Constraints on the ionization fraction in DM Tau. Results from the steady state approximation are shown with: observational values, black dashed; modelled values from the entire disk, dark blue solid; modelled values from the HCO⁺ molecular layer, light blue solid; lower limit from Dutrey et al. (2007), light orange solid and lower limit from Öberg et al. (2011b), dark orange solid. All errors, dominated by the values from the chemical modelling, are a factor of 3.