exceeds a detection threshold can be made and plotted as shown for three cosmological models in Fig. \[cosmcompare.fig\]. The detection level was chosen to be 1 mJy for 100 km s$^{-1}$, consistent with feasible integration times with existing radio telescopes. In Fig. \[cosmcompare.fig\], the horizontal axis is chosen to be observed frequency and the counts are binned according to expected detection rate per bandwidth of radio spectrum. These are natural units for use in planning surveys. In computing this diagram, the assumption was made that there is no evolution in the comoving number densities or luminosities of the OH megamasers with redshift. Curves for showing the detection rate for the confusing signals from normal galaxies emitting the 21cm hydrogen line are shown for comparison. Computations of the detection rate for a range of sensitivities is given in Fig. \[ratecombo.fig\].

Confusion with 21cm line emission from normal galaxies
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The expected number of detections of normal galaxies in the 21cm line of neutral hydrogen can be computed in a similar manner to that for OH megamasers. The HI luminosity function $\phi(M_{HI})$ observed for nearby galaxies (Zwaan et al 1997) can be represented as a function of HI mass, $M_{HI}$ $$\phi(M_{HI})\,  dM_{HI} 