Learn about units of measure in PI AF

PI System Explorer provides numerous, standard units-of-measure (UOM) classes and conversion factors. You can extend these classes by adding new UOM, as well as new measurement classes. The implementation of the units of measure feature in PI Asset Framework (AF) is based on the International System of Units (SI).

UOM database

All PI AF databases use the same set of UOMs, which are defined in the UOM database. All UOMs have the Read permission set, but other permissions can be set for the UOM database as a whole, as described in Configure security for the UOM database.

Beginning with PI AF 2017 R2, users with administration privileges can review audit trail data on the UOM database by selecting File > Audit Trail Events after they have clicked Unit of Measure in the navigator. For more information, see Track PI AF changes with audit trail.

UOM classes and canonical units

In PI AF, each unit of measure is based on a UOM class. Classes represent measurable properties, such as temperature, length, time, and mass. Each class has a canonical unit. This is the base unit from which PI AF converts values to other units when required. For example, the canonical unit for the Length class is meter, and the abbreviation is m.

UOM conversions in client applications

The UOM feature enables automatic unit conversions in client applications. For example, suppose a PI AF attribute has a UOM of meter. A user who is viewing that attribute value can choose to view the value in a different unit, such as foot. PI AF automatically converts the data from meters to feet.

UOM abbreviations

By default, UOM abbreviations within PI AF are case insensitive. Beginning in PI AF 2015 R2 (v2.7.5), you can configure the PI System to support case-sensitive UOM abbreviations, which enables you to define abbreviations that differ only by case, such as MV (megavolt) and mV (millivolt), or s (second) and S (Siemens).

Note: All PI AF clients must be upgraded to PI AF 2015 R2 or later before they can access a PI AF server on which case-sensitive UOMs are enabled.

Time integral UOMs

Beginning with PI AF 2017, you can view the UOMs associated with a time-weighted total (Automatic and Per-Day) of rate UOMs. Rate UOMs are from classes that contain time in the denominator of their Base Units of Measure (for example, Mass Flow Rate contains Time-1, Pressure contains Time-2). Each time-weighted total property displays the name of the UOM that will result from a summary Total (Per-Day) or TotalWithUOM (Automatic) data call.

UOM groups

Beginning with PI AF 2017 R2, UOM groups such as Metric, and US Customary are available so that you can map UOMs in a UOM class to the setting that is most appropriate for your location. By selecting the Display UOM Group option in Tools > Options, you can then see attribute values displayed in the preferred unit of measure instead of the value selected in an attribute template's Default UOM field. For more information on the US Customary UOM group, see the Wikipedia article United States customary units.

UOM origin

Beginning with PI AF 2017 R2, you can check the origin of a UOM definition in the UOM database. Possible values shown in the Origin column are:

Video

For information on how to edit units of measure, watch this video: