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.