Cables for Temperature Sensing
Cables for temperature sensing are characterized by a loose mechanical coupling between the fibre(s) and their sheath in order to prevent the transfer of mechanical strain thus limiting the Brillouin sensitivity to temperature only. Brillouin properties, operating temperature range, protection from environmental agents such as moisture, rodents, etc. depends on fibre and coating properties as discussed already in the above.
The fibre is typically wet-coupled to the containing tubing using specific silicone gels that have the scope of improving the water resistance and lubricating the fibre-sheath coupling thus reducing possible stress transfer due to friction. At low temperature, the viscosity of the used gel may however decrease (or even freeze) and allow some drag transfer of dynamic stress, limiting the minimum operating temperature of the sensing cable or introducing unwanted Hysteresis.
A key element for proper cable design is fibre over-length, that means that the fibre inside the cable is actually a little bit longer than the final cable length, this to ensure that no strain is applied to the sensing element even in case that the cable is actually subject to some elongation up to the over-length value. Over-length must be taken into account for correct identification of the correspondence between the temperature/length distribution measured by the interrogator and the actual location of the measured temperature distribution along the monitored element. A typical 1% over-length means that, for a 1km cable segment, the fibre actually embedded inside it is long 1010m, so the difference between the measured strain profile and the real position along the cable is zero at the beginning of the cable but becomes 10m at its end.
The cable structure, and in particular the longitudinal thermal conductivity of its constitutive elements, may influence its behaviour in case of temperature transients and discontinuities.
Small "hot spot" applied at the external surface of the cable, due to the combined effect of internal strengthening members with high thermal conductivity and thick protective external layers with low conductivity may result "spread" over a longer region of the internal sensing fibre.
This effect can be reduced by placing the temperature sensing fibres more close to the external surface, however it has not to be necessarily regarded as a negative effect, since the distance resolution limit of Brillouin technology prevents the correct detection of very concentrated temperature changes, so the "spreading" effect increases the chance of detecting "hot spots" even below the distance resolution limit of the interrogator Equipment.