Level Switch Overview; Measurement Principles; Process Characteristics; Vessel Characteristics - Emerson Rosemount 2120 Reference Manual

Reference Manual
00809-0100-4030
2

Level switch overview

2.1

Measurement principles

Using the principle of a tuning fork, a piezo-electric crystal oscillates the forks at their
natural frequency. Changes to the oscillation frequency are continuously monitored by
electronics as it varies depending on the liquid medium in which the forks are immersed.
The denser the liquid, the lower the oscillation frequency.
When used as a low-level point alarm, the liquid medium in the vessel (tank) or pipe drains
down past the fork, causing a change of oscillation frequency that is detected by the
electronics and switches the output state i.e. wet-to-dry.
When the level switch is used as a high-level point alarm, the liquid rises in the vessel
(tank) or pipe making contact with the fork and causing the output state to switch i.e. dry-
to-wet.
2.2

Process characteristics

Emerson's vibrating fork technology is virtually unaffected by turbulence, foam, solids
content, coating products, and liquid properties. The natural frequency (1400 Hz) of the
fork avoids interference from plant vibration that may cause false switching to a wet state.
This allows for minimum intrusion into the tank or pipe using a short fork.
2.3

Vessel characteristics

The level switch should be mounted using its process connection, and in a horizontal or
vertical orientation so that the liquid medium can flow freely in the gap between the forks.
A vessel (tank) or pipe can be almost any shape or type, but check that the process
conditions are within the operating limits of the level switch.
Avoid installing near agitators and inlet pipes where the forks are likely to be splashed and
cause a false switch to a wet state. False switching events can be minimized by an
adjustable time delay on the electronics cassette and allows time for the fork to dry.
Never force the level switch into a vessel (tank) or pipe space. Any contact with the
opposite wall, or in-tank objects, could damage the forks and other wetted-process parts.
Extended length versions require supports at regular spaced intervals.
Related information
Required supports for extended fork
Rosemount 2120 Level Switch
Level switch overview
March 2022
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