Following is a definition of terms frequently used in the instrumentation, industrial automation and test & measurement communities.
Radiation: See Infrared
Random Access Memory (RAM): Memory that can be both read and changed during computer operation. Unlike other semi-conductor memories, RAM is volatile-if power to the RAM is disrupted or lost, all the data stored is lost.
Range: Those values over which a transducer is intended to measure, specified by its upper and lower limits.
Rangeability: The ratio of the maximum flowrate to the minimum flowrate of a meter.
Rankine (°R): An absolute temperature scale based upon the Fahrenheit scale with 180° between the ice point and boiling point of water. 459.67°R = 0°F.
Rate Action: The derivative function of a temperature controller.
Rate time: The time interval over which the system temperature is sampled for the derivative function.
Ratiometric Measurement: A measurement technique where an external signal is used to provide the voltage reference for the dual-slope A/D converter. The external signal can be derived from the voltage excitation applied to a bridge circuit or pick-off supply, thereby eliminating errors due to power supply fluctuations.
Read Only Memory (ROM): Memory that contains fixed data. The computer can read the data, but cannot change it in any way.
Real Time: The time interval over which the system temperature is sampled for the derivative function.
Record: A collection of unrelated information that is treated as a single unit.
Recovery Time: The length of time which it takes a transducer to return to normal after applying a proof pressure.
Redox Potential: The potential developed by a metallic electrode when placed in a solution containing a species in two different oxidation states.
Reference Junction: The cold junction in a thermocouple circuit which is held at a stable known temperature. The standard reference temperature is 0°C (32°F). However, other temperatures can be used.
Reference Mark: Any diagnostic point or mark which can be used to relate a position during rotation of a part to its location when stopped.
Reference Plane: Any plane perpendicular to the shaft axis to which an amount of unbalance is referred.
Refractory Metal Thermocouple: A class of thermocouples with melting points above 3600°F. The most common are made from tungsten and tungsten/rhenium alloys Types G and C. They can be used for measuring high temperatures up to 4000°F (2200°C) in non-oxidizing, inert, or vacuum environments.
Register: A storage device with a specific capacity, such as a bit, byte or word.
Relay (Mechanical): An electromechanical device that completes or interrupts a circuit by physically moving electrical contacts into contact with each other.
Relay (Solid State): A solid state switching device which completes or interrupts a circuit electrically with no moving parts.
Remote: Not hard-wired; communicating via switched lines, such as telephone lines. Usually refers to peripheral devices that are located a site away from the CPU.
Repeatability: The ability of a transducer to reproduce output readings when the same measurand value is applied to it consecutively, under the same conditions, and in the same direction. Repeatability is expressed as the maximum difference between output readings.
Reserved Word: A word that has a defined function in the language, and cannot be used as a variable name.
Residual (Final) Unbalance: Residual unbalance is that unbalance of any kind that remains after balancing.
Resistance Ratio Characteristic: For thermistors, the ratio of the resistance of the thermistor at 25 °C to the resistance at 125 °C.
Resistance Temperature Characteristic: A relationship between a thermistor’s resistance and the temperature.
Resistance: The resistance to the flow of electric current measured in ohms (1/2) for a conductor. Resistance is function of diameter, resistivity (an intrinsic property of the material) and length.
Resolution: The smallest detectable increment of measurement. Resolution is usually limited by the number of bits used to quantize the input signal. For example, a 12-bit A/D can resolve to one part in 4096 (2 to the 12 power equals 4096).
Resonant Frequency: The measurand frequency at which a transducer responds with maximum amplitude.
Response Time (time constant): The time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilize at 600 of the step change value.
Response Time: The length of time required for the output of a transducer to rise to a specified percentage of its final value as a result of a step change of input.
Reynolds Number: The ratio of inertial and viscous forces in a fluid defined by the formula Re = rVD/µ, where: r = Density of fluid, µ = Viscosity in centipoise (CP), V = Velocity, and D = Inside diameter of pipe.
RFI: Radio frequency interference.
Rheostat: A variable resistor.
Rigid Rotor: A rotor is considered rigid when it can be corrected in any two (arbitrarily selected) planes [see “Correction (Balancing)Plane”] and after that correction, its unbalance does not significantly exceed the balancing tolerances (relative to the shaft axis) at any speed up to maximum operating speed and when running under conditions which approximate closely to those of the final supporting system.
Rise Time: The time required for a sensor or system to respond to an instantaneous step function, measured from the 10% to 90% points on the response waveforms.
Room Conditions: Ambient environmental conditions under which transducers must commonly operate.
Root Mean Square (RMS): Square root of the mean of the square of the signal taken during one full cycle.
Rotor: A rotor is a rotating body whose journals are supported by bearings.
RTD: Resistance temperature detector.