection element as the sensor may be provided on the first substrate. The magnetic field detection element may be provided at a position overlapping with the magnet when viewed along a center axis of the case. This allows providing the differential pressure detection unit and the antenna unit inside one measurement unit. 
     Here, the case may have a fifth hole, a sixth hole, a seventh hole, and an eighth hole that is a through-hole. The fifth hole may open to the first hole. The sixth hole may open to the fifth hole. The seventh hole may communicate between a side surface of the case and the sixth hole. The eighth hole may have an end opening to the fourth hole and another end opening to the fifth hole. The antenna unit may have a diameter a same as an inner diameter of the first hole. The antenna unit may have a seventh hole, which penetrates in a direction crossing an axial direction of the antenna. A strain gauge as the sensor may be provided on a bottom surface of the fifth hole. In the eighth hole, a cable having one end connected to the strain gauge may be provided. This allows providing a pressure detection unit and the antenna unit inside one measurement unit. 
     Here, the fourth hole may have a bottom surface adjacent to a bottom surface of the first hole or the first end face. A temperature detection element as the sensor may be provided in a vicinity of the bottom surface of the fourth hole. This allows providing a temperature detection unit and the antenna unit inside one measurement unit. 
     Here, the antenna may have a circular ring shape in a plan view. The case may have a first hole and a second hole. The first hole may open at a second end face as an end face on a side opposite to the first end face. The second hole may be a through-hole having an end opening to the first hole and another end opening at the first end face. The antenna unit may include a cover. The cover may be made of an elastically deformable material and internally includes the antenna unit. The cover may have a protrusion portion inserted into a hollow portion of the antenna. The protrusion portion may be provided in the first hole to provide the cover on the first end face. The antenna unit may be provided adjacent to the first end face. The protrusion portion may have a third hole. The third hole may penetrate penetrates in a direction crossing an axial direction of the protrusion and communicate between outside of the case and the tenth hole. At least the sensor or a first substrate to which the sensor is connected may be provided inside the first hole. As a result, the antenna unit, the antenna wire, and the sensor can be provided at different positions, and both functions of the antenna and the measurement portion can be achieved. In addition, the antenna is covered with the cover and the antenna does not contact liquid, and thus the IC tag is easily read. 
     A filter device according to the present invention includes, for example, the measurement unit, a filter case, a head, and a filter element. The filter case has a bottomed tubular shape with an open upper end. The head is provided on the case so as to cover the upper end of the filter case. The filter element is provided in an internal space formed by the filter case and the head. The filter element includes a filtration member and an upper plate. The filtration member has a tubular shape. The upper plate is provided so as to cover an upper end face of the filtration member. The IC tag is provided on the upper plate. The measurement unit is provided on the head such that the antenna unit is exposed to a space formed by the filter case, the filter element, and the head. 
     According to the filter device according to the present invention, the head is provided on the case so as to cover the opening of the upper end of the filter case having the bottomed tubular shape. The filter element is provided in the internal space formed by the filter case and the head. The IC tag is provided on the upper plate of the filter element. The measurement unit is provided on the head such that the antenna unit is exposed to the space formed by the filter case, the filter element, and the head. This allows measurement (such as a pressure and a temperature) of the filter device and reading of the IC tag simultaneously by simply mounting the measurement unit. Additionally, since the antenna is exposed to the space formed by the filter case, the filter element, and the head, recording information of the IC tag can be accurately read even with the metallic head. 
     Here, the upper plate may include a plate-shaped portion and a protruding portion. The filtration member may abut on a lower side of the plate-shaped portion. The protruding portion may be provided on the plate-shaped portion. The IC tag may be provided inside the protruding portion. The protruding portion may project upward from the plate-shaped portion. The protruding portion may project outside the plate-shaped portion when viewed along a center axis of the filter element. The head may have a projection to position the protruding portion in a circumferential direction. The measurement unit may be provided in a vicinity of the projection. With the filter element inserted into an inside of the case, the IC tag may be adjacent to the antenna unit. In this way, the IC tag and the antenna can be disposed as close as possible, and the recording information of the IC tag can be accurately read even with the use of an IC tag (for example, an HF band) having a short transmission distance (a detection distance). 
     Here, the IC tag may have a hollow circular plate shape. The IC tag may have an inner diameter greater than an inner diameter of the filtration member. The IC tag may be provided on the upper plate with an adhesive that bonds the filtration member and the upper plate together. This eliminates a need for positioning the IC tag and the antenna and facilitates assembly of the return filter. In the present embodiment, since positioning of the IC tag and the antenna is unnecessary, the measurement unit can be provided at various positions. 
     According to one or more embodiments of the present invention, the measurement portion and an antenna assembly are allowed to be provided in the filter device by simply mounting one member. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an outline of a return filter  1  as an embodiment of the present invention. 
         FIG. 2  is a perspective view when a plate  24  and a tubular portion  31  of the return filter  1  are viewed obliquely from above and is a partially enlarged view. 
         FIG. 3  is a cross-sectional view illustrating an outline of a measurement unit  2  as an embodiment of the present invention. 
         FIG. 4  is a diagram illustrating an outline of the measurement unit  2  when viewed from a distal end side. 
         FIG. 5  is a cross-sectional view illustrating an outline of a measurement unit  3  as an embodiment of the present invention. 
         FIG. 6  is a cross-sectional view illustrating an outline of a pressure detection unit  80 . 
         FIG. 7  is a plan view of a diaphragm portion  82 . 
         FIG. 8  is a cross-sectional view illustrating an outline of a measurement unit  4  as an embodiment of the present invention. 
         FIG. 9  is a cross-sectional view illustrating an outline of a measurement unit  5  as an embodiment of the present invention. 
         FIG. 10  is a cross-sectional view of a return filter  6  as an embodiment of the present invention and is a partially enlarged view. 
         FIG. 11  is a plan view of a filter element  20 A and is a drawing in which a plate  24 A is partially cut off. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. While the following embodiments are described with an example of a return filter provided in a tank that stores hydraulic oil, a filter device of the present invention is not limited to the return filter, and, for example, can be used for a fuel filter. In the present embodiment, while hydraulic oil is described as an example of liquid to be filtered, the liquid to be filtered is not limited to the hydraulic oil. 
     First Embodiment 
       FIG. 1  is a cross-sectional view illustrating an outline of a return filter  1 .  FIG. 1  partially omits hatching indicating a cross section. 
     The return filter  1  mainly includes a measurement unit  2 , a filter case  10 , a filter element  20 , a head  30 , and an IC tag  40 . The IC tag  40  is a small electronic component that can communicate with an antenna  51   a  (described in detail later) and contactlessly reads and writes data in a built-in memory using radio waves received from the antenna  51   a.    
     The filter case  10  is made of a metal having high corrosion resistance (for example, stainless steel). The filter case  10  is provided inside the tank so as to protrude from a top surface of the tank to a lower side (−z side). 
     The filter case  10  has a bottomed substantially tubular shape and has an open upper end face. The filter case  10  is hollow inside and internally includes the filter element  20 , and the like. 
     The filter case  10  has a bottom surface  11 . An outflow portion  12  is provided so as to penetrate the bottom surface  11 . The outflow portion  12  causes a space (a space S 2 ) inside the filter element  20  to communicate with a space outside the filter case  10 . 
     The outflow portion  12  mainly includes a cylindrical portion  12   a  having a substantially bottomed cylindrical shape and a back pressure valve  12   b.    
     The cylindrical portion  12   a  has a substantially bottomed cylindrical shape as a whole, and includes a first cylindrical portion  12   c , a second cylindrical portion  12   d , and a bottom surface  12   e . The second cylindrical portion  12   d  is provided on a lower side of the first cylindrical portion  12   c . An outflow hole  12   f  is formed in the first cylindrical portion  12   c , and an outflow hole  12   g  is formed in the second cylindrical portion  12   d . The outflow holes  12   f  and  12   g  are formed on side surfaces of the first cylindrical portion  12   c  and the second cylindrical portion  12   d  so as to penetrate the first cylindrical portion  12   c  and the second cylindrical portion  12   d , respectively. 
     The back pressure valve  12   b  is provided inside the cylindrical portion  12   a . The back pressure valve  12   b  mainly includes a valve body  12   h  having a substantially plate shape and an elastic member  12   i  provided between the bottom surface  12   e  and the valve body  12   h . The elastic member  12   i  pushes the valve body  12   h  to a valve seat  12   j  provided between the first cylindrical portion  12   c  and the second cylindrical portion  12   d.    
     The filter case  10  includes a mounting member  13  provided in the vicinity of an upper end thereof. The filter case  10  is provided in a tank  100  via the mounting member  13 . Furthermore, the mounting member  13  integrates the filter case  10  and a tubular portion  31  (described in detail later). 
     An inflow portion (not illustrated) is provided on a side surface of the filter case  10 . The inflow portion causes hydraulic oil to flow into a space (a space SD inside the filter case  10  and outside the filter element  20 . 
     The filter element  20  is a member having a bottomed substantially tubular shape and provided in an internal space formed by the filter case  10  and the head  30 . The filter element  20  mainly includes a filtration member  21 , an inner tube  23 , a plate  24 , a plate  25 , and the IC tag  40 . 
     The filtration member  21  is a member having a substantially cylindrical shape and having openings at both ends. The filtration member  21  is formed by pleating a filter paper using, for example, synthetic resin or a paper, and connecting both ends of the pleated filter paper to form a cylindrical shape. The inner tube  23  where holes through which the hydraulic oil passes are formed in a substantially entire area is provided inside the filtration member  21 . An outer tube having holes through which the hydraulic oil passes may be formed in a substantially entire area outside the filtration member  21 . 
     The plate  24  made of resin is provided on an end on the upper side of the filtration member  21 . The plate  24  covers upper end faces of the filtration member  21  and the inner tube  23 . An adhesive  45  is applied between the plate  24  and the filtration member  21 , and the plate  24  and the filtration member  21  are bonded with the adhesive  45 . Although the adhesive  45  is liquid when applied, the adhesive  45  hardens over time to bond the plate  24  and the filtration member  21  together. Here, various types of organic adhesives using resin, rubber, and elastomer as the main material can be used as the adhesive  45 . 
     The plate  24  mainly includes a plate-shaped portion  24   a  having a substantially circular plate shape provided along an upper end face of the filtration member  21  such that the filtration member  21  abuts on a lower side of the plate-shaped portion  24   a , a tubular portion  24   b  provided on outer peripheral edge of the plate-shaped portion  24   a , a tubular portion  24   c  provided on an inner peripheral edge of the plate-shaped portion  24   a , and a protruding portion  24   d  provided in the plate-shaped portion  24   a . The tubular portion  24   b  protrudes downward (in the −z direction) from the plate-shaped portion  24   a . The tubular portion  24   c  protrudes upward (in the +z direction) from the plate-shaped portion  24   a.    
     The protruding portion  24   d  protrudes upward from the plate-shaped portion  24   a . Also, the protruding portion  24   d  protrudes outside the plate-shaped portion  24   a  when viewed along a center axis ax 1  of the filter element  20  (when viewed along the z-direction). 
     The IC tag  40  is coated with resin. The IC tag  40  is provided on the plate  24  so as to not contact the hydraulic oil. In the present embodiment, the IC tag  40  is provided inside the protruding portion  24   d.    
     The plate  25  is provided on an end on the lower side of the filtration member  21 . The plate  25  is a member having a substantially hollow circular plate shape that covers lower end faces of the filtration member  21  and the inner tube  23 . A recessed portion  25   a  into which the filtration member  21  is inserted is formed in a surface of the upper side of the plate  25 . The adhesive  45  is applied between the recessed portion  25   a  and the filtration member  21 , and the recessed portion  25   a  and the filtration member  21  are bonded with the adhesive  45 . 
     The outflow portion  12  is inserted into a hole  25   b  formed in the substantially center of the plate  25 . The hole  25   b  and the outflow portion  12  are sealed by a sealing member (for example, an O-ring)  46 . 
     The head  30  is provided on the outside of the tank. The head  30  is provided on the filter case  10  (here, the mounting member  13 ) and the plate  24  (here, the tubular portion  24   c ) so as to cover the opening of the upper end face of the filter case  10 . 
     The head  30  is made of a metal having high corrosion resistance (for example, stainless steel). The head  30  mainly includes the tubular portion  31 , a cover  32 , and a mounting portion  33 . The tubular portion  31  has a substantially cylindrical shape and is fixed to the filter case  10  (here, the mounting member  13 ). The cover  32  is a substantially plate-shaped member and provided on the upper side (+z-side) of the tubular portion  31  so as to cover a hollow portion of the tubular portion  31 . The cover  32  is detachably provided to the tubular portion  31 . 
     The tubular portion  31  has an inner diameter greater than an outer diameter of the plate  24  (the plate-shaped portion  24   a  and the tubular portion  24   b ). The tubular portion  31  has a side surface on which a hole  31   a  penetrating the side surface is provided. By inserting and fixing the measurement unit  2  to the hole  31   a , the measurement unit  2  is provided on the tubular portion  31 . In the present embodiment, the measurement unit  2  is provided in the hole  31   a  by screwing an external thread portion  50   q  (see  FIG. 3 ) provided on the outer peripheral surface of the measurement unit  2  and an internal thread portion (not illustrated) formed in the hole  31   a . The measurement unit  2  includes an antenna  51   a . Accordingly, the antenna  51   a  can be easily mounted to the head  30 . Note that  FIG. 1  omits detailed illustration of the internal structure of the measurement unit  2 . 
     The mounting portion  33  is provided on the cover  32 . The mounting portion  33  is a substantially tubular member, and protrudes to the lower side (−z-side) of the cover  32 . A valve  47  is provided on a lower side of the mounting portion  33 . The valve  47  is usually closed. When the filtration member  21  becomes clogged and a pressure inside the filter case  10  increases, the valve  47  opens to flow the hydraulic oil from the space S 1  to the space S 2  to reduce damage of the return filter  1 . Since the valve  47  is already known, a description thereof is omitted. 
     The mounting portion  33  is inserted into the tubular portion  24   c , and the valve  47  is inserted into the space S 2 . The mounting portion  33  and the tubular portion  24   c  are sealed by a sealing member (for example, an O-ring)  48 . 
     A flow path  34  for air and hydraulic oil is provided in the head  30 . The flow path  34  is constituted by a hole formed in the mounting portion  33 , a hole formed in the cover  32 , and a hole formed in the tubular portion  31 , and communicates between the internal space of the tank  100  and the space S 2  via a hole  100   a  formed in the tank  100 . 
     Further, the head  30  has a hole  35  that communicates between the space S 2  and the hole  31   a . The hole  35  is constituted by a hole formed in the mounting portion  33 , a hole formed in the cover  32 , and a hole formed in the tubular portion  31 , and has one end opening to the side surface of the hole  31   a . The hole  35  communicates with a hole  50   f  (see  FIG. 3 , described in detail later) provided in the measurement unit  2 , and communicates between a space S 4  (see  FIG. 3 , described in detail later) inside the measurement unit  2  and the space S 2 . 
       FIG. 2  is a perspective view when the plate  24  and the tubular portion  31  are viewed obliquely from above and is a partially enlarged view. 
     The substantially cylindrical IC tag  40  is provided inside the protruding portion  24   d . Avoiding a contact of the IC tag  40  with liquid allows reducing a failure of the IC tag  40 . 
     Into the hole  31   a , which penetrates the side surface of the tubular portion  31 , the measurement unit  2  is inserted. The antenna  51   a  (described in detail later) is exposed from an opening on the inner peripheral surface side of the hole  31   a.    
     Two projections  31   b  are formed on the side surface of the tubular portion  31 . The projections  31   b  are formed on an inner peripheral surface of the tubular portion  31  so as to protrude inward in a radial direction. The projections  31   b  are formed in the vicinity of the hole  31   a , here, on both sides of the hole  31   a . That is, the measurement unit  2  is provided in the vicinity of the projections  31   b.    
     The projections  31   b  position the protruding portion  24   d  in the circumferential direction. The protruding portion  24   d  is disposed between the two projections  31   b . Therefore, the IC tag  40  and the antenna  51   a  are provided adjacent to one another and disposed as close as possible, and thus sensing can be reliably performed. 
     Especially, since the protruding portion  24   d  protrudes upward from the plate-shaped portion  24   a  in side view and protrudes outward of the plate-shaped portion  24   a  in plan view, in the state where the filter element  20  is inserted into the inside of the filter case  10 , the distance between the IC tag  40  and the antenna  51   a  is close, and the end face of the IC tag  40  faces the antenna  51   a . Therefore, the IC tag  40  and the antenna  51   a  are disposed as close as possible, and thus sensing can be more reliably performed even the use of the inexpensive IC tag  40 . 
     Next, the measurement unit  2  is described.  FIG. 3  is a cross-sectional view illustrating an outline of the measurement unit  2 .  FIG. 3  partially omits hatching indicating a cross section. 
     The measurement unit  2  mainly includes a case  50 , an antenna unit  51 , and a differential pressure detection unit  60 . The differential pressure detection unit  60  mainly includes a detection unit  61 , a spool  62 , a magnet  63 , and an elastic member  64 . 
     The case  50  has a substantially columnar shape, and is provided with the antenna  51   a  (described in detail later) and a magnetic field detection element  61   c  (equivalent to a sensor of the present invention). 
     Additionally, the case  50  has a hole  50   a  (equivalent to a first hole of the present invention), a hole  50   b  (equivalent to a second hole of the present invention), holes  50   c  (equivalent to a third hole of the present invention), a hole  50   d  (equivalent to a fourth hole of the present invention), a hole  50   e  (equivalent to a fifth hole of the present invention), and the hole  50   f  (equivalent to a sixth hole of the present invention). The case  50  has respective end faces  50   m  and  50   n  at both ends. The hole  50   a  opens to the end face  50   m , and the hole  50   b  opens to the end face  50   n . The holes  50   c  have both ends opening to the hole  50   a  and the hole  50   b . The hole  50   d  opens to the hole  50   b , and the hole  50   e  opens to the hole  50   a . The hole  50   f  penetrates the case  50  in the radial direction, and has both ends opening to the vicinity of the bottom surface of the hole  50   e  and an outer peripheral surface of the case  50 . The hole  50   a , the hole  50   b , the hole  50   c , the hole  50   d , the hole  50   e , and the hole  50   f  each have a substantially cylindrical shape. 
     The antenna unit  51  is provided in the hole  50   a . The antenna unit  51  has a substantially plate shape and a diameter substantially the same as an inner diameter of the hole  50   a . The antenna unit  51  has a hole  51   d  that penetrates in the thickness direction, and the hole  50   e  communicates with the hole  50   a  via the hole  51   d.    
     A recessed portion  50   g  is provided on a bottom surface of the hole  50   a , and a sealing member (here, an O-ring)  53  is provided on the recessed portion  50   g . Additionally, a recessed portion  50   h  is provided on the side surface of the hole  50   a , and a retainer ring  52  is provided on the recessed portion  50   h . The sealing member  53  has elasticity and pushes the antenna unit  51  toward the retainer ring  52  side, and thus the antenna unit  51  is provided inside the hole  50   a.    
       FIG. 4  is a diagram illustrating an outline of the measurement unit  2  when viewed from a distal end side (the end face  50   m  side). The antenna  51   a  is provided on the surface on the end face  50   m  side of the antenna unit  51 . The antenna  51   a  is a wiring pattern (an antenna coil pattern) formed on one surface of the antenna unit  51 , and antenna terminals  51   b  and  51   c  are provided on both ends. An antenna wire  54  (see  FIG. 3 ) is connected to the respective antenna terminals  51   b  and  51   c.    
     The antenna unit  51  is provided inside the hole  50   a  such that the antenna  51   a  is exposed from the opening of the hole  50   a . As a result, when the measurement unit  2  is provided on the head  30 , the antenna  51   a  is exposed to a space formed by the filter case  10 , the filter element  20 , and the head  30 . 
     The description will now return to  FIG. 3 . The two holes  50   c  are provided adjacent to one another in the case  50 , and the antenna wire  54  is provided to the respective holes  50   c . A substrate  55 , which is an IC tag reader substrate (equivalent to a second substrate of the present invention), is provided in the hole  50   b . The antenna wire  54  has one end provided on the antenna unit  51  and the other end provided on the substrate  55 . 
     A filler  58  is provided inside the holes  50   c  so as to cover the antenna wire  54 , and an insulating pipe  59  is provided outside the filler  58 . The filler  58  is provided so as to cover the insulating pipe  59 . The filler  58  is, for example, an adhesive having a high polymer compound as a main component. Filling the inside of the holes  50   c  with the filler  58  in this manner avoids the liquid to leak from the holes  50   c  to the hole  50   d.    
     Note that in the present embodiment, the insulating pipe  59  is provided separately from the antenna wire  54 , but the insulating pipe  59  is unnecessary as long as the antenna wire  54  has an insulating function. In this case, the holes  50   c  may be filled with the filler  58  so as to cover the antenna wire  54 . 
     For example, an IC chip (not illustrated) is mounted on the substrate  55 . When the substrate  55  receives a radio wave from the IC tag  40 , the substrate  55  generates a reception signal via the antenna wire  54 , and outputs a signal to the outside of the measurement unit  2  via a signal line  71 . 
     The detection unit  61  is provided in the hole  50   d . When viewed along a center axis ax 2  of the case, the holes  50   c  and the hole  50   d  are disposed so as not to overlap. 
     The detection unit  61  has a substantially cylindrical shape, and an external thread portion  61   a  is formed around the detection unit  61 . By screwing the external thread portion  61   a  into an internal thread portion  50   r  formed in the hole  50   d , the detection unit  61  is provided inside the hole  50   d , and a height of the detection unit  61  (a position in the direction along the center axis ax 2 ) can be adjusted. 
     A substrate  61   b  (equivalent to a first substrate of the present invention) is provided on a distal end side (the end face  50   m  side) of the detection unit  61 . A magnetic field detection element  61   c  is provided on the substrate  61   b . In other words, the substrate  61   b  and the magnetic field detection element  61   c  are provided in the hole  50   d . The magnetic field detection element  61   c  detects a change in magnetic field formed by the magnet  63 . As the magnetic field detection element  61   c , for example, a reed switch and a Hall element can be used. Since the reed switch and the Hall element has been already known, description of the reed switch and the Hall element will be omitted. 
     The spool  62 , the magnet  63 , and the elastic member  64  are provided inside the hole  50   e . When viewed along the center axis ax 2  of the case, the holes  50   c  and the hole  50   e  are disposed so as not to overlap. 
     The spool  62  is slidably provided inside the hole  50   e . The spool  62  divides the hole  50   e  into the space S 4  and a space S 5 . The space S 4  communicates with the space S 1  via the hole  50   f . The space S 5  communicates with the space S 2  via the hole  50   a.    
     The elastic member  64  is, for example, a coil spring, and has one end provided on the spool  62  and the other end provided on an E-ring  65  provided on the end face  50   m  side of the hole  50   e . The elastic member  64  urges a force in a direction heading from the space S 5  toward the space S 4  on the spool  62 . 
     Note that in the present embodiment, the elastic member  64  is provided on the end face  50   m  side with respect to the spool  62 , but the elastic member  64  may be provided on the end face  50   n  side with respect to the spool  62 . In this case, the elastic member  64  urges a force in a direction heading from the space S 4  toward the space S 5  on the spool  62 . 
     The magnet  63  is provided on a surface facing the bottom surface of the hole  50   e  of the spool  62 , that is, on the surface on the end face  50   n  side of the spool  62 . The magnetic field detection element  61   c  is provided at a position overlapping with the magnet  63  when viewed along the center axis ax 2  of the case  50 . 
     When, for example, clogging of the filtration member  21  does not occur, and a pressure difference between the space S 1  (the space S 4 ) and the space S 2  (the space S 5 ) is less than or equal to a threshold value, the spool  62  is at a position close to the magnetic field detection element  61   c  by the urging force of the elastic member  64 . 
     When pressure in the space S 1  (the space S 4 ) increases due to, for example, clogging of the filtration member  21 , the spool  62  moves to the end face  50   m  side against the urging force of the elastic member  64 . In accordance with this, the magnet  63  also moves toward the end face  50   m  side, and the magnet  63  moves away from the magnetic field detection element  61   c . The magnetic field detection element  61   c  detects a change in magnetic flux density due to the movement of the magnet  63 . 
     The detection result of the magnetic field detection element  61   c  is output to outside the measurement unit  2  via a signal line  74  having one end provided on the substrate  61   b . The signal line  71  and power cables  72  and  73  are provided adjacent to the signal line  74 . 
     The signal lines  71  and  74  and the power cables  72  and  73  penetrate a cover  56  covering the hole  50   b . The power cables  72  and  73  supply power to the substrate  61   b  and the substrate  55 , and are connected to the substrate  61   b  and the substrate  55  (not illustrated). This allows supplying the power to the two substrates (the substrate  61   b  and the substrate  55 ) with one set of the power cables  72  and  73  and saving a space. 
     Next, the functionality of the return filter  1  thus configured will be described. The dashed-two dotted line arrows in  FIG. 1  indicate a flow of the hydraulic oil. 
     When an operation of an internal combustion engine (an engine) of a work machine stops, the hydraulic oil is not contained in the filter case  10 . Thus, as illustrated in  FIG. 1 , the back pressure valve  12   b  is in the closed state. 
     While the engine is idling, a flow rate of the hydraulic oil is low, and therefore the back pressure valve  12   b  is in the closed state. Thus, the hydraulic oil flows into the filter case  10  from the inflow portion to rise the oil surface. 
     When the engine of the work machine operates, the flow rate of the hydraulic oil increases, the inside of the filter case  10  is filled with the hydraulic oil, and the pressure inside the filter case  10  becomes sufficiently high. As a result, the hydraulic oil pushes down the valve body  12   h  against the urging force of the elastic member  12   i.    
     The hydraulic oil flowing into the space S 1  flows from the outside to the inside of the filtration member  21 , and the filtration member  21  removes, for example, dust in the hydraulic oil. The hydraulic oil after filtration flows out to the space S 2 . After that, the hydraulic oil after filtration flows out from the outflow portion  12  to the inside of the tank. During idling, the hydraulic oil after filtration flows out from the outflow hole  12   f , and during engine operation, the hydraulic oil after filtration flows out from the outflow hole  12   f  and the outflow hole  12   g.    
     Note that when the engine of the work machine stops, the flow rate of the hydraulic oil decreases and the back pressure valve  12   b  is closed. When a certain amount of period has passed after the work machine stops, the air flows into the inside of the filter case  10  through the flow path  34 , thereby lowering the oil surface with respect to the bottom surface  11  without removing the cover  32  from the tubular portion  31 . 
     As illustrated in  FIG. 3 , in a state where the pressure of the high-pressure side (the space S 1 ) is low, the spool  62  is pushed to the back side (the high-pressure side, the end face  50   n  side) of the hole  50   e  due to the urging force of the elastic member  64 , and the magnet  63  is at a position closest to the bottom surface of the hole  50   e.    
     When pressure in the space S 1  increases due to, for example, clogging of the filtration member  21 , the spool  62  moves to the distal end side (the low pressure side, the end face  50   m  side) against the urging force of the elastic member  64 . The detection unit  61  detects a change in the magnetic field due to the movement of the magnet  63 , and transmits the detection result to an external device. In a case where the detection result of the differential pressure between the space S 1  and the space S 2  is a certain amount or more, that is, the clogging of the filtration member  21  exceeds a predetermined amount, the external device performs display prompting replacement of the filter element  20 . 
     Since the clogging of the filtration member  21  is substantially proportional to the operating time of the filter element  20 , the operating time of the filter element  20  is measured by the IC tag  40 , the antenna  51   a  reads the IC tag  40 , and the measurement unit  2  can transmit the read result to the external device. When the operating time of the filter element  20  has elapsed for a certain period of time, the external device provides a display prompting replacement of the filter element  20 . The filter element  20  after replacement is provided with the IC tag  40  different from that of the filter element  20  prior to the replacement, and the operating time of the filter element  20  after the replacement is measured by reading the IC tag  40 . In addition, when, for example, an imitation in which the IC tag  40  is not provided is used as the filter element after replacement, needless to say, the IC tag  40  cannot be read. The external device can perform error display or disable the operation of the filter device. For example, by reading the IC tag  40  provided on the filter element  20  after the replacement, the external device can determine that a filter element other than the predetermined filter element is provided. 
     According to the present embodiment, since the measurement unit  2  includes the differential pressure detection unit  60  and the antenna unit  51 , the differential pressure detection unit  60  and the antenna  51   a  can be provided in the return filter  1  by simply mounting the measurement unit  2  to the return filter  1 . In addition, since the configurations of the case  50  and the differential pressure detection unit  60  do not change from the conventional differential pressure detection device, not only the detection of the differential pressure between the space S 1  and the space S 2  but also reading the IC tag  40  provided in the filter element  20  is possible by only replacing the conventional differential pressure detection device with the measurement unit  2 . In particular, since the differential pressure detection unit  60  and the antenna unit  51  are disposed at different positions, the respective function of the differential pressure detection unit  60  and function of the antenna unit  51  are not limited. 
     In addition, with the present embodiment, the antenna  51   a  is exposed from the opening on the distal end side (the end face  50   m  side) of the measurement unit  2 . Therefore, disposing the measurement unit  2  in the hole  31   a  allows exposing the antenna  51   a  from the opening on the inner peripheral side of the hole  31   a  and accurately reading recording information of the IC tag  40  even with the metallic head  30 . Furthermore, since the IC tag  40  and the antenna  51   a  are adjacent to one another, the IC tag  40  and the antenna  51   a  can be disposed as close as possible, and the recording information of the IC tag  40  can be accurately read even with the use of an IC tag (for example, an HF band) having a short transmission distance (a detection distance). 
     In addition, according to the present embodiment, the antenna unit  51  overlaps with the magnetic field detection element  61   c  when viewed along the center axis ax 2 , and thus the measurement unit  2  can be downsized. 
     Note that, although the plate  24  is made of the resin in the present embodiment, the material of the plate  24  is not limited to the resin. The material of the plate  24  only needs to have high corrosion resistance, and may be, for example, a metal such as stainless steel. When the plate  24  is made of metal, it is preferred to expose an end face of the IC tag  40  from the plate  24  or form an opening in the plate  24  from which the IC tag  40  is exposed and close the opening with, for example, a resin. 
     Note that in the present embodiment, the substrate  55  and the substrate  61   b  are provided in the measurement unit  2 , but the substrate  55  is not essential. In a case where the substrate  55  is not provided, the antenna wire  54  is bent in a substantially U shape inside the hole  50   b , and an end not provided on the antenna unit  51  is provided on the substrate  61   b.    
     In addition, in the present embodiment, when viewed along the center axis ax 2 , the antenna unit  51  (the antenna  51   a ) overlaps with the magnetic field detection element  61   c , but when viewed along the center axis ax 2 , the antenna unit  51  need not overlap with the magnetic field detection element  61   c . For example, the antenna unit  51  may be reduced in size and the hole  50   a  may be configured so as not to overlap with the hole  50   e  when viewed along the center axis ax 2 , and the bottom surface of the hole  50   e  may open to the end face  50   m . However, to downsize the measurement unit  2 , when viewed along the center axis ax 2 , the antenna unit  51  preferably overlaps with a temperature detection element  92 . 
     In addition, in the present embodiment, the measurement unit  2  is provided on the return filter  1 , which is an example of the filter device, but the measurement unit  2  is provided not limited to the return filter  1 . For example, the measurement unit  2  may be provided in various filter devices having filtration members. The filter device also includes, for example, an air breather. The air breather has two filtration members provided concentrically. When the measurement unit  2  is provided in the air breather, the differential pressure detection unit  60  measures a differential pressure between the pressure inside the inner filtration member and an atmospheric pressure. 
     Second Embodiment 
     The second embodiment of the present invention has a configuration in which a pressure gauge is provided in a measurement unit. Hereinafter, a measurement unit  3  according to the second embodiment will be described. The measurement unit  3  is provided on the return filter  1 , similarly to the measurement unit  2 . Note that the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted. 
       FIG. 5  is a cross-sectional view illustrating an outline of the measurement unit  3 .  FIG. 5  partially omits hatching indicating a cross section. 
     The measurement unit  3  mainly includes a case  50 A, the antenna unit  51 , and a pressure detection unit  80 . The pressure detection unit  80  mainly includes a detection unit  81 , a diaphragm portion  82 , a fixing member  83 , and a mounting member  84 . 
     The case  50 A has a substantially columnar shape, and is provided with the antenna  51   a  and a strain gauge  82   c  (described in detail later, equivalent to the sensor of the present invention). 
     Additionally, the case  50 A has the hole  50   a , the hole  50   b , the holes  50   c , the hole  50   d , a hole  50   j , a hole  50   k  (equivalent to a seventh hole of the present invention), and a hole  50   j  (equivalent to an eighth hole of the present invention). The hole  50   k  penetrates the case  50 A in the radial direction, and has both ends opening to the vicinity of the bottom surface of the hole  50   j  and an outer peripheral surface of the case  50 A. The hole  50   j  opens to the hole  50   a . The hole  50   l  has both ends opening to the hole  50   d  and the hole  50   j.    
     When viewed along the center axis ax 2  of the case, the hole  50   c  is disposed not overlapping with the hole  50   j , and the hole  50   c  is disposed not overlapping with the hole  50   l.    
     The detection unit  81  is provided in the hole  50   d . The detection unit  81  has a substantially cylindrical shape, and an external thread portion  81   a  is formed around the detection unit  81 . By screwing the external thread portion  81   a  into the internal thread portion  50   r  formed in the hole  50   d , the detection unit  81  is provided inside the hole  50   d , and a height of the detection unit  81  (a position in the direction along the center axis ax 2 ) can be adjusted. A substrate  81   b  (equivalent to the first substrate of the present invention) is provided on a distal end side (the end face  50   m  side) of the detection unit  81 . In other words, the substrate  81   b  is provided in the hole  50   d.    
     The hole  50   j  has a hole  50   ja  (equivalent to the fifth hole of the present invention) having one end opening to the hole  50   a  and a hole  50   jb  (equivalent to the sixth hole of the present invention) provided on the back side (the end face  50   n  side) of the hole  50   ja  and having one end opening to the hole  50   ja . The diaphragm portion  82 , the fixing member  83 , and the mounting member  84  are mainly provided inside the hole  50   ja.    
     The diaphragm portion  82  is provided on a bottom surface of the hole  50   ja . The fixing member  83  is a substantially columnar-shaped member and abuts on the diaphragm portion  82 . The mounting member  84  is a substantially columnar-shaped member and abuts on the fixing member  83 . By screwing the external thread portion formed on the outer peripheral surface of the fixing member  83  into the internal thread portion  50   r  formed on the inner peripheral surface of the hole  50   ja , the diaphragm portion  82  and the fixing member  83  are provided inside the hole  50   ja , and the heights of the diaphragm portion  82  and the fixing member  83  can be adjusted. When the mounting member  84  presses up the diaphragm portion  82  and the fixing member  83 , the fixing member  83  is pressed against the diaphragm portion  82 , and the diaphragm portion  82  is pressed against the bottom surface of the hole  50   ja.    
     The fixing member  83  has a hole  83   a  substantially along a center axis ax 3  of the hole  50   ja , and the hole  83   a  is provided so as to penetrate the fixing member  83 . Furthermore, the mounting member  84  has a hole  84   a  substantially along the center axis ax 3 , and the hole  84   a  is provided so as to penetrate the mounting member  84 . As a result, the end face on the distal end side (the end face  50   m  side) of the diaphragm portion  82  communicates with the space S 2  (see  FIG. 1 ) via the hole  83   a , the hole  84   a , the hole  51   d , and the hole  50   a , and the liquid after filtration is guided to the end face on the distal end side of the diaphragm portion  82 . The diaphragm portion  82  will be described in detail later. 
     The hole  50   jb  has one end opening to the bottom surface of the hole  50   ja . One end of the hole  50   k  opens to the hole  50   jb , and thus an end face on the back side (the end face  50   n  side) of the diaphragm portion  82  communicates with the external space of the measurement unit  3 . 
     When viewed along the center axes ax 2  and ax 3 , the hole  83   a  and the hole  84   a  are provided at a position overlapping with the hole  50   jb  Thus, the diaphragm portion  82  can be deformed by a minute amount according to the pressure in the space S 2  to the atmospheric pressure. 
     The hole  50   l  is a hole substantially parallel to the hole  50   j , and has one end opening to the hole  50   ja  and the other end opening to the hole  50   d . When viewed along the center axis ax 2 , the hole  50   j  does not overlap with the hole  50   l . Cables  85   a  and  85   b  are provided inside the hole  50   l , and the cables  85   a  and  85   b  transmit electrical signals generated in the strain gauge  82   c  (described in detail later) to the detection unit  81  (the substrate  81   b ). 
       FIG. 6  is a cross-sectional view illustrating an outline of the pressure detection unit  80 .  FIG. 6  partially omits hatching indicating a cross section.  FIG. 7  is a plan view of the diaphragm portion  82 . In  FIG. 7 , the main parts of the diaphragm portion  82  are transparent. 
     The diaphragm portion  82  has a substantially circular plate shape, and is sandwiched between the bottom surface of the hole  50   ja  and the fixing member  83 . The diaphragm portion  82  mainly includes plates  82   a  and  82   b , the strain gauge  82   c , and an annular member  82   d.    
     The plates  82   a  and  82   b  are made of metal (for example, stainless steel), and are provided such that the strain gauges  82   c  and the annular member  82   d  are sandwiched between both sides. Here, the “plate-shaped member” refers to a member having a thickness with which, for example, bending and winding are difficult (for example, approximately 0.1 mm or more in the case of a metal plate), and thicker than a sheet and a film that can be bent or wound. In the present embodiment, the thicknesses of the plates  82   a  and  82   b  are approximately from 0.2 mm to approximately 0.3 mm. This allows a minute amount of deformation when subjected to pressure from the liquid, and can maintain the strength of the diaphragm portion  82 . 
     The strain gauge  82   c  is provided on the plate  82   a  or the plate  82   b  (the plate  82   a  in the present embodiment). The strain gauge  82   c  is provided with a metal wire therein, the metal wire expands and contracts in accordance with the extension and shrinkage of a measured object (here, the plate  82   a  or the plate  82   b ), and a change in an electrical resistance of the metal wire is measured to obtain the extension and shrinkage (a strain) of the measured object. The cables  85   a  aa  85   b  are connected to the metal wire. The diameter of the strain gauge  82   c  is preferably less than or equal to the diameters of the hole  50   j a and the hole  83   a.    
     The strain gauge  82   c  is provided in a hollow portion of the annular member  82   d . The annular member  82   d  is a plate-shaped member having a substantially circular ring shape, and has a thickness thicker than the strain gauge  82   c . Accordingly, an interval between the plate  82   a  and the plate  82   b  is wider than the thickness of the strain gauge  82   c . This allows reducing the strain gauge  82   c  from collapsing and accurately detecting amounts of deformation of the plate  82   a  and the plate  82   b  by the strain gauge  82   c.    
     In the annular member  82   d , a notch  82   e  through which the cables  85   a  and  85   b  pass is formed along the radial direction of the annular member  82   d . The cables  85   a  and  85   b  are guided to outside the diaphragm portion  82  via the notch  82   e  and a hole  82   f  formed in the plate  82   a.    
     A filling member  82   g  is filled between the plate  82   a  and the plate  82   b . Although the thickness of the strain gauge  82   c  is thinner than the thickness of the annular member  82   d  (the interval between the plate  82   a  and the plate  82   b ), filling the filling member  82   g  between the plate  82   a  and the plate  82   b  allows detecting the strain of the plate  82   b , in addition to the strain of the plate  82   a  by the strain gauge  82   c . The filling member  82   g  is a polymer material composed of a high molecular substance (typically having a molecular weight of 10000 or more), and contains, for example, resin and rubber. 
     A groove  50   o  is provided in the case  50 A so as to surround the hole  50   jb . In addition, a groove  83   c  is provided in a surface  83   b  abutting on the diaphragm portion  82  of the fixing member  83  so as to surround the hole  83   a . Elastically deformable annular packings  86  (for example, O-rings) are provided in a groove  11   c  and the groove  83   c . Additionally, a groove  83   e  is provided in a side surface  83   d  of the fixing member  83 , and an elastically deformable annular packing  87  (for example, an O-ring) is provided in the groove  83   e . The annular packings  86  and  87  avoid a liquid to leak from around the diaphragm portion  82 . 
     The description will now return to  FIG. 5 . The detection result of the strain gauges  82   c  is transmitted to the substrate  81   b  by the cables  85   a  and  85   b  and is output to outside the measurement unit  2  via the signal line  74  having one end provided on the substrate  81   b.    
     According to the present embodiment, since the measurement unit  3  includes the pressure detection unit  80  and the antenna unit  51 , the pressure detection unit  80  and the antenna  51   a  can be provided in the return filter by simply mounting the measurement unit  2  to the return filter  1 . In addition, since the configurations of the case  50 A and the pressure detection unit  80  do not change from the conventional differential pressure detection device, not only the detection of the pressure but also reading the IC tag  40  provided in the filter element  20  is possible by replacing the conventional differential pressure detection device with the measurement unit  3 . 
     Note that in the present embodiment, the substrate  55  and the substrate  81   b  are provided in the measurement unit  3 , but it is only necessary to provide either one of the substrate  55  and the substrate  81   b . In a case where the substrate  55  is not provided, it is only necessary that the antenna wire  54  is bent in a substantially U shape inside the hole  50   b  and an end not provided on the antenna unit  51  is provided on the substrate  81   b . In a case where the substrate  81   b  is not provided, it is only necessary to provide ends on the side not provided on the strain gauge  82   c  of the cables  85   a  and  85   b  on the substrate  55 . 
     In addition, in the present embodiment, when viewed along the center axis ax 2 , the antenna unit  51  (the antenna  51   a ) overlaps with the diaphragm portion  82  (the strain gauge  82   c ), but when viewed along the center axis ax 2 , the antenna unit  51  need not overlap with the diaphragm portion  82 . For example, the antenna unit  51  may be reduced in size and the hole  50   a  is configured to so as not to overlap with the hole  50   j  when viewed along the center axis ax 2 , and the bottom surface of the hole  50   j  may open to the end face  50   m . However, to downsize the measurement unit  3 , when viewed along the center axis ax 2 , the antenna unit  51  preferably overlaps with the diaphragm portion  82 . 
     Third Embodiment 
     The third embodiment of the present invention has a configuration in which a temperature detection element is provided in a measurement unit. Hereinafter, a measurement unit  4  according to the third embodiment will be described. The measurement unit  4  is provided on the return filter  1 , similarly to the measurement unit  2 . Note that the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted. 
       FIG. 8  is a cross-sectional view illustrating an outline of the measurement unit  4 .  FIG. 8  partially omits hatching indicating a cross section. 
     The measurement unit  4  mainly includes a case  50 B, the antenna unit  51 , and a temperature detection unit  90 . The temperature detection unit  90  mainly includes a detection unit  91 , the temperature detection element  92 , and cables  93 . 
     The case  50 B has a substantially columnar shape, and is provided with the antenna  51   a  and the temperature detection element  92  (equivalent to the sensor of the present invention). 
     Additionally, the case  50 B has the hole  50   a , the hole  50   b , the hole  50   c , and a hole  50   p  (equivalent to the fourth hole of the present invention). The hole  50   p  opens to the hole  50   b . When viewed along the center axis ax 2  of the case, the hole  50   c  and the hole  50   p  are disposed so as not to overlap. 
     The hole  50   p  includes a hole  50   pa  opening to the hole  50   b  and a hole  50   pb  provided on the back side (the end face  50   m  side) of the hole  50   pa  and opening to the hole  50   pa . The hole  50   pa  and the hole  50   pb  have center axes that are substantially matched, and a center axis ax 4  is substantially parallel to the center axis ax 2 . 
     The detection unit  91  is provided in the hole  50   pa . The detection unit  91  has a substantially cylindrical shape, and an external thread portion  91   a  is formed around the detection unit  91 . By screwing the external thread portion  91   a  into an internal thread portion formed in the hole  50   pa , the detection unit  91  is provided inside the hole  50   pa , and a height of the detection unit  91  can be adjusted. A substrate  91   b  is provided on a distal end side (the end face  50   m  side) of the detection unit  91 . In other words, the substrate  91   b  (equivalent to the first substrate of the present invention) is provided in the hole  50   d.    
     The bottom surface of the hole  50   pb  is adjacent to the bottom surface of the hole  50   a . The temperature detection element  92  is provided in the vicinity of the bottom surface of the hole  50   pb . The temperature detection element  92  measures a temperature of a liquid flowing into the hole  50   a . Note that, as the temperature detection element  92 , a thermistor, such as an NTC thermistor and a PTC thermistor, or various thermocouples can be used. 
     The cables  93  are provided in the hole  50   p . The cable  93  has one end provided on the temperature detection element  92  and the other end provided on the substrate  91   b . The cables  93  transmit electrical signals generated in the temperature detection element  92  to the detection unit  91 . The detection result of the temperature detection element  92  is output to outside the measurement unit  2  via the signal line  74  provided on the substrate  91   b.    
     According to the present embodiment, since the measurement unit  4  includes the temperature detection unit  90  and the antenna unit  51 , the temperature detection unit  90  and the antenna  51   a  can be provided in the return filter by simply mounting the measurement unit  4  to the return filter  1 . In addition, since the configurations of the case  50 B and the temperature detection unit  90  do not change from the conventional differential pressure detection device, not only the detection of the pressure but also reading the IC tag  40  provided in the filter element  20  is possible by replacing the conventional differential pressure detection device with the measurement unit  4 . 
     Note that in the present embodiment, the diameter of the hole  50   a  is large, and the hole  50   a  overlaps with the hole  50   p  when viewed along the center axis ax 2 . However, when viewed along the center axis ax 2 , the hole  50   a  and the hole  50   p  may be provided in the case  50 B such that the hole  50   a  does not overlap with the hole  50   p . That is, in the present embodiment, when viewed along the center axis ax 2 , the antenna unit  51  (the antenna  51   a ) overlaps with the temperature detection element  92 , but when viewed along the center axis ax 2 , the antenna unit  51  need not overlap with the temperature detection element  92 . At this time, the bottom surface of the hole  50   p  is preferably adjacent to the end face  50   m . However, to downsize the measurement unit  4 , when viewed along the center axis ax 2 , the antenna unit  51  preferably overlaps with the temperature detection element  92 . 
     Additionally, in the present embodiment, the substrate  55  and the substrate  91   b  are provided in the measurement unit  4 , but it is only necessary to provide either one of the substrate  55  and the substrate  91   b . In a case where the substrate  55  is not provided, it is only necessary that the antenna wire  54  is bent in a substantially U shape inside the hole  50   b  and an end not provided on the antenna unit  51  is provided on the substrate  91   b . In a case where the substrate  91   b  is not provided, it is only necessary to provide ends on the side not provided on the temperature detection element  92  of the cables  93  on the substrate  55 . 
     Fourth Embodiment 
     The fourth embodiment of the present invention has a configuration in which a configuration of a differential pressure detection unit provided in a measurement unit is different. Hereinafter, a measurement unit  5  according to the fourth embodiment will be described. The measurement unit  5  is provided on the return filter  1 , similarly to the measurement unit  2 . Note that the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted. 
       FIG. 9  is a cross-sectional view illustrating an outline of the measurement unit  5 .  FIG. 5  partially omits hatching indicating a cross section. Further,  FIG. 9  partially omits illustration of cables. 
     The measurement unit  5  mainly includes a case  50 C, an antenna unit  51 A, the substrate  55 , a cover  57 , and a differential pressure detection unit  60 A. 
     The case  50 C has a substantially tubular shape, and has a hole  50   s , a hole  50   t , a hole  50   u , a hole  50   v , and a hole  50   w . The hole  50   w  opens to the hole  50   t . The hole  50   s  has one end opening to the bottom surface of the hole  50   w  and the other end opening to the end face  50   m  to communicate between the hole  50   t  and an external space of the measurement unit  5 . The hole  50   v  penetrates the case  50 C in the radial direction, and has both ends opening to the hole  50   w  and an outer peripheral surface of the case  50 C. 
     The substrate  55  is provided in the hole  50   t , and the differential pressure detection unit  60 A is provided in the holes  50   t  and  50   w.    
     The antenna unit  51 A has a substantially circular ring shape and is provided at the distal end of the case  50 C. Here, the end face  50   n  side of the case  50 C is referred to as a root side, and the end face  50   m  side of the case  50 C on the side opposite to the root side is referred to as a distal end side. 
     The antenna unit  51 A mainly includes an antenna (for example, an antenna coil pattern)  50   e  and the cover  57 . An antenna  51   e  is a plate-shaped member having a substantially circular plate shape in plan view, and can communicate with the IC tag  40 . One end of an antenna wire (not illustrated) is provided on the antenna unit  51 A. This antenna wire is provided inside the hole  50   u , and the other end of the antenna wire is provided on the substrate  55 . 
     The substrate  55  is provided inside the hole  50   t  and mounts, for example, an IC chip (not illustrated). When the substrate  55  receives a radio wave from the IC tag  40 , the substrate  55  generates a reception signal and outputs a signal via a signal line (not illustrated). 
     The filler  58  is provided inside the hole  50   u  so as to cover the antenna wire. The filler  58  is, for example, an adhesive having a high polymer compound as a main component. Filling the inside of the hole  50   u  with the filler  58  in this manner avoids a fuel to leak from the case  50 C. 
     The cover  57  is formed of an elastically deformable material, and internally includes a substantially bottomed tubular insertion portion  57   a  in which the antenna  51   e  is provided. Additionally, the cover  57  includes a protrusion portion  57   b  inserted into an inside of a hole  57   d  at the center of the insertion portion  57   a.    
     The insertion portion  57   a  has an open upper end, and insertion of the protrusion portion  57   b  into the hole  50   s  covers the upper end of the insertion portion  57   a  with the case  50 C. In this way, the antenna  51   e  is disposed adjacent to the end face  50   m , and a contact of hydraulic oil with the antenna  51   e  can be avoided. Further, since a sealing member (for example, an o ring)  53 A is provided between the protrusion portion  57   b  and the hole  57   d , the antenna unit  51 A is fixed to the distal end of the case  50 C. 
     A mounting member  66  is provided in the holes  50   t  and  50   w . The mounting member  66  includes a hole  66   a  provided on the root side (the substrate  55  side), and a hole  66   c  provided on the distal end side (the antenna unit  51 A side). The detection unit  61  of the differential pressure detection unit  60 A is provided in the hole  66   a.    
     The differential pressure detection unit  60 A mainly includes the detection unit  61 , the spool  62 , the magnet  63 , and the elastic member  64 . 
     By screwing the external thread portion  61   a , which is formed around the detection unit  61 , into an internal thread portion  66   b  formed in the hole  66   a , the detection unit  61  is provided inside the hole  66   a , and a height of the detection unit  61  (a position in the direction along the center axis ax 2 ) can be adjusted. 
     The substrate  61   b  is provided on the distal end side of the detection unit  61 . The magnetic field detection element  61   c  is provided on the substrate  61   b.    
     In other words, the substrate  61   b  and the magnetic field detection element  61   c  are provided in the hole  50   t  via the mounting member  66 . 
     The spool  62  slides along an inner peripheral surface  67   a  of a sliding member  67  of the tubular portion provided inside the hole  50   w.    
     The elastic member  64  is, for example, a coil spring, and has one end provided on the spool  62  and the other end provided on the bottom surface of the hole  50   w . The elastic member  64  urges the force in the direction of pushing up the spool  62  on the spool  62 . 
     A space between the bottom surface of the hole  50   w  and the spool  62  communicates with the space S 1  (see  FIG. 1 ) via a hole  57   c  of the cover  57 . The space between the spool  62  and the hole  66   c  communicates with the space S 2  (see  FIG. 1 ) via the hole  50   v.    
     The magnet  63  is provided on a surface facing the bottom surface of the hole  50   w  of the spool  62 , that is, on the surface on the detection unit  61  side of the spool  62 . When viewed along the center axis ax 2  of the case  50 C, the magnetic field detection element  61   c  is provided at a position overlapping with the magnet  63 . 
     When pressure in the space S 2  (see  FIG. 1 ) increases due to, for example, clogging of the filtration member  21 , the spool  62  moves to the lower side in  FIG. 9  against the urging force of the elastic member  64 . In accordance with this, the magnet  63  also moves toward the lower side in  FIG. 9 , and the magnet  63  moves away from the magnetic field detection element  61   c . The magnetic field detection element  61   c  detects a change in magnetic flux density due to the movement of the magnet  63 . 
     According to the present embodiment, since the measurement unit  5  includes the differential pressure detection unit  60 A and the antenna unit  51 A, the differential pressure detection unit  60 A and the antenna can be provided in the return filter  1  by simply mounting the measurement unit  5  to the return filter  1 . In addition, since the configurations of the case  50 C and the differential pressure detection unit  60 A do not change from the conventional differential pressure detection device, not only the detection of the differential pressure between the space S 1  and the space S 2  but also reading the IC tag  40  provided in the filter element  20  is possible by replacing the conventional differential pressure detection device with the measurement unit  5 . 
     In addition, according to the present embodiment, the antenna unit  51 A is covered with the cover  57  and the antenna unit  51 A does not contact the hydraulic oil, and thus the IC tag  40  is easily read. 
     Fifth Embodiment 
     The fifth embodiment of the present invention is a configuration in which a filter element has a substantially plate-shaped IC tag. Hereinafter, a return filter  6  according to the fifth embodiment will be described. Note that the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted. 
     Differences between the return filter  6  according to the fourth embodiment and the return filter  1  according to the present embodiment are only a shape of the IC tag and a shape of a lid body.  FIG. 10  is a cross-sectional view of the return filter  6  and is a partially enlarged view.  FIG. 10  partially omits hatching indicating a cross section.  FIG. 10  omits detailed illustration of the internal structure of the measurement unit  2 . 
     The return filter  6  is provided inside the tank and mainly includes the filter case  10 , a filter element  20 A, a head  30 A, an IC tag  40 A, and the measurement unit  2 . 
     The filter element  20 A mainly includes the filtration member  21 , the inner tube  23 , a plate  24 A, the plate  25  (not illustrated in  FIG. 11 ), and the IC tag  40 A. A difference between the plate  24  and the plate  24 A is presence/absence of the protruding portion  24   d , and the plate  24 A does not have the protruding portion  24   d.    
     The adhesive  45  is sealed between the filtration member  21  and the plate  24 A, and the filtration member  21  and the plate  24 A are bonded with the adhesive  45 . In addition, the IC tag  40 A is provided on the plate  24 A (here, the plate-shaped portion  24   a ) with the adhesive  45 . 
       FIG. 11  is a plan view of the filter element  20 A and is a drawing in which the plate  24 A is partially cut off. 
     The IC tag  40 A has a substantially hollow circular plate shape. The IC tag  40 A has an inner diameter greater than the inner diameter of the filtration member  21 . Accordingly, the IC tag  40 A is provided inside the adhesive  45 , which bonds the filtration member  21  and the plate  24  together. 
     The description will now return to  FIG. 10 . Into the hole  31   a , which penetrates the side surface of a tubular portion  31 A, the measurement unit  2  is inserted. The antenna  51   a  is exposed from an opening on the inner peripheral surface side of the hole  31   a . The tubular portion  31 A differs from the tubular portion  31  in that the projection  31   b  is not provided. 
     With the present embodiment, because the IC tag  40 A has the substantially hollow circular plate shape, positioning of the IC tag  40 A with the antenna  51   a  is unnecessary, and the assembly of the return filter  6  is easy. 
     Note that, in the present embodiment, the measurement unit  2  is provided in the tubular portion  31 A, but the measurement unit  2  is not limited to be provided in the tubular portion  31 A. For example, the measurement unit  2  may be provided on the cover  32 . In the present embodiment, since positioning of the IC tag  40 A and the antenna  51   a  is unnecessary, the measurement unit  2  can be provided at various positions. However, when the measurement unit  2  is provided on the cover  32 , the center axis ax 2  of the measurement unit  2  is preferably substantially parallel to the center axis ax 1  to cause the IC tag  40 A to face the antenna  51   a.    
     The embodiments of the invention are described above in detail with reference to the drawings. However, specific configurations are not limited to the embodiments and also include changes in design or the like without departing from the gist of the invention. For example, in the examples described above, detailed description is made to facilitate understanding of the present invention, and the examples are not necessarily limited to examples including all the configurations described above. In addition, the configuration of an embodiment can be replaced partially with the configurations of other embodiments. In addition, addition, deletion, replacement or the like of other configurations can be made on the configurations of the embodiments. 
     In addition, the term “substantially” is not to be understood as merely being strictly the same, and is a concept that includes errors and modifications to an extent that does not result in loss in identity. For example, the “substantially cylindrical shape” is a concept that includes a case where, for example, the shape can be placed in the same category as the cylindrical shape, and is not strictly limited to the cylindrical shape. Further, simple expressions such as orthogonal, parallel, and identical are not to be understood as merely being strictly, for example, orthogonal, parallel, and identical, and include being, for example, substantially parallel, substantially orthogonal, and substantially identical. 
     In addition, the term “vicinity” means to include a range of regions (which can be determined as desired) near a position serving as a reference. For example, the term “a vicinity of an end” refers to a range of regions in the vicinity of the end, and is a concept indicating that the end may or need not be included. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  6 : Return filter 
           2 ,  3 ,  4 ,  5 : Measurement unit 
           10 : Filter case 
           11 : Bottom surface 
           11   c : Groove 
           12 : Outflow portion 
           12   a : Cylindrical portion 
           12   b : Back pressure valve 
           12   c : First cylindrical portion 
           12   d : Second cylindrical portion 
           12   e : Bottom surface 
           12   f : Outflow hole 
           12   g : Outflow hole 
           12   h : Valve body 
           12   i : Elastic member 
           12   j : Valve seat 
           13 : Mounting member 
           20 ,  20 A: Filter element 
           21 : Filtration member 
           23 : Inner tube 
           24 ,  24 A,  25 : Plate 
           24   a : Plate-shaped portion 
           24   b ,  24   c : Tubular portion 
           24   d : Protruding portion 
           25   a : Recessed portion 
           25   b : Hole 
           30 ,  30 A: Head 
           31 : Tubular portion 
           31 A: Tubular portion 
           31   a : Hole 
           31   b : Projection 
           32 : Cover 
           33 : Mounting portion 
           34 : Flow path 
           35 : Hole 
           40 ,  40 A: IC tag 
           45 : Adhesive 
           46 ,  48 : Sealing member 
           47 : Valve 
           50 ,  50 A,  50 B,  50 C: Case 
           50   a ,  50   b ,  50   d ,  50   e ,  50   j ,  50   ja ,  50   jb ,  50   p ,  50   pa ,  50   pb ,  50   t ,  50   w : Hole 
           50   c ,  50   f ,  50   k ,  501 ,  50   s ,  50   u ,  50   v : Hole 
           50   g : Recessed portion 
           50   h : Recessed portion 
           50   m ,  50   n : End face 
           50   o : Groove 
           50   q : External thread portion 
           50   r : Internal thread portion 
           51 ,  51 A: Antenna unit 
           51   a ,  51   e : Antenna 
           51   b ,  51   c : Antenna terminal 
           51   d : Hole 
           52 : Retainer ring 
           53 : Sealing member 
           54 : Antenna wire 
           55 : Substrate 
           56 ,  57 : Cover 
           57   a : Inserted portion 
           57   b : Protrusion portion 
           57   c : Hole 
           57   d : Hole 
           58 : Filler 
           59 : Insulating pipe 
           60 ,  60 A: Differential pressure detection unit 
           61 : Detection unit 
           61   a : External thread portion 
           61   b : Substrate 
           61   c : Magnetic field detection element 
           62 : Spool 
           63 : Magnet 
           64 : Elastic member 
           65 : E-ring 
           66 : Mounting member 
           66   a : Hole 
           66   b : Internal thread portion 
           66   c : Hole 
           67 : Sliding member 
           67   a : Inner peripheral surface 
           71 : Signal line 
           72 ,  73 : Power cable 
           74 : Signal line 
           80 : Pressure detection unit 
           81 : Detection unit 
           81   a : External thread portion 
           81   b : Substrate 
           82 : Diaphragm portion 
           82   a ,  82   b : Plate 
           82   c : Strain gauge 
           82   d : Annular member 
           82   e : Notch 
           82   f : Hole 
           82   g : Filling member 
           83 : Fixing member 
           83   a : Hole 
           83   b : Surface 
           83   c : Groove 
           83   d : Side surface 
           83   e : Groove 
           84 : Mounting member 
           84   a : Hole 
           85   a ,  85   b : Cable 
           86 ,  87 : Annular packing 
           90 : Temperature detection unit 
           91 : Detection unit 
           91   a : External thread portion 
           91   b : Substrate 
           92 : Temperature detection element 
           93 : Cable 
           100 : Tank 
           100   a : Hole