Experimental and simulation study on thermal gas flowmeter based on fiber Bragg grating coated with silver film

An optical fiber thermal gas flowmeter based on a core-offset fiber Bragg grating (FBG) has been proposed and demonstrated for low-rate gas flow measurement. Light from a pump laser within the fiber was coupled by the core-offset structure into the downstream fiber cladding and was absorbed by the silver layer coated on the fiber surface over the FBG. The FBG was heated by this absorption and the produced heat would be partially taken away as air flowed by, which caused a Bragg wavelength shift with a dependence on the air flow rate. A stainless steel capillary tube with a convection slot was designed for encapsulation of the FBG sensor to increase its mechanical strength and usage durability. We modeled the heated fiber flowmeter to acquire the temperature fields at various flow velocities by using the finite element modeling (FEM) method, and calibrated the flowmeter with a Standard Bell Prover. Experimental results revealed a monotonous power function relationship between the gas flow and the Bragg wavelength with a fitting error less than 3% and a gas flow detection limit of ∼0.178 m3/h in a range up to 32 m3/h. Moreover, the proposed flowmeter is of high security, easy-integration and low cost.