Fisher 2100 - Principle of Operation

Instruction Manual
D200119X012
2100 and 2100E Liquid Level Switches
June 2017
9
Principle of Operation
The 2100 switch (figure 4) is an external, cage‐mounted, pneumatic liquid level switch. When the switch is in the
normal position with the flapper against the nozzle, output pressure cannot bleed off and remains the same as full
supply pressure. Rising liquid level exerts a buoyant force on the displacer, producing a torque in the torque tube.
When this torque exceeds the torque exerted on the flapper by the magnet, the flapper snaps away from the nozzle.
This action allows output pressure to bleed through the nozzle faster than supply pressure can enter through the bleed
orifice.
Figure 4. Principle of Operation for High‐Level Fisher 2100 Switch
A3619‐2
DISPLACER
FLAPPER
MAGNET
TORQUE TUBE
NOZZLE
OPTIONAL
STREET TEE
OPTIONAL BLEED ORIFICE
SUPPLY
OUTPUT SIGNAL
SHUTDOWN VALVE
(OR ALARM SYSTEM)
The reduced pressure in the output signal line activates the shutdown or alarm system. When the liquid level lowers,
the falling displacer forces the flapper into the field of the magnet, letting the magnet snap the flapper against the
nozzle and causing output pressure to build to full supply pressure.
On applications where low‐level switch operation is required, the nozzle, flapper, and magnet are positioned on the
opposite side of the torque tube so that downward displacer travel moves the flapper away from the nozzle.
The 2100E switch is an external, cage‐mounted, electric liquid level switch. Rising liquid level exerts a buoyant force on
the torque tube that either activates or deactivates an electrical SPDT (single‐pole, double‐throw) or DPDT
(double‐pole, double‐throw) switch depending on the required switching action. Falling liquid level deactivates or
activates the same switch depending on the required action.