This screening method does not affect the operation life of the sensor.
A method has been developed to screen commercial electrochemical oxygen sensors to reduce the failure rate. There are three aspects to the method: First, the sensitivity over time (several days) can be measured and the rate of change of the sensitivity can be used to predict sensor failure. This method has been demonstrated in ongoing tests. Second, an improvement to this method would be to store the sensors in an oxygen- free (e.g., nitrogen) environment and intermittently measure the sensitivity over time (several days) to accomplish the same result while preserving the sensor lifetime by limiting consumption of the electrode. Third, the second time derivative of the sensor response over time can be used to determine the point in time at which the sensors are sufficiently stable for use.
Commercial electrochemical oxygen sensors are a limited-lifetime item because the sensor electrode is consumed during normal operation. Basically, a given sensor at the time of manufacture has a finite lifetime, which can be quantified in terms of ppm-hours so that an exposure to a given concentration for a given time reduces the lifetime by the product of those two factors. Common practice is to simply replace a sensor that fails within the vendor-specified lifetime. In applications requiring long operational life with no replacement option, screening of the sensors is advantageous to reduce the sensor failure rate. Prior art for screening is unknown given the commercial nature and application of these sensors. The simple and obvious method for screening would be to measure the sensor response at a known oxygen concentration (i.e., initial sensitivity) and determine a statistical threshold for excluding a sensor from use. However, this does not guarantee acceptable lifetime performance.
The benefits of the invention are reduced failure rate, which is especially advantageous in applications with long operational life requirements and no replacement option. In addition, this provides a screening method that does not affect the operation life of the sensor to accomplish this screening. Finally, the method provides a criterion for determining sensor acceptability prior to system level integration. These benefits significantly improve on the common practice, which cannot predict failure at any point in time beyond the initial screening. The oxygen-free environment method would prevent unnecessary reduction in the available lifetime of the sensor.
This work was done by Daniel Gonzales of Hamilton Sundstrand for Johnson Space Center. For further information, contact the JSC Innovation Partnerships Office at (281) 483-3809. MSC-24916-1