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What is meant by contact spacing?

If the process variable varies within a fixed interval about the setpoint - the contact spacing Xsh - then neither of the outputs is active. Exception: 3-state controllers with I and D components. Within the contact spacing, only the proportional component is inactive.
This contact spacing is necessary to prevent continual switching between the two manipulating variables, e.g. heating and cooling registers, when the control variable is unsteady. The contact spacing is also commonly called the dead band. Too small a dead band can lead to a pointless waste of energy in a plant.