The subject matter of this specification can be embodied in, among other things, a method that includes evaluating an analog electrical current waveform, determining a Boolean waveform based on the evaluated analog electrical current waveform, determining a collection of one or more first durations based on one or more on times identified from the Boolean waveform, determining a collection of one or more second durations based on one or more off times identified from the Boolean waveform, comparing the first durations and the second durations, determining, based on the comparing, an event based on the comparing, identifying a time at which the event occurred, and providing the time as a completion time.