An ultrasonic device may include an electromechanical ultrasonic system defined by a predetermined resonant frequency and include an ultrasonic transducer coupled to an ultrasonic blade. A method of delivering energy to the device may include applying energy to the blade at a first power level via the transducer coupled to the blade, measuring a complex impedance of the transducer, receiving a complex impedance feedback data point, comparing the complex impedance feedback data point to a reference complex impedance characteristic pattern, and determining that the blade is contacting a vessel based on the comparison. The method may also include disabling the power applied to the transducer and switching to a lower power level. The method may further include generating a warning that the blade is contacting a vessel, such as a light or a sound. An ultrasonic surgical instrument may effect the method.
