Every instrument has at least one input and one output. For a pressure sensor, the input would be some fluid pressure and the output would (most likely) be an electronic signal. For a loop indicator, the input would be a 4-20 mA current signal and the output would be a human-readable display. For a variable-speed motor drive, the input would be an electronic signal and the output would be electric power to the motor.
Calibration and ranging are two tasks associated with establishing an accurate correspondence between any instrument’s input signal and its output signal. Simply defined, calibration assures the instrument accurately senses the real-world variable it is supposed to measure or control. Simply defined, ranging establishes the desired relationship between an instrument’s input and its output.