The behaviour of a system or process with time. All processes are inherently unstable and dynamic as their properties vary with time. It is therefore necessary to control the system or process to disturbances, and to restore the controlled variable back to its design or desirable value. This is done by using mathematical rate equations to describe the process such as material, energy, and momentum equations. Dynamic analysis (p. 299) is then used to determine how the associated process variables change with time and how their behaviour changes in response to disturbances.