Watlow Controls 8LS, User Manual

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Glossary
124 8LS User’s Guide
Bumpless Transfer
A smooth transition from Auto (closed loop) to
Manual (open loop) operation. The control output
does not change during the transfer.
C
Calibration
The comparison of a measuring device (an
unknown) against an equal or better standard.
Celsius (Centigrade)
Formerly known as Centigrade. A temperature
scale in which water freezes at 0
°
C and boils at
100
°
C at standard atmospheric pressure. The for-
mula for conversion to the Fahrenheit scale is:
°
F=(1.8x
°
C)+32.
Central Processing Unit (CPU)
The unit of a computing system that includes the
circuits controlling the interpretation of instruc-
tions and their execution.
Circuit
Any closed path for electrical current. A configu-
ration of electrically or electromagnetically-con-
nected components or devices.
Closed Loop
A control system that uses a sensor to measure a
process variable and makes decisions based on
that feedback.
Cold Junction
Connection point between thermocouple metals
and the electronic instrument.
Common Mode Rejection Ratio
The ability of an instrument to reject electrical
noise, with relation to ground, from a common
voltage. Usually expressed in decibels (dB).
Communications
The use of digital computer messages to link
components.
See Serial Communications.
See Baud Rate.
Control Action
The response of the PID control output relative to
the error between the process variable and the
setpoint. For reverse action (usually heating), as
the process decreases below the setpoint the out-
put increases. For direct action (usually cooling),
as the process increases above the setpoint, the
output increases.
Control Mode
The type of action that a controller uses. For
example, On/Off, time proportioning, PID, Auto-
matic or manual, and combinations of these.
Current
The rate of flow of electricity. The unit of mea-
sure is the ampere (A).
1 ampere = 1 coulomb per second.
Cycle Time
The time required for a controller to complete
one on-off-on cycle. It is usually expressed in
seconds.
Cyclic Redundancy Check (CRC)
An error checking method in communications. It
provides a high level of data security but is more
difficult to implement than Block Check Charac-
ter (BCC).
See Block Check Character.
D
Data Logging
A method of recording a process variable over a
period of time. Used to review process perfor-
mance.
Deadband
The range through which a variation of the input
produces no noticeable change in the output. In
the deadband, specific conditions can be placed
on control output actions. Operators select the
deadband. It is usually above the heating propor-
tional band and below the cooling proportional
band.
Default Parameters
The programmed instructions that are perma-
nently stored in the microprocessor software.