now referring to the drawings , the electrochemical sensing cell s of the present invention will be described in detail . the sensing cell s is illustrated in the drawings and will be described as a sensing cell based on galvanic operation as a result of the choice of an active anode material for use in the electrochemical sensing cell . it will be recognized by those skilled in the art that such sensing cells may also be readily adapted to function as potentiostatic and polarographic sensors and sensors which function on oxidation as well as reduction of the chemical reactants . the sensor s is housed in a generally , electrically insulative housing 10 that may be constructed of an electrically insulative plastic material , such as an acrylic plastic . the housing 10 , as illustrated , has a generally u - shaped configuration with an open top end and which open end is secured by a cover member 11 which may be a stainless steel member that is sealed to the insulative housing 10 proper , including by means of the o - ring 12 , as illustrated . one side wall of the housing 10 , the left hand wall , is provided with a circular aperture 10a for mounting a cathode electrode k therein for exposure to an electrolyte solution 13 . the housing 10 has an internal cavity 10c for storing the electrolyte solution 13 . the cavity 10c for the housing 10 also mounts a circular anode electrode a , illustrated as overlying the bottom inside wall of the sensor s . the aperture 10a in the side wall of the housing 10 is enclosed by a circular cover element 14 that is secured to the housing 10 proper in a conventional fashion by means of a plurality of fasteners 15 spaced around the periphery of the circular element 14 . the aperture 10a is sealed by the provision of an o - ring 16 secured between the outer wall of the container 10 and the inner wall of the element 14 at the defined shoulder . the cover member 14 is provided with a pair of spaced apertures thereon for receiving a pair of gas tubes for conveying the gas to be analyzed past the back side of the cathode electrode k and then is conveyed outside of the container 10 . for this purpose , a gas inlet tubular element 20 is arranged to distribute gas near the bottom of the aperture 10a for conveying the gas to be analyzed into the aperture 10a for traversing the adjacent backing layer of the cathode k . a gas exit tubular element 21 is arranged upwardly of the opening 10a for receiving a gas moving past the cathode k and conveying the gas by means of the tube 21 outside of the sensor s . the source of gas to be analyzed is illustrated in block form in fig1 and is identified by the reference numberal 22 . the source 22 has a tubular element that is normally coupled for delivering gas to the tubular element 20 for analysis by the sensor s . the bottom wall of the sensor s secures an external cathode terminal k - 23 and an external anode terminal a - 23 that are electrically connected to the corresponding cathode k and anode a electrodes within the sensor s proper , as will become more evident hereinafter . an important consideration of the present invention is the utilization of a novel cathode electrode k in the sensor s that is generally characterized as a high surface area metal catalyst gas diffusion electrode . electrodes that are characterized and utilized in the present invention are commercially available from prototech company of newton , mass ., and other suppliers . specifically , the structural organization of such an electrode is well - known for use in fuel cells and is disclosed in detail in u . s . pat . no . 4 , 647 , 359 and assigned to said prototech company . the commercial embodiments of such electrodes as disclosed in the prototech patent are available from the prototech company and useful in the sensor s . the presently preferred embodiment of the electode that is commercially available is identified as a gas diffusing electrode on toray paper and has been utilized in the sensor s . the disclosure of u . s . pat . no . 4 , 647 , 359 is incorporated herein by reference . although such electrodes are utilized in fuel cells for generating electrical power , there is no known utilization of such electrodes in electrochemical gas analyzers of the type which is the subject of the present invention and , particularly , in sensing oxygen traces in the parts per billion range . the use of a high surface area metal catalyzed gas diffusion electrode affords the possibility of achieving much higher effective surface areas for the sensing cathode electrode of an electrochemical gas analyzing cell without increasing its geometric area . the typical effective surface area of a gas diffusion electrode is usually 100 times greater than its geometric area , when compared to a smooth , metal screen electrode where the effective area is usually twice the geometric area . for the purposes of the present invention , the basic characteristics and structural organization facilitating the understanding of the gas diffusion cathode k will be explained in conjunction with the diagrammatic representation in fig3 of the composite structure defining the gas diffusing cathode k . the cathode k is a composite structure having a gas diffusing polytetrafluoroethylene - carbon backing layer on one side thereof bonded to a relatively thin layer of high surface area catalyst metal dispersed on a high surface area carbon support ; see fig3 . the gas diffusing portion of the cathode k is illustrated to the left in fig3 and consists of a microporous structure of polytetrafluoroethylene and carbon mixture . the microporous structure functions as gas wicks to convey the gas subjected thereto through the microporous , electrically conductive , hydrophobic structure of the polytetrafluoroethylene - carbon mixture to the catalyst surface . this gas receiving and diffusing layer permits the conveyance of the gas therethrough by means of the gas wicks formed by polytetrafluoroethylene - carbon structures . the catalyst layer of the cathode is a relatively thin layer having approximately 1 / 10th of the thickness of the backing layer . this layer can be termed the catalyst layer as it is exposed to the electrolyte solution in the cell s . the catalyst layer consists of a high surface area metal catalyst , such as silver , platinum , gold and the like metals , dispersed on a high surface area carbon support . a relatively small amount of polytetrafluoroethylene or a polymeric , fluorinated hydrocarbon material is used as a binder of the metallic catalyst to the carbon support . the backing layer is also characterized as being a hydrophobic porous layer which permits the gas to be analyzed to be conveyed therethrough to the catalyst layer but blocks or prevents the electrolyte solution from traveling beyond the catalyst layer and thereby exiting the sensor s . the cathode electrode k has a circular configuration and the presently preferred embodiment has an area of approximately 1 square inch . the thus defined gas diffusing backing layer of the sensing cathode k allows the gas to be analyzed to diffuse through i & amp ; s microporous structure and reach the metallic catalyst surfaces where the electrochemically active gas , such as oxygen , reacts . since the gas to be analyzed is diffused through the gas wicks of the backing layer , only the electrolyte immediately adjacent the catalyst metal surface is exposed to the gas . as a result of the very high effective surface area of the catalyst provided for the cathode k , most of the reacting gas is consumed at the cathode k surface . it is thought that only a small portion of the unreacted dissolved gas passes through the catalyst layer to the bulk of the electrolyte 13 which allows it to be rapidly removed , as will be explained hereinafter . referring back to the sensor structure of fig1 it will be seen that the cathode k is mounted in the circular cavity 10a on the side wall of the sensor housing 10 , as illustrated . for this purpose , the cathode k is held in position by means of an insulative , holding ring which may be a plastic polyethylene ring and is identified by the reference numeral 25 . the polyethylene ring is mounted between the outer surface of the housing 10 and the outer surface of the cathode k , as best illustrated in fig1 . along with the holding ring 25 , a very thin silver ring ( not shown ) is arranged between the outer surface of the cathode k and the insulative ring 25 . the silver ring is in electrically conducting contact with the cathode electrode k . a number of electrical lead wires , thin silver wires , are connected between the conductive silver ring and the external cathode terminal k - 23 , although only one lead wire 30 is illustrated . the one lead wire 30 is electrically connected to the silver ring and is passed through a suitable aperture in the inside of the side wall of the housing 10 and through an enlarged aperture 10w to extend outside the lower end of the housing 10 . the outer end of the silver wire 30 is electrically connected to the external cathode terminal k - 23 , as illustrated . the cover member 14 for the sensor s is mounted so that the distance between the outside surface of the gas exposed layer of the cathode k and the inside wall of the plate 14 is typically greater than 1 / 2 inch . the gas to be analyzed is conveyed from the source 22 through the tubular conduit 20 and enters the space between the cathode k and the inside surface of the plate 14 and after being subjected to the cathode , the remaining amount of gas exits from the second tubular element 21 secured to the plate 14 . during the process , the gas to be analyzed contacts the backing surface of the cathode k and diffuses through the gas wicks provided to arrive at the catalyst surface of the electrolyte layer . a typical surface area of the catalyst metal is 150 m 2 / g . the gas to be analyzed is circulated past the backing layer of the cathode k at atmospheric pressure and it is preferred to be maintained at such a pressure . the rate of diffusion of the gas through the gas wicks of the backing layer for the cathode k is insensitive to the rate of flow of the gas exposed to the backing layer provided the gas pressure remains substantially unchanged . therefore , variations in the gas flow rate will not affect the electrical signal output from the sensor s . in addition , minor mechanical vibrations causing stirring of the electrolyte solution will not affect the integrity of the electrical output signal from the sensor s since the gas mass transport occurs through the diffusing cathode electrode and not the electrolyte solution , as in the prior art sensors discussed hereinafter . this mounting arrangement of the cathode electrode k substantially limits the contact of the electrolyte solution to the gas mixture undergoing analysis . the anode electrode a , which may be either constructed of lead , cadmium or the like , is a circular disc mounted on the inside bottom wall of the cavity 10c for the housing 10 . an electrically conductive stud 31 is connected onto the bottom side of the anode wall of the housing 10 and is electrically connected to an external anode terminal a - 23 and secures the anode electrode to the housing 10 . the open end of the housing 10 is sealed by means of a stainless steel plate 11 having three apertures for accommodating three stainless steel tubular elements that are welded thereto . the left hand tubular element , as illustrated in fig1 is identified by the reference numeral 35 and extends upwardly or outwardly of the plate 11 and downwardly into the electrolyte solution 13 for the sensor s . the tubular element 35 may have a fine pore glass frit attached thereto for bubbling a purging gas into the electrolyte solution 13 . when a frit is utilized , the length of the frit stem is defined so that the gas emerges near the bottom of the electrolyte solution . for this purpose , the stem of the frit may have a disc at its end for dispersing the purging gas therethrough . the second tubular element 36 is spaced inwardly from the element 35 and the two tubular elements 35 and 36 are connected and controlled by means of a three - way valve 37 . the tubular element 36 extends outwardly of the cover element 11 and a short distance into the space between the bottom wall of the cover 11 and the top surface of the electrolyte 13 . the three - way valve 37 is connected to a source of purging gas 38 , shown in block form , to receive and control the flow of the gas . the function of the three - way valve 37 is to control the flow of the purging gas into one of the tubular elements 35 and 36 and to completely turn off the gas . a third tubular element 39 that is welded to the cover 11 is a short length of tubing which extends a short distance below the bottom surface of the cover 11 and outwardly of the top of the cover and functions as a vent for exhausting the gas in the sensor s proper to the atmosphere . the sensor s is also illustrated with a water port 40 for permitting water to be added to the electrolyte solution 13 . the port 40 is sealed with an o - ring and a polytetrafluoroethylene screw , as illustrated . the aqueous electrolyte solution may be any electrolyte known in the art and in the disclosed embodiment , the electrolyte utilized was a potassium hydroxide , aqueous solution . as mentioned hereinabove , the sensor s of the present invention is particularly adapted for sensing oxygen concentrations that are characterized as trace quantities or quantities in the range between 0 and 100 ppb . as is the usual practice , in order to assure correct output readings from a sensor , the electrolyte solution 13 must have the dissolved oxygen removed therefrom prior to analyzing a gas . in accordance with the present invention , one method of purging the reactive gases from the electrolyte solution is by the provision of the source of purging gas 38 , which may be pure nitrogen , which is bubbled through the electrolyte solution 13 by operating the valve 37 to apply the nitrogen gas from the source 38 directly into the electrolyte solution , as illustrated . this can rapidly remove the oxygen from the electrolyte . once the electrolyte 13 is free of dissolved oxygen , the sensor s is ready to analyze the gas mixture coupled thereto . during the sensing period , the valve 37 may be controlled to cause the nitrogen gas from the source 38 to be removed from the tubular element 35 and coupled to the tubular element 36 . this will cause the nitrogen gas to flow over the top surface of the electrolyte 13 during the analysis period . by passing the nitrogen gas over the electrolyte 13 in this fashion , it will minimize the loss of water from the electrolyte due to the evaporation and improve the overall operation of the sensor s . in the analysis of the gas to be analyzed , the source 22 is coupled to the inlet conduit 20 so as to circulate it past the backing layer of the cathode k to permit it to diffuse through the backing layer and reach the catalyst metal where it reacts . an important consideration in this reaction procedure should be noted at the present time . in prior art sensors , the gas to be analyzed is first caused to be dissolved in the aqueous electrolyte solution and once the gas is dissolved , it diffuses towards the cathode electrode through a relatively thick layer of electrolyte , and once it reaches the electrode , is reduced at the electrode - electrolyte interface . only a fraction of the dissolved gas reacts at the electrode and a substantial amount of the reactive gas stays in the bulk electrolyte . this is in contrast to the present invention where most of the dissolved reactive gas is used up at the high surface area electrode covered by a thin film of electrolyte without diffusion into the bulk electrolyte . the cathode electrode k , as in the prior art , functions as the sensing electrode and when oxygen is being sensed , the sensed oxygen produces a cathodic current at the catalytic metal surface in response to the wetting of the metal and the reception of the gas mixture undergoing analysis thereat , and the cathodic reaction provides an electric current flow externally between the cathode k and the anode a and the anode is oxidized in response to the cathodic reduction . the external electric current , then , is available at the terminals k - 23 and a - 23 on the outside of the sensor s . now referring to fig4 the electrical circuit for processing the electrical signals derived from the sensing cell s at the external cathode terminal k - 23 and the external anode terminal a - 23 will be explored . for this purpose , it should be noted that the output current available at these external terminals is on the order of 10 to 14 microamperes per parts per million of oxygen in a gas to be analyzed when the cathode electrode has the aforementioned one inch square diameter . this electrical output is about ten times higher than the conventional oxygen analyzers presently known for sensing the low oxygen concentrations in gas mixtures . any conventional sensing circuit may be employed by coupling it to the external anode and cathode terminals and in fig4 a conventional operational amplifier amp is illustrated connected to the external anode and cathode terminals with the anode a - 23 connected to the negative input terminal of the amplifier amp . the output circuit for the amplifier amp is connected in series circuit relationship with an output resistor ao connected to a common voltage level or ground . a feedback resistor af is connected between the output terminal common to the resistors ar and ao , to the negative input terminal of the amplifier amp . a meter m may be connected between the output terminal o and ground that is calibrated to read the concentration of the sensed oxygen of the gas undergoing analysis for a direct read - out of the sensor s .