Application of sapphire fibres to IR fibre-optic evanescent field gas sensors

A fibre-optic gas sensor for hydrocarbons in the mid IR range (MIR) suitable for measurements at elevated temperatures, based on sapphire fibres and evanescent field spectroscopy, has been developed. This sensor takes advantage of the high thermal stability, mechanical ruggedness and the chemical resistivity of sapphire fibres. However, the spectral window of the fibres is confined to wavelengths below 4 mu m. Thus, spectroscopic applications in the MIR are focused mainly on the exploitation of C-H, O-H and N-H stretch vibrations. An external fibre-optic set-up was developed, including a flow cell completely enclosed in an oven. A vacuum interface enabled direct coupling of the fibre to an MCT detector. Methane and butane were determined as analyte gases al various concentrations and temperatures. Calibration graphs were obtained For methane in a concentration range from 2 to 20% at temperatures from 20 to 300 degrees C and for butane in a concentration range from 1 to 10% at temperatures from 20 to 200 degrees C, respectively.