Reset Adjustment; Differential Relief Adjustment; Calibration; Precalibration Requirements - Fisher 2502 Series Level-Trol Instruction Manual

percentage of pressure fed back to the proportional
bellows. The adjustment is performed by opening the
controller cover and turning the percent proportional
band knob (just below the raise level dial).

Reset Adjustment

To adjust reset action (figure 11) turn the knob clock-
wise to decrease the minutes per repeat. Turn the
knob counterclockwise to increase the minutes per
repeat. Increasing the minutes per repeat provides a
slower reset action.
The reset rate adjustment dial is calibrated in minutes
per repeat. By definition, this is the time in minutes
required for the reset action to produce a correction
which is equal to the correction produced by propor-
tional control action. This is, in effect, the time in min-
utes required for the controller to increase (or de-
crease) its output pressure by an amount equal to a
proportional increase (or decrease) caused by a
change in control conditions.

Differential Relief Adjustment

The differential relief valve protrudes from the back of
the controller case on a construction with an F in the
type number. Although normally factory-set to relieve
when the differential between the proportional and re-
set bellows reaches 5 psi, the differential may be re-
duced down to 2 psi by turning the adjustment screw
clockwise or increased up to 7 psi by turning the screw
counterclockwise. The minimum differential setting will
yield the minimum set point overshoot during startup.
Depending on the characteristics of the process, the
relief valve can be positioned so that the arrow cast on
the case points either to the letters RE (reset) or to the
letter P (proportional) on the back of the manifold. To
reposition the arrow, see figure 11. Remove the
mounting screws. Reposition the differential relief
valve to RE or P and reinstall the mounting screws.

Calibration

Precalibration Requirements

Calibration of a unit with a displacer de-
signed for interface or density control
must be conducted with the displacer
completely submerged in a liquid of the
specific gravity for which the unit was
designed.
Note
To calibrate a controller, it is necessary to place the
device into operation. This may be done on the vessel
with the actual service liquid. It may also be done in
the shop, but other means of obtaining a displacement
force change must be provided. It must be done in the
shop if the process variable is not available for calibra-
tion or if the process cannot be varied for calibration.
There are two methods of adapting the calibration pro-
cedure to shop calibration: wet and dry.

Wet Calibration

Remove the entire controller and sensor assembly
from the vessel. For caged sensors, pour the liquid
into the cage. For cageless sensors, suspend the dis-
placer to an appropriate depth in a liquid having a spe-
cific gravity equal to that of the process liquid.
If necessary, use water for wet calibration in the shop.
However, this procedure requires compensation for
the difference between the specific gravity of the water
and that of the process liquids. For example, assume
that the process liquid has a specific gravity of 0.7 and
that wet calibration with water (specific gravity of 1.0)
is desired. To simulate a process level of 50 percent of
the input span, a water level of 35 percent is required
(0.7/1.0 x 50 percent = 35 percent).

Dry Calibration

Remove the controller and torque tube arm, as a
single unit, from the cage or vessel. Then, wherever
the standard calibration instructions in this manual re-
quire a specific process variable for input to the sen-
sor, simulate that variable by suspending the proper
weight (such as a can of sand) from the end of the
displacer rod. Complete the following Controller and
Torque Tube Arm Disassembly and the Determining
Suspended Weight for Calibration sections before pro-
ceeding to the calibration procedure.
Controller and Torque Tube Arm
Disassembly
WARNING
To avoid personal injury from contact
with the process liquid, 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 and torque tube arm from the
2502 Series
11