A basic capacitive liquid level sensor operates on the principle of varying capacitance based on the level of a liquid in a container. Capacitance is the ability of two conductive surfaces to store electrical charge when a voltage is applied across them. In the case of a capacitive liquid level sensor, these conductive surfaces are typically two parallel plates: one attached to the sensor and the other exposed to the liquid.
Here's how it works:
Sensor Setup: The sensor consists of two plates positioned in close proximity to each other, usually separated by the wall of the container holding the liquid. One plate is the sensor electrode, while the other is the container wall or another electrode in the sensor. The space between the plates acts as a dielectric material (insulator), which could be air or the liquid itself.
Capacitance Formation: When there is no liquid in the container, the region between the plates is filled with air (or another dielectric medium), and the capacitance between the plates is relatively low. This is because the dielectric constant of air is lower than that of most liquids.
Liquid Level Change: As the liquid level in the container rises, it comes into contact with the exposed plate, effectively changing the dielectric constant between the plates. Liquids generally have a higher dielectric constant compared to air. This change in dielectric constant causes an increase in capacitance between the plates.
Capacitance Sensing: The sensor is connected to a circuit that generates a small alternating current (AC) signal. This AC signal is applied to one of the plates, and the other plate is connected to the ground or reference potential. The capacitance between the plates allows the sensor to store charge as the voltage alternates.
Capacitance Measurement: The sensor circuit measures the impedance of the capacitor formed by the plates. Impedance is the combination of resistance and reactance (similar to resistance in AC circuits). As the capacitance changes with the liquid level, the impedance of the capacitor also changes. This change in impedance is detected and processed by the sensor's electronics.
Output Generation: The sensor's electronics convert the impedance measurement into a corresponding output signal. This output signal can be used to determine the liquid level. Typically, the sensor provides an analog signal, such as a voltage or current, that corresponds to the liquid level. This analog signal can then be further processed or displayed as required.
In summary, a capacitive liquid level sensor measures changes in capacitance between two plates as the liquid level in a container changes. This change in capacitance is caused by the varying dielectric constant between the plates, resulting in a measurable change in impedance that is then converted into an output signal indicating the liquid level.