Watlow Series 988LF - Manual (Page 57)

Appendix WATLOW Series 988LF Users Manual A.3
NEMA 4X — A NEMA specification for determining resistance
to moisture infiltration. This rating certifies the controller as
washable and corrosion resistant.
on/off — A method of control that turns the output full on
until set point is reached, and then off until the process error
exceeds the hysteresis.
open loop — A control system with no sensory feedback.
output — Control signal action in response to the difference
between set point and process variable.
overshoot — The amount by which a process variable exceeds
the set point before it stabilizes.
P control — Proportioning control.
PD control — Proportioning control with derivative (rate)
action.
PDR control — Proportional derivative control with manual
reset, used in fast responding systems where the reset causes
instabilities. With PDR control, an operator can enter a
manual reset value that eliminates droop in the system.
PI control — Proportioning control with integral (automatic
reset) action.
PID — Proportional, integral, derivative. A control mode with
three functions: proportional action dampens the system
response, integral corrects for droop, and derivative prevents
overshoot and undershoot.
process variable — The parameter that is controlled or
measured. Typical examples are temperature, relative humid-
ity, pressure, flow, fluid level, events, etc. The high process
variable is the highest value of the process range, expressed in
engineering units. The low process variable is the lowest value
of the process range.
proportional band (PB) — A range in which the proportioning
function of the control is active. Expressed in units, degrees or
percent of span. See PID.
proportional control — A control using only the P (propor-
tional) value of PID control.
rate band — A range in which the rate function of a controller
is active. Expressed in multiples of the proportional band. See
PID.
ratio — A method by which the controller measures the flow
of an uncontrolled variable and uses a proportion of it to
control the flow of a second variable.
reference junction — See junction.
reset — Control action that automatically eliminates offset, or
droop, between set point and actual process temperature. Also
see integral.
automatic reset — The integral function of a PI or
PID temperature controller that adjusts the process tempera-
ture to the set point after the system stabilizes. The inverse of
integral.
automatic power reset — A feature in latching limit
controls that does not recognize power outage as a limit
condition. When power is restored, the output is re-energized
automatically, as long as the temperature is within limits.
manual reset — 1) A feature on a limit control that
requires human intervention to return the limit to normal
operation after a limit condition has occurred. 2) The adjust-
ment of a proportional control to raise the proportional band
to compensate for droop.
no key reset — A method for resetting the
controller’s memory (for instance, after an EPROM change).
resistance temperature detector (RTD) — A sensor that
uses the resistance temperature characteristic to measure
temperature. There are two basic types of RTDs: the wire
RTD, which is usually made of platinum, and the thermistor,
which is made of a semiconductor material. The wire RTD is a
positive temperature coefficient sensor only, while the
thermistor can have either a negative or positive temperature
coefficient.
retransmit output — An analog output signal that may be
scaled to represent the process value or set point value.
RTD — See resistance temperature detector.
slidewire feedback — A method of controlling the position of
a valve. It uses a potentiometer to vary resistance ( ) and
indicate position of the valve.
switching sensitivity — In on/off control, the temperature
change necessary to change the output from full on to full off.
See hysteresis.
thermal system — A regulated environment that consists of a
heat source, heat transfer medium or load, sensing device and
a control instrument.
thermocouple (t/c) — A temperature sensing device made by
joining two dissimilar metals. This junction produces an
electrical voltage in proportion to the difference in temperature
between the hot junction (sensing junction) and the leadwire
connection to the instrument (cold junction).
thermocouple break protection — The ability of a control to
detect a break in the thermocouple circuit and take a prede-
termined action.
three-mode control — Proportioning control with integral
(reset) and derivative (rate). Also see PID.
time proportioning control — A method of controlling power
by varying the on/off duty cycle of an output. This variance is
proportional to the difference between the set point and the
actual process temperature.
zero cross — Action that provides output switching only at or
near the zero-voltage crossing points of the AC sine wave. See
burst fire.
zero switching — See zero cross.
Glossary