Measuring Oxygen Concentration in a Fuel Tank Ullage

For many years the Federal Aviation Administration (FAA) has been seeking to improve fuel tank safety, particularly center-wing fuel tanks, which were implicated in four fatal explosions over a 10-year period. Fuel tank inerting technologies have been examined by the FAA in depth and evaluated for potential benefit and cost. In support of this, the William J. Hughes Technical Center Fire Safety Branch has examined and evaluated many technologies for measuring oxygen concentration, some of which have been applied for use in a fuel tank ullage. To evaluate existing methods for measuring oxygen concentration in a fuel tank ullage, three different technologies were examined to determine how well they could duplicate calibration gas values, given the method of application, over a range of relevant conditions. Also, each technology/method was installed in a scale fuel tank test article to gauge the ability of the method to accurately determine the oxygen concentration under simulated commercial transport airplane fuel tank ullage conditions. These technologies and methods are 1) a typical lab-based galvanic cell sensor, using a regulated sample train; 2) a light absorption sensor, using an unregulated sample train; 3) an optical fluorescencebased sensor that is used in situ (in place). Preliminary data has illustrated that the light absorption method as well as the optical fluorescence method of sensing can represent calibration gases in a variable environment with some accuracy, but additional work is needed to adequately compare the capability of each method.