Watlow Series 988 Enhanced - Manual (Page 11)

WATLOW Series 988 Enhanced Software Features 11
Duplex
Requirements
The duplex control feature requires enhanced software and a process out-
put.
Overview
Certain systems require that a single process output control both heating
and cooling outputs. 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.
NOTE:
Duplex applications
require a special
valve.
Sample Application
The system outlined above uses a three-way valve for heating and cooling
a fluid sample. Coils surround the container holding the fluid. When the
temperature 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.
Figure 9.11 - Fluid
sample container
with duplex control.