Principle Of Operation - Emerson Fisher FIELDVUE L2t Instruction Manual

Instruction Manual
D104782X012

Principle of Operation

The operation of the L2t's mechanical sensor is based on Archimedes' Principle, which states that a body immersed in
a liquid will be buoyed up by a force equal to the weight of the fluid being displaced. The net weight of the displacer in
the liquid is transmitted to the controller and amplified by the lever arm ratio. The amount of force amplification is
increased by adding extensions in the arm, and/or by mounting the displacer horizontally, since the effective length of
the vessel side of the lever arm is the radial distance from the pivot to a vertical line through the center of gravity of the
displacer. (Mounting the displacer horizontally distributes the total buoyancy change over a much smaller vertical
level change. It also introduces non-linearity, because the change in submerged volume of a horizontal cylinder is
much higher for small level changes around the 50% point than it is near the top and bottom limits.)
Changes in the resultant force are transmitted through lever B to move the magnet (refer to figure 11). As fluid level
rises, the magnet moves away from the switch and vice versa. The lower switch point (at which the dump valve is to
close) is set by balancing out the net force at that position with the zero-spring setting.
The magnet is attached to lever B, which has a fixed return spring bringing lever B down toward the zero spring. Lever
B is adjusted at the factory so that when it is on one travel stop, it is connected with a fixed pivot. The movement of
the magnet is defined during factory calibration.
Figure 11. Principle of Operation Schematic
DISPLACER
LEVER ARM
FIXED PIVOT
LEVER B
ZERO SPRING
L2t Liquid Level Controller
October 2023
MAGNET
23