Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 39)

Optimizing Your Process System with the WATLOW Series 988
Enhanced Software Features
4.6
TL W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
Duplex
Overview
Certain systems require that a single process
output control both heating and cooling out-
puts. A Series 988 controller configured with
enhanced software and a process output can
function as two separate outputs. With a 4 to
20mA output the heating output will operate
from 12 to 20mA (0 to +100 percent) and the
cooling output will operate from 12 to 4mA (0
to -100 percent). In some cases this type of
output is required by the device that the 988 controls, such as a three-way valve that opens
one way with a 12 to 20mA signal and opens
the other way with a 4 to 12mA signal. This
feature reduces the overall system cost by
using a single output to act as two outputs.
Requirements
The duplex control feature requires the en-
hanced software and a process output. Duplex
applications require a special valve.
Fluid Sample Container
temperature transmitter
input 1 output 1
cold water
hot water
Sample Application
The system outlined below uses a three-way valve for heating and cooling a
fluid sample. Coils surround the container holding the fluid. When the tem-
perature needs to be raised, the signal to the valve will be between 12 and
20mA, sending hot water through the coils. When cooling is required, the
signal will be between 12 and 4mA, sending cold water through the coils.