CompDev, CompDevPercent, CompMax, and CompMin
- Last UpdatedOct 02, 2024
- 2 minute read
Data Archive uses compression specifications to filter the data that is passed from the snapshot to the archive. By filtering out unneeded data, you get more efficient archive storage.
While exception reporting filters data with a simple deadband method, compression testing filters data with a more complex method that follows the slope of the data (known as the swinging-door compression algorithm). When a new snapshot arrives, the previous one is evaluated according to the compression specifications to determine if it is a significant event. Significant events are sent to the event queue while insignificant events are discarded. For more information, see Compression testing.
You can specify the following compression-testing attributes.
|
Specification |
Attribute |
Description |
|---|---|---|
|
Compression deviation |
CompDev |
Specifies the compression deviation in the point's engineering units. Typically, set CompDev to the precision of the instrument. |
|
CompDevPercent |
Specifies the compression deviation as a percent of the point's Span attribute. 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. When you change CompDev or CompDevPercent, the other is automatically updated to be compatible. If you changed both at once, CompDevPercent overrides CompDev. For non-numeric tags, CompDev and CompDevPercent are set to zero and ignored. |
|
|
Compression maximum |
CompMax |
Specifies the maximum time in seconds between events in the archive. If the time since the most recent snapshot event is greater than or equal to the point's CompMax value, the current snapshot value is recorded in the archive. This attribute is usually set to one value for all points in the system. The recommended maximum time specification is 8 hours. Duplicate values are archived if the elapsed time exceeds CompMax. Note: The CompMax specification does not guarantee that a value will be written to the archive within a certain time. The archive waits for events to be sent to it. It does not check to see if a point has timed out. It does not create new values. |
|
Compression minimum time |
CompMin |
Specifies the minimum time in seconds between events in the archive. If the time since the last recorded event is equal to or greater than the point's CompMin value, the current snapshot value is recorded in the archive. For any point coming from an interface that does exception reporting, set CompMin to zero. Typically, you use CompMin to prevent an extremely noisy point from using a large amount of archive space. For a data acquisition system with a slow scan time, this parameter is not important. |