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WATLOW Series 945 User's Manual
Appendix
Glossary, D-P
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 consist-
ing 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 corre-
sponding to temperature.
Thermocouple break protection: Fail-safe opera-
tion that assures output shutdown upon an open
thermocouple condition.
Time Proportioning Control: Action which varies
the amount of ON 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.
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 switch-
ing only at the zero voltage crossing points of the AC
line.
ON/OFF control: Control of temperature about a set
point by turning the output full ON below set point
and full OFF above set point in the heat mode.
Open loop: System with no sensory feedback.
Output: Action in response to difference between
set point and process variable.
Overshoot: Condition where temperature exceeds
set point due to initial power up or process changes.
Parameter: A physical property whose value
determines the response of an electronic control to
given inputs.
PID control: Proportioning control with auto-reset
and rate. Also known as 3 mode control.
Process variable: Thermal system element to be
regulated, such as time, temperature, relative
humidity, etc.
Proportional band: Span of temperature about the
set point where time proportional control action takes
place.
Rate: Anticipatory action that senses the rate of
change of temperature and compensates to minimize
overshoot. Also "derivative."
Reset: Control action that automatically eliminates
offset, or "droop," between set point and actual
process temperature. Also "integral."
RTD: Resistance Temperature Detector. Resistive
sensing device displaying resistance versus tem-
perature characteristics. Displays positive tempera-
ture coefficient.
Glossary, O-Z
