Jun 01 2021 Free to read Unlock A primary mercury gas standard was developed at VSL to establish an SI-traceable reference point for mercury concentrations at emission and background levels in the atmosphere. The primary gas standard has been developed as a mercury gas generator to establish metrological traceability of mercury concentration measurement results, based upon a gravimetric approach, for ambient air levels as well as higher concentrations. The working principle of the primary mercury vapour generator is based on diffusion according to ISO 6145-8:2005. Specially developed stainless-steel diffusion cells are filled with approximately 2 mL of elemental mercury. The mercury vapour diffuses from the cell through a capillary. To obtain several diffusion ranges, cells with different capillary diameters are used. By weighing the diffusion cells at regular time intervals on a high-resolution analytical balance the mercury mass flow rate (diffusion) is determined gravimetrically.