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Chapter 11:
Temperature Sensors
One of the most frequently measured physical parameters is temperature. Often times, temperature changes can be an indicator of physical changes occurring in other parameters of interest. Furthermore, since temperature is an interfering parameter in the measurement of other physical variables, it also becomes necessary to measure temperature to correct any temperature-induced effects that could lead to measurement errors of other measurements such as strain, pressure, displacement, etc. In addition, several other considerations drive the need for the utilization of fiber optic temperature sensors instead of conventional ones [such as thermistors, thermocouples, resistance temperature detectors (RTDs), or similar]. Thermometers are often required to operate in the presence of strong electromagnetic fields, while in contact with human patients, or to be interrogated over long distances. Sensors with metallic leads will experience eddy currents in such environments, which will create both noise and the potential for heating of the sensor, which in turn causes inaccuracy in the temperature measurement. In medical applications, it is imperative to offer electrical isolation and to be able to function in conjunction with magnetic resonance imaging (MRI) and computed tomography (CT) scan-type equipment. Fiber optic temperature sensors, which do not use metallic transducers to perform their conversion, allow for minimized heat dissipation by conduction and provide quick response. Since they are less perturbing to the environment, they have the potential for extreme accuracy. Several fiber optic sensing concepts have been exploited to develop thermometers as well as to perform temperature measurements based on reflectance, microbending, fluorescence, absorbance, and blackbody radiation, as well as intensity-, phase- or wavelength-modulated approaches. In addition, a unique temperature-sensing capability of the optical fiber is to perform truly distributed temperature sensing along its length through the use of Raman and Brillouin scattering effects. In the sections to follow, a review will be made of the most common fiber optic temperature sensing techniques.
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