Xylem PC320 Process Controller - Manual (Page 9)

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Since in normal use the water level will never fall below zero, the Lower Threshold
will have no effect and not trigger the relay.
Hysteresis is a percentage change of the full span of the sensor required to turn a relay
off again once it has been triggered. This is used to keep the relays from turning on
and off too often when a sensor reading is very near a threshold level. Note that the
percentage is of the span the sensor is calibrated for, not necessarily the full range of
the sensor. If there is a pH sensor capable of measuring from 0-14, but is calibrated at
a pH of 4 and 10, the span is considered 10-4 or 6 pH units. If the Hysteresis is set to
10%, that is equivalent to 10% of 6 or 0.6 pH units. The maximum hysteresis is 25%.
Example: A water level sensor monitors the level in a tank and the PC320 is
connected to a pump; which will pump water out of the tank. The Upper Threshold is
set to 10 feet and the Hysteresis is set to zero. When the level rises above 10 feet, the
pump will turn on. When the level falls to 9.999 feet, the pump will turn off. Thus;
the pump will turn on and off repeatedly trying to maintain the level at exactly 10 feet.
The level sensor has a total calibrated span of 15 feet and 10% of that is 1.5 feet. If the
Hysteresis is set to 10%; the relay will trigger when the level rises to 10 feet, pumping
water from the tank. The pump will not turn off again until the level falls to 10 feet
minus 10% or 8.5 feet. The level in the tank will be maintained between 8.5 and 10
feet, saving the pump from turning on and off so often and wearing it out.
Example: A temperature sensor has a span of -50ºC to +50ºC, or 100 degrees
centigrade. The controller is set to turn on a heater when the temperature falls below
25ºC (Lower Threshold). The Hysteresis is set to 5% or 5ºC (5% of 100º span). The
heater will turn on just below 25ºC but not turn off until the temperature rises to 25C +
5% or 30C. The normal temperature range in the tank will be 25ºC to 30ºC.
The Delay Time is the amount of time required for a sensor reading to be above or
below a threshold level before a relay is turned on. This can be used to prevent a relay
from triggering when a sensor reading briefly falls outside of a normal range.
Maximum delay time is 99 seconds.