Liquid-level measurement ultrasonic method

Density measurements are closely akin to liquid level measurements because both are often required simultaneously to establish the mass contents of a tank, and the same physical principle may often be used for either measurement. Thus, the methods of density determination include the techniques of direct weighing, buoyancy, differential pressure, capacitance, optical, acoustic, ultrasonic, momentum, rotating paddle, transverse momentum, nuclear radiation attenuation, and nuclear magnetic resonance. Each of the principles involved will be discussed along with its relative merits and shortcomings. 

The theory of sonic-electronic level measurement is fundamentally based on a sound wave emission source from a transmitter, and a reflection of the sonic wave pulse to a receiver. Measurement of the transit time of this sound pulse and its correlation with electrical impulses provide a means for liquid level detection. Two basic designs operating on this principle use the vapor phase and the liquid phase methods. As most of the attention is currently devoted to the latter type system, this discussion deals exclusively with ultrasonic gaging over a liquid path. 

Other methods are available to measure liquid level. One of them employs an ultrasonic transducer located at the bottom of the tank which generates pulses and receives them back after a delay proportional to the liquid level by reflection from the interface between the liquid level and the air space above. 

Another interesting alternative is ultrasonic velocimetry. Sound travels more quickly in solids than liquids, and it is possible to measure the solids content of an oil sample by measuring the time needed for an ultrasonic pulse to move through a fixed pathlength of oil as a function of temperature (McClements and Povey, 1987). Comparisons of ultrasonic with NMR methods have shown the former perform at least as well as the latter, and perform better in the case of low levels of solids (McClements and Povey, 1988). However, because this technique has not been widely adopted or received detailed review by the AOCS or other professional organizations, it is not given the status of a recommended method here. 

The ultrasonic pulse echo (UPE) method is used to measure the wall thickness of vessels. UPE can be adapted to distinguish chemical munitions from conventional types and to determine the filling level of shells. Differentiation between munition types is based on the fact that, besides the normal reflexion signal from the front wall, the detection of a rear-wall or burster-tube echo clearly indicates the presence of a liquid as filling, whereas with conventional munitions the original pulse is reflected only from the front wall. 

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