Strain and temperature sensors

Distributed Strain and Temperature Sensors (DSTS) by optical fibres are quite promising methods for the long-term (a few decades typically) structure health monitoring. As responsible operators, Andra and EDF are evaluating the feasibility to deploy such systems in their respective facilities: prospective underground radioactive waste repositories in the one hand, and hydraulics facilities (dams, dikes, water pipe-lines) in the other hand.

In the works performed within the frame of the EMRP project ENV54 MetroDecom (2014-2017), LNE has successfully developed thermal characterization facilities suitable to the Raman-DTS systems (which are only dedicated to the temperature measurements). Strengthened by this experience, LNE in association with Andra and EDF proposed new solution for the in situ calibration of such Raman-DTS. These original technologies will be designed and implemented in several exploitation sites of Andra and EDF where Raman-DTS systems are already in operation.

The main actions that will be performed within MetroDecom 2 consists in developing new experimental facilities dedicated to the metrological evaluation of DSTS systems based on the Brillouin scattering in sensing optical fibres. LNE designs a six meters length tubular furnace that will be used in particular for the characterisation of the spatial resolution of DSTS systems (DSTS systems are expected to have spatial resolutions in the order of one meter). This specific furnace will enable to operate in the temperature range from 0 to 60 °C by ensuring thermal homogeneity and stability within 0.1 °C. The DSTS systems are sensitive to mechanical strain (Fig 1.a) and temperature (Fig 1.b). Then, tubular furnace will be associated to a stretching device in order to apply controlled strains to a reference optical fibre as is illustrated in Fig 2.

Figure 1: a) Typical Brillouin frequency measured along a sensing optical fibre over 16 km of length at four temperature levels; b) Typical Brillouin frequency shifts measured on a located section of a thermal-stabilized sensing optical fibre at different stretching levels (expressed as micro-strain unit and noted µ which reports to µm/m in the SI).

Scheme of the facility developed by LNE for the mechanical/thermal characterizations of Brillouin DSTS devices

LNE will propose experimental procedures enabling the mechanical and thermal characterizations of Brillouin DSTS systems, with the aim to contribute to the standardization initiatives in progress at the international level (i.e. IEC TC/SC 86C/WG2).