Type of electrical noise that can affect electronic circuits adversely.
A programmed rate of rise or decline in the setpoint,
and hence, the process variable.
Time for a setpoint ramp to reach the next or final setpoint.
A steadily rising or falling voltage.
The area between two limits in which a quantity or value
is measured. It is usually described in terms of lower and
Same as the derivative or "D" part of PID controllers. See
Rate Action (Derivative Control)
Produces a corrective signal proportional to rate at which controlled
variable is changing. Produces corrective action faster than proportional action
In the action of a proportional-plus rate or proportional-plus
reset-plus-rate controller, the time by which rate action advances proportional
action on the controlled device.
A device that converts alternating current into direct
current. The part of a sensor (photoelectric or ultrasonic)
which accepts the emitter signal and provides an output.
Consists of two thin magnetic strips (reeds). When a coil close to the reeds
is energised, they are magnetised and drawn together making a connection between
leads attached to the reeds.
Technology where the reed contacts are designed to be actuated by a magnet.
When a magnetic field is brought close to the reed contacts, the contacts are
drawn together to make the circuit.
The junction in a thermocouple circuit which is held at
a stable known temperature (cold junction). The standard
reference temperature is 0ºC (32ºF). However, other
temperatures can be used.
A scanning technique in which the light source is aimed at a reflective
surface to illuminate the photosensor. Retroreflective, specular, diffuse scan
and convergent beam are all reflective scan techniques.
A data storage location in a PLC.
The ratio of voltage extremes due to loading or line fluctuations. The
process of holding constant a quantity such as voltage by means of a system that
automatically corrects errors. For example, as more current is drawn from a
battery or power supply, the output voltage tends to decrease (load regulation).
With a power supply derived from AC, the DC output voltage can vary with the
variation in AC voltage (line regulation).
A controller changing the a output variable to move the process variable
back to the set point
How accurately a change in signal is measured.
Relative Humidity (rH)
Ratio of amount of water vapor contained in air at given temperature and
pressure to maximum amount it could contain at same temperature and pressure
under saturated conditions. The moisture content of air, in relation to the
maximum it can contain at that same pressure and temperature.
Device with contacts that open and/or close when its coil is energized or
de-energized in response to a change in conditions of electrical circuit.
Operation of contacts affect the operation of other devices in same circuit or
Noise due to rapid opening and closing of relay contacts.
Relay - Mechanical
An electromagnetic device that completes or interrupts a
circuit by physically moving electrical contacts into
contact with each other.
A power switching device in which mercury,
displaced by a plunger, completes the electric
circuit across contacts.
Relay - Solid-State (SSR)
A solid-state switching device which completes or
interrupts a circuit electrically with no moving parts.
The level to which force on the plunger must be reduced to allow contacts to
snap from the operated contact position to the normal contact position.
That position of the plunger at which the contacts snap from the operated
contact position to the normal contact position.
Time period required for an output to change state after
the object being sensed is removed.
As an operating characteristic of a switch, the distance through which the
plunger moves when traveled from the release point to the free position. As a
characteristic of the actuation applied to the switch, the distance the plunger
is released past the release point.
A device that accepts a signal from a master controller.
A feature that allows setpoint setting from a remote
location using an analog signal. The setpoint generated
externally from controller or recorder.
Remote Switching (Digital Input)
Detects state of external contacts but responds according to how switching
input is configured.
Remote Terminal Unit (RTU)
A data acquisition device at a remote location which transmits data back to,
and accepts commands from, a central PC (or other controller).
Repeat accuracy means the difference between the
measured values of successive measurements within a period
of 8 hours at an ambient temperature of 23°C ± 5°C.
(1) The ability of an instrument to give the same reading under repeated
identical conditions. (2) Ability of a sensor to reproduce output readings when
the same value is applied to it consecutively in the same direction, for a
specified number of cycles, or specified time duration. (3) The variation in
outputs for the same change of input.
Adjusts the controller’s output in accordance with both size of the
deviation (SP-PV) and time it lasts.
Number of repeats per minute or minutes per repeat that proportional
response of a two or three mode controller to a step input is repeated by the
initial integral response.
Integral action continuing to change the controller output value after the
actual output reaches a physical limit.
Opposition to flow of electricity in an electric circuit measured in ohms.
Resistance Temperature Detector (RTD)
Resistance temperature devices (or detectors) rely on the principle that the
resistance of a metal increases with temperature. When made of platinum, they
may be known as platinum resistance thermometers (PRTs).
(1) Minimum detectable change of some variable in a measurement system. A
measure of the smallest change that can be detected. (2) Magnitude of output
step changes as the pressure is continuously varied over the range. This term
applies primarily to potentiometric sensors. Resolution is best specified as
average and maximum resolution. Usually expressed in percent of full scale
The time a system takes to respond to a given input. The sum of the sensor,
amplifier, and output response is the total response time. For example: the time
between software sending a message to an instrument and the instrument sending a
reply, or the time a sensor takes to indicate a change in conditions.
With the emitter and receiver in the same housing this
sensor transmit light which is reflected back from a
reflector. The sensor switches when the light beam is
Reverse Acting Control
Control in which value of output signal decreases as value of input
(measured variable or controlled variable) increases.
Reverse Polarity Protection
Circuitry, usually a diode which prevents current from flowing into the
control in case of accidental miswiring of the plus (+) or minus (-) terminals,
preventing damage to the unit.
The alternating component of voltage from a rectifier or
generator. A slight fluctuation in the intensity of a steady
current. The voltage supplied to the sensor should always be
within the specified range for proper operation. Within this
range a 10% ripple (VR) is allowed.
A measure (10% to 90%) of the time required for an output voltage to rise
from a state of low voltage to a high voltage level, once a level change has
Root mean square. The square root of the sum of the squares of a set of
quantities divided by the total number of quantities. Used when monitoring AC
(alternating current) signals. Many power supplies, for example, issue an AC
signal. This needs to be converted to a DC (direct current) signal for the
computer interface. The solution is a signal conditioning input that produces a
dc signal proportional to the RMS of the amplitude of the input signal. The RMS
operation means the reading will always be positive.
A loop that is relatively insensitive to process changes. A less robust loop
is more sensitive to process changes.
A European Union directive aimed at restricting the use
of certain hazardous substances commonly found in electrical
and electronic equipment. A product that is RoHS-compliant
has none (or very little) of these hazardous substances.
The maximum mechanical RPM is dependent on the bearing;
an RPM of 12,000 should not be exceeded. The maximum
electrical RPM is defined by the maximum frequency of the
internal electronics and the user interface.
An EIA (Electronic Industries Association) standard that defines a protocol
for serial data communications. An RS232 link will run at up 38400 baud (bits
per second) over short distances, and at lower speeds as the distance increases.
You can plug the RS232 lead directly into a computer's serial (COM) port.
Another EIA protocol for serial communications. Allows several devices to be
connected to a single cable, distributed over a wide area.
Resistance Temperature Detector.
time after ignition and before operating setpoint is reached during which
the main burner is firing. (In a flame safeguard programming control, the timer