Rosemount 2120 Series Fork Level Switch Vibrating Level Switch, Direct Load Output, Side or Top Mount, Aluminium Body,

Rosemount 2120 Vibrating Level Switch

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.

Vibrating Liquid Level Switches