New implementation of high-precision and instant-response air thermometer by ultrasonic sensors

In this article, a new single-chip microprocessor-based, non-contact thermometer with high-precision and instant-response is proposed. It uses ultrasonic sensors made of piezoelectric devices to detect changes of the speed of sound in the air and calculate the temperature of the bulk air. The changes of speed of sound are computed from combining two variations, time-of-flight (TOF) from a binary frequency shift-keyed (BFSK) ultrasonic signal and phase shift from two frequency continuous waves (TFCW). This technique can work in a wider range than using phase shift alone and is more accurate than TOF. In our experiment, in a temperature-controlled chamber, we placed two 40 kHz ultrasonic sensors face to face with a fixed distance in between. We use BFSK signals and TFCW of 40 kHz and 42 kHz. A single-chip microprocessor-based signal generator and phase detector are used to record and calculate TOF, phase shift and temperature. The calculated results of temperatures are then sent to either an LCD display or to a PC for calibration and examination. In a proof-of-concept experiment, it is accurate to ±0.3 °C from 0 °C to 60 °C with 0.05% resolution and temperature changes are instantly reflected within 100 ms. The advantages of our system are high accuracy, instant detection, non-contact, low cost and easy implementation.