Watlow Series 94 - Manual (Page 32)

A . 8 ■ A p p e n d i x W a t l o w S e r i e s 9 4
M
manual mode A selectable mode that has no automatic
control aspects. The operator sets output levels.
N
NEMA 4X A NEMA specification for determining resis-
tance to moisture infiltration and corrosion resistance.
This rating certifies the controller as washable and corro-
sion resistant.
O
on/off A method of control that turns the output full on
until set point is reached, and then off until the process
error exceeds the hysteresis.
open loop A control system with no sensory feedback.
output Control signal action in response to the differ-
ence between set point and process variable.
overshoot The amount by which a process variable
exceeds the set point before it stabilizes.
P - Q
parallel circuit A circuit configuration in which the same
voltage is applied to all components, with current divided
among the components according to their respective
resistances or impedances.
parameter A variable that is given a constant value for
a specific application or process.
process variable The parameter that is controlled or
measured. Typical examples are temperature, relative
humidity, pressure, flow, fluid level, events, etc. The high
process variable is the highest value of the process
range, expressed in engineering units. The low process
variable is the lowest value of the process range.
programmed display data Displayed information that
gives the operator the intended process information,
such as intended set point, intended alarm limit, etc.,
corresponding to temperature.
prompt A symbol or message displayed by the con-
troller that requests input from the user.
R
reference junction See junction.
resistance temperature detector (RTD) A sensor that
uses the resistance temperature characteristic to mea-
sure 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 materi-
al. The wire RTD is a positive temperature coefficient
sensor only, while the thermistor can have either a nega-
tive 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
set point The desired value programmed into a con-
troller. For example, the temperature at which a system
is to be maintained.
switching sensitivity In on/off control, the temperature
change necessary to change the output from full on to
full off. See hysteresis.
T - Z
thermal system A regulated environment that consists
of a heat source, heat transfer medium or load, sensing
device and a control instrument.
thermocouple (t/c) A temperature sensing device
made by joining two dissimilar metals. This junction pro-
duces an electrical voltage in proportion to the difference
in temperature between the hot junction (sensing junc-
tion) and the leadwire connection to the instrument (cold
junction).
thermocouple break protection The ability of a control
to detect a break in the thermocouple circuit and take a
predetermined action.
Appendix