Comparison between the sandwich and TUH models. Left: volume density of the inner layer $n_{{\text{H}}_2\text{, \hspace{0.17em}in}}$ estimated with the sandwich model as a function of the H₂ column density. Middle and right: comparison of the mean abundance estimated with the sandwich model as a function of the H₂ column density and the model proposed by Tafalla et al. (2021). The second and third panels show this comparison for ¹²CO and HCO⁺, respectively. In all panels, joint histograms show our data. The black lines show the power law fits and their uncertainty intervals. The pink lines show the Tafalla et al.’s model. The dashed vertical lines mark the N(H₂) corresponding to a visual extinction at 3, 7, and 16 mag.