PD control:
Proportioning control with rate
action.
PI control:
Proportioning control with auto-reset.
PID control:
Proportioning control with auto-
reset and rate.
Process varlable: 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.
See “Actual displayed data.”
Proportlonal band:
Span of temperature about
the set point where time proportional control
action takes place.
Proportloning 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 elimi-
nates 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 me-
dium, 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 differ-
ence 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 “Programmed 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.
Appendix
WATLOW Series 980/985
User’s Manual
45
