Three-Dimensional Gas-Source Localization in Indoor Environment
Authors: Matsukura Haruka , Yoshikawa Keita , Ishida Hiroshi 
Published: February 2020

Gas sensors are used to detect the presence of gaseous chemical species in air. Once the target gas is detected, it is often required to track the gas and locate its source. In principle, the gas source location can be determined by tracking the spatial gas concentration gradient, which can be measured using a spatially aligned array of gas sensors. However, it is not an easy task as it appears to be. Diffusion of gas molecules into air is generally extremely slow. Even in a closed room with no ventilation, there exists convective airflow, and its velocity dominates the molecular diffusion. Therefore, the gas molecules released from their source are carried by airflow and form an aerial trail. Since the gas plume thus formed has a shape elongated in the downwind direction, the gas concentration gradient along the airflow direction is extremely small. Moreover, the airflow in environments of practical interest is almost always turbulent. Since the airflow direction and velocity fluctuate randomly, the gas concentration measurements are unstable.
In order to overcome these issues, we have proposed a directional gas-plume detector that consists of an anemometer in addition to an array of gas sensors. Here we report the experimental results on three-dimensional tracking of a gas plume under various conditions in an indoor environment. The main element of the gas-plume detector is an ultrasonic anemometer (Model 81000, Young). It allows accurate measurement of the three-dimensional direction of the airflow for a wide range of velocities ranging from 1 cm/s to 40 m/s. Six metal-oxide gas sensors (TGS2620, Figaro Engineering) are aligned around the anemometer, and are used to estimate the three-dimensional gas concentration gradient vector. The direction of the gas source is obtained by taking a linear combination of the airflow velocity vector and the gas concentration gradient vector. Owing to the elongated shape of the gas plume, the gas concentration gradient vector points not directly to the location of the gas source but rather toward the centerline of the plume. Meanwhile, if the presence of a gas plume is detected by the gas sensors, its source is estimated to be in the upwind direction. Therefore, the airflow velocity vector and the gas concentration gradient vector allow us to track the gas plume in the upwind direction while staying near the plume centerline.
In the gas-source localization experiments, a smoke generator (SLN-200, Nissho Electric Works) was used as a gas source. White smoke of glycol mist was released from the smoke generator, and a smoke plume was formed in an experimental room. The smoke is detectable with the gas sensors, and at the same time, allows us to visually check the extension of the plume. The plume detector was fixed on a tripod, and was first placed in the downstream part of the plume. Then, the sensor readings were measured for 60 s, and the detector was manually moved by 20 cm in the estimated direction of the gas source. By repeating this process, the plume detector succeeded in tracking the gas plume three-dimensionally. As the detector approached the gas source, the plume becomes narrower. Therefore, a slight change in the airflow direction causes large fluctuations in the gas concentration at the locations of the gas sensors. As a result, the standard deviations of the gas sensor responses in the 60-s time period increased as the detector approached the gas source. These standard deviations can be used as indices showing the proximity of the gas-plume detector to the gas source.