Onboard alarms which become active on the meter will only remain present in the Active Alarm Window in SMS for the period of time which is set in the SMS Configuration Tool for the parameter "Onboard Alarm Check". By default, this parameter is set to 60 seconds.
This means that every 60 seconds, SMS will go out and check the Alarm Log onboard the meter. If there are any new alarms present in the Alarm Log since the last check, SMS will display these alarms in the Active Alarm Window, they also populate in the Unacknowledged Alarms Window. The alarm will remain in the Active Alarm Window until SMS goes back out again in 60 seconds to check the onboard Alarm Log for new alarms. If the alarm has remained active for the duration of the 60 seconds and hasn't dropped out yet, SMS will recognize that it has already identified and reported this specific onboard alarm on it's previous check and consequently will not report it again (every entry in the onboard Alarm Log has a unique identifier). This results in the alarm being removed from the Active Alarm Window in SMS. Once the dropout for the specific onboard alarm has occurred, SMS will detect this in the onboard Alarm Log at the next 60 second interval that it goes out and checks. The dropout will be populated in the Active Alarm Window and the Unacknowledged Alarm Window in SMS. The dropout alarm will only remain in the Active Alarm Window for 60 seconds, until SMS goes out and checks the onboard Alarm Log again.
The alarm entries will remain in the Unacknowledged Alarm Window until manually cleared.
Additionally, this behavior is only unique to onboard alarms. PC-based alarms will remain present in the Active Alarm Window for the duration of the period in which the alarm is active.