New sample cell technologies for mid infra red methane detection
Tunable diode laser spectroscopy (TDLS) is an important measurement technology for gas detection, offering both high sensitivity and specificity to the target gas. With this technique, much of the engineering requirement and achievable field performance is attributable to the gas cell in which the light interacts with the target gas. We present two new technologies for gas cells, each with different attributes, used for methane detection in different applications.
The first technology has been developed for in-flight measurement of methane on board light aircraft, with the potential for further development for UAVs. Deployment on a light aircraft offers the potential for greater manoeuvrability and lower flight speeds than larger atmospheric research aircraft, allowing measurements to be made closer to gas emission sources and with higher spatial resolution. However, these aircraft typically experience higher levels of vibration and g-loading than larger platforms. The emission wavelength of an interband cascade laser (ICL) is scanned through methane lines at 3.313µm. An integrating sphere was chosen as a multipass gas cell as this offers an extended pathlength in a compact form and is robust against vibration or misalignment caused by g-loading. A limit of detection for methane of 0.3ppm has been achieved using low cost, light weight and low power components. The instrument is currently being certified for deployment on a two seater propeller aircraft, a Scottish Aviation Bulldog, owned and operated by Cranfield University.
Our second instrument uses a novel, low volume gas cell with a long interaction length (5-10m). This has been constructed from hollow silica waveguides, which have an internal bore of 300-1000µm, in combination with a quantum cascade laser (QCL) operating at 7.8µm. The low volume cell may be used where low flow rates are required, for example for detecting headspace gases released from biological samples, or at high flow rates where short (<1s) response times are needed. We have engineered entry / exit points for the light and the gas with no dead volume, and mechanical supports for the coiled gas cell itself. Through a combination of optomechanical engineering and use of the fast intra-pulse modulation technique, issues of vibration and drift for these cells are removed, again making them robust, and a limit of detection of 0.26ppm has been achieved.
Performance results for both technologies will be presented, and common themes of sensor systems design, certification and practical issues will be discussed.
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Dr Jane Hodgkinson (Cranfield University)
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