by Philip Taupe (AIT Austrian Institute of Technology), Martin Litzenberger (AIT Austrian Institute of Technology), Dieter Rothbacher (CBRN Protection GmbH) and David Monetti (Skyability GmbH)
Unmanned Aerial Vehicle (UAV)-borne gas detection and measurement can drastically reduce operational risks for the involved first responders in scenarios where hazardous gases may be present, e.g. accidents in industrial plants or gas leaks/accumulations in urban settings. However, measurement accuracy and detection abilities of gas sensors can be severely affected by the UAV’s rotor downwash, thus leading to erroneously low gas-concentration readings. We have investigated the applicability and operational parameters of probing tubes extending from under the UAV to escape downwash effects and thereby improve measurement results.
Multicopter UAVs are becoming an important tool for first responders in disaster situations to quickly develop an operational picture and improve situational awareness for decision makers. Especially in the case of (urban) search and rescue scenarios involving (partially) collapsed buildings, as well as in the case of emergency response in an industrial complex, UAVs are valuable assets. In such scenarios, the detection and measurement of the concentration of hazardous gases is sometimes also a critical requirement of the mission. If gas leaks and associated plumes occur only locally (i.e. the gas plume is relatively small in relation to both the UAV and the area of operation), UAV-borne gas sensors are severely affected by rotor downwash. The UAV’s rotors create a strong airflow close to and especially below the vehicle, which can affect detection and measurements of such gas plumes in two ways: 1. Dilution, by mixing with ambient air, and 2. Shielding, by moving the gas plume away from the gas sensor. The boundaries between both effects are fluid; however, both lead to uncertain or invalid detection and measurement results.
Previous studies on the airflow effects on concentration measurements using a UAV-mounted gas monitor were conducted with the UAV hovering stationary in a chamber with a constant NOx (nitrogen oxide) concentration, but without considering highly local gas point sources. . A MEMS metal-oxide (MOX) sensor has been evaluated for UAV-based gas concentration measurement, with a bottle of propylene alcohol as a tracer substance in an outdoor environment . The experiment was done under uncontrolled environmental/wind conditions and therefore the effect of gas dilution by the UAV rotor downwash could not be evaluated. Indoor tests with ethanol as a tracer gas and a MOX gas detector mounted at the top of the fuselage of a small UAV have been presented in . In this investigation, the airflow patterns around the UAV and with respect to the gas sensor have also been analysed using dry ice. Probing tubes have not been investigated as the MOX sensor was mounted on top of the fuselage. McKinney et al. have installed an array of short sampling tubes for VOC under the frame of the professional-grade UAV . Pressure distribution around the UAV has been investigated using computational fluid dynamics simulations to assess the impact of rotor downwash and airflow turbulence on the sampling efficiency. The impact of the downwash effect on gas detection and measurements is evident and one possible solution is to install a long probing tube to move the gas intake well below the downwash zone. The effectiveness of this solution has, however, not been addressed in any of the above-mentioned publications.
We have evaluated the use of probing tubes for the improvement of UAV-based gas detection and concentration measurements. The aim was to experimentally validate the approach and derive operational parameters for first responders in the field. We also investigated when downwash effects would not only impair quantitative concentration measurements, but also qualitative gas detections against an ambient background.
The experiments to detect CO2 have been conducted inside a tall building to reduce the influence of environmental conditions, such as wind. Figure 1(A) shows the used professional-grade UAV hovering over the centre of the test basin. The basin (2m×2m footprint, 40cm in height) was used to retain some CO2 close to the sensor (Sniffer4D v2, Soarability). We monitored the CO2 concentration and the level of the basin’s upper edge. The UAV started in a hover position 15m above the sensor and subsequently lowered its position in 2m intervals. At each hover position, the basin was flooded with a burst of gasous CO2 using a fire extinguisher to simulate an active gas leak.
Upticks in the measured gas concentration relative to the ambient baseline have been recorded (Figure 1(B)). At altitudes of 15m and 13m, a clear response to the CO2 burst is visible (subject to sensor response time), before the measured concentration falls back to the ambient level (note that the ambient level increases over time as more CO2 is generally present). Response levels decreased with decreasing altitude, and at 11m no response was recorded. This indicates a clear boundary at which not even a qualitative assessment of the presence of hazardous gases is possible and misleading gas monitoring could pose a serious threat to first responders.
Figure 1: (A) Test setup with UAV hovering above gas basin. (B) Measured CO2 concentration at varying hover altitude in response to CO2 bursts from a fire extinguisher. Blue line (lhs): UAV altitude; Green line (rhs): CO2 concentration; Shaded areas: Duration of CO2 bursts.
We have investigated gas plume dilution/shielding by UAV rotor downwash in a realistic test setup using a CO2-filled basin and a professional-grade UAV together with a CO2 sensor. Our method has shown that the ability to detect increased gas concentrations against an ambient background is severely impaired below a certain minimum hover altitude (in the range of 11 to 13m for the specific UAV used during the experiment), suggesting that a probing tube should always have a length that is considerably longer than this absolute minimum altitude.
This study was carried out as part of the research project “UAV-Rescue” which has been funded by the Austrian security research programme KIRAS of the Federal Ministry of Agriculture, Regions and Tourism (BMLRT). We would also like to express our gratitude towards the fire brigade of the city Vienna (Berufsfeuerwehr Wien), who kindly supplied facilities and materials used during the experiments.
 J. L. Brinkman and C. E. Johnson, “Effects of downwash from a 6-rotor unmanned aerial vehicle (UAV) on gas monitor concentrations,” Mining, Metallurgy & Exploration, Bd. 38, Nr. 4, S. 1789–1800, Aug. 2021, doi: 10.1007/s42461-021-00436-5.
 M. Rossi, et al., “Gas-drone: portable gas sensing system on UAVs for gas leakage localization,” in Proc. IEEE SENSORS 2014, pp. 1431–1434. doi: 10.1109/ICSENS.2014.6985282.
 J. Burgués, et al., “Smelling nano aerial vehicle for gas source localization and mapping,” Sensors, Bd. 19, Nr. 3, S. 478, Jan. 2019. doi: 10.3390/s19030478.
 K. A. McKinney et al., “A sampler for atmospheric volatile organic compounds by copter unmanned aerial vehicles,” Atmos. Meas. Tech., Bd. 12, Nr. 6, S. 3123–3135, 2019. doi: 10.5194/amt-12-3123-2019.
Philip Taupe, AIT Austrian Institute of Technology, Austria