What is time lag in process control?

Industrial temperature measurement processes can have the characteristics of delaying and holding back changes in the values of the process variables. Time lags in process controls typically describes these process delays and the lags are caused by three properties of the process. These properties are resistance, capacitance, and transportation time. 


Resistance is the part of the process that opposes the movement of energy between capacities. For example, in a lubrication oil cooling system, the walls of the lubrication oil cooler work in opposition with the transfer of heat from the lubrication oil within the tubes to the cooling water outside the tubes. 


Capacitance is the ability to store energy during a process. Being able to store energy means the process can be held back. 

Transportation time 

This is the time required to carry a change in a process variable from one location to another in the process. The time lag not only delays or blocks a change, but it is also during the time delay that no change is happening at all. 

Control system stability 

The part of a control mode that helps a process variable go back to being a steady value after a disturbance is known as “stability”. In other words, stability is the control loop being able to return a controlled variable back to a steady, non-cyclic value, after a disruption. 

Control loops can be either stable or unstable. Instability is the result of a mixture of process time lags (resistance, capacitance, and transportation time) and natural time lags within a control system. This causes slow response to changes in the controlled variable. Therefore, the controlled variable will constantly cycle around the set point value. 

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