[Fe ii] 1.64 μm/[S ii](6716+6731 Å) intensity ratio, as predicted by NLTE statistical equilibrium calculations, as a function of the electron density and for temperatures of 10 000 K and 20 000 K. Solar abundances from Asplund et al. (2006) are assumed. The dashed areas indicate the range of extinction corrected values measured in the internal knots A and the external knots from C to L. The reported ranges also include the uncertainties from both image calibrations and extinction. The measured ratios are always below the predicted value, irrespective of the assumed temperature and density, which indicates a Fe abundance lower than solar.