CD Automation srl
pag. 36 di 121
14.3.2
Alarm Hysteresis
The purpose of the hysteresis is to prevent, when the measurement is near the thresholdand there are
disturbances on the measure, the alarm state changes continuously from ON to OFF and vice versa.
To do this it’s possible to define a "safety margin" so that the alarm goes ON when the measure reaches a
specified value (A) but does not go OFF before the measure reaches another value (B) more closely to the
optimal condition.
Value (A) Is used as the threshold and value (B) as the threshold more or less hysteresis;
The following example will clarify the foregoing.
Considering that we want to set a Low alarm process that goes ON at least 300 ° C (value A) and returns to
OFF only when the measure has risen to 360 ° C (B value).
In this case the setting of the threshold is 300 (° C)
In questo caso il valore di soglia da impostare è 300 (°C) while the hysteresis have to be set equal to 60(°C).
note. The hysteresis value is expressed on the instrument in % of full scale.
14.3.3
Alarm Out
Generally the state of an alarm is made visible to the user and can also produce physical actions on the
system.
The status indication of an alarm can be:
1) Visual indication (a LED on the front of the instrument panel)
2) Software indication(state of a boolean variable on the serial communication)
3) State of a physical Out (normally a relay)
Note:
a) These three indications are present both individually than simultaneously.
b) At" State of a physical Out " are associated also physical action on the system.
