Principle Of Operation - Emerson Fisher L2e Instruction Manual

L2e Controller
June 2022

Principle of Operation

The operation of the L2e'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 levers A and B to move the magnet (refer to figure 8 and 9). 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.
Figure 8. Principle of Operation Schematic
LEVER ARM
DISPLACER
E1775
The magnet is attached to a plunger on lever B, which has a fixed return spring forcing the plunger toward the
electronic switch. The hysteresis between magnet positions that cause the electronic switch to change states is fixed.
The magnet plunger is adjusted at the factory so that when lever B is on its travel stop, the switch contacts are in the
state required to close the dump valve, and the full hysteresis required to change to the state that will open the dump
valve is available for utilization. Levers A and B are connected by a movable pivot so that the change in buoyant force
required to produce a given amount of movement between the magnet and the switch can be adjusted. This span
adjustment controls the amount of buoyancy change required to change the switch state, thus setting upper switch
point and the static value of the differential gap.
12
LEVER A PIVOT AND
LEVER B TRAVEL STOP
LEVER B
PIVOT
Instruction Manual
SPAN
ADJUSTMENT
MAGNET
NON-CONTACT
LEVER B
SWITCH
RETURN
SPRING
ZERO
SPRING
D103531X012
LEVER B
PIVOT
LEVER A