In an ideal world, the interface would apply some sort of logic or test to data collection. This Exception Test is often referred to as “Reporting by Exception”. It works by removing values that are not changing with time or values whose change is insignificant and below the instrument’s accuracy threshold.
For example an interface that reads an instrument accurate to within 1/100, and receives the following values: 1.00000, 0.99999, 1.00001, 0.99999, will only store the 1.00000 value and show a straight line at 1.00 after that, because all changes were below the instrument’s accuracy threshold.
The exception reporting in the PI Data Archive uses a simple deadband algorithm to determine whether to send events to the PI Server. For each point, you can set exception reporting specifications that create the deadband. The interface ignores values that fall inside the deadband.
How do I know what the deadband measurement is?
The deadband is created by applying the tag attribute ExcDev in both a plus and minus direction.
In the illustration above, the following values will be sent to the PI Server: A, E and F. Why do we need the previous value?
We send the previous value for a very basic reason: without the previous value it would not be possible to correctly draw the historical trend. Consider the series of points below. Draw one trend using just the initial value A and the value that jumps out of the deadband, B. Then draw a trend line that includes not only those two points but the value previous, C.
Of the two trend lines you just drew, which trend is more accurate? But what if my values never go outside the deadband!
No problem. The ExcMax attribute sets a limit on how long the interface can go without reporting a value to PI. After the ExcMax time period, the interface sends the next new value to PI, regardless of whether the new value is different from the last reported value.
Note: Some interfaces do not support exception reporting. See the documentation for your interface to determine whether it supports this capability.
Think of an instrument being monitored by your PI System. Find the resolution of the instrument and then estimate what an appropriate ExcDev would be for its associated tag. Now search for the tag and inspect the actual ExcDev in this tag if one is available. Does it match your estimate? Do you feel the configured ExcDev is appropriate? Repeat the exercise for two other tags that you consider important for your operations.
You should be proud of yourself, there is only one more Module and you will be done with PI System Basics. Go to Module 10 or back to the Workshop Outline.
MORE LEARNING OPTIONS
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