## 3 Results and Discussion
### Temperature Test Results  
The results of temperature distribution between the layers of the fabric system with heating insert B (Figure 12a) demonstrated that temperature T1 (between underwear U1 and material O2 with the insert--at the site of its placement) and T2 (between material O2 with the insert and down package Z4) rose gradually with increasing electrical power of the insert, with a significant increment noted only after exceeding the power of 0.63 W. As it can be seen from the graph (Figure 12a), temperature T1 at the highest power of 3.94 W did not exceed 43 \({}^{\circ}\)C, which is in accordance with the requirements of the standard PN-EN 60335-2-17 (max. temp. 50 \({}^{\circ}\)C) [38]. Therefore, it can be deemed that the applied heating insert will not cause burns to the user's skin. When using the maximum power of the insert, the measuring plate of the \"skin model\" was turned off (the power of the plate was 0 W). The thermal resistance of the system then increased to 3.5 m\({}^{2}\)-K/W (Figure 12b).  
Temperature T1\({}^{\prime}\) (between underwear U1 and material O2 with the insert--next to the insert), regardless of the insert power, was at the level of approx. 30 \({}^{\circ}\)C, which indicates that the heat generated by the insert was mainly concentrated in the area of the insert, while the side areas of the garment were underheated. Analyzing the temperatures recorded by thermocouple T2, it can be seen that they were slightly lower than T1 at all powers of the heating insert, which indicates that most of the heat was retained at the \"skin.\" This is also evidenced by the very small changes of temperature T3 (between down package Z4 and laminate W2) at the max. level of 2.4 \({}^{\circ}\)C, with increased heating insert power.  
Figure 12: _Cont._  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variant** & **Applied** & **Power, W** &
\begin{tabular}{c} **Electrical** \\ **Resistance, \(\Omega\)** \\ \end{tabular} \\ \hline New & & 1.95 & 6.92 \\ after exposure to sweat (wet, 5 min) & 4 V & 2.00 & 6.87 \\ after exposure to sweat (dry, 24 h) & & 1.89 & 6.98 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Electrical properties of heating insert (variant B) after exposure to sweat.  
Studies conducted by [PERSON] et al. [39] have also demonstrated that the heating efficiency of a heating insert depends on the combination of the layers of the clothing worn. Research on volunteers showed that the use of a t-shirt with a heating insert on the inside powered with 5 V resulted in an increase in skin temperature to 38.8 \({}^{\circ}\)C. However, in the case where an additional sweater was worn, skin temperature increased to 39.3 \({}^{\circ}\)C. At a lower voltage of the insert, i.e., 3 V, skin temperature did not exceed 36.5 \({}^{\circ}\)C when wearing a two-layer clothing set (t-shirt + sweater).  
Tests of the temperature of the external surface of the heating insert using a thermal imaging camera made it possible to assess heat loss to the environment. Depending on the supply voltage level, the surface temperature of the insert was in the range of 27.74-36.38 \({}^{\circ}\)C. It was suggested that these losses could be reduced by increasing the thermal insulation properties of the outer layer of the heating insert or by using layered clothing. Our research has confirmed the latter hypothesis because, as mentioned previously, the temperature above the heating insert was lower than that between the underwater and the insert. At the highest voltage, i.e., 5.5 V (corresponding to a power of 3.95 W), the temperature on the surface of the insert was approx. 40 \({}^{\circ}\)C and approx. 4 \({}^{\circ}\)C higher than the surface temperature of the insert developed by [PERSON] et al. [39].