High precision, fast ultrasonic thermometer based on measurement of the speed of sound in air

This study presents a microcomputer-based ultrasonic system which measures air temperature by detecting variations in the speed of sound in the air. Changes in the speed of sound are detected by phase shift variations of a 40 kHz continuous ultrasonic wave. In a test embodiment, two 40 kHz ultrasonic transducers are set face to face at a constant distance. Phase angle differences between transmitted and received signals are determined by a FPGA digital phase detector and then analyzed in an 89C51 single-chip microcomputer. Temperature is calculated and then sent to a LCD display and, optionally, to a PC. Accuracy of measurement is within 0.05 °C at an inter-transducer distance of 10 cm. Temperature variations are displayed within 10 ms. The main advantages of the proposed system are high resolution, rapid temperature measurement, noncontact measurement and easy implementation.