The optical fiber serves as the distributed temperature sensor for LIOS DTS systems and is typically surrounded by a protective layer. It is preferably encased in a stainless steel tube which significantly increases the mechanical stability of the sensor. Fibre encapsulated in stainless steel tube or short “Fiber In Metal Tube (FIMT)” is a hermetically sealed rugged construction for very long lengths of optical fibres. Furthermore, it is particularly effective in protecting against the hydrostatic pressures, high temperature effects and corrosive environments.
The inside of the FIMT metallic tube may lined with gel to ensure that the sensor cable remains permanently water-proof. This viscous gel protects the fibres from many environmental concerns, prevents damage from microbending conditions and helps to minimize the forces applied during spooling and deployment.
An important parameter to consider with FIMT is the Excess Fiber Length. It is defined by the percentage of excess fiber loaded into the metal tube during processing to relieve induced stresses directly related to the differing thermal coefficient of expansions of each material contained in the construction. FIMT is exposed to drastic and often rapid changes in temperature and pressure, so the Excess Fiber Length must be considered to ensure continuous, robust performance across all operational conditions.
FIMT constructions contain individual or multiple fibers (single mode or multi mode) and are available in sizes ranging from 1 mm to 3 mm outer diameter, variations in effective wall thickness are possible as well. FIMT serves as the core for various fiber optic sensor cable constructions or may be integrated into high voltage cable designs directly.
The FIMT is manufactured from a special stainless steel strip. During the manufacturing process this strip is formed to a tube and welded along its length. The stainless steel tubes must be hermetically sealed for all applications. For this reason the complete weld seam is subjected to a leak proof test by means of eddy current. By means of a drawing process the desired final diameter is attained on the one hand and an increase in strength through cold conversion on the other. During the manufacturing process the optical fibers are inserted into the tube. They are clearly differentiated by means of a specific colour code. At the end of the production process the tubes are subjected to a complete optical attenuation measurement and then documented for back-tracing. Additionally, each length produced is subjected to a weld penetration test, each weld seam undergoes a leak test, and the Excess Fiber Length is checked.
LIOS offers a variety of FIMT constructions perfectly suitable for distributed temperature sensing applications along power cable transmission systems.