Comparison of the spectra obtained using offline and online opacity calibration for the 183 GHz H₂O line towards VY CMa, plotted at 0.5 km s^-1 resolution. Use of the offline opacity correction increases peak flux density of the 183 GHz water line in VY CMa by 48%, where atmospheric transmission varies significantly across the sub-bands making up the USB. It makes little difference to the SiO lines that are in a region of flatter atmospheric transmission in the LSB, as expected. Throughout this paper values and figures from the offline calibration are used.