Abstract:
A gas sensor device including a housing provided with an inlet for introducing a gas to be measured, a sensor element capable of detecting oxygen gas or the like contained in the gas and a base element supporting the sensor element within the interior of the housing. The base element is integrally assembled to the housing by means of a seal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates a gas sensor device which detects a particular gas that is introduced into the device. More particularly, the present invention relates to a gas sensor device which may be employed in various equipment used for medical treatment, gas analysis and gas inspection which require accurate and reliable gas detection. 
     BACKGROUND OF THE INVENTION 
     Gas sensors for detecting a given gas introduced therein include those in which a housing accommodating a sensor element is separately assembled to a printed circuit board or the like which constitutes the measuring circuit. For example, the gas sensor requires an assembly step in which, after the sensor element has been soldered to the printed circuit board, the housing is secured to the printed circuit board with screws or the like. 
     However, such separate assembly of the sensor element and the housing to the printed circuit board or the like involves cumbersome, time-consuming and inefficient assembly operations. 
     Furthermore, to enhance the accuracy of the sensor, it is necessary to provide a barrier between the space in which the sensor element operates and the external atmosphere. Hence, it is necessary to provide a gas-tight or hermetic seal between the printed circuit board and the housing. Accordingly, the housing must be mounted on the printed circuit board using a packing member such as an O-ring, increasing the number of component parts of the device and rendering assembly even more cumbersome. 
     Still further depending on the operating situation of the sensor device, there is a risk of hermetic seal deterioration between the printed circuit board and the housing and fluctuation in the volume of space in which the sensor element operates, thereby adversely influencing the sensitivity and accuracy of the sensor. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a gas sensor device having a reduced number of component parts, requiring reduced time for assembly and mounting on a printed circuit board. 
     It is another object of the present invention to provide a gas sensor device which can reduce irregularities in sensitivity and measurement accuracy. 
     The above objects of the present invention have been achieved by providing a gas sensor device for detecting the presence of a gas introduced into the device, the device including: 
     a housing and an inlet and an outlet communicating with the interior of the housing for introducing and discharging said gas to and from the said interior; 
     a sensor element disposed within said interior for detecting the presence of said gas; and 
     a base element supporting the sensor element within the interior of said housing, said base element being integrally assembled to said housing. 
     According to the invention, the base element which supports the sensor element in the housing is integrally assembled to the housing suitably by means of a seal such as a hermetic seal. Such a construction ensures complete segregation between the inside and outside of the housing, and it is unnecessary to separately assemble the housing and the base element to a circuit board or the like. Furthermore, the space in which the sensor element operates is hermetically sealed by the housing and the base element. Thus, no deterioration of the hermetic seal between the housing and the base element and no fluctuation in the volume of the space within the housing can occur, even when inconsistencies occur in the assembly conditions of the housing and base element into the circuit board. 
     In a first embodiment of the invention, one of the inlet and the outlet is formed by a hollow cylindrical body which extends outwardly from the housing, and the other of the inlet and the outlet is formed by at least one aperture in the housing. 
     According to this embodiment, the inlet or outlet is formed in a cylindrical shape, and extends outwardly of the housing. Such a construction, for example, enables tubular gas piping to be attached to the inlet or outlet, ensuring that a constant through flow of gas can be achieved in order to guarantee accurate and continuous monitoring. 
     In a second embodiment of the invention, each of the inlet and the outlet is formed by a hollow cylindrical body. 
     According to this second embodiment, the inlet and the outlet are provided on the housing at two locations. The inlet serves to introduce the gas into the interior of the housing and the outlet serves to discharge the gas to the outside of the housing. Due to such a construction, a feedback path can be formed to return the gas to its source, avoiding gas wastage. 
     Preferably, the device also includes a porous body disposed within the interior of the housing intermediate of the inlet and the sensor element. 
     The provision of the porous body, suitably a filter element, in the path of the gas entering the device through the inlet, ensures that the advancing gas is dispersed by passing through the porous body. This construction prevents the gas from directly impinging on the sensor element, thereby avoiding unnecessary fluctuations in sensitivity and accuracy, for example, caused by cooling of the sensor element. 
     Suitably, the porous body is sandwiched or clamped between the housing and the base element. This prevents movement of the porous body within the housing while avoiding its direct fixture to either the housing or the base element, for example, with an adhesive. 
     Suitably, the sensor element is provided with a heat generating component. 
     The heat generating component enables the sensor element to be self-heating. Such a construction permits use of a sensor element of a type having enhanced sensitivity upon heating or of a type which acts as a detector only upon heating, increasing the utility of the sensor device to a broader range of gases. 
     Preferably, the sensor element is an oxygen ion conductor enabling it to be used for monitoring the presence and concentration of oxygen in an oxygen-containing gas. It can therefore be used for detecting CO, CO 2 , NO x  or the like, as well as O 2  itself, and may be used in medical equipment, diving equipment, gas analyzers, exhaust monitors and the like. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross sectional view showing the gas sensor device to the first embodiment of the present invention. 
     FIG. 2 is a perspective view showing the external appearance of the gas sensor device of the first embodiment. 
     FIG. 3 is a schematic perspective view of the sensor element of the gas sensor device of the invention. 
     FIG. 4 is a schematic cross sectional view showing the gas sensor device according to a first modification of the first embodiment. 
     FIG. 5 is a schematic cross sectional view showing the gas sensor device according to a second modification of the first embodiment, wherein 
     FIG.  5 (A) shows an overall view and 
     FIG.  5 (B) an enlarged view of a part of FIG.  5 (A). 
     FIG. 6 is a schematic cross sectional view showing the gas sensor device according to the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The construction of the gas sensor device according to the first embodiment is explained by reference to FIGS. 1 to  3 . The gas sensor device  20  is provided for measuring the concentration of oxygen or the like, and it is used, for example, as an oxygen sensor for medical equipment for supplying oxygen to a patient suffering from a respiratory disease. 
     As shown in FIG. 1, the gas sensor device  20  includes a housing  22 , a sensor base  24 , a filter element  26  and a sensor element  28 . The presence and the concentration of oxygen are detected by the sensor element  28  from the composition of gas introduced into an internal filter space  21   c  from a gas inlet passage  21   a  in the housing  22  by way of the filter element  26  and a space gap  21   b.    
     The housing  22  is suitably made of heat resistant resin (for example, MC nylon, Mono Costs® polyamide resin, registered trade mark of Polymer Co., USA, substantially the same as nylon 6, and available from Polypenco Co., Japan) and together with a sensor base  24  defines and forms a measuring space. The housing  22  includes a dome portion  22   a  which is formed in a cylindrical dome shape, an inlet nozzle  22   b  which is formed at the top of the dome portion  22   a  such that it protrudes externally of the dome, and an inner peripheral flange  22   c  which is formed on the inner peripheral wall of the open end of the dome portion  22   a.    
     The inlet nozzle  22   b  formed at the top of the dome portion  22   a  has a cylindrical shape which is tapered towards its distal end to facilitate connection of rubber tubing or the like (not shown) for introducing the oxygen-containing gas into the gas inlet passage  21   a.  Furthermore, by providing an annular bead (not shown) on the outer peripheral surface of the inlet nozzle  22   b,  the rubber tubing or the like is prevented from accidental disconnection. 
     Inside the inlet nozzle  22   b,  the gas inlet passage  21   a  is formed as a through hole. Furthermore, a gas outlet port  21   d  for discharging the gas to the outside of the housing  22  is formed in the peripheral wall of the dome portion  22   a  close to the position of the inner peripheral flange  22   c,  the gas outlet port  21   d  being formed as a through hole. In this embodiment, two gas outlet ports  21   d  are formed in the dome at diametrically opposed positions as shown in FIG.  1 . 
     The gas which is introduced into the device by way of the gas inlet passage  21   a  flows in from the top portion of the filter  26  toward the sensor base  24 , that is, in a generally axial direction, and thereafter flows over the sensor base  24  in a generally radial direction, and finally flows out of the housing  22  through the gas outlet ports  21   d  formed in the peripheral wall of the dome portion  22   a.  The continuous flow of gas through the sensor device  20  enables the oxygen in the gas to be monitored on a continuous basis. 
     The flow direction of the gas may be opposite that shown by the arrows in FIG.  1 . That is to say, the gas may be introduced through the gas ports  21   d  to flow in a radial direction over the sensor base  24  and thereafter flow in an axial direction through the filter  26  and to the outside of the housing  22  through the gas passage  21   a.    
     The sensor base  24  which functions as a support for the sensor element  28 , is also made of heat-resistant resin (for example, MC nylon) and is formed as a disc having a convex-shaped axial cross section and includes a larger diameter portion  24   a  and a smaller diameter portion  24   b.  The outer diameter of the larger diameter portion  24   a  is slightly less than that of the open diameter of the dome portion  22   a  of the housing  22 , while the outer diameter of a smaller diameter portion  24   b  is slightly greater than the diameter of the filter element  26 , as explained below. The sensor base  24  therefore defines and forms the measuring space together with the housing  22  while supporting the sensor element  28 . 
     As shown in FIG. 2, sensor pins  27   a,    27   b  and heater pins  27   c,    27   d,  and a positioning pin  27   e  which are made of metal pierce the sensor base  24 . Of these pins, four (sensor pins  27   a,    27   b  and heater pins  27   c,    27   d ) support the sensor element  28  as shown in FIG.  3 . Furthermore, as shown in FIG. 1, the sensor base  24  also serves to clamp the filter element  26 , an external flange portion  26   b  of the filter element  26  being clamped between the larger diameter portion  24   a  of the sensor base  24  and the inner flange  22   c  of the housing  22 . 
     The filter element  26  which constitutes the porous body is made of fluororesin (for example, Teflon®) and is formed in a cylindrical shape. The filter element  26  includes a cup portion  26   a  and the external flange portion  26   b  which extends around the entire periphery of the element. The filter element  26  restricts the incoming gas from directly impinging on the sensor element  28 . This construction enhances the measurement accuracy of the gas sensor  20 , while preventing foreign material such as minute dust particles from infiltrating into the space around the sensor element  28 . The filter element  26  preferably has a shape which maintains a constant distance between the inner peripheral wall of the housing  22  and the outer surface of the filter element  26 . 
     The gas introduced through the gas inlet passage  21   a  is diffused by the porous filter element  26 , and measurement errors which would otherwise be caused by direct impingement of the gas on the sensor element  28  can be restricted. Accordingly, measurement sensitivity and measurement accuracy can be increased. Although the filter element  26  in the above embodiment is made of a fluororesin, the filter element  26  may be formed of, for example, a fine metal mesh. 
     The inner diameter of the open end of the filter element  26  is almost the same as the outer diameter of the smaller diameter portion  24   b  of the sensor base  24 . Hence, by covering the filter element  26  with the smaller diameter portion  24   b,  the filter element  26  can define and form the internal filter space  21   c.  As described above, at the time of assembling the gas sensor device  20 , the flange  26   b  is clamped between the larger diameter portion  24   a  of the sensor base  24  and the flange  22   c  of the housing  22 . Hence, the filter element  26  is securely located inside the housing  22 . 
     As shown in FIG. 3, the sensor element  28  is made of a ceramic laminated substrate which is produced by laminating an oxygen ionic conductive solid electrolyte layer  28   a  (hereinafter referred to as “the zirconia solid electrolyte layer”) such as zirconia ceramic, for example, which is molded in a rectangular shape 5 mm×3 mm, for example, and an insulation layer  28   b  such as alumina, for example. Sensor pins  27   a,    27   b  are respectively electrically connected to the zirconia solid electrolyte layer  28   a  by way of leads made of platinum wire  29   a,    29   b.  Furthermore, on the surface of the insulation layer  28   b,  a resistance pattern of metal such as platinum wire is formed and the heater pins  27   c,    27   d  are respectively electrically connected to the resistance pattern by way of leads  29   c,    29   d  made of platinum wire. With such a construction, when electric energy is supplied from the heater pins  27   c,    27   d,  the resistance pattern heats the laminated zirconia solid electrolyte layer  28   a  to a given temperature (for example, several hundreds degrees C). The heated zirconia solid electrolyte layer  28   a  has sensor output characteristics which correspond to the oxygen density and hence, the sensor portion  28  outputs the oxygen concentration information to circuitry which is connected to the sensor pins  27   a,    27   b.    
     To assemble the sensor element  28 , the sensor pins  27   a,    27   b,  the heater pins  27   c,    27   d,  and the positioning pin  27   e  are heated and fitted into the molded resin sensor base  24  under pressure. Then, the leads  29   a,    29   b,    29   c,    29   d  connected to the ceramic substrate on which the zirconia solid electrolyte layer  28   a  and the insulation layer  28   b  are laminated are connected to the pins  27   a,    27   b,    27   c,    2   d  by wire bonding. 
     The filter element  26  is then located in the resin molded housing  22 , and the filter element housing assembly is placed over the sensor element  28 . The inner periphery of the filter element  26  is fitted to the outer periphery of the smaller diameter portion  24   b  of the sensor base  24  to prevent radial and axial movement of the filter element  26 . Finally, the inner peripheral wall of the housing  22  and the outer peripheral wall of the larger diameter portion of the sensor base  24  are hermetically sealed by way of a joint  25 , by an adhesive, heat sealing or the like. 
     As shown in FIG. 4, a first modification of the device differs from that described above in that there is no inner peripheral flange  22   c  at the open end  36   b  of the housing. Instead, a plurality of protrusions  32   c  project from the inner top portion of the housing  32 , and a flange portion is not provided at the open end of the filter element. A dome portion  32   a  and an inlet nozzle  32   b  of the housing  32  and a cup portion  36   a  of the filter element correspond to the dome portion  22   a  and the inlet nozzle  22   b  of the housing  22  and the cup portion  26   a  of the filter element  26  described above. In FIG. 4, numeral  35  indicates a joint which hermetically seals the housing  32  to the sensor base  24  by means of an adhesive or heat sealing. Other components which are substantially the same as those of the gas sensor device  20  are indicated by the same numerals. 
     The protrusions  32   c  are formed on the inner top portion of the housing  32 , that is, adjacent the entry of the gas inlet passage  21   a  into the housing  22 . These protrusions  32   c  are formed around the opening of the gas inlet passage  21   a  at four locations circumferentially spaced apart at intervals of 90°. The height of the protrusions  32   c  corresponds to the spacing between the top of the filter element  36  and the interior of the housing  32  after assembly of the gas sensor device  30 . The protrusions  32   c  therefore contact the top of the filter element  36  and prevent axial movement of the filter element  36  within the housing. The filter element  36  is therefore clamped between the housing  32  and the sensor base  24  and is firmly located in position. Accordingly, the inner flange  22   c  on the peripheral wall of the housing  22  of the gas sensor  20  is unnecessary and not present, and the outer flange  26   b  of the filter element  26  is also unnecessary and not present in this embodiment. Therefore, the forming molds for the filter element  36  and the housing  32  can be simplified as a result. 
     In an alternative design, the protrusions  32   c  are formed into a cylindrical shape. With protrusions having this cylinder shape the gas inlet passage  21   a  is effectively extended. Hence, the gas introduced through the inlet nozzle  32   b  passes directly into the inside of the filter  36 . Accordingly, leakage of the gas to the outside of the filter element  36  is eliminated. 
     As shown in FIGS.  5 (A) and  5 (B), a second modification of the device  40  differs from that described above in that the open end  46   b  of the filter element  46  is flared and formed with an external peripheral bead  46   c  having an external tapered surface  46   d  and an inner tapered surface  46   e  instead of being formed with a flange. An annular groove  42   d  which matches with the shape of an external tapered surface  46   d  of the bead  46   c  is formed on an inner peripheral flange  42   c  of the housing  42 , and an outer peripheral surface  44   c  which matches the shape of the internal tapered surface  46   e  of the bead  46   c  is formed on the smaller diameter portion  44   b  of the sensor base  44 . 
     The dome portion  42   a  and inlet nozzle  42   b  of the housing  42 , and the cup portion  46   a  of the filter element  46  correspond to the dome portion  22   a  and the inlet nozzle  22   b  of the housing  22  and the cup portion  26   a  of the filter element  26  of the gas sensor device  20  described above. In FIGS.  5 (A) and  5 (B), numeral  45  indicates a joint which secures the housing  42  and the sensor base  44  hermetically by an adhesive or heat sealing. Constituent parts which are substantially the same as those of the gas sensor  20  are indicated by the same numerals. 
     The filter element  46  is formed so that its diameter increases towards its open end  46   b,  that is, the open end  46   b  flares like a skirt. Furthermore, the annular bead  46   c  is formed on the outside of the open end  46   b  so that it protrudes. The bead  46   c  has both inner and outer peripheral surfaces  46   e  and  46   d  provided with a tapered shape. 
     In this manner, the filter element  46  can be engaged with the housing  42  by means of the flange  42   c  having the annular groove  42   d.  Furthermore, when the sensor base  44  is assembled with the housing  42 , the outer peripheral surface  44   c  of the sensor base  44  can push the inner tapered surface  46   e  of the filter element  46  in a radial outward direction. Accordingly, the filter element  46  can be clamped between the housing  42  and the sensor base  44  and supported at a large diameter stepped portion  44   a  of the sensor base  44 . Hence, even when a radial molding error is present at the open end  46   b  of the filter element  46 , for example, the open end  46   b  of the filter element  46  is pushed and expanded toward the housing  42  by means of the sensor base  44  so that the filter element  46  is firmly engaged with the flange  42   c.  Accordingly, the engagement of the filter element  46  is further assured irrespective of the presence of molding errors of the filter  46 . 
     Due to such a construction, without using packing materials such as O-rings, the seal between the interior and the exterior of the sensor device  20  can be assured. Accordingly, the number of component parts is reduced. 
     Furthermore, the housing  22  and the sensor base  24  are assembled as a single unit before the device is attached to a circuit board or the like. Accordingly, the time required for assembling to the circuit board or the like is reduced. 
     Still further, the hermetic seal between the housing  22  and the sensor element  24  does not deteriorate at the time of assembling the gas sensor device  20  to the circuit board or the like, and there is no volumetric fluctuation of the measuring space. Accordingly, measurement sensitivity and the measurement accuracy of the sensor are improved. 
     As shown in FIG. 6, a gas sensor device  60  according to the second embodiment includes a housing  62 , a sensor base  24 , a filter element  26  and a sensor element  28 . 
     The gas sensor device  60  differs from the gas sensor device  20  of the first embodiment in that an inlet nozzle  62   b  and an outlet nozzle  62   c  of the housing  62  are positioned diametrically opposite to each other and the gas which is discharged through the outlet nozzle  62   c  is recovered. A dome portion  62   a  and a flange  62   d  of the housing  62  correspond to the dome portion  22   a  and the flange  22   c  of the housing  22  of the gas sensor device  20 . In FIG. 6, numeral  65  indicates a joint which hermetically secures the housing  62  to the sensor base  24  by means of adhesive or heat sealing or the like. Furthermore, constituent parts which are substantially the same as those of the gas sensor  20  are indicated by the same numerals. 
     The inlet nozzle  62   b  of the housing  62  protrudes in a radial outward direction from the cylindrical portion of the dome portion  62   a  which constitutes the housing  62 . Inside the inlet nozzle  62   b  there is a gas inlet passage  61   a  which allows gas to enter a space gap  61   b  into a measuring space  63   c  from the outside of the housing  62 . Similarly, the outlet nozzle  62   c  which protrudes in a radial outward direction from the dome portion  62   a,  is disposed diametrically opposite to the inlet nozzle  62   b.  Inside the outlet nozzle  62   c  there is a gas outlet passage  61   d.    
     By connecting a tube or the like which supplies gas to the distal end of the inlet nozzle  62   b  and connecting a tube or the like which returns the gas to the supply source to the distal end of the outlet nozzle  62   c,  the gas is introduced into the measuring space by way of the gas inlet passage  21   a  and thereafter is fed back to the supply source by way of the gas outlet passage  21   d.  That is to say, the gas is recovered without being discarded and is returned without waste. The gas sensor device of this embodiment differs from the gas sensor device  20  of the first embodiment which does not recover the gas after the gas is introduced into the measuring space. 
     Although preferred embodiments of the invention and its construction and operation have been described herein, it will be evident to those skilled in the art that other applications and embodiments may be devised. Such applications and embodiments, and modifications thereof, are within the knowledge of those skilled in the art and are encompassed by the spirit and scope of the invention.