Optical and fibre-optic sensors for vapours of polar solvents

An optical sensor is described which continuously measures the concentration of vapours of polar organic solvents such as alcohols, ethers, esters and ketones, but does not respond to hydrocarbons and chlorinated hydrocarbons. The detection is based on reversible decolorization of the blue thermal printer paper used in graphic plotters. Two sensing principles are exploited. In the first, a sensor sheet is placed in a flow-through cell. An LED acts as a source of yellow light and a phototransistor measures the light transmitted. In the second, the sensing membrane is placed at the end of a bifurcated fibre-optic and its diffuse reflectance is measured. The sensitivity of the devices towards various kinds of vapours has been studied and the detection limits vary from 10 to 1000 ppm for some of the most common technical solvents.