Emerson Fisher 249VS Instruction Manual

Emerson Field Instrumentation Instruction Manual - Fisher 249VS.
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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 Fisher

r 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:

FB

SGU)Hdisp *

]

[

=

* VD *

(1)

ρw

(SGL - SGU )

Where:

FB

= buoyant force

ρw

= density of water at 4

_C, 1 

atmosphere = 1.0000 Kg/liter
(0.03613 lb/in3)

VD

= displacer volume

Hdisp

= height of interface on displacer,

normalized to displacer length

SGU

= specific gravity of upper fluid

(0.0 for Level)

SGL

= 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

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