A basic capacitive liquid level sensor operates on the principle of capacitance, which is the ability of a system to store an electric charge. In this context, the sensor uses changes in capacitance to determine the level of a liquid within a container. Here's how it generally works:
Basic Setup: The sensor consists of two main components: a probe and a reference electrode. These components are usually made of conductive materials like metal. The probe is installed inside the container, while the reference electrode remains outside or at the top of the container.
Capacitance Variation: When the container is empty or the liquid level is low, there is a significant air gap between the probe and the surface of the liquid. In this state, the effective capacitance between the probe and the reference electrode is relatively low.
Capacitance Increase: As the liquid level rises and comes into contact with the probe, the dielectric constant (a measure of a material's ability to store electric charge) of the medium around the probe changes. Liquids generally have a higher dielectric constant than air. As the liquid fills the gap between the probe and the liquid surface, the effective capacitance between the probe and the reference electrode increases.
Capacitance Measurement: The sensor is connected to a capacitance measurement circuit. This circuit applies a small alternating current (AC) voltage to the probe and measures the resulting current flow. The capacitance of the system affects how quickly the charge builds up and discharges, which is reflected in the current flow.
Signal Processing: The measured current is then converted into a voltage signal, and this voltage is proportional to the capacitance between the probe and the reference electrode. The capacitance is, in turn, related to the liquid level in the container.
Calibration: Before the sensor can accurately determine the liquid level, it often needs to be calibrated. This involves establishing a correlation between the measured capacitance (voltage) and the actual liquid level in the container. Manufacturers provide calibration procedures or data for specific sensor models.
Output: The output of the sensor can be in various forms, such as analog voltage levels, digital signals, or even communication protocols like 4-20 mA current loops. This output can be further processed by control systems, microcontrollers, or other devices to display, monitor, or control the liquid level.
It's important to note that variations of capacitive liquid level sensors can include additional features for compensating for factors like changes in temperature, conductivity of the liquid, and material buildup on the probe. Overall, capacitive liquid level sensors are widely used due to their simplicity, reliability, and relatively low cost.