This paper presents a general discussion on the parameters influencing the accuracy of ultrasonic instruments operating on transit-time principles. A particular emphasis is given to the transit-time ultrasonic flow meters fixed on closed pipes. The properties of solids suspended in fluids together with acoustic-ray theory is used to simulate reflection, refraction, diffusions, scattering and absorption of ultrasonic waves propagating inside a pipe. The simulation results clearly demonstrate the changes in the ultrasonic waves propagating in clean fluids in comparison to the fluids containing particles. It is shown that in the presence of particles the wave front scatters and weakens considerably. The simulation results are verified by real time experiments conducted on slurries flowing in closed pipes. As a result of this study, it is suggested that the flow meters should be made from multiple transmitters and receivers. This approach will enable accurate, reliable and consistent results in time and space-variant processes.