  We demonstrate phase correction of 3 mm VLBI observations using the scanning
18 GHz to 26 GHz water vapour radiometer at Effelsberg and we demonstrate an
absolute accuracy of 15 mm in zenith path delay by comparing with GPS and
radiosondes. This accuracy should provide significant improvement in
astrometric phase referencing observations. It is not good enough for geodetic
VLBI to replace the tropospheric delay estimation but could be used to remove
short-term path-length fluctuations and so improve the geodetic observables. We
discuss lessons learned and opportunities for further improvement.
