Today, Wuxi Rihuan Sensing Technology Co., Ltd. will introduce to us: When using capacitive proximity switches to measure liquid levels, will foam and wall deposits cause false alarms? - How to adjust the sensitivity to avoid interference?

In the liquid level detection site, foam in the water tank and wall deposits on the oil tank inner wall are the two most common "false alarm culprits" for capacitive proximity switches. Even if the liquid level has not reached the limit, the switch may act prematurely; or even if the liquid level has dropped, the switch may not reset in time. Many engineers think that the sensor is broken, and they repeatedly replace it but the problem persists - in fact, the problem lies in the change of dielectric state, while the sensitivity remains at the factory setting.

1. Why does foam cause false alarms? - The difference in dielectric constant is the key

The detection principle of capacitive proximity switches is to measure the change in capacitance between the probe and the container wall (or reference ground). The capacitance value depends on the dielectric constant (ε_r) between the two plates.

The dielectric constants of different media vary greatly:

The dielectric constant of water is approximately 80

The dielectric constant of air is approximately 1

Most foams have a dielectric constant close to that of air (about 1-2)

When the probe contacts the real water body, the capacitance value significantly increases, and the sensor triggers the output. While when the probe is covered by foam, due to the main component of foam being air, the capacitance change is extremely small.

The problem lies here: If the sensitivity is set too high, the sensor will judge the weak capacitance change caused by foam as "liquid", thus triggering falsely. Conversely, if the sensitivity is too low, the real liquid level may not be detected even when it has reached. The key to setting the sensitivity is to find the "just right threshold point" to distinguish foam from liquid.

2. Why does wall deposits cause false alarms? - Residues make the sensor "think" the liquid level is still there

The wall deposit problem is particularly prominent in the liquid level detection of high-viscosity liquids (such as glue, lubricating oil, paint, syrup, etc.). Residues remain on the container wall or the probe surface, even if the real liquid level has dropped, the residues are still recognized by the sensor as "liquid".

The essence of wall deposits is: The residues replace the liquid and persist within the sensor's sensing range, preventing the capacitance value from returning to the unloaded state. The sensor is not "stuck", but the residues are indeed still being detected.

3. How to adjust the sensitivity? - Three steps to find the "optimal threshold"

Step 1: Lower the sensitivity in the unloaded state

In the state where the container is empty (no liquid, no foam, no wall deposits), gradually increase the sensitivity from the lowest to the highest until the sensor does not trigger. This step ensures that the sensor is stable in the OFF state in the unloaded condition.

Step 2: Verify triggering in the liquid state

Fill the container with liquid (the real liquid level reaches the probe position), observe whether the sensor triggers stably. If it triggers, it indicates that the sensitivity setting is effective; if it does not trigger, the sensitivity needs to be appropriately increased until the liquid reaches and reliably acts.

Step 3: Confirm no false alarms in foam/wall deposit states

Reduce the liquid level to the foam layer or wall deposit area, observe whether the sensor triggers falsely. If it triggers, it indicates that the sensitivity is still too high, and it needs to be appropriately adjusted back until the foam or wall deposits no longer trigger.

Core principle: The sensitivity should be based on the critical value that can stably detect the liquid while not responding to foam and wall deposits. Different liquids and container wall thicknesses have different optimal sensitivities, and they must be debugged with actual working condition media on-site.

Most capacitive proximity switches can be adjusted for sensitivity through an internal potentiometer knob (turn clockwise to increase, counterclockwise to decrease), and some high-end models support teaching (Teach-in) buttons for one-click calibration. When adjusting, it is necessary to observe the sensor status in real time with an indicator light.

4. Other anti-false alarm measures

1. Cone-shaped probe anti-wall liquid design

Some capacitive level sensors adopt a conical head design, which reduces the contact area between the liquid and the detection surface compared to the ordinary cylindrical probe. Due to the lack of sufficient flat adhesion force, the liquid naturally drips under the influence of gravity and does not form liquid droplets on the probe surface, effectively avoiding false judgments caused by liquid accumulation.

2. Sensor with foam/hanging wall compensation function

Some high-end capacitive sensors have built-in compensation functions for foam and viscous media, which can automatically distinguish the true liquid level from foam, films, or residues. For working conditions with severe foam or hanging walls, it is recommended to choose models with such functions rather than ordinary capacitive proximity switches.

3. Changing installation position

If foam or hanging walls are concentrated in specific areas (such as the middle of the container, near the outlet), the sensor can be installed in a position where hanging walls or foam are less likely to occur. For equipment prone to condensation, the conditions for false alarms can also be analyzed in combination with temperature and humidity changes.

4. Multi-point detection to build "liquid level profile"

In scenarios requiring high-level control, multiple vertically arranged electrodes can be used to construct a "liquid level profile" - comprehensively judging the states of multiple detection points instead of relying solely on the output of a single sensor. This method can effectively reduce false judgments caused by foam or hanging walls at a single point.

Summary

Interference Type Reason Solution

Foam false alarm When the foam dielectric constant is close to air but the sensitivity is too high, it will still trigger Reduce the sensitivity to set a threshold of "measuring liquids but not foams"

Hang-up false alarm Residues persist in the sensing area, and the capacitance cannot return to the unloaded state Reduce the sensitivity, choose a conical probe or a model with compensation function

Empty-load false alarm Sensitivity is set too high, the container wall or air is also detected Adjust the sensitivity from the lowest level gradually

Capacitive proximity switch for liquid level measurement, the sensitivity is not always the higher the better. If set too high, both foam and hang-up will become "false liquid levels"; if set too low, the real liquid level will not act even when it reaches it. The optimal state is: act when the liquid is present; do not act when foam or hang-up is present.

Rihuan Sensor provides a series of capacitive proximity switch products, supporting manual adjustment of sensitivity and teaching calibration functions, suitable for various liquid level detection scenarios such as water tank liquid level, oil tank oil level, chemical storage tanks, etc. Welcome to follow Rihuan Sensor to obtain professional liquid level detection selection guidance and technical support.