A Distributed Fibre Optic Sensing System for Humidity Measurement

Abstract

A novel multi-point distributed humidity sensing system has been constructed based on the principle of using the absorption spectrum of a colorimetric reagent (cobalt chloride) immobilised on the surface of the core of a multimode optical fibre by employing a thin gelatin film. Two-wavelength detection is employed to provide intensity referencing. Light attenuation at the reference wavelength is only 0.2 dB. The basic design, construction and testing of an experimental sensor in the humidity region 20 to 80 % RH and temperature range 25 to 50°C is described. Resolution can be better than 2% RH and time constant 1 second. Repeatability is at least as good as the resolution of the measurement. A twowavelength optical time domain reflectometer (OTDR) has been developed and a foursensor network has been built. The resolution of measurement on the network can be better than 4 % RH, but because of noise in the OTDR the resolution value is dependant on averaging time in the instrument.