A method includes providing electrical energy to a transducer for sealing a vessel, where a frequency of the electrical energy is in an ultrasound range, controlling the electrical energy to achieve a predetermined velocity of an end effector coupled to the transducer, when the end effector is grasping the vessel, sensing parameters of the electrical energy when the end effector achieves the predetermined velocity, calculating power of the electrical energy based on the sensed parameters and estimating a size range of the vessel based on the power, and controlling the electrical energy to achieve a target velocity, which is determined based on the estimated size range of the vessel, to seal the vessel.