1. Field of the Invention
The present invention relates to sensors capable of detecting a gas in a fluid and, more particularly, to sensors capable of measuring the presence and the amount of oxygen (O.sub.2).
2. Description of the Prior Art
Analytical methods for the determination of O.sub.2 in a fluid include the amperometric and galvanic methods. These methods are quite rapid, simple in operation, and are specially suited for determining O.sub.2 in either its gaseous state or as it may be found dissolved in a liquid.
In the amperometric method, a sensor such as that shown in FIG. 1 is employed. In such a sensor, generally indicated as 10, body 12 is provided having a reservoir 14 therein containing a center post 16 for supporting a gold cathode 18. A membrane holder 20, cap 22, and O-ring 23 are provided to hold a polymeric membrane material 24, such as polytetrafluoroethylene of a thickness of from 0.00025 inches to 0.002 inches, stretched over the gold cathode thereby completely enclosing reservoir 14. Reservoir 14 is then filled with an electrolyte 26, typically a 5% KCl solution, either buffered or unbuffered. When the sensor is to be used in an application where temperatures in excess of 100.degree. F. are generally encountered, the electrolyte is generally mixed with typical antifreeze compounds such as ethylene glycol. Additionally, an anode 28, typically silver, is disposed within reservoir 14 in contact with electrolyte 26. A potential of about 750 millivolts is applied between the anode 28 and the cathode 18 by means 29 connected thereto. In stretching the membrane 24 over cathode 18, a very minimal amount of electrolyte 26 is contained between gold cathode 18 and membrane 24. As sample fluid is brought in contact with membrane 24, O.sub.2 diffuses through membrane 24 to contact the gold cathode 18 in the presence of the electrolyte 26. A current flow results which is linear with the partial pressure of O.sub.2 being sampled. Thus, this current can be measured and correlated to the amount of O.sub.2 in the sample.
In the galvanic method, a sensor such as that shown in FIG. 2 is employed, wherein corresponding numbers indicate corresponding parts. The sensor employed in the galvanic method typically has a silver cathode 18', a lead anode 28' and a readout device 29', e.g., an ammeter.
While these methods are satisfactory in many applications, they suffer drastically in atmospheres of high carbon dioxide (CO.sub.2) such as encountered in monitoring automobile exhaust or stack gas emissions. Such sensors as that of FIG. 1 and FIG. 2, as described above, become less sensitive to O.sub.2 upon even brief exposure to high concentrations of CO.sub.2 and may take several hours to recover so as to indicate the proper value of O.sub.2. The response of such sensors is highly dependent on the pH of the electrolyte at the interface between the gas diffusing through the membrane and the cathode. At low, or acid, pH levels the response is low. At high, or base, pH levels the response is higher. Typical responses by a prior art sensor are shown in FIG. 3 and FIG. 4. In FIG. 3, the sensor was first exposed to ambient air containing approximately 21% O.sub.2. It was then exposed to pure nitrogen (N.sub.2). FIG. 3 shows the response which was both expected and achieved. Upon exposure to N.sub.2, the response dropped to the zero line. Upon exposure to air, the response climbed to the level indicating approximately 21% O.sub.2. This cycle was repeatable without problem. Referring now to FIG. 4, the expected and actual response of a prior art sensor is shown when the sensor was exposed to ambient air and then exposed to a mixture of 15% CO.sub.2 plus 3% O.sub.2 and the balance nitrogen. When exposed to the mixture, the expected response is for the output to drop to the 3% level, being an indication of the 3% O.sub.2 content of the mixture. Upon exposure to ambient air, it is expected that the output will climb to the 21% O.sub.2 level of the ambient air. The actual response, however, was not as anticipated. When the sensor was exposed to the mixture, the response fell to the expected 3% O.sub.2 level. When the sensor was subsequently exposed once again to the ambient air sample, the output overshot the 21% level, then reversed and undershot the 21% level, and then slowly approached the 21% level asymptotically. It was found that the recovery period required for the output to attain the actual 21% level varied depending both on the duration of exposure to CO.sub.2 and the amount of CO.sub.2 in the sample. For example, when using such a prior art sensor on an automobile exhaust, an exposure for a period one minute to the exhaust gases resulted in a recovery period on the order of two to three hours before an accurate ambient response could be attained.
This phenomenon is a result of the small volume of electrolyte trapped adjacent to the gold cathode by the membrane. This is typically on the order of 1 microliter. As previously mentioned, the response of such a cell is dependent on the pH of the electrolyte. When CO.sub.2 is introduced, carbonic acid is formed which, when mixed with such an extremely small volume of basic electrolyte, results in a change of the pH of the electrolyte adjacent the gold cathode. In the typical electrolyte having a pH of approximately 13.5, the introduction of carbonic acid having a pH in the order of 4.5 results in a change of pH of the electrolyte trapped adjacent the gold cathode to a level of approximately 9. The response of the electrode will be correspondingly reduced until such time as the pH can attain its normal value by diffusion of normal electrolyte into the space between the membrane. The initial overshoot observed is, presumably, caused by the sudden change in pH and the unsettling of the electrolyte in the cathode area.
Attempts at improving the performance of O.sub.2 sensors are not new in the art. It is well known that changes in pH of the electrolyte adjacent the cathode will change the response of the electrode. On the other hand, it is known that the thickness of the electrolyte in this same area affects the sensitivity of the electrode to oxygen. Thus, ideally, the spacing between the membrane and the cathode is kept minimal while means are provided for allowing the free movement of the electrolyte through the space. Thus, in the prior art, it has been suggested to roughen the surface of the gold cathode, provide channels therein for the movement of electrolyte, and depose porous materials between the membrane and the cathode to provide channels for the movement of the electrolyte.
Such prior art suggestions have resulted in O.sub.2 sensors of marginal sensitivity and poor response times for certain applications. In particular, automobile exhaust analysis and flue gas analysis provide environments imposing restrictions beyond the capabilities of prior art O.sub.2 sensors employing such techniques. In the field of automobile exhaust gas analysis, the ability to cycle and recover at rapid rates is imperative in "assembly line" type testing environments.
Accordingly, it would be highly desirable to have an electrochemical O.sub.2 sensor capable of yielding accurate and reproducible date in applications wherein high CO.sub.2 levels are encountered at both ambient, or low, and elevated temperatures. Such a sensor would be highly useful for monitoring the O.sub.2 content of automobile exhaust and stack gas emissions.