Efficient and Automated Generation of Orthogonal Atmospheres for the Characterization of Low-Cost Gas Sensor Systems in Air Quality Monitoring

This article presents a compact continuous-flow automaton for the metrological characterization of an array of low-cost gas sensor systems (up to 17 devices) used in air quality monitoring. The automaton readily generates homogeneous gas mixtures of carbon monoxide (CO), nitrogen monoxide (NO) and dioxide (NO2), and ozone (O-3) in the parts-per-billion range (ppb, nmol mol(-1)) that are stable, that is, dispersion of 1 pph (NO, NO2, O-3) and 9 ppb (CO), with response times of 2 min (CO, NO, O-3) and 30 min (NO2). The resulting mixtures, which are traceable to the International System of Units (SI) due to the usage of calibrated high-grade reference instruments, can be humidified [0%-60% relative humidity (RH)] and, in addition, the devices under test (DUTs) can be heated (to temperatures between 5 degrees C and 30 degrees C) to systematically simulate different atmospheric environments. The application of fractional factorial designs makes the protocol efficient and leads to orthogonal variables. With the presented installation, ten low-cost gas sensor systems are calibrated and an uncertainty estimation is performed. The average values of the relative standard uncertainties across all DUTs are estimated as 52% (CO), 61% (NO), 22% (NO2), and 21% (O-3).