Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 16)

Optimizing Your Process System with the WATLOW Series 988
2.6
Basic Control Strategies and Terms
• What is frequent? In order to tightly control a
process, the more frequent the better. Unless
absolutely necessary, mechanical relays should
not be used as the control output. A typical
mechanical relay application cycles ON and
OFF 86,400 times over a period of one month
(30 second cycle time, 24 hours per day). The
mechanical relay option is only warranted for
100,000 cycles.
• With a solid-state output a variable-time-base
burst fire option can provide the tightest load
control. Make sure the power control is com-
patible with the burst-fire output. (page 3.3)
What actuator interfaces are required? (page
6.8)
• Typically the output of the Series 988 does not
directly control the load. Instead it acts as a
pilot-duty output interfacing with another
device that actually controls the load. If this is
the case in your application, make sure that
the output of the Series 988 is compatible with
the input of the external device.
What alarm or annunciation outputs are
required? (page 5.1)
• The alarm options in the Series 988 can be
used to alert an operator to a system malfunc-
tion. These outputs should not be used as
safety limits to protect system components or
personnel. A separate safety limit device should
be used in any system where damage or injury
could occur due to a system component failure.
How should the alarms operate? (page 5.1)
• The alarm options can be field-configured to
function in any fashion. A process alarm can
be used to warn the operator of impending
equipment damage. A deviation alarm can be
used to warn that the system is operating
outside of the desired specifications. Each
alarm can be set for latching or non-latching,
silencing and reverse or normal logic.
Should you monitor the load current? (page
3.7)
• While this option is available as an input, it is
important to note that this feature in the Series
988 is not currently available when using a
process output (4-20mA, 0-5VDC, etc.). Also, it
limits the minimum on-time of the output.
Does the process require a retransmit output?
(page 3.12)
• Output 3 can be used as either a 4-20 or 0-
20mA output or as a 0-5, 1-5 or 0-10VDC
output to retransmit the process value of either
of the inputs or the set point value. It allows
the controller to function as a master controller
in a master-remote application or to retransmit
a process value to a chart recorder.
What types of monitoring of the outputs is
required? (page 3.4)
• The retransmit option is one way to monitor the
system status.
• A serial communications option can be used to
monitor and/or change all the parameters in
the controller. With the EIA-485 option up to
32 controllers can be monitored from a single
communications port on a personal computer,
over distances of 4000 feet. The RS-422 option
can address up to 10 controllers over 4000
feet, and the RS-232 option can address a
single control over a distance of 50 feet. Make
sure the computer or other monitoring device is
equipped with the correct serial communica-
tions card and that the baud rate and other
capabilities are compatable.
Controller Environment
There are several factors to consider concern-
ing the environment that the Series 988 con-
troller operates in:
What are the physical dimensions of the
enclosure? (page 6.4)
• The Series 988 family of controls has a behind-
panel depth of 4.06 inches, allowing the control
to fit an enclosure with a depth of 6.0 inches.
When mounting more than one controller in a
panel we recommend a minimum spacing of
1.66 inches between controllers.
What is the ambient temperature inside the
enclosure? (page 6.8)
• The Series 988 can operate in ambient tem-
peratures as low as 32 °F (0°C) and as high as
150 °F (65°C). The enclosure requires a non-