A method of controlling the temperature of an ultrasonic blade includes applying a power level to an ultrasonic transducer to achieve a desired temperature at an ultrasonic blade coupled to the transducer via an ultrasonic waveguide, inferring a temperature of the blade based on a voltage Vg(t) signal and a current Ig(t) signal applied to the transducer, comparing the inferred temperature of the blade to a predetermined temperature； and adjusting the power level to the transducer based on the comparison. In some aspects, the method includes measuring a phase angle ϕ between the voltage Vg(t) and the current Ig(t) and inferring the temperature of the blade from the phase angle ϕ. In some aspects, the method includes measuring an impedance Zg(t) equal to a ratio of the voltage Vg(t) to the current Ig(t) and inferring the temperature of the blade from the impedance Zg(t).
