Understand expression analyses

Expression analyses calculate one or more output values from specified functions, operators, and input values. You specify the inputs and calculations for these analyses in expressions. You can map any output that an expression calculates to an attribute. You must map at least one output to an attribute.

You can map the output to an existing attribute or you can create a new attribute when you map the output. If you create a new attribute when you map the output (from the Analyses tab in PI System Explorer), you must choose whether or not to save the output history; the system creates the appropriate type of attribute:

• If you choose to save output history, the system creates an attribute with a PI point data reference and creates a PI point in the specified Data Archive. With the output saved in a PI point, you can use visualization tools to see the trend associated with the calculation, and you can retrieve the value for any time that the analysis performed the calculation. The analysis calculates and stores a value as specified by the analysis schedule.

• If you choose not to save output history, PI System Explorer creates an attribute with an analysis data reference. An analysis only calculates the value for expressions mapped to attributes with an analysis data reference when requested. For best performance, map output attributes to attributes with analysis data references if you only need to know the most recent value and do not need previous values.