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Type S thermocouples were assembled in situ by applying high intensity electric arc discharges to the contact junction of two platinum (Pt) and Pt-10% rhodium (Pt-10% Rh) wires, inserted on a silica capillary. The electrically insulated thermocouples built in this way were afterwards employed to estimate the temperature of an optical fiber subjected to arc discharges. For typical values of the arc discharge parameters used to arc-induce long-period fiber gratings (electric current I=9 mA and arc duration t=1 s), a capillary peak temperature value of 1420°C±40°C was obtained by extrapolation of the experimental data for the limit situation of having a thermocouple with negligible diameter. The temperature profiles in the capillary and in an optical fiber were calculated based on a heat transfer model implemented by a finite element algorithm and fitted to the experimental temperature distribution in the Pt and Pt-10% Rh wires. The correspondent peak temperatures computed for the capillary and for the fiber were 1450°C and 1320°C, respectively. A good agreement between the capillary temperature values determined graphically and numerically was obtained.