Radio frequency: The frequency range between ultrasonic and infrared. AM broadcast frequencies range from 540 to 1,800 kHz, while FM broadcasts from 88 to 108 MHz.
Radio frequency interference (RFI): Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring an instrument signal.
Ramp: The total (transient plus steady-state) time response resulting from a sudden increase in the rate of change from zero to some finite value of input stimulus.
Range: The region between the limits within which a quantity is measured, received or transmitted, expressed by stating lower and upper range values.
Reactance: The opposition to the flow of ac current, which is created by either inductance or capacitance. In such a circuit, total impedance is therefore the sum of reactance and resistance. The unit is the ohm.
Reference input: An external signal serving as a setpoint or as a standard of comparison for a controlled variable.
Reliability: The probability that a device will perform its objective adequately for the period of time specified, under the operating conditions specified.
Repeatability: The maximum difference between output readings when the same input is applied consecutively; the closeness of agreement among consecutive measurements of an output for the same value of input under the same operating conditions, approaching from the same direction, usually measured as non-repeatability and expressed as percent of span.
Reproducibility: The closeness of agreement among repeated measurements of an output for the same value of input made under the same operating conditions over a period of time, approaching from both directions. It includes hysteresis, deadband, drift and repeatability.
Resistance; Resistivity: The opposition to the flow of current in a dc circuit. The unit is the ohm, which is defined as the resistance that will give a one-ampere current flow if a one-volt potential difference is applied in a circuit Resistivity is the reciprocal of conductivity; its unit is the ohm/cm.
Resolution: The smallest change in input which produces a detectable change in output. This is the smallest increment of change that can be detected by a measurement system. Resolution can be expressed in bits, in proportions, in percent of actual reading or in percent of full scale. For example, a 12-bit system has a resolution of one part in 4,096 or 0.0244% of full scale.
Resonance: A condition of oscillation caused when a small amplitude of periodic input has a frequency approaching one of the natural frequencies of the driven system.
Response time: An output expressed as a function of time, resulting from the application of a specified input under specified operating conditions.