Comparison of NEFD estimates for three observation campaigns. The measured NEFD using the scatter method is plotted as a function of the line-of-sight atmospheric opacity (τ_νx) for the 1 mm (left) and 2 mm (right) channels. Data points are NEFD estimates in mJy s^1/2 for N2R9 (blue), N2R12 (orange) and N2R14 (chartreuse). We also show the expected NEFD evolution with the line-of-sight atmospheric opacity in the plots as solid curves using the median zenith atmospheric opacity derived from all the scans acquired during a campaign.