Also called the delay time. It is the time, which the output signal of a 1st order delay element requires to reach 63% of the end value. After 3 · T 95 % and at 5 · T almost 100 % of the end value (Figure Z 3).
The term time constant is –– not completely correctly –– used if systems (e.g. ) can be assumed to be 1st order systems (e.g. if the natural frequency is somewhat dampened and the valve behaviour in the subsystem does not play a role) or in the case of comparable elements with an increase according to an exponential function. The time constant results as the inverse of the loop gain:
Figure Z 3: Transfer function of a PT1 element