Watlow CAS - Manual (Page 96)

Glossary
86 CAS User’s Guide
RS-485 — An Electronics Industries of America
(EIA) standard for electrical characteristics of
generators and receivers for use in balanced digi-
tal multipoint systems. This is usually used to
communicate with multiple devices over a com-
mon cable or where distances over 50 feet are
required.
Setpoint (SP)
The desired value programmed into a controller.
For example, the temperature at which a system
is to be maintained.
Shield
A metallic foil or braided wire layer surrounding
conductors that is designed to prevent electro-
static or electromagnetic interference from exter-
nal sources.
Signal
Any electrical transmittance that conveys infor-
mation.
Solid State Relay (SSR)
See Relay, Solid State.
Span
The difference between the lower and upper lim-
its of a range expressed in the same units as the
range.
Spread
In heat/cool applications, the +/- difference
between heat and cool. Also known as process
deadband.
See deadband.
Stability
The ability of a device to maintain a constant out-
put with the application of a constant input.
T
T/C Extension Wire
A grade of wire used between the measuring
junction and the reference junction of a thermo-
couple. Extension wire and thermocouple wire
have similar properties, but extension wire is less
costly.
TD (Timed Derivative)
The derivative function.
Thermistor
A temperature-sensing device made of semicon-
ductor material that exhibits a large change in
resistance for a small change in temperature.
Thermistors usually have negative temperature
coefficients, although they are also available with
positive temperature coefficients.
Thermocouple (T/C)
A temperature sensing device made by joining
two dissimilar metals. This junction produces an
electrical voltage in proportion to the difference
in temperature between the hot junction (sensing
junction) and the lead wire connection to the
instrument (cold junction).
TI (Timed Integral)
The Integral term.
Transmitter
A device that transmits temperature data from
either a thermocouple or RTD by way of a two-
wire loop. The loop has an external power supply.
The transmitter acts as a variable resistor with
respect to its input signal. Transmitters are desir-
able when long lead or extension wires produce
unacceptable signal degradation.
U
Upscale Break Protection
A form of break detection for burned-out thermo-
couples. Signals the operator that the thermocou-
ple has burned out.
Undershoot
The amount by which a process variable falls
below the setpoint before it stabilizes.