High accuracy ultrasonic air temperature measurement using multi-frequency continuous wave

This article describes a highly accurate and instant-response ultrasonic system for measuring air temperature using multi-frequency continuous waves (MFCW). The proposed system uses a method heretofore applied only to radio frequency distance measurement but not to air-based ultrasonic systems. The method presented here is based upon the comparative phase shifts generated by three continuous ultrasonic waves of different but closely spaced frequencies. It uses the ultrasonic measurement of the changes in the speed of sound in the air to determine the temperature of bulk air. The changes in the speed of sound are calculated from phase shift of multi-frequency continuous wave. In our experiment, in a temperature-controlled chamber, we placed two 40 kHz ultrasonic transducers face to face with a constant distance in between. We use MFCW of 40/41.8/42 kHz. A single-chip microprocessor-based signal generator and phase detector are used to record and calculate phase shift and temperature. In a proof-of-concept experiment, it is accurate to ±0.4 °C from 0 to 80 °C with 0.05% resolution and temperature changes are instantly reflected within 100 ms.