US3160577A - Sensing unit - Google Patents

Description

United States Patent Oihce 3,16,577 Patented Bec. 8, 1964 3,169,577 SENSING UNH' Roger W. Nolan, r., Davenport, Iowa, assigner to The Bendix gCorporation, Davenport, iowa, a corporation or Delaware Filed Mar. 24, 1961, Ser. No. $98,646 '1 Slaim. (Cl. 294-195) This invention relates to polmographic units for sensing or measuring theV presence of selected materials in fluids, both liquids and gases.

Among the objects of the invention are the provision of means for reliably and rapidly detecting the presence of selected mateiis. Anotherk object is to provide a novel unit capable of ready manufacture in compact form and .operablein all attitudes. While the invention has applications to sensing other materials and in other iields, one of the objects of the invention is to provide an improved unit by which to detect the amount of oxygen in an atmosphere such, for example, as the atmosphere in an aviators breathing mask.

Other objects and advantages of the invention are apparent in the following specification and the accompanying drawing, in cross section, of a unit embodying the invention.

it is to be understood that the embodiment selected for illustration in the drawing may be modified, and that other embodiments'ot the invention are possible without departing from the spirit of the invention and the scope of the appended claims.

Certain of the objects of the invention are realized by the provision of two electrodes bridged by an electrolytic medium, one of the electrodes being interposed between the medium and the material to be measured and arranged to retain the medium while permitting passage to the medium of the substance, or a constituent thereof, to be measured.

One advantage of the invention lies in the fact that it employs known polarographic principles in its operation. Selection of suitable materials for the electrolytic medium and the electrodes is well within the skill of workers in the art. However, the employment of certain of these materials provides added advantage in the invention and these will be described.

in operation of the unit, the electrolytic medium provides for electron ow in the polarographic processes electro-reduction and electro-oxidation. oxygen detection these processes are described by the equations:

When a continual indication is required, means are provided to insure availability of a continual supply of electrons. To this end and to a maintain the sensitivity and calibration or" the unit for extended periods, suitable buffers and pl-l control agents are added to the electrolyte and materials having high hydrogen overvoltage are selected for the cathode electrode. In most cases the electrolytic medium is an electrolyte but it is possible to employ a non-conductive material which becomes an electrolyte only upon combination with constituents of the material analyzed. T he term electrolytic medium is intended to define both of these types of material.

Particularly in oxygen detection an advantageous com- .bination of materials is a chloride electrolyte together with a gold, platinum, or silver cathode and a cadmium anode or, when the anode is one of gold or platinum, a

In the case ofV silver or silver-silver oxide cathode. The chloride may be one of potassium, sodium, or hydrogen.

A unit embodying the invention is illustrated in the drawing. It comprises an anode electrode 11, a cathode electrode 10, and an electrolytic medium 12. The unit is incased in a housing, in this case a drawn aluminum cup 13 having a large central opening 14 formed in the cup bottom. This opening is closed by a disc of insulating material 15 which because of its character and thickness is relatively impermeable to gases. The disc 15 is formed of dense polyethylene plastic and it is held in place by an aluminum tube 16. The other diameter of tube 16 is substantially the same as the inner diameter of the cup 13 and is press-fitted into the cup intoA engagement with disc 15. The upper rim of the tube 16 is slightly lower than the upper rim of the cup 13 to form an annular ledge. The circula anode electrode 11 rests on this ledge and is soldered in place to form one Wall of the completed housing.

The insulating disc 15 is formed with a central opening into which is threaded a terminal member 20 whose in er end is plated with cathode material, here the gold 21. The terminal member is threaded into the housing until its gold end engages the electrode 11 and then it is turned oppositely a predetermined amount to provide a selected spacing between the electrodes. A small spacing, about one sixty-fourth of an inch, is preferred if fast reaction time is desired.

The electrolytic medium 12 is introduced into the unit through an opening 23 in the insulating disc and the opening is melted closed as at 24 when llling is complete.

The electrode 11 is arranged to contain the electrolytic medium within the unit while permitting constituents of the material to which the electrode is exposed to pass through the electrode into contact with the medium. It is required that the electrolytic medium pass through the electrode less readily than the material to be detected. While this can be controlled by selectionof mediums that will not readily Wet the electrode material and such devices, it is usually possible to accomplish the required result simply by making the electrolytic medium more viscous than the material to be detected. A Wide variety of thickening and jelling agents may be added to the various electrolytic mediums without affecting their operation. For example the unit shown in the drawing contains a mixture of potassium chloride, hydrochloric acid and agar to form a thick gel which is easily contained by an electrode that will readily admit gases and non-viscous liquids.

The electrode 11 itself may have a wide variety of specific forms. It may, for example, comprise a sintered plate or a mesh of cadmium Wires pressed until cold Welded together. A section of electrode 11 is shown enlarged in FIG. 2. It comprises a mesh of tine steel wires pressed together into a cold welded grid 39. This grid is pressed partly into a larger mesh oi steel wires 31. The whole electrode is cadmium plated to a small thickness. The openings 32 of the grid 36 are sulliciently large to admit gases and non-viscous liquids but are too small to permit escape of the more viscous electrolytic medium 10 in any deletan'ous amount. the openings 32 are of live micron size. When measuring most of the thinner liquids, ten micron sized openings are adequate.

ln operation of the unit shown, a voltage of about 0.6 volt appears between the electrodes when no oxygen is present adjacent to the electrode 11. When the electrode is exposed to oxygen which passes throughthe electrode to the electrolytic medium, this voltage increases substantially linearly with the amount of oxygen.

A meter M is connected in circuit with the unit and the deflection of its pointer above 0.6 volt is a measure of In the case of electrode 11 the quantity of oxygen in the space adjacent the electrode 11. If it is desired that the meter read zero in the absence of oxygen a counter voltage of 0.6 Volt is included in the circuit. This voltage may be provided by a battery 40 as shown. The range of the electrical output of the unit may be calibrated to the meter scale by varying the amount of measured material that is permitted to contact the electrolytic medium. This is easily accomplished by altering the size of the area through which the substance measured may contact the electrolytic medium. Advantageously, as shown, this is accomplished by painting a relatively impermeable substance 41 over areas of the electrode 11 While the unit is in an atmosphere of known oxygen content until meter M provides the proper corresponding indication.

The response time of the unit, for a given electrolytic medium is determined by the spacing of the electrodes and may be adjusted by rotating the terminal member 20.

I claim:

In a polarographic sensing unit a housing, a metallic cathode electrode Within said housing and insulated therefrom, a metallic anode electrode formed of a mesh Wire including cadmium surfaces forming an exterior wall of said housing, and an electrolytic medium contained within said housing surrounding said cathode and bridging said cathode and said anode, said anode being formed with openings of not more than 10 microns to contain said medium while permitting the passage of gas therethrough.

References Cited in the file of this patent UNITED STATES PATENTS 2,382,735 Marks Aug. 14, 1945 2,531,747 Steam NOV. 28, 195() 2,797,254 Schumacher et a1 June 25, 1957 2,805,191 Hersch Sept. 3, 1957 2,806,982 Holik et al Sept. 17, 1957 2,830,109 Iusti et al. Apr. 8, 1958 2,898,282 Flock et al Aug. 4, 1959 2,900,317 Keidel Aug. 18, 1959 2,913,386 Clark Nov. 17, 1959 2,939,827 Jacobson et al. June 7, 1960 2,992,170 Robinson July 11, 1961 3,000,804 Cahoon et al. Sept. 19, 1961

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