D8 Series User’s Guide
167
R
Ramp
A programmed change in the temperature of a
set point system.
Range
The area between two limits in which a quantity
or value is measured. It is usually described in
terms of lower and upper limits.
Recipe
See Job.
Relay
A switching device.
Electromechanical Relay –
A power
switching device that completes or interrupts
a circuit by physically moving electrical
contacts in and out of contact with each
other. Not recommended for PID control.
Solid State Relay (SSR) –
A switching
device with no moving parts that completes
or interrupts a circuit electrically.
Reset
See Automatic Reset, Manual Reset.
Resistance
Opposition to the flow of electric current,
measured in Ohms.
Resistance Temperature Detector (RTD)
A sensor that uses the resistance temperature
characteristic to measure temperature. There
are two basic types of RTDs: the wire RTD,
which is usually made of platinum, and the
thermistor, which is made of a semiconductor
material. The wire RTD is a positive temperature
coefficient sensor only, while the thermistor can
have either a negative or positive temperature
coefficient.
Reverse Action
An output control action in which an increase in
the process variable causes a decrease in the
output. Heating applications usually use reverse
action.
RTD
See Resistance Temperature Detector.
S
Serial Communications
A method of transmitting information between
devices by sending all bits serially over a single
communication channel.
Set Point (SP)
The desired value of the process variable. 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
electrostatic or electromagnetic interference
from external sources.
Signal
Any electrical transmittance that conveys
information.
Solid State Relay (SSR)
See Relay, Solid State.
Span
The difference between the lower and upper
limits of a range expressed in the same units as
the range.
Stability
The ability of a device to maintain a constant
output with the application of a constant input.
T
Thermistor
A temperature-sensing device made of
semiconductor 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).
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