Comparison between high-J HIFI and low-J JCMT spectra for the low-mass source Ser SMM1. ¹²CO J = 10–9 spectra (left-top) and C¹⁸O J = 9–8 line (right-top) observed with HIFI and for the ¹²CO J = 3–2 and C¹⁸O J = 3–2 lines (left-bottom and right-bottom respectively) observed with JCMT. The spectra have been resampled to 0.27 km s^-1 and shifted to zero velocity. The green line indicates the baseline subtraction.