RADEX fit for H₂CO in IRS7B, given T = 40 K (from the rotational diagram fit of the full IRS7B survey). The solid line shows the measured 3₀₃ → 2₀₂ line intensity, and the dashed line is the measured 3₀₃ → 2₀₂/ 5₀₅ → 4₀₄ line ratio. 3σ errors are shown as grey lines, but can barely be distinguished from the measured values. When we estimate uncertainties, the errors on the fitted temperatures are also taken into account.