From a control perspective, it is highly desirable that the process gain be constant. If the process gain is constant, then the same proportional gain may be used over the entire operating range of the control loop. However, if for example the valve characteristic has not been selected based on the process requirements, then the installed characteristic could be non-linear as illustrated below.
From this plot, the process gain may be determined at any operating point by dividing the percent change in the controlled parameter by the percent change in the manipulated parameter. As illustrated in this example, the process gain varies from 0.5 to 4, that is, the process gain changes by a factor of eight. Even if the loop tuning is selected to have a high gain margin, such a large range in process gain can cause slow response in the low gain region and potentially unstable operation in the high gain region. If the process gain is known to change over the operating range of the final control element, then loop tuning should be established in the high gain region to ensure that stable operation will be achieved over the entire operating region. From a control perspective, the product of controller gain and process gain should be a constant value over the entire operating region of the final control element. If the process gain is changing, then the impact is the same as changing the control loop gain of a process with constant gain. Thus, changes in process gain can have a large impact on control performance.