On a recent project our team had a long conversation with the client about hysteresis on his fuel tanks.  There were a number of alarms that came in to his SAMS when his tanks was right at the high level and at low level alarm points, so our integrator suggested to implement the hysteresis feature on the tanks to avoid running back and forth, as his vessel was pitching and rolling.  Hysteresis is defined by: the phenomenon exhibited by a system, often a ferromagnetic or imperfectly elastic material, in which the reaction of the system to changes is dependent upon its past reactions to change.  This means when a level is reached to activate an alarm point, and the alarm is silenced, history normally shows it will auto-reset and reactivate.  In this case we implemented the hysteresis, with a definable range in percent, so that when the high alarm activated, and was silence, it would have to drop an extra level before it could re-activate.
The example was, that he wanted a high alarm at 95% tank level, which equated to 2750 gallons of fuel.  When the alarm was silenced, his hysteresis would require the level to drop 3% before it to auto-rest.  Alarm came in at 2750 Gallons, had to drop to 2663 Gallons, before a new high alarm could come in.
It was easy to implement, and worked like a charm, “right out of the box”
The control room became much quieter after it was implemented, and he had a smile on his face that told us we had given him something he thought was unattainable.
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