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How can I detect liquid level through an insulating wall?

Capacitive sensors can be used for sensing most materials but are particularly useful for sensing non metallic substances like plastic, wood, paper, glass etc which are not detectable with inductive sensors. They tend to work on the mass of material so are very useful in hoppers etc where contamination with dust and small particles is unlikely to effect their operation.

These characteristics also mean that they can sense material through a thin walled vessel, without having to penetrate the wall physically to install the sensor.

A good example is water level in a plastic tank:
In such a case, mount the sensor as close to the outside wall as possible without detecting the tank itself. In the case of a sensor with adjustable sensitivity it can be mounted very close or even touching the tank and the sensitivity turned down until it JUST fails to detect the tank. If the tank wall is thin enough or of material that sensor simply does not  pick it up then the sensor can be left on a high sensitivity setting but check to make sure this cannot then be triggered by small amounts of dust or debris collecting on or around the sensor.

Water level rising in the tank to the level of the sensor will have a dramatic effect on the capacitive sensing of the sensor and will  turn on the sensor

Schneider Electric sell a range of capacitive proximity sensors, please search for OsiSense XT
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