Flow sensor using optical fibre strain gauges

A novel technique for the measurement of air flow velocity using an optical fibre sensor is reported. The sensor measures the deformation of a rubber cantilever beam when subjected to the stresses induced by drag forces in the presence of the airflow. Tests performed in a wind tunnel have indicated a sensitivity of 2iW/(m/s). A qualitative model based on fibre mode propagation has been developed which allows the sensor to be characterised in terms of optical losses. A single 1 mm diameter polymer fibre is mounted on the rectangular section rubber cantilever (section 14 mm x 6 mm) and six grooves are etched into the fibre which extend into the core of the fibre. As the beam deviates the surface deforms (stretches or contracts) and the fibre is subjected to strain. As the strain is increased the grooves become wider and the amount of light transmitted through the fibre is reduced due to increased losses. The sensor described has all the advantages of optical fibre sensors including electrical noise immunity and intrinsic safety for use in hazardous environments. However, its simple construction, robustness, versatility for a number of different fluid flow applications as well as relatively low cost make it attractive for use in a wide variety of measurement applications e.g. wind velocity measurement where airborne moisture or chemicals are present.