The behaviour of a process to a disturbance such as a change in flow, temperature, pressure, etc. It is measured using an instrument and the information used by a controller to return the process back to its desired value. A step response is the behaviour of the controlled process to adjust from one steady-state condition to another. A dynamic response is the behaviour of the controlled process with respect to time. The response time is the time for a process or instrument to adjust to a disturbance and return to a new steady-state value. It is expressed as the time required to fall within a specified percentage of the change final steady-state value. This is often taken as 5 per cent or occasionally 3 per cent.