Hach-Lange POLYMETRON 9240 Operator Manual - Manual (Page 86)

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User Setup
7.2.6 Graph time base
7.3 Alarms
For graphical displays, enter the number of hours as the base
line for the graph. This can be 4, 8, 12, 16, 20 or a 24 hour
period.
Select the ALARMS option to set up the parameters for all the
alarms including the system and warning alarms.
Select the alarm to set up.