Purity analysis of parent gases used to produce reference gas mixtures is fundamental to assure the metrological traceability of the certified gas composition, and the use of purity data in the calculation of the mixture composition should be performed in accordance with the requirements of international standards. Purity analysis can be difficult to realize since limited measurement standards are available for the determination of trace levels of gaseous compounds. The first step of purity analysis is the definition of the impurities considered critical or significant to the final composition of a mixture. In this work, we present the activity carried out for the identification and quantification of impurities of carbon dioxide and water in some ultrapure gases used for the preparation of primary certified reference gas mixtures of carbon dioxide at atmospheric amount fraction (400–800 µmol·mol⁻¹), by means of different spectroscopic techniques (Fourier Transform IR, Non-Dispersive IR and Cavity Ring-Down). Dynamic dilution was used for the generation of reference mixtures for the calibration of the analyzers by using calibrated Mass Flow Controllers. The certified reference gas mixtures produced with the tested pure gases will also be applied to characterization studies and calibration protocols for gas sensors used both for outdoor and indoor monitoring.

Low-cost sensors are increasingly used in atmospheric monitoring to provide spatially distributed measurements of gas concentrations, often through sensor networks. However, their application is still limited by the lack of metrologically robust characterization procedures. This work addresses a metrological characterization of SCD30 (Sensirion) non-dispersive infrared (NDIR) low-cost sensors for atmospheric carbon dioxide measurements, tested against an NDIR reference analyzer. A dedicated experimental facility and a systematic characterization procedure were developed using a dynamic dilution method in an isolator, covering a concentration range of approximately (350–950) µmol/mol, representative of typical ambient conditions. The analysis focused on sensor performance, calibration functions, uncertainty evaluation, and statistical indicators. Results show that all sensors exhibit good linearity but significant systematic deviations. The uncertainty evaluation highlights reproducibility as the dominant contribution (>85% of the uncertainty budget). The results demonstrate that, after applying calibration, root mean square error (RMSE) and mean absolute error (MAE) are reduced below 20 µmol/mol, demonstrating a substantial improvement in accuracy. The Bland–Altman analysis shows a good agreement between the reference instrument and the low-cost sensors. The proposed methodology provides a robust framework for the metrological evaluation and calibration of low-cost sensors, which can be extended to other atmospheric gases.

This paper presents the metrological characterization of low-cost sensors integrated into a multi-sensor system for Indoor Environmental Quality monitoring, developed within the MIRABLE project. The analysis focuses on two domains: the thermal domain, using Sensirion SHT45 and SEN55 temperature sensors; and the Indoor Air Quality (IAQ) domain, using an Infineon photoacoustic spectroscopy (PAS)-based sensor for carbon dioxide (CO₂). All tests were conducted under controlled laboratory conditions using calibrated reference instruments. In the thermal domain, the influence of sensor integration within the device case was investigated at temperature (T) between 15 °C and 35 °C and relative humidities (RH) between 30 %rh and 60 %rh. The results revealed self-heating effects in the desk unit, causing temperature biases of up to 0.6 °C. In the IAQ domain, the repeatability and the impact of T and RH on CO₂ measurements were evaluated. T was identified as the main influencing factor; whereas, RH had a negligible effect. These results were supported by statistical analysis ANOVA. A correction strategy based on concentration intervals is proposed for operation between 15 °C and 25 °C, with an expanded uncertainty (k = 2) of (3.26–6.42) ppm for the with-case configuration. These results support the reliable use of the MIRABLE system.