Ultrasonic Measurements of Temperature in Aqueous Solutions: Why and How

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. 1. Introduction to Ultrasonic Evaluation of Temperature Ultrasonic evaluation is used for various objects and media, especially when they are opaque and thus impenetrable by electromagnetic radiation. It involves excitation of ultrasonic waves by some transducers and reception of these waves after they have passed through the whole or a part of the object under evaluation. The measured decrease in the wave’s amplitude determines the ultrasound attenuation whilst the measured propagation delay specifies the ultrasound velocity. These parameters differ for various materials and also depend on the environmental conditions such as temperature (e.g., [1]). Ultrasonic spectroscopy is concerned with the ultrasound attenuation and velocity across a range of frequencies. Ultrasonic evaluation normally employs low intensity ultrasound, and can be considered non-invasive in most cases. However, the emitted energy needs to be constrained if it can cause changes to the object under evaluation. Ultrasonic evaluation is realised by using various arrangements for the wave excitation and reception (Figure 1) that have their specific advantages and limitations (Table 1). Table 1: Features of various ultrasonic evaluation methods. Figure 1: Experimental arrangements for various