## 2 A Design of the Experimental Equipment  
The line of the facepiece of the half mask must copy, as accurately as possible, the wearer's face. Due to variability of human faces, the mask must exhibit hardness and flexibility that will ensure that it optimally fits. Despite the efforts aimed at designing an optimal shape of the facepiece of the half mask, during breathing, the air was still leaking inside the mask through the gaps which may be formed between the mask and a human face. The measurements of the total inward leakage were carried out in a chamber while the mask was actually applied on a control object, and a focus of the analysis was the inward leakage of sodium hydroxide aerosol. The certification of half masks requires the determination of the total inward leakage of aerosol through all connections and contact surfaces of a mask. According to the EN 1827 + A1 standard, the inward leakage of the test aerosol through the valves (if on the mask) and the skin-facepiece interface of the mask must not exceed 2%. In order to create an analogue measurement, but without the necessity for using aerosol, an experimental stand was constructed and used for the measurements of the total air flow through the leaks in the facepiece area of the mask, and it was proportionally compared to the total air flow during inhalation. The key hypothesis of the analogy was that there was equality between the total penetration of aerosol and the ratio of air leakage to the total air flow during inhalation or exhalation. In order to create the most unfavourable conditions in which the air leakage was at a maximum level, it was necessary to create adequate negative pressure during inhalation and positive pressure during exhalation. According to the EN 1827 standard, the maximum negative pressure during inhalation is 420 Pa at the air flow rate of 95 L-min\({}^{-1}\) for particle-filtering masks of the FMP3 class. During exhalation at the flow rate of 160 L-min\({}^{-1}\), positive pressure in the mask must not exceed 300 Pa. The target parameter of the measuring stand was the determination of leakage during inhalation below 0.2%; at the flow rate during inhalation of 95 L-min\({}^{-1}\), it was therefore necessary to measure the leak flow rate of 0.19 L-min\({}^{-1}\) (or 3.2 mL-s\({}^{-1}\)). Measuring such a low flow rate at pressures approaching the atmospheric pressure is significantly difficult. That is why a measuring system was designed with an indirect measuring method, i.e., measuring the air pressure in the container of a defined size (Figure 1). With a known volume of the container and actual changes in the pressure over time, it was possible to identify the development of inward leakage through the facepiece over time. During the measurements, an analysis was focused on the isothermal pressure drop in the container, which was caused by changes in the weight of the air. The following equation was used:  
\[\mathrm{d}m=\frac{V}{r\cdot T}\mathrm{d}p\quad\mathrm{(kg)} \tag{1}\]  
wherein \(\mathrm{d}m\) is the elementary change in the weight of the air in the system, which was caused by the leakage (kg); \(V\) is the total internal volume of the measuring system, including the distribution systems and the internal volume of the mask (m\({}^{3}\)); \(r\) is the specific gas constant (J-kg\({}^{-1}\cdot\)K\({}^{-1}\)); \(T\) is the average temperature of the measurement system (K); and \(\mathrm{d}p\) is the elementary change in the air pressure (Pa). The mass flow rate of the air leaking through the respirator, as divided by time differential \(\mathrm{d}\tau\), was calculated using the following formula:  
\[Q_{m}=\frac{V}{r\cdot T}\cdot\frac{\mathrm{d}p}{\mathrm{d}\tau}\quad\mathrm{ (kg\cdot s^{-1})} \tag{2}\]  
Figure 1: Scheme of individual components of the system measuring the respirator inward leakage in the facepiece area.