Researchers are exploring the use of unmanned aerial vehicles (UAV) to measure concentrations of gas emissions in situations where such measurements are otherwise extremely difficult to obtain, such as NO and NO2 (NOx) emissions from surface mine blasting. Recently, countries have imposed or are planning to impose surface blasting gas pollution regulations, which creates a need for an improved understanding of UAV-based gas measuring systems. The experiments detailed in this paper were performed to determine if airflow generated by the rotors, also known as downwash, unnaturally alters concentration measurements. Downwash turbulence could create locations of varying pressure, which could affect density-based concentrations as measured by some governing entities, including the Occupational Safety and Health Administration (OSHA). A UAV fitted with gas sensors was attached to a stand in a controlled environment containing a mixture of NOx. The gas-sensor measuring points were placed at various distances on and from the UAV body to evaluate downwash effects on gas concentration. Results showed with 95 percent statistical confidence that the presence of UAV downwash caused no significant changes to measured concentration values. Additionally, there was 95 percent statistical confidence that changing the measuring point location within the downwash region, including directly attached to the UAV, does not cause significant changes to measured concentration values. These conclusions apply to distances of 0.1 to 3.5 m vertically below the UAV, 0.1 to 0.8 m horizontally from the UAV propeller tip, and directly on top of the UAV body.
Turbulence was expected to be the most probable source of concentration fluctuation, yet the measured concentration values showed no correlation with the turbulent regions discovered by our previous work. Because no significant concentration changes were observed, and concentrations had no correlation with turbulence, it is improbable that the presence of rotor downwash or the measuring point location within the downwash region had any effect on concentration values. Based on these results, it is possible to use a UAV-mounted gas monitor to measure concentrations without concern for downwash interference, provided the UAV and measuring point are entirely contained within the gas cloud.
Mining, Metallurgy & Exploration (2021) 38:1789–1800, https://doi.org/10.1007/s42461-021-00436-5