Demonstration of a novel tracer gas method to investigate indoor air mixing and movement

This paper reports on equipment and procedures that enable the application of the pulsed tracer method to study air movement, contaminant transport, and mixing in rooms. We use ethanol as a non-toxic tracer and a network of low-cost, fast response (2 s) metal oxide sensors to measure airborne concentrations at high frequency. The method was demonstrated in a 158 m³ room of the FLEXLAB facility at Lawrence Berkeley National Laboratory, with an overhead HVAC system with controllable supply airflow and temperature. The room was configured as a meeting space with 8 simulated occupants. The sensors were mounted in a 3 × 4 grid in the upper room (0.3 m from the 2.74 m ceiling), in the middle height of the room at 1.1–1.4 m, and at several locations 0.1–0.4 m from the floor. Vaporized ethanol was released in pulses of 20 s. Sensors were cross-calibrated in-situ to provide quantitative information about relative concentrations and exposures. Results show that the method provides quantitative information about air movement patterns and mixing. For example, mixing throughout the room took 3–4 min with high supply airflow at neutral temperature and 7.5–9 min with heated supply air provided at a lower rate. The test can be used to evaluate whether air movement from the occupied zone to the upper room is fast enough to achieve the extremely high air cleaning rates that are possible with upper room germicidal ultraviolet disinfection (GUV) systems under ideal mixing conditions.