Fiber Optic Oxygen sensor using Fluorescence Quenching for aircraft inerting fuel tank applications Allen Panahi* Accro USA LLC, Pasadena, CA91117 ABSTRACT On July 18, 2008, the FAA mandated that new aircraft are to include inerting technology to significantly reduce the potential for flammable vapor spaces in center wing fuel tanks. All passenger aircraft constructed since 1991 must also be retrofitted with this technology. This ruling is the result of 18 aircraft that have experienced fuel tank flammable vapor ignition incidents since 1960. Included in these are the TWA 800 and Avianca Flight 203 incidents that resulted in 337 total fatalities. Comprised of heavier hydrocarbon components, jet fuel is much less volatile, with Jet A having a flash point of approximately 100°F and JP-4 having a flash point of approximately 0°F. In contrast, straight-run gasoline has a flash point of approximately -40°F. The flash point is the minimum temperature where a liquid fuel can generate enough vapor to form a flammable mixture with air. If the temperature is below the flash point there isn’t enough fuel evaporating to form a flammable fuel-air mixture. Since jet fuel and gasoline have similar flammable concentration limits, gasoline must produce much more vapor at a given temperature to have such a low flash point; hence gasoline is much more volatile than jet fuel. In this paper we explore Fluorescence Technology as applied to the design and development of O2 sensors that can be used for this application and discuss the various test and measurement techniques used to estimate the O2 gas concentration. We compare the various intensity based approaches and contrast them with the frequency domain techniques that measure phase to extract fluorescent lifetimes. The various inerting fuel tank requirements are explained and finally a novel compact measurement system using that uses the frequency heterodyning cross correlation technique that can be used for various applications is described in detail while the benefits are explored together with some test data collected. Key Words: Fiber optic Oxygen, Fluorescence Quenching, inerting, aircraft fuel tanks
1. INTRODUCTION TO INERTING FUEL TANKS FOR COMMERCIAL AIRCRAFTS: Like gasoline, jet fuel is generally safe under normal operating conditions. What is concerning the aviation industry is abnormal circumstances, for instance where the fuel is heated by reject heat from the plane air conditioning system, increasing the vapor generated such that the flammable range is reached. With the new mandate, the FAA is stating that this hazard is substantial enough to require additional measures (inerting technology) to reduce the risk of fuel system explosions. The FAA has decided that it is time to reduce the hazards of these abnormal conditions by requiring gas inerting equipment for aircraft.
2. FLUORESCENCE LIFETIME MEASUREMENTS IN TIME AND FREQUENCY DOMAINS Fluorescence lifetimes in the nanosecond range have typically been measured by either of two methods, Time Correlated Single Photon Counting (TCSPC) or phase modulation. Phase modulation methods have enjoyed widespread success in the past for the relative simplicity of the instrumental setup. TCSPC, on the other hand, has been seen as the more complex method, relying on expensive detectors and complex electronics. Furthermore, the TCSPC method was generally implemented with either flash lamp sources (100 KHz pulse rates) which were very slow in data collection or with complex fast-pulsed lasers which were restricted to several distinct wavelength regions. In effect, TCSPC was a complex method confined (when using lasers) to large well-equipped laser laboratories and relied on a large degree of expertise. Recently, however, the emergence of new LED and laser diode technologies along with miniaturized electronics has enabled Time Correlated Single Photon Counting (TCSPC) methods to gain more widespread use. Such miniaturization of the electronics for TCSPC onto a single PC board with software control has at a stroke made the method much more accessible. ________________________________________________ * Corresponding author e-mail address:
[email protected]
Photonics in the Transportation Industry: Auto to Aerospace II, edited by Alex A. Kazemi, Bernard C. Kress Proc. of SPIE Vol. 7314, 73140D · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.821732
Proc. of SPIE Vol. 7314 73140D-1
Semiconductor LEDs and lasers as fluorescence excitation sources have improved dramatically in the past decade and will revolutionize fluorescence lifetime measurement systems. LED devices can now offer excitation wavelengths of between 380 nm and 600 nm with pulse rates in excess of 1 MHz and pulse widths of ~1 ns, which is ideal for mainstream biological fluorescence lifetime studies. A further development has been the emergence of commercial violet GaN laser diodes in the past two years. These lasers are compact, capable of pulse widths less than 100 ps, and are easily incorporated into fluorescence lifetime systems. The use of fluorescence lifetimes as a sensor method has attracted widespread interest in the past decade. Analytes such as halides, oxygen, and carbon dioxide, have been monitored using fluorescence lifetimes. Fluorescence lifetime based techniques have several advantages over more traditional fluorescence intensity methods which are susceptible to changes in excitation light intensity, to photobleaching, variation in light scattering and absorption of the sample.
2.1 Fluorescence Quenching Technology for O2 Sensor The partial pressure of oxygen is inversely proportional to the intensity of the fluorescence exhibited and is described by the Stern-Volmer equation: I0 / I = 1+ k (PO2) where I0 is the fluorescence intensity of a standard at zero pressure of oxygen; I is the fluorescence intensity of a standard with a pressure within the high end of the working range and k is the Stern-Volmer constant, which is dependant on the chemical composition of the sensor. This equation functions well for lower values of partial pressure, but for samples over 20 kPa, the second order polynomial algorithm is required, which is as follows: I0 / I = 1+ k1 (PO2) + k2 (PO2)
3. BASIC SETUP FOR FLUORESCENCE LIFETIME MEASUREMENT TECHNIQUES The fluorescence measurement is a simple experiment, but intrinsic and basic information can be directly measured and analyzed [1]. Among the specific techniques it offers are fluorescence lifetime and fluorescence resonance energy transfer [2,3]. Fluorescent samples have a characteristic time behavior between excitation and emission, which is determined by the time dependent quantum mechanical overlap between states [4]. For many fluorophores, this behavior can be simply represented by a single exponential decay function with a characteristic fluorescence lifetime [5]. Historically, there are two complementary techniques of lifetime measurement: time-domain and frequency domain. In frequency-domain, the fluorescent sample is excited by a modulated source of light. The fluorescence emitted by the fluorophore has a similar waveform, but is modulated and phase-shifted from the excitation source spectrum. The fluorescence lifetime can be calculated from the observed modulation depth and phase shift. Traditional fluorescencelifetime measurement in frequency-domain has been done by a cross- correlation phase-modulation technique. In the cross-correlation technique, the fluorescence emission is detected by a photomultiplier tube modulated at the same base radio frequency as the modulation source plus a low cross-correlation frequency. The FM mixing technique is adapted to analyze lifetime information. Recently, for frequency-domain methods there has been proposed and constructed a FD spectrometer with a blue light-emitting diode (LED) as an excitation source and a photodiode as a photodetector. There has been constructed a direct LED driving circuit for radiofrequency modulation as an excitation source and a high bandwidth oscilloscope to record both excitation and emission spectra. P. Harms et al. have proposed and constructed a frequency-domain lifetime spectrometer using a RF lock-in amplifier. The RF lock-in directly provided the ac signal used to modulate the intensity of the blue LED excitation source and the emission was measured by a photomultiplier tube connected to the RF lock-in. In this paper, we have designed and constructed a simple frequency-domain (FD) fluorescence-lifetime measurement system using a Blue-violet laser diode, a photomultiplier tube, and an oscilloscope system. Both excitation and emission intensity have been recorded by oscilloscope and analyzed by using the custommade FFT algorithm in the Matlab program. Therefore, the fluorescence lifetime can be easily calculated from the result of a frequency-modulated experiment. In order to determine the fluorescence lifetime, we designed and constructed a simple but accurate spectrometer using a violet laser diode.
Proc. of SPIE Vol. 7314 73140D-2
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(c) Figure 1. Sensors use Fluorescence quenching technology to measure O2 gas concentrations that respond to O2 partial pressure: this follows the Stern-Volmer relationship as shown in the above figures a, and b. Figure c is a pictorial of the fluorescence quenching in action. Rhuthenium based Sol-gel sensors are exposed to Blue light from LED or Lasers @ 470 nm and the reflected Fluorescence optical signal is measured with a orange filter centered at 600nm.
3.1 Time Domain Fluorescence lifetime measurements We present an alternative approach to real-time lifetime determination, which is based on the fact that for a single exponential decay, the fluorescence lifetime is equal to the average time lag between an excitation laser pulse and the subsequent detection of a fluorescence photon. For each photon, a time-to-amplitude converter ~TAC! generates a pulse of an amplitude proportional to the time lag between fluorescence photon and excitation pulse. The resulting sequence of pulses, with exponentially distributed amplitudes and inter-pulse times, is then subjected to a count rate independent fluorescence intensity at different time delays after the pulse. As a result, an averaging procedure start. It utilizes a pulsed light source (an LED, a laser diode or a nitrogen/dye laser) and measures Fluorescence Lifetime Measurements • There exist two methods to measure fluorescent lifetime: – Time domain: using pulse fluorimetry • pulse length is
