Differential gap control is a refinement of on/off control. Instead of changing output from on (100%) to off (0%) at a single setpoint, differential gap action changes output at high and low limits called boundaries. As long as the measurement remains between the boundaries, the controller holds the last output. This extends the period and limits the amplitude of the controlled variable oscillations (see Figure 300-6). On many controllers the size and position of the differential gap is adjustable, permitting fine-tuning.
Differential gap control is suitable for some continuous automatic feedback control loops. It slows the rapid cycling of on/off control, reducing wear on the final control element while maintaining much of the simplicity of on/off control. A typical application of differential gap control is the operation of a dump valve or pump to keep a vessel level within an acceptable range.