The 2120 is a popular choice for its simplicity, ease-of-use, and reliability, in high and low level alarm applications.
When used as a low level alarm, the liquid in the tank or pipe drains down past the fork, causing a change of natural frequency that is detected by the electronics and switches the output state.
When used as a high level alarm, the liquid rises in the tank or pipe, making contact with the fork which then causes the output state to switch.

8/16 mA (’Off’/’On’), Direct Load, PNP, Relay, or NAMUR switched output electronics (depending on version)
Threaded process connection for side, top, or bottom mounting
2 off M20 conduit entries for supplied cable gland and blanking plug (cable not supplied)
Easy selection of Dry ’On’ or Wet ’On’ Mode and Switching Delay
Magnetic test point makes functional test easy

Vibrating Liquid Level Switches