RADEX models of HCN(1–0)/¹²CO(1–0) line ratios plotted against n_H2, and assuming [CO]/[HCN] = 10⁵. Here the ΔV = 20 km s^-1 models are illustrated. The upper limit HCN(1–0)/¹²CO(1–0) ratio for NGC 1140, shown as a (black) filled circle, has not been corrected for beam dilution because of the similar beam sizes for the two transitions. The approximate best-fit RADEX model for NGC 1140 is shown as a (purple) solid curve that is consistent with the non-detection of HCN(1–0). Also shown are three additional parameter combinations of ¹²CO column density N_12CO and volume density n_H2.