Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 20)

Optimizing Your Process System with the WATLOW Series 988
2.10
Basic Control Strategies and Terms
reference junction — synonymous with cold
junction. Also see “cold junction.”
retransmit — an analog signal representing a
control variable, either the process values or
the set point values.
RTD — resistance temperature detector. Resistive
temperature-sensing device that displays a
positive temperature coefficient.
slidewire feedback — closed-loop, valve-actuator
control using a potentiometer to indicate valve
position.
switching sensitivity — in ON/OFF control, the
process value change necessary to change the
output from full on to full off.
thermal system — a regulated environment
consisting of a heat source, heat transfer
medium, sensing device and a process variable
control instrument.
thermocouple — temperature-sensing device
constructed of two dissimilar metals that
generates a measurable, predictable voltage
that corresponds to its temperature.
thermocouple break protection — fail-safe
operation that assures output shutdown upon
an open thermocouple condition.
three-mode control — proportioning control
with integral (reset) and derivative (rate). Also
see “PID.”
time proportioning control — action that varies
the amount of on and off time when “close” to
the set point, i.e., in the proportional band.
This variance is proportional to the difference
between the set point and the actual process.
In other words, the amount of time the output
relay is energized depends on the system
process value.
transmitter power supply — When Option “T” is
ordered for output 2, 3 or 4, the Series 988 can
supply power to external signal conditioners,
transducers or transmitters. With internal DIP
switches, the user selects between 5, 12 or
20VDC at 30mA ratings.
zero-cross — Action that provides output switch-
ing only at the zero-voltage crossing points of
the AC sine wave. Also see “burst fire.”