Capacitive Proximity Sensors can be used to detect non-metal objects, such as liquids and plastics.
There are 6 products of Capacitive.
Long-distance Capacitive Sensor with Adjustable Sensitivity
General-purpose Threaded Capacitive Sensor
Flat Capacitive Sensor with a Thickness of Only 10 mm
Liquid Level Sensor That Is Unaffected by the Color of the Pipe or Liquid
Fluororesin-coated Capacitive Sensor with Sensitivity Adjuster
Flat Capacitive Sensor with Separate Amplifier Ideal for Mounting on Robot Hands.