the excitation of He, as measured by the / $\lambda\lambda 4686/5876$ ratio. This is shown in the second panel of Figure \[fig6\]. Below $v_s \sim 200$km/s, this ratio is determined by the pre-ionisation of the shock, and the temperature structure of the post-shock region, and is multi-valued. However, above this velocity, it provides a useful diagnostic, albeit somewhat sensitive to $\log D_{\rm Fe}$. This sensitivity is caused by changes in the cooling length with gas-phase Fe abundance, which changes the ionisation structure of He in the shock. In what follows, we will use both the He and \[\] diagnostic ratios as spectroscopic indicators of the shock velocity.

BPT diagnostic diagrams
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For shocks, the position of the observations on the standard @BPT81 and @Veilleux87 diagnostic diagrams (hereafter BPT Diagrams), is indicative of the mode of excitation. However, they are not really useful as diagnostics of the detailed shock conditions. This is demonstrated in Figure \[fig7\]. It should be noted that the range in observed line ratio and in the theoretical grid is very small on these diagrams, compared with the range covered by either HII regions or active galaxies and Narrow Line Regions. All these diagrams serve to show is 