In this paper we describe application of fiber-optic distributed temperature sensing to the process monitoring and control environment. The measurement technique utilizes a modified optical time-domain reflectometer (OTDR) arrangement, where an excitation pulse is launched into the fiber, and a temperature-sensitive property of the backscatter is analyzed as a function of time. From this information we deduce the temperature profile along the length of the fiber. The fiber itself is used as the sensor, and the temperature- sensitive property measured is the spontaneous anti-Stokes Raman scattering of the fiber core material. The optical fiber is installed throughout the environment to be monitored, and temperature profiles are logged as a function of time. The fiber used is standard communication-grade, graded-index multimode fiber, although certain applications require fibers with special coatings. Typically, a 2 km loop is measured in 12 seconds to a point resolution of ± 1 C (2.5 sigma) with a sample separation of 5 m. The performance characteristics of the distributed temperature measurement system are described including optical design, fiber properties, splice, connector and coiling effects. Installation design principles are discussed together with the interplay between the physical installation and the performance of the instrument. This discussion includes the relationships between spatial resolution, temperature resolution and accuracy, coil length, measurement time, and the environment. Use of distributed temperature sensing in a simulated production environment is discussed, including process capability study measurements and on-line process monitoring and control. A brief application review is also given.
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