Oxygen Sensors: Materials and Applications
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Sensor Criteria The success ofa material ina particular application is based on its ability to meet 44
sensing criteria such as sensitivity, selectivity, stability, size, response time, reversibility, reliability, and dynamic ränge.9 Sensitivity is the measure of a sensor's ability to detect the presence of a gas (in this case, oxygen) in its environment and is often presented as the "detection lim it." Various levels of sensitivity, from a few percent 10 down to parts-per-billion (ppb) concentrations, 11 may be required. Sekctivity is the sensor's ability to meas ure only one particular gas (oxygen) without responding to, or experiencing interference from, other chemical Com pounds. Oxygen is often difficult to de tect because cross-sensitivity may cause interactions with other chemical species, some of which can cause false Signals or reduce the oxygen Signal intensity. The long-term stability of the sensor material, and hence its signal, is impor tant because drift can attenuate sensing responses and may necessitate frequent instrument recalibration. A sensing ma terial must interact with oxygen on a molecular level but retain its original ma terial properties over a long time. Often, a reference electrode is necessary to ac count for changes in environmental parameters including temperature, pH, light, and moisture, any of which may affect the sensing signal. Ideally, the ref erence electrode is "blind" to oxygen con centrations and registers only changes in the other conditions. Sensor size can play an important role because it may determine the deployment location, power requirement, and response time. The location of a sensor is critical: it usually must be as close as possible to the area of interest without interfering with its surroundings. 1 2 Power limitations in battery- or solarcell-operated devices for remote or por table applications drive the technology toward miniaturization. Microfabrica-
tionand microelectromechanical Systems (MEMS) technology can significantly lower the power requirements for signal measurement, processing, and temperai ture control. Physical characteristics such as mass also affect sensor response, especially if the sensing mechanism depends on diffusion or operating temperature. The re sponse time is the m i n i m u m duration required for oxygen to interact with the sensor and to create a signal. Also, a criti cal time can be defined, which depends greatly on the Situation—the control of manufacturing processes or automotive emissions need small time constants, while for medical or biological appli cations, longer response times may be acceptable. Reversibility is often one of the most difficult requirements to meet. The interaction of oxygen with the sensor should be strong, but also completely reversible. This often requires increased device sophistication to account for factors such as signal base-line offset. A device is usually calibrated so that it is accurate over an appropriate concen tration ränge. For example, devices that detect oxygen at ppm levels
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