Comparisons of R_D(HCO⁺) values. The dashed black line shows the observationally derived value in DM Tau and the blue solid lines show the chemical model values at different time steps. Typical errors are a factor of 3. They grey vertical bars show where the observationally derived column densities of HCO⁺ and DCO⁺ are extrapolated beyond the inner and outer edges found in parameterisation. The orange dash-dotted line shows the value observed in TW Hya (Qi et al. 2008), the red dotted line, R_D(HCO⁺) in HD 163296 (Mathews et al. 2013), and the yellow dashed region, values from a survey of prestellar cores (Butner et al. 1995). The top y-axis shows the temperature of the DCO⁺ layer derived from our parametric fitting.