Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips

In order to perform gas velocity measurements using a single optical probe, an optimisation of the probe geometry has been undertaken. The responses of conical probes, analysed in a previous article, were found to be strongly sensitive to deviations from an ideal geometry. To render the technique much sounder, two new shapes, namely, cone + cylinder (2C) and cone + cylinder + cone (3C) are considered. They are both effective for simultaneous gas detection and gas velocity measurements, but the latter, free of proximity detection, appears to be the most promising due to a calibration curve, i.e. the relationship between signal rise time and interface velocity, weakly sensitive to uncontrollable parameters such as the interface impact angle. In addition, its latency length can be controlled during the manufacturing process, allowing good reproducibility of probe tips. Analysis of the signal transients is used to provide guidelines for effective signal processing. Finally, possible extensions of the monofiber technique are discussed as well as remaining limitations. (C) 1998 Elsevier Science Ltd. All rights reserved.