I measured the CO2 levels in one room using several different CO2 monitors - under similar conditions (steady-state with one person in the room, windows partially open) on different days; Aranet readings were the most stable and consistent, while matching the "expected ppm".
I also tested the monitors by exhaling directly on them, and confirming the change in readings.
I also tested the monitors by exhaling directly on them, and confirming the change in readings.