N(SH⁺)/N(CH⁺) column density ratio as a function of the CH⁺ mean abundance. The CH⁺, SH⁺, and H column densities are computed from CH⁺, ¹³CH⁺, SH⁺, HI, HF, and CH opacities integrated over the velocity intervals given in Tables 3 and 4. The blue and purple points are from the absorption lines observed along the Galactic centre sight lines (SgrA*+50 and SgrB2(N)) and along the other sight lines (DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51), respectively. The red line corresponds to a least-squares fit of the latter data.