Principle Of Operation - Emerson Fisher L2 Instruction Manual

L2 Controller
July 2015

Principle of Operation

The operation of L2 controllers in combination with the 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 liquid displaced. The buoyant force
and resultant movement of the displacer in the liquid is transmitted to the controller which delivers a pneumatic signal
to a control valve.
Figure 6 shows a simple schematic of the controller and sensor. In its normal position, the counterclockwise moment
due to the weight of the displacer about pivot point O is balanced by the clockwise zero spring moment and the
counterclockwise relay zero force moment applied through lever A to the displacer rod. The weight of the displacer
decreases when the liquid level increases and the subsequent buoyant force increases causing a force imbalance
between the zero spring, relay, and displacer forces. This force imbalance is transmitted to the relay by levers A and B.
The relay compensates for the force imbalance by converting it to a pressure output to a control valve and bringing the
forces back into equilibrium.
Figure 6. Operational Schematic
DISPLACER ROD
PROPORTIONAL
BAND ADJUSTMENT
A5592
For throttling control, the relay pressure output will be proportional to the buoyant force. For on/off control, the relay
pressure output will be either zero or equal to the supply pressure over the range of liquid level change. The liquid level
change required to fully operate the relay is adjusted by sliding the proportional band adjustment along lever A to vary
the lever ratio between levers A and B.
With reverse‐acting proportional control, the principle of operation remains the same as that for direct action;
however, the controller delivers an increasing pneumatic signal to the control valve when the liquid level falls.
10
LEVER B
PIVOT
POINT O
DISPLACER
RELAY
LEVER A
Instruction Manual
D103032X012
ZERO
SPRING