Noncontact capacitive measurement

Fig. 11. Schematic of the noncontact capacitance measurement setup. (Courtesy of ADE Corporation, Westwood, MA.)...

Capacitance probes (Fig. 18.18(a)) are widely used in liquid level measurements. It is possible, when the tank waUs are metal, to use a single bare or insulated metal rod as one capacitor plate and the tank walls as the other. More frequently, capacitance probes consist of a metal rod within a concentric cylinder open at the ends, which makes the transducer independent of the tank construction. An interesting appHcation of this type of capacitance probe is in aircraft fuel quantity indicators. Capacitance switches can be utilized as depicted in Fig. 18.18(b) to provide noncontact point measurements of Hquid level. 

Information on the dielectric properties of a material might also be obtained using noncontact scanning techniques. These can include rf or microwave measurements made with an electrode that is held in the vicinity of the surface. This approach has the advantage that samples remain undamaged by the measurement and thus can be reevaluated or subjected to other studies as needed. However, this approach cannot provide accurate values for the capacitance, in very thin films such that d/e < 4 A. Moreover, the current-voltage measurements are still needed in order... 

The second type of conductance measurement utilizes noncontacting sensors and depends on inductive or capacitive effects to measure conductance. 

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