## 3 Results and Discussion  
This section presents the results of the numerical and analytical studies and compares them with the results of experiments. Figures 5 and 6 show typical out-of-plane deformations of the analyzed IGU in the case of series 2 and 6 (Table 1), respectively. For clarity, a \"view cut\" was used in the figures with a vertical plane passing through the model's center, so the figures show only half of the models. It can be seen that the deformations are symmetrical due to the dimensions of the model. Both glass panes of the IGU deform as a plate simply supported on four edges. The deformation of the spacer between the glass panels is negligible for the change of the gas volume inside. Figures 5 and 6 show only deformation in \(Z\) (U3) direction \(Z\) (U3) to better show the concept of deformation. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **E [GPa]** & \(\mu\) **[-]** & \begin{tabular}{c} **Volume Density** \\ **[kg/m\({}^{3}\)]** \\ \end{tabular} & \
\begin{tabular}{c} **Inelastic Heat** \\ **Friction [-]** \\ \end{tabular} &
\begin{tabular}{c} **Specified Heat** \\ **Capacity [\(J\)/(kg\(\cdot\)K)]** \\ \end{tabular} \\ \hline Glass & \(70\times 10^{9}\) & 0.23 & 2500 & 0.9 & 720 \\ Spacer & \(1\times 10^{9}\) & 0.49 & 1000 & 0.9 & 113 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Material properties used in FE analyses. Figure 4: Reference FE model: (**a**) FE mesh; (**b**) mechanical boundary conditions. The maximum value of displacement reaches approx. 0.240 mm, which is 0.83% different from the analytical model. For the presented case, the difference between deflection obtained in the numerical calculation and experiment is 0.02 mm, which equals 17%. The difference in results was also observed in measured and estimated pressure values in the gap. Considering the law of mass conservation, the quantity of mass of fluid inside the gap is only a constant parameter of the gas. The initial and additional air mass was obtained using Equation (1), assuming the atmospheric pressure of 100 kPa. In the numerical model, the parameter \"MCAV\", which refers to the mass of the fluid in the gap, was controlled for the first and last load steps. Figure 7 shows a qualitative comparison of the experimental and numerical results in terms of the pressure difference (difference in gap pressure and atmospheric pressure), deflection, and stress in the glass in the middle of the sample. Although the measurements were carried out for 5 min, the results are presented only up to 3 s of the test for clarity. The difference in the stabilized measured values for 3 s and 300 s was not more than 2%. Since the pane always deformed in opposite directions, the results for each series were averaged from two measurement points (LVDTs and strain gauges) located on opposite sides of the gap. Additionally, the figures show the average values of five repetitions for each test series. Figure 5: Deformations UZ [m] of the model in the case of injection of 100 mL of air (test series 2) to the gap. Figure 6: Deformations UZ [m] of the model in the case of withdrawal of 100 mL of air (test series 6) from the gap. As mentioned, a slight decrease in all measured values was observed after reaching the maximum value. This is due to the adiabatic transformation [43] of gas in the gap. A sudden injecting or withdrawal of gas into/from the gap causes an increase or decrease in the pressure difference, respectively, and thus a change in the gas temperature. After a while, heat is exchanged between the gas inside the gap and the glass panes. This causes the temperature to equilibrate, the pressure in the gap to stabilize, and consequently the measured values to decrease slightly. The adiabatic transformation happens very quickly. Figure 7: Comparison of the results of the experimental and numerical tests in terms of pressure difference, deflection of panes and stress in glass for: (**a**) injecting gas into the gap, (**b**) withdrawing gas from the gap.