Ratio of the flux from the v = 2 and v = 1 levels of COversus the inferred radius of emission for observational data and DALI model results. Line connect the dots in order of inner model radius. Labels indicate the different assumptions for the excitation calculation. All models have a gas-to-dust ratio of 100. The model with the largest cavity is always at the largest CO radius. Different assumptions on the excitation are tested.