Material investigations and simulation tools towards a design strategy for an ultrasonic densitometer
Abstract: The purpose of the research described in the thesis has been to improve the accuracy, precision and reliability of an ultrasonic densitometer for liquids in general. Its practical application is a stand-alone device or, in combination with an ultrasonic volumetric flow meter, an all-ultrasonic mass flow meter. The work shows that the densitometer is reliable at different though steady temperatures. Changes in temperature alter the characteristics of the densitometer. Recognition of one of these alterations enables one to compensate for the thermal influence. The presence of thermal gradients in the densitometer due to thermal transients can be recognized and handled properly. A simulation tool is refined to aid in the design of the densitometer to include spectral and thermal dependencies of the sensor materials. The model described by the simulation tool is compared successfully with actual ultrasonic systems. The simulation tool is used to predict the received electrical signal of a two-transducer ultrasonic mass flow meter. The simulation tool is also used to investigate the effects of transducer variations on flow measurement errors in a volumetric flow meter. When the two transducers are not identical, there is a difference in the times of flight of the ultrasonic pulses used to calculate the flow velocity, which should not occur when the fluid is stationary. The simulation tool is based on the electronic simulation software SPICE, which aids in the design of the associated electronics. To incorporate the spectral and thermal dependencies of polymers used in the densitometer into the simulation tool, their characteristics have been explored. The superposition of frequency and temperature of their characteristics is used and parametric surfaces are proposed. The problem however is that those characteristics are not commonly found for most polymers. Signal processing techniques are applied to two densitometers for solids to illustrate the extraction of information from the measured signals as well as their drawbacks. The same signal processing methods provide the densitometer for liquids a means to achieve better accuracy, precision and reliability.
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