Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

In this study we investigate the capabilities of Fiber Bragg Gratings (FBG) sensors to
measure temperature variations in the bulk of liquids. In particular, we want to show to what extent
the utilisation of optical glass fibers without encapsulation can fulfill the requirements with regard to
robustness and measurement accuracy. We built an experimental test setup as a benchmark, in which
23 FBG measuring positions are instrumented in parallel with thermocouples for validation. We
present a special assembly procedure in which the fiber is placed under a defined tension to improve
its stiffness and immobility for given flow conditions. We propose the application of a single FBG
sensor as a strain reference measuring the strain effect in real time and allowing for accurate relative
temperature measurements at the other FBG sensor points. The further addition of a well-calibrated,
strain-independent thermometer at the reference FBG provides satisfactory absolute temperature
data. We apply this method on two test cases: (i) a stable gradient in the liquid metal GaInSn and (ii)
a local source of heat in water. We succeeded in accurately tracking the temperature variation both
spatially and temporally.