Sensing Methane Using a Tuneable Diode Laser Spectroscopy Method
Authors: McFadden Jeff 
Published: June 2014

The use of a fiber-based backbone to perform a number of functions in an installation with the potential of an explosive atmosphere has significant potential. The intent of this paper is to describe a method of sensing methane using a Tunable Diode Laser Spectroscopy method.

The method currently in filed trials, utilize discrete passive sensors connected to a central sensing unit with fiber optic cable. The system utilizes a low power (10mW) diode laser in conjunction with a distributed fiber network and a series of ruggedized sensor modules. It is largely unaffected by common environmental conditions which can cause erroneous readings with many other sensor types including high humidity, anaerobic conditions, and the presence of other non-target gases. The system is tolerant of dust contamination, and fully functioning at signal losses of up to 90%. It can be configured as a distributed 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 cable is utilized. Unlike conventional methane detection systems currently employed, our system does not require calibration at the remote sensor. Various alarm schemes can be configured to look for rising methane levels, over-limit conditions, as well as breaks in the sensing fiber. Key attributes of the system include:Real-time methane gas monitoring is enabled at hundreds of points over long distance (up to 20km) fiber optic networksSystem is totally electrically passive outside the central controller.Rugged sensor modules, consist of a perforated stainless steel sensor tube, junction box, and drip shield. They have rapid response and are unaffected by anaerobic conditions, catalytic poisoning, other non-target gases, water sprays and excessive humidityGas sensing technique is self-referencing, which provides inherent calibration stability and ensures that the full system only requires a single, one-off calibration at the central controllerAutomatic system self-checking function at central controller gives advance warning of any potential controller, network or sensor problems facilitating a predictive maintenance protocolThe system offers high gas sensitivity, wide measurement range (0.05% to 100% CH4), exceptional gas selectivity and no gas cross-sensitivityPlug-in modular format offers flexibility, easing component replacement and system expansionSensors have many years of proven, continuous, stable operation in hostile environments, require no scheduled maintenance and allow constant condition monitoring at the central controllerSensor can tolerate up to 90% signal loss, providing good immunity to high dust conditions

The focus of this paper will be to describe the current efforts to minimize the gas cell portion of the system. This will facilitate the inclusion of the gas cell into the fiber bundle, enabling the use of the sensing system in down-hole applications. The performance characteristics of the system will remain unchanged.

The design and construction of the embedded gas cell will be described, as well as the interface and connection to the optical fiber. The hardening of these components to withstand the rigors of the down-hole environment will be discussed in detail.