Rockwell Automation 1756-HIST2G Historian ME 2.1 - Manual (Page 95)

5 • M ANAGING P OINTS
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C OMPRESSION F ILTERING P OINT P ROPERTIES
When a new Snapshot arrives, the previous one is evaluated according to the
compression specifications to see if it is a significant event. If so, it is sent to the
Event Queue. If not, it is discarded. The result is that only significant data is written
to the archive. This process is called compression.
The Compression Filtering section contains the following fields:
Field Description
Compression On This flag should be turned ON for all real-time points in the
system.
With compression OFF, every value sent to the Snapshot
subsystem is saved in the archive.
Compression Min Value that indicates events are archived if the elapsed time since
the previous event is greater than or equal to the minimum time,
and the event value has changed by more than the deviation.
The value is expressed as dd hh mm ss.
Typically, the Compression Min should be 0.
Compression Max Value that indicates events are archived if the elapsed time is
greater than the maximum time.
The value is expressed as dd hh mm ss.
The recommended maximum time specification is one work shift
(such as, eight hours). Duplicate values will be archived if the
elapsed time exceeds the Compression Max. Under no
circumstances does this cause the FactoryTalk Historian ME
server to generate events; it only filters events that are externally
generated.
Compression
Deviation
Value for the most important compression specification.
Setting this value too low causes too little data compression and
wastes space in the archive. Setting this value too high causes
loss of useful data. For most flows, pressures, and levels, use a
deviation specification of 1% or 2% span. For temperatures, the
deviation should usually be one or two degrees.