Observed H₂O  and CO line ratios (black dots) are compared with the predictions of slow C-type (red lines) and J-type (green lines) shock models (Flower & Pineau Des Forêts 2010). The solid lines indicate the errors due to the line signal-to-noise, while the dashed lines indicate the total error, obtained by adding the 30% error in the PACS flux calibration. The crosses along the red and green curves correspond to increasing shock velocity, from 10 km s^-1  to 40 km s^-1  (C-type shocks) or from 10 km s^-1  to 30 km s^-1  (J-type shocks). At the lowest velocity (V_shock = 10 km s^-1), the pre-shock density is given.