Hysteresis
Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of the moment often form a loop or hysteresis curve, where there are different values of one variable depending on the direction of change of another variable. This history dependence is the basis of memory in a hard disk drive and the remanence that retains a record of the Earth's magnetic field magnitude in the past. Hysteresis occurs in ferromagnetic and ferroelectric materials, as well as in the deformation of rubber bands and shape-memory alloys and many other natural phenomena. In natural systems it is often associated with irreversible thermodynamic change such as phase transitions and with internal friction; and dissipation is a common side effect. (Citing source: Wikipedia)
LORRIC paddle wheel flowmeter takes advantage of Hysteresis for alarm setting which makes on-site operation more stable.
The relationship between output and flow rate:
Under the original signal route (blue line in image below), assume 200 LPM as the trigger value for the red light alarm, 0 (open) makes the red light turn off and 1 (close) turns the red light on.
If the flow data swings between 199 and 201 LPM with no hysteresis, the red light will turn off when the value is 199 Lpm; the red light will blink at 201 LPM, and the light will continue to change, blinking, turning off, blinking, turning off, and so on, which is likely to interfere with field operations. If a hysteresis percentage is set to 10%, the signal must be lower than 10% of 200 LPM, that is, 180 LPM (red line in image below) to turn off red light.
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