Abstract:
A galvanic-cell gas sensor includes casings containing a diaphragm permitting the permeation of gas to be detected but having a water-repelling property, a cathode provided on the back side of the diaphragm, an anode formed by electrolytically coating a layer of lead (Pb) on an electrically conductive material having a corrosion resistance to electrolyte so as to leave a portion thereof uncoated to serve as a lead, and a sheet impregnated with electrolyte before assembly and disposed between the cathode and anode, with the diaphragm positioned so as to be exposed to the atmosphere.

Description:
FIELD OF THE INVENTION 
     This invention relates to a galvanic-cell gas sensor comprising a cathode and an anode adapted to come into contact with a gas to be detected and sealed in an electrolyte. 
     DESCRIPTION OF THE INVENTION 
     A conventional galvanic-cell gas sensor comprises a holder serving as a container to hold an acid electrolyte and having a window and a diaphragm of a water-repellent film of a macromolecular fluorochemical sealed therein, with a cathode of gold (Au) disposed in a segment holding the electrolyte and an anode of lead (Pb) disposed apart from the cathode. 
     This type of sensor holding an electrolyte has been required to have a liquid-tightness that in turn calls for a complex sealing structure. The use of lead as the unnecessarily large anode has made it extremely difficult to reduce the size of the container. 
     The anode A has a rod portion B at one end thereof formed by casting or other method or a rod B′ of a metal having a high corrosion resistance to the electrolyte and planted on the anode, as shown in FIG.  5 . Therefore, the anode has been disproportionally sized considering the service life of the sensor, and has hampered the size reduction of the sensor. 
     SUMMARY OF THE INVENTION 
     This galvanic-cell gas sensor eliminates the need to form the anode lead segment on the anode from lead (Pb) sheet, while permitting a reduction in the sensor size by coating an appropriate amount of Pb proportional to the sensor life on the anode through accurate control of coating time. 
     This galvanic-cell gas sensor eliminates the need to form the lead segment on the anode from lead sheet while permitting to reduce the sensor size by coating an appropriate amount of lead proportional to the sensor life on the anode through accurate control of coating time. 
     Thus, the object of this invention is to provide a galvanic-cell gas sensor of smaller size by making the anode size proportional to the sensor life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an embodiment of a galvanic-cell gas sensor according to this invention. 
     FIG. 2 is a perspective view of component elements, notably an electrode-holding frame, of the same gas sensor. 
     FIGS.  3 ( a ) and  3 ( b ) show the top and bottom conducting patterns of a circuit board mounted in the same gas sensor. 
     FIGS.  4 ( a ) and  4 ( b ) are cross-sectional views of a substrate making up the anode and an embodiment of the anode. 
     FIG. 5 is a perspective view showing an example of the conventional anode. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a galvanic-cell gas sensor according to this invention. An electrode-holding frame  1  consisting of a closed-end cylinder has a through hole  3  at the center of the top thereof. The through hole  3  having a diameter substantially equal to the outside diameter of a cathode  2  is adapted to hold the cathode  2  therein with the top of the cathode  2  protruding somewhat above the top edge thereof, as shown in FIG. 2. A cathode  2  whose top surface  2   a  is convexly shaped is put in the upper part of the through hole  3  in the electrode-holding frame  1 . A lead  4  is run from the cathode  2  to the bottom of the electrode-holding frame  1  over the top and peripheral surfaces thereof. The lead  4  is pulled out to run in the nonradial direction over the top surface  1   a  of the electrode-holding frame  1  and held in contact therewith over a relatively long distance. 
     A sheet  5  to hold an electrolyte and an anode  6  described later are laminated on the rear side of the electrode-holding frame  1 . A lead  7  is pulled from anode  6  through a through hole  1   b  in the electrode-holding frame  1  so as to run somewhat off the radial direction as shown in FIG.  2 . 
     To the top surface of the electrode-holding frame  1  is fastened a gas-permeable water-repellent film  8 , such as a film of propylene fluoride copolymer, whose diameter is somewhat smaller than the diameter of the electrode-holding frame  1  and larger than that of the through hole  3  with an adhesive bond applied to the outer edge thereof. An annular groove  1   c  provided outside the through hole  3  takes in any excess adhesive bond so as to prevent the adhesive bond from flowing into the cathode  2 . 
     A closed-end cap  10  having a center through hole  9  of electrically insulating material, such as rubber, is placed over the water-repellent film  8 . The rear surface of the cap  10  is concaved with respect to the top surface of the electrode-holding frame  1 , while the cathode  2  having a substantially convex surface is placed so as to lie substantially flush with the top surface  1   a . As such, the water-repellent film  8  becomes spherically curved when pressed under the cap  10 . 
     In this state, the lead  7  of the anode  6  pulled out through the through hole  1   b  in the top surface  1   a  of the electrode-holding frame  1  and a through hole  10   a  in the top corner of the cap  10  is run over the top surface of the cap  10  to a point opposite the lead  4  of the cathode  2  (as designated by reference numeral  7 ′ with a dot-dash line in FIG.  2 ). Then, the lead  7  is bent so as to run along the peripheral surface of the cap  10  and taken in through a notch  10   b . The lead  7  thus detoured establishes a definite conductive relationship with the metal cap proper  13  by increasing the area of contact therewith to a maximum. 
     A sealing film  12  of macromolecular substance, such as a film of tetrafluoroethylene copolymer, fit against a stepped part  1   d  formed in the opening of the electrode-holding frame and held by the metal cap proper  13  having a window  13   a  facing the cathode  2  defines a space to hold the electrolyte outside of which a circuit board  14  is provided. 
     The circuit board  14  has conducting patterns  15  and  16  on the surface thereof facing the anode  6  that are adapted to be connected to the leads  4  and  7  of the cathode  2  and anode  6  as shown in FIG.  3 ( a ). The reverse side of the circuit board  14  has an annular conductive pattern  16 ′ to connect to the anode  6  along the periphery thereof and a terminal conductive pattern  15 ′ to connect to the cathode  2  in the center thereof. A relay terminal  17 , to which a temperature-compensating thermal sensing resistor R 1  and an output standardizing relay R 2  are connected, is provided between the conductive patterns  15 ′ and  16 ′. 
     A metal bottom cap  18  is fit in the cap  10  whose inside diameter is substantially equal to the outer periphery of the metal bottom cap  18 . An annular electrically insulating plate  19  is provided around the metal bottom cap  18  to caulk the opening in the metal cap  13 . 
     Thus, the anode  6  establishes a conductive relationship with the metal cap proper  13  by means of the lead  7  that comes into contact with the bottom and inner wall and the bottom of the top surface of the metal cap proper  13 , while the cathode  2  establishes a conductive relationship by means of the metal bottom cap  18  that resiliently comes into contact with the annular conductive pattern  16 ′ on the circuit board  14 . Therefore, the sensor output can be taken out from the side and bottom of the cap, as with a button cell. 
     FIG. 4 shows an embodiment of the anode  6  described earlier. A sheet of material having a corrosion resistance to the electrolyte, good workability and electrical conductivity, and having no electrochemical action on the anode, such as nickel, is formed, by stamping or etching, into a piece comprising an annular electrode segment  20  and the lead  7 , as shown in FIG.  4 ( a ). Then, a layer of lead  21  of amount appropriate for the sensor life is coated on the electrode segment as shown in FIG.  4 ( b ). 
     This layer of lead  21  is formed by using a plating bath comprising, for example, 200 parts PbCO 3 , 100 parts HBF 4 , 15 parts H 3 BO 3 , and 0.2-0.5 part gelatin, with the portion to be used as the lead  7  not immersed in the bath or kept out of contact therewith by using a resist medium. The anode  6  having the lead  7  not coated with lead is formed by controlling the amount of lead coated on the electrode segment  20  by passing an electric current for a time corresponding to the amount of electric charge appropriate for the sensor life. 
     The electrolyte is either preliminarily impregnated into the sheet  5  or poured into a hollow space  11  through an opening in the electrode-holding frame  1  before the circuit board  1  is placed in position.