Top: absolute difference between the expected ΛCDM(3ν) TT CMB spectrum and a spectrum with the same values of the cosmological parameters except for Σm_ν = 0.3eV (computed with CLASS (black)) in the linear regime. The shaded area is the original ΛCDM(3ν) spectrum rescaled to 1/1000. The differences introduced by the non-linear effects for Σm_ν = 0.3eV are shown for CLASS in orange and CAMB in red and green (cf. text). Bottom: same differences relative to the uncertainties of the hlpTT spectrum are shown.