Ultrasonic technology has been widely used to measure the thickness of lubricant film. The time delay between multiple reflections reflects the thickness of the liquid layer. As the thickness of liquid layer decreases, the measurement becomes more difficult owing to the overlapping of waveforms. Unlike the mainstream methods focusing on the characteristics of amplitude, a phase spectrum method is proposed in this paper to determine the thickness, which is much easier to operate and doesn’t need any extra noise reduction. It provides another point of view to solve the problem of film thickness measurement. This algorithm has been verified to test the liquid layer thickness ranging of 10–200 μm in a self-made experimental platform. Experimental results show that the thickness can be obtained accurately and the error is within the allowable range.

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