2120D1AV1NAAA0000 2120 Series Fork Level Switch Vibrating Level Switch. The Rosemount Liquid Level Switch is designed using the principle of a tuning fork. A piezo-electric crystal oscillates the forks at their natural frequency. The frequency of the vibrating fork sensor changes depending on the medium in which it is immersed. The denser the liquid, the lower the frequency. 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 2110 is 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.