Scale down of a dynamic generator of VOC reference gas mixtures

A system for the dynamic preparation of reference gas mixtures based on the diffusion technique has been developed by the National Metrology Institute of Italy i.e. the Istituto Nazionale di Ricerca Metrologica (INRIM). The gravimetric method used for the estimation of the diffusion rate and consequent concentration, gives the system the property to be a primary standard. The system can generate mixtures with low uncertainty and high stability in the 20 nmol.mol-1 - 2.5 µmol.mol-1 concentration range with a 5% (k = 2) expanded uncertainty for mixtures of acetone in air. Based on this system, a transportable device for the generation of VOC reference gas mixtures to be used as calibration standard was designed and developed. The methodology used for the scale down included several steps. An initial characterization and modelling of the primary device was done using computational tools. Based on the response of the computational model to the different physical quantities, a set of design parameters was identified. The thresholds for this set of parameters were established and translated into a set of design criteria to consider in order to keep the metrological performance target. After the design and development of the transportable device, a metrological characterization was carried out, to verify its capabilities. The metrological characterization of the generator was done in the Dutch National Metrology Institute i.e. the Van Swinden Laboratory (VSL) trough Cavity Ring Down Spectroscopy (CRDS) analyses to evaluate the linearity, the reproducibility and the short term stability. The results for the generation of methanol mixtures with molar fractions in the 80-150 nmol.mol-1 range, were 99.6% linear, with a reproducibility after 3 days within 2,9% and a short term stability better than 1% per hour. Repeatable measurements of the generated concentration were obtained for three different molar fractions, with the use of both CRDS (VSL) and the GC/FID (INRIM). A flow of the desired dry pure carrier gas can be connected to the device. The presence of water in the system has not been taken into account and further analyses should be done before introducing it to the system. Water presence might affect the adsorption rate, and consequently the flushing time before normal operation. This transportable device is able to perform in-situ calibration of instruments and has been designed to generate gas mixtures of up to four species at a time.