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Like the strain gauge, differential capacitance sensors use a change in electrical characteristics to infer pressure. Here a change in capacitance is used to infer pressure measurement. The capacitor is a device that stores electrical charge. It consists of two metal plates separated by an electrical insulator. The metal plates are connected to an external electrical circuit through which electrical charge can be transferred from one metal plate to the other.
The capacitance of a capacitor is a measure of its ability to store charge. The capacitance of a capacitor is directly proportional to the area of the metal plates and inversely proportional to the distance between them. It also depends on a characteristic of the insulating material between them. This characteristic, called permittivity is a measure of how well the insulating material increases the ability of the capacitor to store charge. Mathematically this can be put as:
C = ε A/d,
here C = capacitance, A = area of plates, d = distance between plates of capacitor. ε = is the permittivity of the insulator between capacitor plates.
In modern electronic transmitters using the differential capacitance sensor technology, capacitive pressure measurement involves sensing the change in capacitance that results from the movement of a diaphragm. In this design, the sensing element is a taut metal diaphragm located equidistant between two stationary metal surfaces, forming a complementary pair of capacitances. An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion from the isolating diaphragms to the sensing diaphragm, and also doubles as an effective dielectric for the two capacitors. The diagram below shows the internal parts of a modern differential capacitance sensor transmitter from Rosemount:
Any difference in pressure across the cell will cause the diaphragm to flex in the direction of least pressure. This results in a change of capacitance across the cell. A high frequency capacitance detector circuit connected to this cell uses a high-frequency AC excitation signal to measure the difference in capacitance between the two halves, translating that into a 4-20mA DC signal which then becomes the signal output by the instrument representing pressure. Differential capacitance sensors are highly accurate, stable and rugged. They have a wide operating range.
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Their solid frame bounds the motion of the two isolating diaphragms such that the sensing diaphragm cannot move past its elastic limit. This gives the differential capacitance sensor excellent resistance to over pressure damage. Differential capacitance sensors are widely used for determining the level in a tank or vessel. Rosemount has a line of electronic pressure transmitters using differential capacitance sensors as the pressure sensing element. You might check it out here Rosemount pressure transmitters
