Actuation & Instrumentation
Dead Band
Dead band is the range a control signal can change in either direction without producing any change in the valve position. It is not the same as hysteresis: hysteresis gives different positions for the same signal depending on direction, while dead band means no response at all until the signal change exceeds the band.
Dead band is the number-one installed-loop killer. Sources, roughly in order of likelihood: packing friction (especially over-tightened or dry-out graphite), stiction at the plug-seat interface, backlash in linkages and shafts on rotary valves, undersized positioner spool capacity, and worn actuator parts.
Tests: a slow ramp-and-return of signal versus measured travel reveals both dead band and hysteresis on one X-Y plot (IEC 60534-6 style test). Modern smart positioners measure and report valve signatures automatically.
Rules of thumb: a throttling valve should show dead band below ~0.5 % with a positioner (many datasheets specify ≤0.25–1 %); anything above ~2 % will usually be visible in loop performance.