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Capacitive Level Sensor basics | Capacitive sensor

This page covers basics of capacitive level sensor. It mentions capacitive level sensing operational working with diagram. The capacitive sensor is used for liquid level sensing using capacitive to digital converters (i.e. CDCs).

Capacitance refers to ability of body to store electrical charge. It is expressed by following equation.
C = Q/V,
Where,
Q = Charge on the capacitor
V = Voltage across capacitor

Capacitance of two parallel plates with area 'A' and separated by distance 'd' is expressed as follows.
C = ε0 * εR * (A/d),
Where,
C = Capacitance in unit of farads
A = Area of overlap of plates i.e. a x b
d = distance between two plates
ε0 = permittivity of free space = 8.854 x 10-12 F/m
εR = Relative permittivity of dielectric material between two plates

Liquid level sensing using capacitive sensor and capacitance to digital converters

capacitive level sensing

The figure-1 depicts working operation of capacitive level sensing. The simple technique to monitor liquid level is shown in the figure using two parallel plate capacitive sensors.

One pair of capacitive sensor (S2) is used as reference. The other plate of sensor (S1) is used to measure it in reference to the first one. As shown in the figure, if the liquid level changes, the amount of dielectric material between plates of S1 changes. This causes the capacitance to vary accordingly.

Due to the fact that εR (Water) is quite greater than εR (Air), the capacitance of sensor can be roughly taken as capacitance of the submerged liquid section. From the fromulas of C1 and C2 of sensors S1 and S2 respectively; level of liquid can be derived as C1/C2 (i.e. ratio of C1 with C2).

The capacitance to digital converters are used to read capacitance and convert it to digital value for further processing or to have any action according to the level i.e. switch off or switch on of the motor.

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