Watlow ANASOFT - Manual (Page 69)

Appendix A: PID Tuning and
ANASOFT User’s Guide 59
However, a process which uses only Proportional control may settle at a
point above or below the setpoint; it may never reach the setpoint at all.
This behavior is known as “offset” or “droop”. (This diagram shows a
process under proportional control only.)
Proportional and Integral Control
For Proportional and Integral control, use the Integral term, or Reset,
with Proportional control. The Integral term corrects for offset by
repeating the Proportional band's error correction until there is no error.
For example, if a process tends to settle about 5
º F below the setpoint,
use Integral control to bring it to the desired setting. (The next diagram
shows a process under proportional and integral control.)