Emerson Fisher 249 249B 249BF 249C 249K & 249L User's Guide

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1. Note that this document does not consider the effects of the thermal expansion of
the moment arm, or the thermal expansion of displacer volume.
Simulation of Process Conditions for Calibration
of Fisherr Level Controllers and
Transmitters—Supplement to 249 Sensor
Instruction Manuals
Displacer / torque tube sensors are transducers that convert a buoyancy change into a shaft rotation. The change in
buoyancy is proportional to the volume of fluid displaced, and the density of the fluid. The change in rotation is
proportional to the change in buoyancy, the moment arm of the displacer about the torque tube, and the torque rate.
The torque rate itself is a function of the torque tube material, the temperature of the material, the wall thickness, and
the length. If the density of the process fluid, process temperature, and torque tube material of the sensor are known,
simulation of process conditions may be accomplished by one of the following means
(1)
:
1. Weight or Force Method:
The interface application is the most general case. The level application can be considered an interface with the upper
fluid SG = 0, and the density application can be considered as a variable SG application with the interface at the top of
the displacer. The buoyancy for a given interface level on the displacer is given by:
F
B
SG
U
)H
disp
*
]
[
=
* V
D
*
(1)
ρ
w
(
SG
L
- SG
U
)
Where:
F
B
= buoyant force
ρ
w
= density of water at 4_C, 1
atmosphere = 1.0000 Kg/liter
(0.03613 lb/in
3
)
V
D
= displacer volume
H
disp
= height of interface on displacer,
normalized to displacer length
SG
U
= specific gravity of upper fluid
(0.0 for Level)
SG
L
= specific gravity of lower fluid
Figure 1. Cutaway View of Fisher 249 Displacer
Sensor
LIQUID DISPLACER
TORQUE TUBE
W2141-1
SUSPENSION ROD
DRIVER ROD
Instruction Manual Supplement
D103066X012
249 Sensors
March 2014