Marimba bar tuning 

Not exactly an industrial instrumentation video, but relevant to frequency and frequency measurement nonetheless. Note the use of a "strobe" type of tuner, which uses flashing light (triggered by the microphone input) to "freeze" marked tracks on a spinning wheel. When the frequency is correct, a pattern of light/dark squares comes into view on the wheel. Multiple tracks on the spinning wheel have light/dark squares matching different octaves of each note. Recall that octaves are harmonics, related to each other by a frequency factor or two.