Rockwell Automation 1756-HIST2G Historian ME 2.1 - Manual (Page 118)

F ACTORY T ALK H ISTORIAN ME M ODULE U SER ’ S G UIDE
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change at all uses very little archive space. A compression maximum time of one
work shift (for example, 8 hours) is often a good choice.
Use the CompMin to prevent an extremely noisy point from using a large amount of
archive space. This parameter should be set to zero for any point coming from Data
Collection that does exception reporting. In this case, the exception minimum time
should be used to control particularly noisy points. For a data acquisition system with
a slow scan time, this parameter is not important. There are few cases where you want
to use a non-zero compression minimum time.
The most significant compression parameter is the deviation specification, CompDev.
This parameter is often adjusted after the point is defined. A reasonable starting point
is one or two percent of span for transmitters and 0.5 to 1.0 degrees for
thermocouples. Look at trend displays to find points for which the reproduction of the
data is not acceptable. The goal is to filter out instrument and process noise and still
record significant process changes. The effect of changing the compression deviation
is not predictable.
For digital points, any change is a significant change. Only the compression
maximum and minimum time are important. The compression deviation specification
is ignored for digital points.