Most real process variables are purposes of time. Typically, standards fluctuate round a usual worth, occasionally slightly higher, occasionally lower. This long time value is one facet of steady-state. When we are commanding a scheme, we want to make little compensatory changes to flows, etc., to try and pull the method back to setpoint. It makes more sense to calculate the change required rather than assessing the new valve place from rub every time. This is made simpler if we hold pathway of how a variable differs from its steady-state worth rather than ...