Oxygen sensor

A method apparatus for constructing an oxygen sensor in a layered construction including an anode element, a cathode element and an intermediate electrolyte-retentive insulator, wherein the layered anode element has a central flat metallic plate with two narrower outer plates bound against respective surfaces of the plate by an electrical conductor wrap, and the cathode element is uniformly compressed toward the anode along an elongate length by elongated support struts.

BACKGROUND OF THE INVENTION 
The present invention relates to oxygen sensors, and to a method for making 
an oxygen sensor, particularly adapted for detecting small quantities of 
oxygen in a gas flowing through the sensor. The invention is used 
primarily in connection with instruments for measuring the permeability of 
films and membranes, wherein oxygen is passed into a chamber, one wall of 
which is enclosed by a material membrane, and a second chamber on the 
other side of the membrane is coupled to the sensor. Oxygen which 
permeates through the membrane is detected by the sensor, which generates 
an electrical signal proportional to the quantity of oxygen detected. 
An oxygen sensor of the type generally related to the present invention is 
disclosed in U.S. Pat. No. 3,223,597, issued Dec. 14, 1965, to Hersch. The 
Hersch patent discloses a general construction of an oxygen sensor, and 
shows the plurality of layers of materials which are or may be utilized to 
construct a workable sensor. The principles of the Hersch invention are 
further elaborated upon in a construction disclosed in U.S. Pat. No. 
4,085,024, issued Apr. 18, 1978, to Lawson. The Lawson patent discloses a 
particular construction and method of making the oxygen sensor, utilizing 
many of the same materials which are the subject of the present invention. 
SUMMARY OF THE INVENTION 
The present invention is an improved construction, and method of making the 
construction, which is an advance in the art over the Hersch and Lawson 
disclosures, which has evolved from the applicant's experience in 
constructing oxygen sensors. The method of making the oxygen sensor has 
been improved by manufacturing steps which improve the electrical 
connections to the anode plate and assure a uniform and continuous surface 
area contact between the cathode and the anode via the 
electrolyte-retentive material wrapped around the anode. The size of the 
anode material is carefully restricted to be smaller than the size of the 
blade upon which it is mounted to eliminate electrical conductivity 
problems; the cathode is uniformly compressed over the anode and 
electrolyte- retentive material by elastomeric bands to assure uniform 
migration of ions and electrons between the anode and cathode. 
It is the principal object of the present invention to provide an oxygen 
sensor having a construction to assure accurate measurements of oxygen 
content in gases. 
It is another object of the present invention to provide an oxygen sensor 
having a fast response time to measure oxygen content. 
It is another object of the present invention to provide an oxygen sensor 
having a construction for eliminating electrical conductivity problems 
between the various active elements of the sensor. 
It is another object of the present invention to provide an oxygen sensor 
having high sensitivity and reliability over an extended period of use.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, a prior art oxygen sensor is illustrated, which 
is constructed in large part according to the principles and techniques 
illustrated in U.S. Pat. No. 3,223,597, Hersch, and U.S. Pat. No. 
4,085,024, Lawson. An oxygen-free envelope 10, preferably constructed of 
glass, is used to house the active elements of the oxygen sensor. Envelope 
10 has an outlet 12 and an inlet 14 to permit the passage of gas 
therethrough. A rubber stopper 15 provides an effective gas seal at the 
enlarged open end of envelope 10. The active elements of the oxygen sensor 
comprise a paddle-shaped blade member 16 which is centrally located within 
envelope 10. Blade 16 has anode sheet materials 18 and 20 across each 
facing surface (see FIG. 2), and anode materials 18, 20 are held in close 
facing contact with blade 16 by means of a criss-cross wrap of wire 17. 
The anode materials 18, 20 are the same width, or slightly wider, than 
blade member 16. The respective ends of wire 17 are typically unconnected 
to any terminus, but are merely cut close to blade 16. The entire 
combination of blade member 16, anode materials 18, 20, and wire 17 
constitute the oxygen sensor anode 23. An insulating material 22 is 
wrapped over the anode materials, and a second wire 19 is wrapped in 
criss-cross fashion over insulating material 22. An end of wire 19 is 
brought to the exterior through the stopper 15 to serve as an active 
conductor for the oxygen sensor. The metallic inlet tube 21 is affixed to 
blade member 16 and serves as the second active conductor of the oxygen 
sensor, and usually is connected to circuit ground. An exterior blanket of 
carbon felt material 24 is wrapped over the assembly including the 
insulating material 22, and a nylon thread 25 is wrapped about the entire 
assembly outside of carbon material 24. In operation, the insulator 
material is partially saturated with a chemical solution such as potassium 
hydroxide (KOH), and a test gas is passed through the sensor via inlet 14 
and outlet 12. The oxygen content in the gas which flows through the 
sensor causes a very small current to be generated between the cathode 24 
and the anode 23, and this current is collected as a current flow in wire 
19 to an external circuit and returned therefrom to metallic tube 21. In 
order for the operation to be reliable it is important that a good surface 
contact be maintained between all of the internal components of the 
device, while at the same time electrically insulating cathode 24 from 
anode 23. 
The present invention is an improvement in the construction of the oxygen 
sensor to achieve an improved surface contact between the active elements 
of the device, while at the same time to minimize the possibility of 
electrical shorting currents arising as a result of construction of the 
device. 
FIG. 2 illustrates some of the problems with prior art devices. The anode 
materials 18, 20 are applied against the surfaces of the blade 16, and a 
wire wrap 17 is tightly applied about the entire subassembly over the 
length of the blade 16. The anode materials 18, 20 are either equal in 
width or slightly wider than blade 16. The tight wrapping of the anode 
materials about blade 16 by wire 17, results in wire 17 biting into the 
edge corners of anode materials 18, 20. This produces roughened edge 
portions, as are shown in FIG. 5, which can contribute to short circuiting 
of the active elements. FIG. 5 illustrates the construction of an anode 
material 20 in facing relationship with blade 16, according to the prior 
art. Anode material 20 is preferably formed of a cadmium mixture which is 
impregnated into and around a nickel wire screen 26. If the anode material 
20 is made with a surface equal to or wider than blade 16, and then wire 
17 is tightly wound around the entire assembly, wire 17 will cause the 
anode material to degrade at respective crossing points 28, 29, as 
illustrated in FIG. 5. This causes some of the anode material to deform or 
fall away and exposes the wire ends of the interior screen 26. The exposed 
wire ends are typically relatively short sections of nickel wire with 
sharp points, and it is relatively easy for these sharpened ends to 
penetrate through insulator 22 during subsequent assembly steps. If any of 
the wire ends of screen 26 penetrate through insulator 22 and come into 
contact with cathode 24, there is created an electrical short circuit 
between the anode and cathode, and the performance of the oxygen sensor is 
degraded or, in some cases, destroyed. It is possible to deal with this 
problem by using protector inserts between the anode and cathode, 
preferably along the edges where the material degradation is likely to 
occur. These protectors could take the form of plastic edge guards which 
overlay the edges of the anode, or of the insulator, to electrically 
isolate any potential conductive paths between the anode and cathode. 
However, the use of such protector inserts does reduce the overall contact 
surface area between the anode and cathode, and also introduces additional 
constructional elements which must be used during assembly. 
FIG. 3 illustrates a cross-sectional view of one feature of the invention 
which tends to overcome this problem. In this example, blade 16 is 
constructed to be wider than either anode material 18 or anode material 
20. When wire 17 is now tightly wound about the assembly the principal 
contact point for wire 17 is against the outer edge 29 of blade 16 and the 
opposite outer edge 30 of blade 16. This relieves the biting force into 
anode elements 18, 20, and thereby reduces the amount of damage to the 
anode elements. This reduces the likelihood of exposure of any of the fine 
wire ends of screen 26. FIG. 4 illustrates a partial side view of the 
construction of FIG. 3, to show two representative wire crossing points 
29a, 29b, each of which crosses at the edge of blade 16. 
A further problem with the prior art is illustrated in FIG. 2, with respect 
to the voids which inherently exist between the active elements of the 
sensor. Because of the prior art wrapping techniques, insulator 22 becomes 
wrapped about the interior anode elements by virtue of wrapping wire 19, 
but the principal force of the wrapping wire is against the corners of 
insulator 22. This provides very little inward force across the center 
surface area of insulator 22, thereby tending to create a void 32a along 
the upper surface of the anode 23 and a void 32b along the lower surface 
of the anode 23. These voids can reduce the effective contact area of the 
electrolyte with respect to the anode, and can cause degraded performance 
of the oxygen sensor. Similarly, the cathode 24 is wrapped by a nylon 
thread 25 about insulator 22 and the other interior components. The 
primary force of this wrapping is also directed against the corners of 
cathode 24, and to some extent across the line contact of the nylon thread 
as it bridges over the wider surface area. However, the cathode 24 is 
typically made from soft, spongy material, and therefore a plurality of 
voids 36a, 36b may be formed between portions of the cathode 24 and the 
insulator 22. These voids also reduce the electrolyte contact area and can 
seriously degrade the performance of the oxygen sensor. 
FIGS. 6 and 7 illustrate another feature of the invention which is directed 
to reducing the contact surface area voids which have occurred in the 
prior art construction. FIG. 6 shows a front elevation view of the 
interior oxygen sensor components, and FIG. 7 shows a right side elevation 
view of the same. The cathode material 24 is overlaid over the active 
components interior of the sensor, and support struts 40a, 40b are 
overlaid along the length of cathode 24. Support struts 40a, 40b are 
compressed inwardly against cathode 24 by a plurality of elastic bands 42. 
Elastic bands 42 are stretched so as to provide a relatively uniform 
inwardly-directed force, and this force is applied against support struts 
40a, 40b to provide a uniform inwardly-directed force along virtually the 
entire length of the oxygen sensor active elements, along the centerline. 
The inwardly-directed force provided by this construction compresses 
cathode 24 uniformly inwardly along its length, and also compresses 
insulator 22 uniformly inward along its length. Therefore, the internal 
voids which have existed in prior art constructions are eliminated, by the 
relatively constant, uniform, inward force directed along the length of 
the active elements of the sensor. Support struts 40a, 40b may be made 
from plastic or other materials which are impervious to the chemical 
reactions which occur inside of the envelope 10, and the elastic bands 42 
may be made from rubber or other materials which are impervious to the 
same chemicals. 
An alternative form of construction which has some utility is to utilize 
the construction of FIGS. 6 and 7, but instead of utilizing elastic bands 
42 to exert an inward force against the cathode 24, the wire 19 could be 
wrapped about the exterior of cathode 24 and support struts 40a, 40b. In 
this construction, wire 19 becomes not only the conductive path making 
contact with cathode 24, but also serves as the wrapping assembly to 
secure cathode 24 in close contact against the inner components. 
FIGS. 6 and 7 also illustrate a further improvement in electrical 
construction. The internal anode wire 17 is positively connected to a 
grounding bracket clamped against tube 21. This connection eliminates 
stray currents and assures a positive circuit ground connection to the 
anode. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof, and it is 
therefore desired that the present embodiment be considered in all 
respects as illustrative and not restrictive, reference being made to the 
appended claims rather than to the foregoing description to indicate the 
scope of the invention.