Qualification and optimisation of a gas mixing apparatus for complex trace gas mixtures

A gas mixing apparatus (GMA) can provide welldefined gas mixtures, which are necessary to calibrate gas sensors for quantitative measurements. This article presents a novel modular GMA and especially the representative measurements carried out to qualify and quantify its performance. New methods were developed to optimise the GMA's settling speed and to enable self-monitoring. The GMA is able to provide up to 14 individual test gases, each in a large concentration range from sub-parts-per-billion to hundreds of parts-per-millions. Additionally, the relative humidity can be set. The system is able to provide stable gas mixtures in approximately 6 min. The entire control software is realised in Python, which determines the control setpoints of all relevant system components based on the concentration input of the user. A major focus was placed on minimising the time of a gas mixture exchange to confirm design and additional software optimisation. Also, a number of techniques are shown for the analytical quantification of the system, including analysis of thermal desorption tubes by an accredited laboratory, showing maximal contaminations of e.g. only 4.18 ppb ethanol. Finally, self monitoring of the system using photoionization detectors (PID) is demonstrated as an addition to flow readback for internal leakage detection.