Sensing Methane Using a Tuneable Diode Laser Spectroscopy Method

摘要

The use of a fiber-based backbone to perform a number of functions in an installation with the potentialof an explosive atmosphere has significant potential. The intent of this paper is to describe a method ofsensing methane using a Tunable Diode Laser Spectroscopy method.The method currently in filed trials, utilize discrete passive sensors connected to a central sensing unitwith fiber optic cable. The system utilizes a low power (10mW) diode laser in conjunction with adistributed fiber network and a series of ruggedized sensor modules. It is largely unaffected by commonenvironmental conditions which can cause erroneous readings with many other sensor types includinghigh humidity, anaerobic conditions, and the presence of other non-target gases. The system is tolerant ofdust contamination, and fully functioning at signal losses of up to 90%. It can be configured as adistributed network with hundreds of sensing points at distances of up to 20 km from a central control unit.Methane levels from 0.5 to 100% can be accurately measured. Standard communication grade fiber cableis utilized. Unlike conventional methane detection systems currently employed, our system does notrequire calibration at the remote sensor. Various alarm schemes can be configured to look for risingmethane levels, over-limit conditions, as well as breaks in the sensing fiber. Key attributes of the systeminclude:1. Real-time methane gas monitoring is enabled at hundreds of points over long distance (up to 20km)fiber optic networks2. System is totally electrically passive outside the central controller.3. Rugged sensor modules, consist of a perforated stainless steel sensor tube, junction box, and dripshield. They have rapid response and are unaffected by anaerobic conditions, catalytic poisoning, othernon-target gases, water sprays and excessive humidity4. Gas sensing technique is self-referencing, which provides inherent calibration stability and ensuresthat the full system only requires a single, one-off calibration at the central controller5. Automatic system self-checking function at central controller gives advance warning of any potentialcontroller, network or sensor problems facilitating a predictive maintenance protocol6. The system offers high gas sensitivity, wide measurement range (0.05% to 100% CH4), exceptionalgas selectivity and no gas cross-sensitivity7. Plug-in modular format offers flexibility, easing component replacement and system expansion8. Sensors have many years of proven, continuous, stable operation in hostile environments, require noscheduled maintenance and allow constant condition monitoring at the central controller9. Sensor can tolerate up to 90% signal loss, providing good immunity to high dust conditionsThe focus of this paper will be to describe the current efforts to minimize the gas cell portion of thesystem. This will facilitate the inclusion of the gas cell into the fiber bundle, enabling the use of thesensing system in down-hole applications. The performance characteristics of the system will remainunchanged.The design and construction of the embedded gas cell will be described, as well as the interface andconnection to the optical fiber. The hardening of these components to withstand the rigors of thedown-hole environment will be discussed in detail.