Results of population diagram based on the narrow component (envelope) of H₂CO as observed with JCMT and HIFI (150 K < E_up < 200 K) and the additional IRAM transitions as taken by Maret et al. (2004). The plot presents the resulting excitation temperature (T_ex ~ T_rot), column density (N_tot), optical depth (tau) and size of the emitting area. The red symbols represent the observed data, the green symbols represent the best fitted data, and the blue line represents the rotation diagram fit. The three right panels show the resulting values after using the χ² method to converge to the solution.