Hysteresis is when a device gives a different result depending on which direction you are approaching from - a switch that trips at one level rising but resets at a lower level falling, or a valve that lands in a slightly different spot depending on whether it was opening or closing. It shows up deliberately in switching logic and unwanted in mechanical linkages. This guide explains both faces of hysteresis and how it relates to deadband.
Hysteresis in one line: Hysteresis is a directional dependence in a system's response: the point at which something switches or the position it reaches depends on whether the input is increasing or decreasing. It is used intentionally to stabilize on/off switching and appears unintentionally as mechanical slack in valves and sensors.
Deliberate hysteresis is what keeps an on/off device from chattering. A tank pump that starts at 80% level and stops at 60% has 20 points of hysteresis: the turn-on and turn-off thresholds are different by design. Because the switching points differ by direction, a level jittering around a single value cannot rapidly cycle the pump - it must travel the full gap before the state can flip back.
The same idea protects alarms (an alarm clears at a lower value than it set), thermostats, and any comparator-driven logic. This intentional hysteresis is closely related to deadband, and in everyday plant language the two words are used almost interchangeably, though hysteresis specifically refers to the direction-dependent switching.
The other face of hysteresis is a defect. In a control valve, mechanical slack, packing friction, and linkage backlash mean that when the controller reverses direction, the valve does not begin moving until the signal has changed by some amount - the actuator has to take up the lost motion first. This shows up as a gap between the up-going and down-going stroke curves and degrades control: the loop cannot make small, precise corrections, and it can even drive limit-cycle oscillation.
Sensors and mechanical gauges show hysteresis too - a Bourdon tube or a float linkage may read slightly differently on rising versus falling pressure or level. It is quantified during calibration as the maximum difference between readings taken while increasing and decreasing the input across the range. Valve positioners and good instrument selection minimize it, which is why hysteresis is a standard line item on instrument accuracy specifications.
Deadband is a band of no response around a value in either direction. Hysteresis specifically means the switching or response point depends on the direction of travel - the on point differs from the off point. In casual plant use the terms overlap, but hysteresis is the direction-dependent version.
Both, depending on where it lives. Intentional hysteresis in switching logic and alarms is good - it stops chattering and nuisance cycling. Unwanted mechanical hysteresis in a valve or sensor is bad - it comes from slack and friction and degrades control precision.
Mechanical slack, packing and seal friction, and linkage backlash. When the controller reverses direction, the actuator must take up this lost motion before the valve moves, so the valve position lags the signal differently on opening than on closing. A valve positioner reduces it.
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