Where the measurand (input) changes with time, we observe performance characteristics that are the result of the instrument’s inability to respond instantly. In these situations, the instrument output often lags the instrument input. The time constant, or response time, has been adopted as this dynamic performance characteristic. The inverse of the time constant provides the instrument’s temporal resolution. In the example shown, a time constant is applied to produce the red curve, which corresponds to output values unaffected by hysteresis.
The figure provides an example of hysteresis caused by sensor time lag. The plot shows total temperature measured during a level aircraft speed run, where the airspeed is first increased and then decreased again. 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. Advancing all the measurements of total temperature by 2.32 sec results in the green line that shows no hysteresis. This is how we know that the time constant of this particular sensor is 2.32 s. Image from NCAR/EOL.