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General principles of transducers (sensors)
In the strict sense of the word transducer is a device that converts a change in one quantity changes in the other. In terms of electronic transmitter ... >>>
The principles of the transformation of the measured physical parameters
In each converter, which converts the element is based on certain physical principle, which is associated with the electrical characteristics of the device so that a change in the quantity being measured   >>>
Contacts
Capacitive pressure transducers, thickness

A capacitance transducer is a flat (or other form) capacitor whose capacitance depends on the area of the electrodes, their mutual arrangement, the distance between them, as well as the size of the dielectric and its dielectric constant.
C = εoεS/δ
These converters can be used to measure various non-electrical quantities, the value of which may be associated with a change in one of the above values, determining the capacity of the  transducer.
Fig.1. The principle of operation of the capacitive dynamometer and pressure gauge.

Fig.2. The principle of operation and the scheme of the device of the capacitive Transducer for thickness measurement

Capacitive dynamometers and pressure gauges operate on the principle of changing the air gap (Fig.1) between two capacitor plates under the action of the measured force or pressure.

For fig.2 shows the principle of the device transducer device for measuring the thickness of the rubber band.
The test strip is stretched between two fixed capacitor electrodes.

With a constant thickness of the tape will be constant and the capacitance of the capacitor. When changing the thickness of the tape will change the air gap, and due to the different values of the dielectric constant of rubber and air will change and the capacity of the transducer.
Capacitive transducer moisture meter grain, powder, fiber, yarn is a cylindrical capacitor (Fig.3).

The outer electrode has the shape of a glass, the inner electrode-the shape of the rod. The space between the electrodes is filled to a certain level with the test material. Since the dielectric permeability of water (ε=80) is much greater than the permeability of other substances, the moisture content in the test material dramatically affects the capacity of the transducer.

Capacitance transducerc usually have a small capacity, and therefore at a frequency of 50 Hz huge capacitance and low power. Therefore, measurements in circuits with capacitive transducers are usually made at high or high frequency using tube amplifiers.
Fig.3. Capacitive transducer moisture meter grain.