Remote ambient methane monitoring using fiber-optically coupled optical sensors

• Published in: Applied physics. B, Lasers and optics Vol. 119; no. 1; pp. 133 - 142
• Main Authors: Schoonbaert, Stephen B., Tyner, David R., Johnson, Matthew R.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.04.2015
• Subjects: Calibration; Engineering; Fiber optics; Lasers; Methane; Optical Devices; Optical fibers; Optical sensors; Optics; Photonics; Physical Chemistry; Physics; Physics and Astronomy; Quantum Optics; Strikes; Systems stability; Measurement Standard Deviation; Tunable Diode Laser Absorption Spectroscopy; Fugitive Emission; Wavelength Modulation Spectroscopy; Methane Concentration
• DOI: 10.1007/s00340-014-6001-0
• ISSN: 0946-2171

A tunable diode laser absorption spectroscopy system, employing a 2 f wavelength modulation spectroscopy measurement scheme, was developed for remote monitoring of ambient methane fluctuations by way of fiber-optically connected all-optical sensors. Detection of fugitive methane emissions demands a measurement precision less than 2.0 ppm v and a lower detection limit less than ~1.7 ppm v methane in air. To determine the optimum base system configuration, the influence of the laser driving signal frequency and amplitude was characterized to strike a balance between measurement precision and system sensitivity. In addition, relative to the basic system configuration, a 50 and 96 % reduction in measurement deviation was achieved by way of polarization scrambling and thermal stabilization of critical optical components, respectively. For methane concentrations between 2.0 and 50.0 ppm v in air, the laboratory-based system achieved a measured precision of 1.36 ppm v and a lower detection limit of 1.56 ppm v using a 6.0-m single-mode optical fiber and an averaging time of 1 s. The long-term system stability and system performance were analyzed using datasets acquired 4 and 12 months after the initial system calibration, yielding a difference in measured precision within the uncertainties of the calibration gas mixture. Finally, it was determined that fiber length between individual remote optical sensors can lead to a varying measurement bias, which implies that length-specific calibrations for each remote optical sensor may be required for a field implementation.