The non-linear response function of the Reduction Gas Analyser is determined using calibrated primary reference gases on the MPI-2009 scale for H2 (Jordan and Steinberg, 2011) and NOAA2004 scale for CO. The range of concentration in the primary reference gases is 375 ppb to 1182 ppb for H2 (5 cylinders) and 62 ppb to 503 ppb for CO (6 cylinders). This non-linearity response function is determined every 1 – 2 months. Instrument repeatability is assessed by calculating the deviation in concentration of the working standard from the bracketing two working standards as the concentration of this is known as it is determined at the same time as the non-linear response function. The accuracy of the system is assessed through the analysis of a target gas every 6 hours. The target gas is a cylinder of gas with accurately assigned concentrations of the target species determined at a central calibration laboratory (in this instance MPI). This target gas is introduced to the system as a sample and following analysis the deviation of H2 and CO concentration from the assigned concentrations are calculated. We also take part in International inter-comparison programs (i.e. WMO round-robins, Cucumbers), see links.