JPH01254856A - Electrooptical sensor - Google Patents

Abstract

PURPOSE: To obtain a sensor in which stability and activity are sustained for a long term while shortening the settling time by arranging anode and cathode concentrically in adjacent to an opening communicating between the outside of body and an electrolyte container. CONSTITUTION: An opening 16 is made to communicate between a container 14 and the outside of a body 12 containing an electrolyte container 14 and the opening 16 is sealed with a polymer diaphragm 18 impermeable to the electrolyte. Anode 22 and cathode 24 having working surfaces spaced apart from the diaphragm 18 are formed contiguously to the diaphragm 18 sealing the opening 16 while defining a space for containing the electrolyte with respect to the diaphragm. Furthermore, a noble metal screen 48 is provided on the working surface of anode 22 in order to sustain a high ratio of area (at least about 85:1) between anode and cathode over one interval and temperature range while minimizing the activity loss of electrode. The noble metal screen 48 is made of platinum or an alloy thereof and has 50-1000 mesh.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a sensor that can extract gas from a fluid, and more specifically, to detect the presence or absence and amount of oxygen.
This relates to a sensor that can determine I11.

[Conventional technology]

One electrochemical method for oxygen measurement is amperometric method. This method is extremely quick and simple to operate and is particularly suitable for measuring either gaseous oxygen or dissolved oxygen in liquids. Sensors utilized in amperometric methods of oxygen detection are well known in the art and generally include a hollow body with a container for an electrolyte formed therein, the hollow body being connected to an exterior and a container. It has an opening that communicates between the two. An anode and a # electrode for contacting the electrolyte can be placed inside the hollow body, and the electrolyte does not permeate through it.
A thin polymer diaphragm, which bypasses oxygen, seals all openings in the hollow body. Means are provided for electrically connecting both electrodes to a voltage source and to a measuring means. When a voltage is applied between the anode and the cathode and the sample fluid contacts the diaphragm that seals the entire hollow body of the sensor, the @ element diffuses through the diaphragm and contacts the cathode when the electrolyte is present. A current is generated that is linearly related to the partial pressure of the oxygen being sampled. This current is measured and correlated to the amount of oxygen present in the sample.

Amperometric oxygen sensors generally come in one of two types. For the first type of sensor, an electric current is carried between the anode and the cathode because oxygen is reduced when it contacts the cathode. This type of sensor requires a constant flow of oxygen through the diaphragm to avoid depletion of oxygen within the sensor, since oxygen is reduced to hydroxyl groups at the cathode and is not regenerated at the anode. This type of sensor is very sensitive to the flow rate of the test fluid and usually requires some means of maintaining the full flow of fluid to the sensor. The current is directly proportional to the partial pressure of oxygen within the test sample. Another type of oxygen sensor is a balanced sensor that uses a voltage sufficient to cause all of the oxygen to be reduced from the electrolyte at the cathode and to produce oxygen at the anode. Once operation is stable, an equilibrium condition is established within the sensor that is only disturbed when the partial pressure of oxygen at the outer surface of the diaphragm changes and disturbs the equilibrium balance. Disturbances in equilibrium also result in a current that is a direct measure of the partial pressure of oxygen in the sample.

Although good results are achieved with both types of sensors, the balanced type sensor is independent of the flow of oxygen across the diaphragm surface and the sample as normally required in the first type of sensor mentioned above. This is preferable because it eliminates the need to control the flow rate of the sample fluid or stir the sample fluid in close proximity to the sensor.

In the balanced mode of operation, the operation of the sensor is maximized when the two electrodes are placed concentrically within the sensor body with the working surfaces adjacent to the medial sacral 1 surface of the diaphragm. For concentrically arranged electrodes, the ratio of anode surface area to cathode surface area should be at least on the order of 85:1. If the ratio of anode surface area to cathode surface area is less than about 85:1, the sensor will take an unreasonably long time, on the order of several hours, to stabilize after any step change in oxygen level. Also, the long-term stability of the sensor is poor even at constant oxygen levels, resulting in a constant loss of sensitivity. It has also been found that such sensors lose active gold over a period of time.

[Isaro that invention attempts to solve]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a dissolved oxygen sensor in an amperometric oxygen sensor that has a short stabilization time after a step change in oxygen level, has long-term stability, and does not lose activity over a long period of time. It is something that can be provided.

[Means to solve the problem]

In accordance with the present invention, an improved sensor is provided that includes a body having an electrolyte container and an electrolyte contained therein, and an opening communicating between the exterior of the body and the container. An anode and a cathode are disposed concentrically within the body adjacent the opening, and a thin polymeric membrane permeable to oxygen and impermeable to electrolyte seals the opening. Means are provided for electrically connecting the picture electrode to a voltmeter and current measuring means. The active surfaces of the anode and cathode, ie the surfaces adjacent to the diaphragm, are spaced apart from the diaphragm to form an electrolyte space between the operative surface and the diaphragm. A precious metal screen is disposed on the working surface of the anode. In a preferred embodiment of the invention, the screen completely covers the entire working surface of the anode, and a portion of the screen that would normally overlie the working surface of the cathode is cut away so that the working surface of the cathode is completely covered. It is unobstructed.

In a highly preferred sister embodiment of the senna of the present invention, the cathode is constructed of rhodium metal and the anode and noble metal screen are constructed of platinum blackened platinum.

[For production]

For reasons that are not clearly understood, the use of a precious metal screen over the entire surface of an anode results in a higher resistance over a period of time than a similarly constructed sensor without a precious metal on the anode working surface. The stability of the electrode can be maintained considerably.

[Example of real sister]

Referring to FIG. 1, a preferred embodiment of the present invention is shown as an oxygen sensor, generally designated 10, having a body 12 having an interior defining a container 111 containing an electrolyte. The body 12 has an opening 16 providing communication between the container ill and the exterior of the body. An anode 22 and a cathode 211 are arranged concentrically within the main body 12 adjacent to the opening 16 . A thin polymeric diaphragm 1g seals the opening 16. The diaphragm 8 is preferably formed of either fluoroethylene propylene or tetrafluoroethylene, which is permeable to oxygen but permeable to the electrolyte. Terminal means 20' for electrically connecting the two electrodes to a voltage source 26 and a current measuring means 28;
Equipped with fe. The anode 22 and the cathode 2II are in a container 110
It is supported inside by a glass tube 52. Yin′@L2
It is fused to one end of the glass tube 32. The anode 22 is approximately disk-shaped.

It has a central hole till, in which the cathode 24 is arranged in a concentric circle. The anode 22 is carried by the end of the glass tube 32 and is attached to the glass tube 32 by an adhesive material 4, such as an epoxy adhesive, which is resistant to the electrolyte. Anode 22 and cathode 21 are connected to terminal means 20 by cathode conductor IIO and anode conductor 42.
It is connected to the.

The septum 18 is held in place over the opening 16 in the body by a threaded recessed lid 3G, the annular lid being split in conjunction with a ring 3g which tightens the entire septum in place.

When properly placed within body 12, anode 22 and cathode 21
1 are adjacent to the openings 16 with the lower working surfaces 50 adjacent to the membranes 1g but spaced from the membranes so as to form an electrolyte space 116'i between each working surface and the membranes. can be installed. The threaded plug 52 is connected to the sensor body 1
2. Seal the electrolyte with a corresponding screw-on tag 511'6 on the side for adding to or removing from the entire container.

According to the invention, a noble metal screen II8 is disposed on the working surface 30 of the anode 22. Screen 4g is cathode 21
The working surface of the cathode is provided with a hole 5° aligned with the working surface 30 of the cathode 1 so that the working surface of the cathode is completely exposed to the electrolyte in the electrolytic space +16. but.

The screen covers the entire working surface 30 of the anode 22.

The anode 22 is constructed of a noble metal, preferably platinum with a layer of platinum black deposited thereon. , the cathode 21+ is also.

In the case of oxygen sensors of the type to which the present invention relates, it is highly preferred that the cathode be constructed of rhodium, since rhodium is highly resistant to carbon dioxide interference. The ratio of the surface area of the anode to the surface area of the cathode is.

It must be at least 85:1 and may be as high as 100:1. It is highly preferred to deposit platinum black on the working surface of the anode 22 because the layer of platinum black significantly increases the surface area of the anode, which achieves the desired anode surface area to cathode surface area. This is because the aforementioned surface area ratio is improved without unnecessarily reducing the dimensions of the cathode or increasing the dimensions of the sensor.

Preferably, the noble metal screen is made of the same material as the anode 22, and is preferably coated with platinum black. The screen IIg is attached to the working surface 50 of the anode 2 by any suitable means such as spot welding, and in the preferred embodiment the screen is coated with platinum black so that the screen and the working surface of the anode are coated with platinum black at the same time. Previously mounted on the anode working surface. The screen may vary from 50 meshes to 1000 meshes of darkness. Good results have been obtained with a 100 mesh wire screen in the US sieve series with a wire diameter of about 0.076 m (0.003 inches). The screen size of 100 mesh is approximately 0.17 mesh (0.17 mm), which is sufficient to allow sufficient accumulation of platinum black on the screen and the working surface 50 of the anode 22.
It has a hole of 0,00 finch). Screen 11
When 8 is completely covered with platinum black separately from the 1st field electrode 22, good results can be obtained with a fine mesh size, but the fine mesh holes are easy to tear, so the screen and anode are assembled and then When fully coated with platinum black, the surface area of the active surface of the anode is reduced. On the other hand, mesh sizes coarser than about 50 mesh in the US standard sieve series are finer mesh sizes, i.e. 50 to io.

Does not provide the optimal increase in working surface provided by the mesh.

In a highly preferred embodiment of the invention, the precious metal screen 4g is subjected to a compression operation prior to assembly onto the working surface 50 of the positive &22 screen 48 in position above the working surface 50 of the positive &22. 18 and cathode 211
Although it is not fully understood, the compression operation acts as a spacer between the cathode and the diaphragm 11.
6'f: It will be seen that it can be considered to minimize the overall response of the sensor. However, the aforementioned compression stage
It will be appreciated that this is not critical and may be omitted in the assembly of a sensor according to the invention.

By way of example, an electrode sensor was constructed according to the description and illustrations of FIGS. 1 and 2 without a precious metal screen on the working surface 30 of the anode 22. The cathode is L fused to a glass tube.
OZ G W (0,011 inch) rhodium wire and the anode was a 6,35 m (0,25 inch) platinum disk with a L27 m (0,05 inch) center cutout. The platinum disk was glued to the end of the glass tube, and the cathode was placed concentrically within the hole in the anode. The anode was provided with a layer of platinum black on its working surface 30. Both 'electrodes were sealed with 0.508we (0.02 inch) fluoroethyl neopropylene membrane. The electrolyte was a 2% caustic potash solution.

A sensor so constructed is placed in an environmental chamber containing normal atmosphere and cycled through a series of temperature steps, the first humidity step being a minimum of -1+ to 5 hours.
The temperature was started at 4°C. At the completion of this period, the temperature was increased to 25° C. for a minimum period of 4 hours and then stepped up to 114° C. for a minimum period of 4 hours. Upon completion of the temperature period of llII °C, the temperature is
The temperature was lowered to 4°C and the cycle was repeated. The sensor was subjected to a temperature cycle and the output signal was recorded for a complete cycle.

The results are plotted with line A representing the signal output at various temperatures and line B representing the No. 13 output during the fourth or final cycle. As can be seen in FIG. 4, the signal output of the sensor increases with increasing temperature. More importantly, however, it will be seen that the signal output, regardless of temperature, is significantly smaller upon completion of the fourth cycle. The test represents 4 weeks of use of the sensor.

It can be seen from FIG. 4 that over the test cycle, the sensor exhibits a significant loss of signal strength. In fact, the reduction in signal strength is greater than the reduction allowed by the product specifications and the sensor is not approved for use. Therefore, either the sensor must be replaced or a new platinum black surface must be applied to the working surfaces of both city poles.

At the completion of four cycles, the same sensor was retrieved from the environmental chamber, disassembled once, and placed on a platinum screen with platinum black (100
A mesh) was attached to the working surface of the anode. The sensor was reassembled using the same type of diaphragm and electrolyte, returned to the environmental chamber, and extended over 5 weeks of continuous use.
Subjected to cycle humidity If changes.

The results are shown in FIG. As you can see, the first
There is only a slight decrease in signal strength over the temperature range between cycle aC and the fifth cycle line. 1M
The decrease in signal strength is within the specified singing strength that is acceptable after 5 weeks of continuous use.

While various embodiments and modifications of the present invention have been described in the foregoing description and shown in the drawings, minor changes in details of construction as well as combinations and arrangements of parts may be made in accordance with the invention as claimed in the claims. It will be seen that this can be done without going out of scope. For example, the anode 22 may be made of sintered metal. It should be noted that good results are achieved using a tube-shaped anode rather than the disk-shaped anode shown in FIG. The choice of electrolyte will be apparent to those skilled in the art, and the electrolyte may be buffered or unbuffered as mentioned on each side.

〔Effect of the invention〕

Since the present invention is configured as described above,
This produces the effects described below.

By arranging the screen adjacent to the surface of the anode, the desired high ratio of anode surface area to cathode surface area can be achieved without making the cathode smaller or increasing the size of the sensor. , and the stability of sensor performance can be improved.

[Brief explanation of the drawing]

FIG. 1 shows an illustration of an oxygen sensor constructed according to the present invention.
11] Elevation partial sectional view. FIG. 2 is an enlarged W6 sectional view of a portion of the sensor shown in FIG. FIG. 5 is an end view of the senna shown in FIG. 2, and FIG. 4 is an end view of the senna shown in FIG.
FIG. 5 is a plot of sensor output in microamperes taken over a period of time for an oxygen sensor constructed according to the prior art versus temperature; FIG. This is a plot of the output thirst of the sensor on the side. 12-one main body, 111-one container, 1g-one septum. 22--one anode, 214--one cathode, 30--working surface, 4
6--electrolyte space, 48--screen. Engraving of drawings (no change in content) F/6/ F/θ3
No. 3. Relationship with the person making the amendment Patent applicant 4, agent 7, subject of amendment Drawings

Claims (1)

[Scope of Claims] 1. A main body having an electrolyte container therein; an opening in the main body that communicates between the outside of the main body and the container;
a thin polymeric diaphragm permeable to oxygen and impermeable to electrolyte sealing said opening; terminal means electrically connected to a voltage source and current measuring means; and a concentric circle within said body. two electrodes forming an anode and a cathode arranged in the form of a diaphragm and having working surfaces adjacent to the diaphragm and spaced apart from the diaphragm so as to form an electrolyte space therebetween; Dissolved oxygen sensor comprising a noble metal screen placed on the working surface of the anode to minimize loss of electrode activity and maintain a high ratio of anode area to cathode area over a period of time and temperature range. Dissolved oxygen sensor characterized by: 2. The dissolved oxygen sensor according to claim 1, wherein the noble metal screen is made of platinum and an alloy thereof. 3. The precious metal screen is 50 mesh to 100 mesh.
The dissolved oxygen sensor according to claim 1, which is 0 mesh. 4. The dissolved oxygen sensor according to claim 1, wherein the electrode is rhodium. 5. The dissolved oxygen sensor of claim 1, wherein said noble metal screen comprises an outer layer of platinum black. 6. The ratio of anode area to cathode area is at least about 85:
1. The dissolved oxygen sensor according to claim 1. 7. The dissolved oxygen sensor of claim 1, wherein the noble metal screen is affixed to the working surface of the anode and maintains its position over the anode. 8. The dissolved oxygen sensor of claim 7 according to claim 1, wherein the noble metal screen is spot welded to the anode working surface. 9. The dissolved oxygen sensor of claim 1, wherein the noble metal screen is compressed to thereby minimize electrolyte space between the cathode and the anode. 10. The dissolved oxygen sensor of claim 1, wherein the noble metal screen overlies substantially the entire area of the working surface of the anode.