AN35: Poseidon – Preventing false alarms

Poseidon monitoring unit sends SMS, e-mail, and SNMP traps whenever a sensor value is outside of a safe range. However, when the temperature or humidity fluctuates, the unit can send tens or hundreds of messages that are unnecessary and bother the user. AN35 describes ways of avoiding 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


The Alarm Notification Delay function is supported in the following (and newer) firmware versions. Firmware files are available here: Poseidon model XXXX

  • Poseidon 3262: fw 3.0.6
  • Poseidon 3265: fw 3.0.5
  • Poseidon 3266: fw 3.0.6
  • Poseidon 3268: fw 3.0.7
  • Poseidon 1250: fw 1.9.9
  • Poseidon 2251: fw 2.3.4


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