The flow instruments used in thermal power plants cannot be calibrated directly for the actual process conditions, since no traceable calibration facility with known uncertainty is available. Asystematic investigation of the relevant influence parameters is therefore needed. It was found in earlier investigations that the dominant influences on the measurement uncertainty are the flowvelocity profile and the temperature. In the present work, we report on our experimental study of the temperature and Reynolds number dependence of a new ten-path ultrasonic flow meterprototype. An improved measuring program is developed that allows for a systematic characterization. Special emphasis was placed on producing and validating well defined velocity profiles on aprecision calibration flow rig. It was also for the first time intended and validated to generate fully developed Reynolds-similar velocity profiles for different temperatures so that the two main influence parameters, namely temperature and Reynolds number, can be clearly characterized separately. Since such ideal measurement conditions are not found in practical applications, the approach is also tested for a disturbed flow condition. A well defined disturbance is generated with a new flow disturber.