W31C. Upper: normalised spectra of the ortho 953 GHz and para 1444 GHz lines. Middle: deconvolved spectra where the strongest hfs component is plotted for both transitions. The hot core V_LSR is marked with a dotted vertical line. Lower: the optical depth and column density ratios of the convolved spectra as functions of V_LSR for absorptions larger than 5σ. The horizontal dashed line marks the high-temperature OPR limit of three. (Details are found in Sect. 3.4.)