The use of dynamic dilution by Mass Flow Controllers in the environmental monitoring of CO2: calibration issues and strategies

Gaseous pollutants determination in atmosphere is usually carried out by comparison methods with dedicated analysers, which need to be calibrated with proper certified reference gas mixtures. D ynamic dilution allows the generation of gas mixtures which can be employed either to calibrate the analysers or to validate gas mixtures contained in high pressure cylinders. The dynamic dilution is often achieved by using Mass Flow Controllers (MFCs) due to their ease of use and good level of reliability. The MFC sensors are based on heat exchange equations, therefore the response of these instruments is sensitive to the nature of the flowing gas. The different response can be taken into account through a so-called Gas Correction Factor (GCF), which can be computed when the gas properties are known. Neglecting the use of proper GCFs may result in a bias in the assignment of the molar fraction of dynamic mixtures. The present paper deals with the calculation carried out at INRIM of the GCF in the generation of dynamic mixtures of carbon dioxide in synthetic air at ambient level. The results obtained without using the GCFs are compared with those obtained by application of the correction, showing the presence of a systematic non-negligible bias.