RADEX non-LTE modelling of o-CH₂ toward W3 IRS5. The solid and dashed- red, blue and orange curves represent the observed peak main-beam brightness temperatures of the 70, 68 and 69 GHz, respectively, and their uncertainties. We note that for the 69 GHz o-CH₂ lines being contaminated by NS, we only represent the uncontaminated modelled intensity and an upper limit. The black dashed and dotted lines mark the limits of the brightness temperature ratio between the 70 and 68 GHz lines. The grey shaded region characterises values of <11 corresponding to a probability of 99.9% over the temperature-density plane for models with fixed column density values of 3 × 10¹⁴ cm⁻² (left), 5× 10¹⁴ cm⁻² (centre), and 7 × 10¹⁴ cm⁻² (right).