Investigation of binary liquid aqueous methanol and ethanol mixtures using meander-shaped fibre-optic evanescent-wave absorption sensors

A fibre-optic evanescent-wave field absorption sensor based on a meander-shaped sensing probe is described. The influences of the fibre core diameter and the refractive index of the surrounding medium on the sensitivity are evaluated by using binary liquid mixtures, in particular methanol and ethanol solved in distilled water at concentrations from 0 to 10 volume percent (%vol). The resolution was experimentally proved to be about 5ldr10^-5 refractive index units at a wavelength of 650 nm, which corresponds to a resolution of about 0.2%vol methanol and 0.1%vol ethanol concentration. The LED based design as well as the chosen wavelength (POF suitable) result in a low cost sensing application. Finally, an analytical approach for predicting the refractices indices of liquid mixtures (Lorentz-Lorenz equation) was examined for validity with regards to the mentioned aqueous methanol/ethanol solutions.