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Ultrasonic temperature measurement
Michael Reisinger
Supervision: Univ.-Prof. Dipl.-Ing. Dr. Bernhard Zagar
Measuring the time of flight of sound waves offers innumerable application possibilities in measurement technology. With the pulse-echo method, sound waves are emitted by a piezoelectric ultrasonic transducer and the time of flight of one or more reflections at boundary layers is detected by the same transducer. In addition, the transmission time, meaning the time that a sound wave needs from a transmitting transducer through a certain medium to an opposing receiving transducer, can also be measured. Probably the most important application of this is the measurement of flow velocities or flow rates in pipes based on the entrainment effect. Since the transit time depends not only on the flow velocity but also on the speed of sound, which in turn depends on the temperature, a temperature measurement can also be implemented. Due to the propagation path of the sound wave, it also has a spatial dependency. In the course of this thesis, a measuring system for measuring temperatures at different positions in a tank filled with water was implemented.
Fig. 1: Schematic representation of the measurement setup of a measurement section.
The set-up consists of a PVC pipe in which six measuring sections are mounted in the form of opposing ultrasonic transducers. The measuring sections are parallel to each other and positioned at different heights. A heating coil is mounted underneath and parallel to the measuring sections. By introducing heat to the volume, the heating coil generates a temporally and spatially dynamic temperature profile. The temporal and spatial dynamics in the measured temperature result from convection of warmer water masses and heat diffusion into the volume. By measuring the sonic transit time for the six measuring sections, it is possible to measure the temperature at different heights. The run time measurement is conducted using an evaluation board which is actually intended for measuring flow. As only one pair of transducers can be connected to this evaluation board, however, a plug-in board with multiplexer components was produced, allowing up to 16 transducer pairs to be connected.
The measurement uncertainty of the structure could be determined by statistical analysis of a series of measured values in which 1000 independent measurements were carried out with a static measurement setup. The standard deviation of the normally distributed measured values was calculated to be 355 ps. During the dynamic measurements, the temperature curves at the different heights were recorded by sequential measurements of the measuring sections while a heat pulse was introduced. This represented the temporal and local temperature curve of the heated, rising water masses.
Fig. 2: Time curve of the heating pulse (red) with a power of 500 W and a pulse duration of 1 s, as well as the measured temperature curves (blue) at different heights z above the heating coil.
Keywords: temperature-measurement based on ultrasound, time-of-flight measurement, spatial temperature profile, temperature-dependent speed of sound
March 28, 2023
