Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 37)

Optimizing Your Process System with the WATLOW Series 988
Enhanced Software Features
4.4
Differential
Overview
Differential control allows the Series 988 to
control one process at a difference to another
process. Input 2 acts as a remote set point
input. However the displayed set point indi-
cates the desired difference between input 1
and input 2. The set point to which input 1 will
control to is determined by the equation:
internal set point = input 2 + differential set point
The lower display shows the differential set
point, which can be adjusted with the incre-
ment (up-arrow) and decrement (down-arrow)
keys.
Please note that while in the differential control
mode the set point for input 1 cannot be viewed
and must be calculated with the equation.
Requirements
Two inputs and the enhanced software option
are required.
TL TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
limit
sensor
heating element
input 1
input 2
temperature
sensor
outside
temperature
sensor output 1
limit
Water Boiler
Sample Application
The most common application using differential control is to maintain water
temperature in a boiler at a differential to the outside air temperature. A
thermocouple at input 2 senses the outside air temperature and adjusts the
internal set point to maintain the boiler water temperature 120 degree higher.
Substituting values we have: boiler temperature = outside temperature + 120
°
.
In this application the system uses two, type J thermocouples: one to sense
boiler water temperature (input 1) and one to sense the outside air tempera-
ture (input 2).
To configure the controller, first enable input 2 (set to J). To enable the
differential control algorithm set the control prompt in the Global menu
to differential . Press the DISPLAY key. The lower display will read 0,
indicating no differential between input 1 and input 2. Adjust the set point to
120. The internal set point for input 1 is now equal to the input 2 value plus
120, which will maintain the boiler water temperature 120 degrees higher
than the outside air temperature.