Liquid-level measurement capacitance method

Measurements of liquid density are closely related to quantity and liquid-level measurements since 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, since liquid-level detectors sense the steep density gradient at the liquid-vapor interface. Thus, the methods of density determination include the following techniques direct weighing, differential pressure, capacitance, optical, acoustic, and nuclear radiation attenuation. In general, the various liquid level principles apply to density measurement techniques as well. 

A liquid level control loop, shown in Figure 7.5, is essentially a single dominant capacitance without dead time. Typical hold-up times are from 5 to 15 min. Typically, processes that are dominated by a single, large capacitance are the easiest to control. However, liquid level processes are not necessarily as simple as they first appear to be. In many liquid level control situations, considerable noise in the measurement is present as a result of surface turbulence, stirring, boiling liquids, etc. The fact that this noise exists often precludes the use of derivative action in the controller. Still, some applications use unique methods of level measurement to minimize the noise in the measurement in order to apply derivative action in the controller. 

Previous studies have examined the release of chemical messengers on the whole-cell level through capillary electrophoresis and microcolumn liquid chromatography with electrochanical detection [6]. Although these techniques provide both qualitative and quantitative information about the average vesicular chemical content, they lack the temporal and spatial resolution needed for the precise detection of individual release events [6], Methods to observe and quantitate individual events have traditionally revolved around electron microscopy and patch-clamp capacitance measurements [7],... 

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