## 3 Results
### Experimental Set-up for Thermal Monitoring
Nevertheless, an open question, in the case of the cavity embedded in the inner elements of the mill cage, concerns the effectiveness of measurement. Some preliminary experiments have been performed upon a set of FBG sensors inscribed on a single-mode fiber, with an outer diameter of cladding equal to 125, core diameter of 9 \(\upmu\)m, and total length of 4 mm. Preliminary characterization of FBG sensors has been completed by identifying their thermal sensitivity coefficient through a direct comparison with a thermo-resistor PT-100. The goal of this calibration is the identification of \(\Delta\lambda_{B}\) for a given change of temperature, and thus the determination of \(K_{\rm T}\) as the slope coefficient of the characteristic function \(\Delta\lambda_{B}=\Delta\lambda_{B}(T)\). In this test rig, a Peltier cell, driven by the controller TEC (\"Thermo-Electric Controller\") TEC-1091 (Meerstetter Engineering GmbH, Rubigen, Switzerland), imposes the temperature changes in measurement activity. The controller TEC-1091 has an input voltage of 5 to 24 V (DC), output voltage of 0 to \(\pm\)21 V, and output current of 0 to \(\pm\)4 A. The TEC controller regulates the current feeding the Peltier cell through a PID control, whose feedback acts on the error between the value of temperature defined by the user and that measured by the PT-100. The thermos-resistor RSPro PT-100 (Platinum) operates within the range of \(-\)50 to +250 \({}^{\circ}\)C by resorting to a \"four wires\" configuration, resistance at a melting point is 100 \(\Omega\), sizes are 2 mm (width) and 10 mm (length), while tolerance class is A. The FBG sensors have been connected to the SmartScan Interrogator (Smart Fibers Ltd., Bracknell, UK), which operates in dynamic mode. It analyzes up to 4 fibers and a maximum of 16 FBG sensors through the WDM method (\"wavelength division multiplexing method\"). The experimental campaign exploited a single fiber with one sensor during each test. The main properties of sensor FemtoFiberTech (FemtoFiberTec GmbH, Goslar, Germany), based on fiber SM1250 BI, are the core diameter of 9.8 \(\upmu\)m, cladding diameter of 125 \(\upmu\)m, coating in polyimide, central wavelength of 1565.07 nm, sensor length of 3.2 mm, reflectivity of 71.4%, bandwidth at 3 dB 0.47 nm, and signal-to-noise ratio of 25.6 dB. The communication between interrogator and PC host is assured through the Ethernet and the software SmartSoft SSI v3.2 (Smart Fibers Ltd., Bracknell, UK). The sampling frequency of 2.5 kHz allows reaching a compromise condition between the need of retrieving as many data as possible and the limitation of the frequency range induced by a high number of samples. That frequency allows for detecting signals within the range of 40 nm. The interrogator is aimed at detecting the amplitude of the reflected component of the signal within the frequency range defined. Wavelength stability is assured within \(\pm\)5 pm, within the temperature range \(-\)10 to +80 \({}^{\circ}\)C, optical connectors FC/APC with a diameter of 2.5 mm of bushing. The Peltier cell used in testing is TEC1-12706 with a maximum current of 3 A, maximum voltage of 8.6 V, maximum power of 20 W, maximum difference in temperature of 66 \({}^{\circ}\)C, sizes 23 \(\times\) 23 \(\times\) 3.6 mm, and weight of 26 g.