SmartZone Gateway EPA064 User Manual
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Appendix B: Hysteresis Demystified
When a temperature or humidity limit is reached and the relevant limit has its OFF to ON
Trap enabled, an alarm trap is issued by the SmartZone Gateway unit.
With a zero hysteresis setting, the traps will continue to be generated each time the limit
is reached.
This may be undesirable in a situation where the temperature or humidity level measure
has reduced by only a small amount before rising again and triggering further traps.
The hysteresis function is provided to prevent further alarm traps from being generated
until the measured value has fallen to a satisfactory level.
As shown in image above, the humidity first rises past its upper warning threshold, which
generates an alarm trap.
The humidity then reduces slightly but does not reduce to the hysteresis level, which is
1.5% relative humidity lower than the alarm setting (1.5% relative humidity lower as an
absolute measured value, rather than 1.5% of currently measured value).
Humidity then increases and decreases again. However, on the second decrease of
humidity the level drops below the hysteresis level. The Humidity falling below the hys-
teresis level re-enables alarm traps for the next alarm event. An upper limit of 25 and a
hysteresis threshold of 1.5 yield a threshold limit of 23.5.
The humidity level again begins to rise and again exceeds the upper limit, however this
time an alarm trap is generated again.
