The TPS sensor (Fig. ), known as the hot-disk thermal constants analyzer is fabricated in the shape of a double spiral. It is made of 20 µm thick nickel foil covered on both sides with 25 µm thick Kapton foil for protection and electrical insulation. The sensor used in this study has a diameter of 6.4 mm and consists of 16 circular sources. Its resistance is about 13 Ω and the temperature coefficient of resistance is 0.0048 K -1 [16].