We investigated the possibility of substantially reducing the cost of minimally invasive ultrasonic non-destructive evaluation (NDE) of liquids, in particular, temperature sensing, using inexpensive narrowband transducers. Although designed for operation in air, ultrasonic transducers enclosed in an aluminum case could be submerged in water and were found to be suitable for this application; however, their responses changed substantially when submerged. The test cell developed was complemented by an amplifier to operate as an oscillator and some other support electronics to supervise the sensor's operation. The sensor was tested in temperatures ranging from 26 to 32°C at a nominal central frequency of 330 kHz and showed a sensitivity of around 280 Hz/K.

The paper describes two different approaches to ultrasonic measurements of temperature in aqueous solutions. The first approach uses two narrowband ultrasonic transducers and support electronics that form an oscillating sensor which output frequency is related to the measured temperature. This low-cost sensor demonstrated sensitivity of about 40 Hz/K at the distance of 190 mm and the operating frequency of about 25 kHz. The second approach utilised pulse-echo mode at the centre frequency of 20 MHz. The reflector featured a cavity that was filled with deionised water. The ultrasound propagation delay in the cavity was related to the temperature in the solution. The experiments were conducted for deionised water, and solutions of sodium persulfate, sodium chloride, and acetic acid with concentrations up to 0.5 M. In the experiments (conducted within the temperature range from 15 to 30• C), we observed increases in the ultrasound velocity for increased temperatures and concentrations as was expected. Measurement results were compared with literature data for pure and seawater. It was concluded that ultrasonic measurements of temperature were conducted with the resolution well below 0.1 K for both methods. Advantages of ultrasonic temperature measurements over conventional thermometers were discussed.

An oscillating ultrasonic sensor can be built using a pair of ultrasonic transducers and an amplifier but this configuration is insufficient for getting consistent output readings within a single frequency interval. Inclusion of a band pass filter and phase shifter enables frequency interval limiting and setting the desired frequency at the sensor's calibration point. These circuits are implemented using various construction techniques and components and are discussed in the paper along with two relevant block diagrams, one being convenient for research purposes and the other for production. Experimental results obtained for temperature and tilt sensors are presented.

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We investigated the possibility of linear angle measurement of a vessel using a pair of partially submerged ultrasound transducers mounted on its walls. Measurements of the transfer function at different tilts of the vessel showed noticeable changes both in magnitude and phase responses. These changes were found easiest to track from the output frequency of an oscillator formed by using the transducers in a positive feedback loop. The output frequency depended on the tilt parabolically and its relative scatter was found lower compared to that of the angular output of an industrial accelerometer. The achieved output frequency capture rate of 150 measurements per second enabled meaningful tracking of dynamic changes of the test rig tilt in real time.