Process characterization is the identification of the dynamic responses of a process to changes in its inputs. In Chapter 9 of Control Loop Foundation, we address process characterization. In some cases the process response may be characterized as a pure gain, lag, or deatime. However, many of the basic thermal and physical mechanisms reflected in the design of process equipment–heat transfer, the blending of gas or liquids– tend to provide a response to input changes that may be approximated as first-order. Also, because of physical equipment size, some transportation delay is often seen in the output response. In such equipment, the output response to a change in a process input may be described as a first order plus deadtime response. The dynamic response of a first order plus deadtime process is fully captured by identifying the process gain, deadtime, and time constant.
The time between a step input change and the first detectable change in output is the process deadtime. The process time constant is the time from the first detectable change in output until the output reaches 63% of its final change in value (not 63% of its final value). The process gain is the (percent) change in the process output divided by the (percent)change in the process input after the process has fully responded.
The first workshop for this chapter on process characterization is designed to give the user experience working with a process that may characterized as first order plus deadtime. By accessing the book’s web site, you may complete this exercise on characterization using your web browser. The viewer below may be used to see the solution to this exercise.