Instrument Performance Characteristics » Additional Performance Characteristics
Drift, hysteresis, or nonlinear span errors can all influence an instrument’s true response.
Drift is a form of instrument instability that produces a time-dependent systematic error, often associated with a physical change or deterioration of components.
Hysteresis occurs when the sensor input for a given output depends on whether the input is increasing or decreasing. As an example, hygrometers exhibit hysteresis because the rates of sorption and desorption of water molecules on the surface of the sensor are different. Another example of hysteresis is illustrated by the blue curves in the figure below.
The figure provides an example of hysteresis caused by temperature sensor time lag, which occurs when an aircraft changes airspeed. Dynamic heating causes the total temperature to exceed the ambient temperature of -18°C by 0.5 Cp V2, where V is the airspeed and Cp is the specific heat of air at constant pressure. The blue hysteresis curves look slightly different depending on whether airspeed is increasing, or decreasing. The temperature lags behind the correct value so it is too low as the aircraft speeds up and too high while the aircraft slows down. Image from NCAR/EOL.