A NOVEL TECHNIQUE FOR OPTICAL-FIBER PH SENSING BASED ON METHYLENE-BLUE ADSORPTION

A novel fiber optic pH sensor is reported. The sensor is based upon su rfacic adsorption of methylene blue dye, producing absorption in the e vanescent field surrounding the sensing fiber. The sensor is based upo n a 200 mu m diameter Plastic Clad Silica (PCS) fiber which has its cl adding removed in order to expose its core. The linear range of operat ion is between pH 3 and pH 9 and its response time is considerably sho rter than indicator based probes which measure spectral changes of pH sensitive chemicals. Effects of measurand ionic strength are shown to be negligible which is a further important advantage of this sensor Ov er other optical fiber pH sensors. The sensor design is sufficiently f lexible to allow it to be configured in probe (reflective) or in-line (transmissive) form, The latter allows the application of time domain reflectometry technique which may be used to achieve multipoint (distr ibuted) sensing.