In certain applications, the proximity of an object with respect to the fixed plate of the transducer is desired. Electrical circuits that develop output voltages proportional to the separation between the plates are available. The circuit shown in figure 4.25 uses an operational amplifier of high gain, giving output signal eo proportional .to xO” The moving object is provided with a plane conducting surface, if it ·does not behave’like one. The object. is. earthed and the fixed plate is so designed. As to have much smaller area than the movable surface and is provided with a guard ring as ShOWl~ in figure 4.25. The output signal eo is given ‘by,
Capacitance Moisture. Transducer
The dielectric constant of pure water is about 80 and that of most insulating materials, .solids or liquids . is less than 10, and so it is possible to measure the moisture content of these materials bymeasuring the dielectric constant of the moist solid or solution of the, substance in water. The technique can be extended for application to other combinations, if the variation in the dielectric constant is due to variation of the proportion of one substance in the mixture. The equivalent series on shunt resistance of the capacitor, representing the dielectric losses of the sample, may also be used to indicate the moisture content.
A more practical form of hygrometer employs the arrangement shown in figure 4.27 (a). The central part of the transducer is an aluminium rod acting as one electrode. The rod is oxidized over part of its length over which is, provided a thin layer of-graphiteor of an evaporated metal. Moisture is absorbed through this thin porous layer, by the aluminium oxide, and the equivalent capacitance