Watlow Series 94 - Manual (Page 31)

W a t l o w S e r i e s 9 4 A p p e n d i x ■ A . 7
Glossary
A - B
alarm A signal that indicates that the process has
exceeded or fallen below the alarm set point. For exam-
ple, an alarm may indicate that a process is too hot or
too cold.
alarm hysteresis A change in the process variable
required to re-energize the alarm output.
alarm silence A feature that disables the alarm relay
output.
automatic prompts Data entry points where a micro-
processor-based controller asks the operator to enter a
control value.
C
calibration offset An adjustment to eliminate the differ-
ence between the indicated value and the actual
process value.
CE A manufacturer’s mark that demonstrates compli-
ance with European Union (EU) laws governing prod-
ucts sold in Europe.
CE-compliant Compliant with the essential require-
ments of European directives pertaining to safety and/or
electromagnetic compatibility.
closed loop A control system that uses a sensor to
measure a process variable and makes decisions based
on that input.
cold junction See junction, cold.
cold junction compensation Electronic means to
compensate for the effective temperature at the cold
junction.
compensation, ambient The ability of an instrument to
adjust for changes in the temperature of the environ-
ment and correct the readings. Sensors are most accu-
rate when maintained at a constant ambient tempera-
ture. When temperature changes, output drifts.
control action The response of the control output rela-
tive to the error between the process variable and the
set point. For reverse action (usually heating), as the
process decreases below the set point, the output
increases. For direct action (usually cooling), as the
process increases above the set point, the output
increases.
D - E
default parameters The programmed instructions that
are permanently stored in the microprocessor software.
direct action An output control action in which an
increase in the process variable causes an increase in
the output. Cooling applications usually use direct
action.
display capability In an instrument with digital display,
the entire possible span of a particular parameter or
value.
F - G
Form A A single-pole, single-throw relay that uses only
the normally open (NO) and common contacts. These
contacts close when the relay coil is energized. They
open when power is removed from the coil.
Form C A single-pole, double-throw relay that uses the
normally open (NO), normally closed (NC) and common
contacts.
H
hysteresis A change in the process variable required
to re-energize the control or alarm output. Sometimes
called switching differential.
I
isolation Electrical separation of sensor from high volt-
age circuitry. Allows use of grounded or ungrounded
sensing element.
J - K
junction The point where two dissimilar metal conduc-
tors join to form a thermocouple.
junction, cold Connection point between thermocouple
metals and the electronic instrument. See reference
junction.
junction, reference The junction in a thermocouple cir-
cuit held at a stable, known temperature (cold junction).
Standard reference temperature is 32°F (0°C).
L
limit or limit control A highly reliable, discrete safety
device (redundant to the primary controller) that moni-
tors and limits the temperature of the process, or a point
in the process. When temperature exceeds or falls
below the limit set point, the limit controller interrupts
power through the load circuit. A limit control can protect
equipment and people when it is correctly installed with
its own power supply, power lines, switch and sensor.
Appendix