Watlow Series 988 - Manual (Page 144)

WATLOW Series 988 User’s Manual 9.7 Enhanced Software Features, Chapter 9
Enhanced Software
TL TL W W W WA A
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
outside
temperature
sensor input 1
input 2
output 1
limit
Wa t e r Boile r
limit
sensor
heating element
temperature
sensor
Sample Application
The most common application using differential control is to maintain
water temperature in a boiler at a differential to the outside air tempera-
ture. A thermocouple at input 2 senses the outside air temperature and
adjusts the internal set point to maintain the boiler water temperature
120 degrees 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
temperature (input 2).
To configure the controller, first enable input 2 (set [`In2] to [```J] ). To
enable the differential control algorithm set the control prompt [CntL] in
the Global Menu to differential [diFF]. 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 tem-
perature 120 degrees higher than the outside air temperature.
Figure 9.7 - Water
boiler with differen-
tial control.