Poseidon – hysteresis - Preventing false alarms

Sources of false alarms

• Contact bounces
• Oscillation of the reading around the threshold
• Changing the connections

The problem is particularly bothering if you use Poseidon with a GSM modem that sends a text message (SMS) to your mobile phone upon each and every alarm.

Poseidon supports two functions to avoid false alarms:

• Alarm Notification Delay
• Hysteresis Idle Range

Alarm Notification Delay

The Alarm Notification Delay function prevents an alarm alert from being sent if the alarm lasts only for a very short time. The same delay is applied to alarm activation and alarm deactivation alerts.

• Blue color indicates the behavior when Delay [s] = 0.
• Orange color indicates the behavior when Delay [s] = 20.

Hysteresis / Idle range

The Hysteresis setting defines a tolerance band for suppressing alarm alerts. The function prevents multiple alarm alerts if the reading oscillates around the specified threshold. The range is configured independently for each sensor.

The figure demonstrates two cases. Without the hysteresis idle range of 5°C, the alarm raised in point 8 would end in point 9; however, the hysteresis function keeps the alarm active until the temperature reaches the upper limit of the tolerance band (point 10): 5℃ + (–15℃) = –10℃.

• Hysteresis = 5°C:   
  The unit sends 3 e-mail (SMS) messages
  Alarm active in points 0..4,  8..10, 12 and beyond
   
• No hysteresis (0°C):   
  The unit sends 8 e-mail (SMS) messages
  Alarm active in points 0..1, 2..3, 8..9, 12..13, 14 and beyond