Current commercial oxygen sensors include Clark type electrodes, semiconducting metal oxide-based sensors, and optical oxygen sensors. Clark electrodes generally do not lend themselves to fuel applications because they require frequent calibration; they have long-term stability problems and the membrane swells when exposed to hydrocarbons.
Semiconducting metal oxide electrodes using metal oxides such as ZrO2, TiO2, which are currently used as exhaust gas oxygen sensors in feedback control systems to control the air-to-fuel ratio in combustion processes. Although very stable and sensitive, these electrodes need to be heated between 300 and 800 C to become active and therefore they could not be used directly in a fuel tank.
Optical oxygen sensors are generally divided into two types including near infra-red (NIR) sensors and sensors using photoluminescence transducers. NIR sensors are based on the absorbance of NIR light by oxygen molecules using tunable diode lasers. The drawback of this sensor is the requirement of a long optical path length to obtain sufficient sensitivity and also the overlap of absorption bands of hydrocarbons and water vapor with the oxygen absorption band.
In photoluminescence (i.e. fluorescence) optical sensors, a fluorescent indicator molecule is immobilized in a polymer or sol-gel matrix and coated on an optical fiber or other optical surface, the indicator molecule is excited by low wavelength radiation and fluoresces at a higher wavelength. The excited molecules also release energy in a non-radioactive process when they collide with molecular oxygen that diffuses into the matrix from the local environment. This quenching is detected as a decrease in either the fluorescence intensity or the fluorescence decay time. The amount of intensity or decay time quenching is correlated with oxygen partial pressure. Some of the advantages of optical sensors over electrodes include lack of electrical spark possibility, immunity to EMI interference, no consumption of oxygen, not subject to errors from fouling, fluid dynamics, and other factors that influence diffusion rate, small size, large dynamic range, ease of multiplexing, ease of calibration, and low power consumption.
The most promising solution is a commercial oxygen sensor based on the quenching of a fluorescent transducer. A fluorescent Ruthenium-organic complex is trapped in a sol-gel matrix and applied as a thin film on the tip of an optical fiber. The optical fiber is interfaced to a blue LED for excitation and a miniature spectrometer for detection of the fluorescence intensity spectra. This advancement has happened with the development of extremely stable fluorophores and matrices for probes, which has enabled the use of spectroscopic intensity detection. The system has met with great success in a variety of medical, industrial, environmental applications as well as in aircraft fuel tanks. Adding a protective "overcoat" of silicone provides some protection, especially from wetting by solvents.
There is an ongoing program to improve existing fiberoptic oxygen sensor technology for in-situ monitoring of oxygen in a fuel tank environment. The usefulness of the new sol-gel formulations and the overcoats will require the complete characterization of the effects of these modifications on sensor performance. Generally, the protective coatings will impede the oxygen diffusion and therefore sensor response time. PLASM coating techniques will minimize the overcoat thickness, achieve good mechanical stability, uniformity and adhesion, and optimize the chemistry.
Characterization of different overcoating materials has facilitated design of multi-layer systems, where for example the first layer is chosen for adhesion and mechanical properties and the next layer for chemical inertness. Experience indicates that the accuracy and stability of multiple temperature, multiple concentration calibrations is a direct indication of the mechanical stability and chemical compatibility of sensors. The technology now exists for a completely self-calibrating, temperature-compensating system to detect the oxygen content in the fuel tank environment.
Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Benton City, WA, and Indianapolis, IN. He is on the technical committees of PAMA and NATA and participates in several Aviation Rulemaking Advisory Committees. He holds an A&P certificate with an Inspection Authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.
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