Dry Calibration; Controller/Transmitter And Torque Tube Arm Disassembly; Determining The Amount Of Suspended Weight - Fisher 2500 Series Instruction Manual

Type 2500

Dry Calibration

Remove the controller/transmitter and torque tube
arm, as a single unit, from the cage or vessel. Then,
wherever the standard calibration procedures in this
manual require a specific process variable input to the
sensor, simulate the process variable by suspending
the proper weight (such as a can of sand) from the
end of the displacer rod. Complete the following proce-
dures (Controller/Transmitter and Torque Tube Arm
Disassembly) and (Determining the Amount of Sus-
pended Weight) before proceeding to the Calibrating
Procedure.
Controller/Transmitter and Torque Tube
Arm Disassembly
WARNING
To avoid personal injury from contact
with the process fluid, lower the vessel
level below the sensor torque tube arm,
or shut off the cage equalizing valves
and drain the cage before proceeding.
For closed vessels, release any pres-
sure that may be in the vessel before
removing the sensor assembly.
When removing the displacer from the displacer rod or
removing the controller/transmitter and torque tube
arm from the cage or vessel, refer to the appropriate
249 Series instruction manual for assistance. The
method of removing the displacer or torque tube arm
and attached controller/ transmitter varies with the
type of sensor.
For a caged sensor with top equalizing connection, it
may be appropriate to remove the entire cage from the
vessel before disassembling.
CAUTION
If the displacer is to be disconnected
from the displacer rod before the sensor
assembly is removed from the cage or
vessel, provide a means of supporting
the displacer to prevent it from dropping
and suffering damage. The spuds or
stem end pieces on all displacers have
holes suitable for inserting rods or other
supports.
Additionally, a threaded rod may be
installed into the 1/4-inch 28 UNF
threaded hole in the displacer spud or
stem end piece of top-mounted cageless
and all caged sensors. For some top-
mounted sensors with long displacers,
16
the sensor may be removed through the
access hole in the sensor head.
For Type 249BP sensors with travel
stop, the stem end piece pins will se-
cure the displacer on the travel stop as
long as the travel stop plate is installed
and the sensor head is in position.
Determining the Amount of Suspended
Weight
CAUTION
Avoid overloading a torque tube sized
for interface or density applications.
Consult your Fisher Controls sales of-
fice or sales representative for the maxi-
mum allowable substitute weight, Ws,
that may be used with your particular
construction.
To determine the total weight that must be suspended
from the displacer rod to simulate a certain condition
of fluid level or specific gravity, solve the following
equation:
W W –[(0.0361)(V)(SP GR)]
s
d
where:
W = Total suspended weight in pounds (should never
s
be less than 0.5 pounds). For a unit with a hori-
zontal displacer, make sure the center of grav-
ity of the substitute weight is where it would be
on the actual displacer.
For liquid level control only, simulate
the lower range limit of the input span
by suspending the displacer from the
displacer rod. For other values of input
span, remove the displacer and suspend
the appropriate weight as determined in
the equation above.
W = Weight of the displacer, in pounds (determine by
d
weighing displacer).
Note