## 1 Introduction  
The equipment and tools used in rescue operations, depending on the requirements of the country, may be subject to special testing and certification procedures [1]. With regard to mobile fans, for the moment in Poland there are no requirements for testing and verification of important technical and performance characteristics that determine their effectiveness. However, it should be pointed out that these devices are an important tool for the implementation of rescue operations, allowing, for example, to carry out activities related to the evacuation of people from inside the building covered by fire (techniques: positive pressure ventilation, negative pressure ventilation and positive pressure attack) and other, unconventional actions such as support for activities concerning the elimination of fires of free-standing objects [1]. There are many types of mobile fan units on the market. In 2022, [PERSON] and [PERSON] showed that there are about 60 models of positive pressure ventilators dedicated to rescue operations, with half being electric and the other half driven by an internal combustion engine [2]. Given the nature of the work of such units, it is very important that the parameters that determine the effectiveness of their operation are properly verified. [PERSON] et al. in 2021 [3] showed that the critical parameters that can determine the success of operations are the volumetric flow rate and the area of effective jet distribution (its shape), respectively. Referring to the issue related to the study of volumetric flow rate, various methodologies are used to assess this parameter [4; 5; 6]. It should also be noted that the manufacturers of these devices themselves have their own proprietary methodologies for assessing flow rates. As shown in the study performed by [PERSON] et al. in 2022 [4], it was demonstrated that the method of measuring volumetric flow rate should reflect the actual conditions of use of such units (free-flow operation). Similar conclusions were also made in 2018 by [PERSON] et al., who showed that the volumetric flow rate parameters declared by the manufacturers may vary and be unreliable due to the lack of a unified methodology [7]. Numerous studies presenting experimental research of mobile fans under field conditions are present in the literature. In 2022, [PERSON] et al. [8] determined the flow characteristics of a mobile fan in a fan-building system. During the tests, the team evaluated the effect of selected fan positioning parameters on the achieved volumetric flow rate. As a result of the performed work, the team showed that the ventilation efficiency for the tested fan increased as the distance of positioning the mobile fan increased, but only up to 5 m. The indicated distance was determined as the most favorable distance. Similar tests were performed by [PERSON] [9], who evaluated the effect of the distance parameter (1.2, 1.8 and 2.8 m) of a positioning of a mobile fan on its efficiency. In this case, for the fan he had, the greatest efficiency was achieved when the fan was positioned directly at the door opening. Another team of researchers [10] showed that the flow rate increases as the fan is brought closer to the doorway. The team also noted that greater efficiency can be achieved by using more units. In the available literature related to the subject of testing mobile PPV fans in published conditions, it is necessary to mention two reports of the National Institute of Standards and Technology on the evaluation of the effectiveness of PPV in large-volume buildings. In a report, [PERSON] et al. [11] studied the issues surrounding the use of PPV ventilation in high-rise facilities. An optimal distance range of 1.2 to 1.8 m between the door opening and the fan was found. They also concluded that placing the fans in a V-shape leads to better efficiency than a series configuration. In addition, they stressed that for high buildings, fans located on the ground floor may not be sufficient to achieve positive pressure on the gas exchange path.