The previous sections of this chapter have dealt with the tuning of controllers for self-regulating processes such as temperature and flow. Liquid-level control, however, has characteristics distinctly different from those of the previous loops. Some of the differences are these:

• Liquid level is usually a non-self-regulating (integrating) process.

• Intuitive rules of thumb used for tuning other loops ("If it's cycling too much, reduce the gain.") do not apply, and in fact will usually produce results that are the opposite of those expected.

• Liquid-level control loops, once properly tuned, do not usually go out of tune.

Most processes can be described at best by an approximate process model that must often be determined by process testing. On the other hand, most liquid-level control loops readily yield to an analytical approach. A simple process model can be formulated, desired performance parameters can be established, and from this controller tuning parameters can be calculated. Once this is done, other attributes of the control loop, such as the period of oscillation, can be predicted.

Determining tuning parameters for a liquid-level loop should probably be considered as an engineering activity, rather than being left for field trial-and-error tuning, for two reasons: the counterintuitive nature of liquid-level loops makes tuning by trial-and-error techniques difficult, and liquid-level loops are amenable to an analytical approach. Hence it can be said that liquid-level control loops should be engineered, not tuned.

Many engineering studies start with an ideal model, then incorporate subsequent considerations to account...