Over a decade of experience with velocimeters aboard deep submergence vehicles has demonstrated that accuracies of 0.1 m/sec or better can be obtained with proper procedures. Calibrations and distilled water equations are discussed, along with the new International Temperature Scale adopted in 1968. Calibration drift is described for three types of velocimeters to underscore that accuracy requires frequent calibrations as well as intercomparisons between several instruments during. deep dives. Pressure corrections are computed. Errors due to thermal lags of earlier velocimeters were observed while transmitting thermoclines. Laboratory measurements with a velocimeter in 50 gal of natural seawater of 27.405‰ salinity gave good results, closer to those of V. A. Del Grosso [J. Acoust. Soc. Amer.47, 947 (1970)], than to W. D. Wilson [J. Acoust. Soc. Amer.32, 1357 (L) (1960)].