A high sensitivity fiber optic macro-bend based gas flow rate transducer for low flow rates: Theory, working principle, and static calibration

A novel fiber optic macro-bend based gas flowmeter for low flow rates is presented. Theoretical analysisof the sensor working principle, design, and static calibration were performed. The measuringsystem consists of: an optical fiber, a light emitting diode (LED), a Quadrant position sensitive Detector(QD), and an analog electronic circuit for signal processing. The fiber tip undergoes a deflectionin the flow, acting like a cantilever. The consequent displacement of light spot center is monitored bythe QD generating four unbalanced photocurrents which are function of fiber tip position. The analogelectronic circuit processes the photocurrents providing voltage signal proportional to light spotposition. A circular target was placed on the fiber in order to increase the sensing surface. Sensor,tested in the measurement range up to 10 lmin−1, shows a discrimination threshold of 2 lmin−1, extremelylow fluid dynamic resistance (0.17 Pamin l−1), and high sensitivity, also at low flow rates (i.e.,33 mVmin l−1 up to 4 lmin−1 and 98 mVmin l−1 from 4 lmin−1 up to 10 lmin−1). Experimental resultsagree with the theoretical predictions. The high sensitivity, along with the reduced dimensionand negligible pressure drop, makes the proposed transducer suitable for medical applications inneonatal ventilation.