WATLOW Series 965 User's Manual
35
Appendix
Glossary, D-P
Glossary, P-Z
PD control: Proportioning control with rate action.
PI control: Proportioning control with auto-reset.
PID control: Proportioning control with auto-reset
and rate.
Process variable: Thermal system element to be
regulated, such as time, temperature, relative
humidity, etc.
Programmed display data: Displayed informa-
tion which gives the operator/programmer the
"programmed" or intended process information,
i.e., intended set point, intended alarm limit, etc.
Proportional band: Span of temperature about
the set point where time proportional control action
takes place.
Proportioning control: See Time Proportioning
Control.
Rate: Anticipatory action that senses the rate of
change of temperature and compensates to
minimize overshoot. Also "derivative."
Rate band: A thermal control band that defines
where the rate (derivative) function begins. A
Watlow rate band occurs centered on set point at
one or more times the width of the proportional
band.
Reference junction: Synonymous with cold
junction. See "Cold junction."
Reset: Control action that automatically eliminates
offset, or "droop," between set point and actual
process temperature. Also "integral."
Reset windup inhibit: Synonymous with anti-
reset. See "Anti-reset."
RTD: Resistance Temperature Detector. Resis-
tive sensing device displaying resistance versus
temperature characteristics. Displays positive
temperature coefficient.
Set point: Intended value of the process variable.
Switching sensitivity: In on/off control, the
temperature 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 that
is constructed of two dissimilar metals wherein a
measurable, predictable voltage is generated
corresponding to temperature.
Thermocouple break protection: Fail-safe
operation that assures output shutdown upon an
open thermocouple condition.
Three mode control: Proportioning control with
reset and rate.
Time proportioning control: Action which 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 temperature.
In other words, the amount of time the output relay
is energized depends on the system temperature.
Triac: Solid state switching device.
Upper display data: Displayed information which
gives the operator/programmer real or "actual"
data, i.e., actual process temperature. See "Pro-
grammed display data."
Warm start: Start-up condition where all program
information is remembered by the instrument's
memory back-up protection.
Zero switching: Action that provides output
switching only at the zero-voltage, crossing points
of the
ac
line.
