Glossary of Terms: R

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Radio Frequency Interference (RFI)
Type of electrical noise that can affect electronic circuits adversely.
Ramp
A programmed rate of rise or decline in the setpoint, and hence, the process variable.
Ramp Time
Time for a setpoint ramp to reach the next or final setpoint.
Ramp Voltage
A steadily rising or falling voltage.
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.
Rate
Same as the derivative or "D" part of PID controllers. See Derivative.
Rate Action (Derivative Control)
Produces a corrective signal proportional to rate at which controlled variable is changing. Produces corrective action faster than proportional action alone.
Rate Time
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.
Receiver
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.
Reed Relay
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.
Reed Technology
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.
Reference Junction
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.
Reflective Scan
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.
Register
A data storage location in a PLC.
Regulation Percentage
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).
Regulator
A controller changing the a output variable to move the process variable back to the set point
Relative Accuracy
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.
Relay
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 other circuits.
Relay Chatter
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.
Relay - Mercury Displacement
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.
Release Force
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.
Release Point
That position of the plunger at which the contacts snap from the operated contact position to the normal contact position.
Release Time
Time period required for an output to change state after the object being sensed is removed.
Release Travel
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.
Remote
A device that accepts a signal from a master controller.
Remote Setpoint
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
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.
Repeatability
(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.
Reset Action
Adjusts the controller’s output in accordance with both size of the deviation (SP-PV) and time it lasts.
Reset Rate
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.
Reset Windup
Integral action continuing to change the controller output value after the actual output reaches a physical limit.
Resistance
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).
Resolution
(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 output.
Response Time
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.
Retransmission
See Analog Transmission.
Retro-reflective Sensors
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 interrupted.
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.
Ripple
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.
Rise Time
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 started.
RMS
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.
Robust
A loop that is relatively insensitive to process changes. A less robust loop is more sensitive to process changes.
RoHS
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.
RPM
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.
RS232
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.
RS485
Another EIA protocol for serial communications. Allows several devices to be connected to a single cable, distributed over a wide area.
RTD
Resistance Temperature Detector.
Run Period
time after ignition and before operating setpoint is reached during which the main burner is firing. (In a flame safeguard programming control, the timer stops.)
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