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The response function of the Flame Ionisation Detector (FID) to CH4 is determined on a weekly basis using calibrated primary reference gases on the NOAA2004 scale produced and calibrated at MPI-Jena. The range of concentration in the primary reference gases is 1754 ppb to 2496 ppb (5 cylinders). 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 estimated repeatability using this approach is routinely less the 3 ppb. 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-Jena). This target gas is introduced to the system as a sample and following analysis the deviation of CH4 concentration from the assigned concentrations are calculated. The estimated accuracy achieved using the target tank is better than 3 ppb. We also take part in International inter-comparison programs (i.e. WMO round-robins, Cucumbers), see links.