Ultrasonic oscillating temperature sensor for operation in air

Ultrasonic oscillating temperature sensors (UOTSes) allow sensing temperatures of the medium of interest across the complete ultrasonic pathway and quickly react to the temperature changes. These features are advantageous comparing to the conventional temperature sensors, which need to come to the thermal equilibrium to the environment in order to report the correct temperature, and sense temperature at a particular point only. To date UOTSes were used for temperature measurements in liquids only.

In its simplest form an UOTS requires a pair of ultrasonic transducers and an amplifier, that feeds the signal from the receiver to the transmitter. Positive feedback leads to sustained oscillations with the frequency, which is dependent on the temperature distribution between the transducers as it determines the ultrasound velocity thus the ultrasound propagation delay.

For the reporting protype we used a pair of narrowband 40 kHz ultrasonic transducers and an electronic driver built around a dual operating amplifier (opamp) LM358. The transducers were mounted on an aluminium 2020 slot 5 profile using two rubber lined Munsen rings with backplates and positioned at desirable distances with the help of four roll-in T-nuts. One opamp provided the mid supply reference point, and the other operated as an inverting amplifier with the gain ranging within 10..50. Increasing the distance between the transducers required increasing the gain to sustain the oscillations. When the distance changed, resulting in the change in the propagation delay, the output frequency of the UOTS changed too, confirming the feasibility of measuring air temperature using UOTSes. The frequency changed substantially when the air between the transducers was heated using a hairdryer.

We present details of the mechanical and electronic design of the working prototype and discuss the obtained experimental results.