Compression deviation

Once events are sent to the PI Snapshot subsystem, a compression algorithm can further filter data and reduce storage to only significant values as they are moved into the archive. An event is recorded:

• After a specified minimum duration of time since the previous event, if it exceeds a specified deviation in value from that event.

• After a specified maximum duration of time since the previous event.

• When activated, compression reporting filters events and stores only periodic values (including duplicates), unless an event represents a significant change in the short-term trend of values.

To turn off compression and archive every event that passes exception reporting, disable the Compressing attribute.

Compression Deviation — Enter the deviation in value required to record an event, either as a number of engineering units, or as a percentage of the point's Span value.

For most flows, pressures, and levels, use a deviation specification of 1 or 2 percent of Span. For temperatures, the deviation should usually be 1 or 2 degrees.

Min Time —Enter the minimum time that must elapse after an event before a compressed value can be recorded. The minimum time should be set to 0 if exception reporting is activated for the same point.

Max Time — Enter the maximum time that can elapse after an event before automatically recording the next event as a compressed value. The recommended maximum time is one work shift (for example, 8 hours). If this value is too low, the compression effects are too limited to save significant archive space. If this value is too high, useful data may be lost. Events that reach the Data Archive server in asynchronous order bypass the compression calculation and are automatically recorded to the archive.

The compression specifications consist of a deviation (CompDev), a minimum time (CompMin), and a maximum time (CompMax).

Events are also archived if the elapsed time is greater than the maximum time. Duplicate values will be archived if the elapsed time exceeds CompMax. Under no circumstances does this cause Data Archive to generate events; it only filters events that are externally generated.

The most important compression specification is the deviation, CompDev. For non-numeric tags, CompDev and CompDevPercent are ignored. They will be displayed by applications as zero.

Note: CompDev specifies the compression deviation in engineering units; CompDevPercent specifies the compression deviation in percent of Span. If you change one, the other is automatically changed to be compatible. If you try to change both at once, CompDevPercent takes precedence.