Test observation at 90 GHz of a strong quasar on a ~650  m baseline with ALMA. The red line is the phase of the observed (complex) visibility on this baseline – note that for a quasar (or other point-like) source at the tracking centre of the interferometer we expect a constant phase in time. The blue line is the visibility phase after correction of the data based on the WVR signals and using the wvrgcal program.