How do I manage units of measure in PI System?

How do I manage units of measure in PI System?

PI System handles units of measure (UOM) at multiple levels. Proper UOM configuration prevents calculation errors and ensures consistent displays.

Two UOM Systems

EngUnits (PI Data Archive)

The EngUnits attribute on a PI point is a free-text string:

Just a label — PI doesn't validate or convert
Stored per tag (e.g., "degF", "PSI", "GPM")
Used by PI DataLink and ProcessBook for display

AF Units of Measure (PI AF)

AF has a structured UOM system with conversions:

Defined in UOM Classes (Temperature, Pressure, Flow, etc.)
Each class has a canonical unit (base unit for conversions)
AF automatically converts between units in the same class

Setting Up AF UOMs

1. Browse Existing UOMs

PI AF ships with hundreds of predefined UOMs. In PI System Explorer:

Go to Library > UOM Classes
Browse Temperature, Pressure, Mass Flow, etc.

2. Assign UOMs to Attributes

In an AF template or element:

Set the Default UOM on each attribute
PI Vision and other clients will display values in this unit

3. Create Custom UOMs

If your unit isn't predefined:

UOM Class: Concentration
Canonical Unit: ppm
New UOM: ppb
Conversion: multiply by 1000 (1 ppm = 1000 ppb)

Automatic Conversion

AF converts automatically when:

An analytics expression references attributes with different UOMs in the same class
PI Vision displays an attribute with a UOM different from the source
A user changes display units in PI Vision

Example: if a temperature attribute is stored in Fahrenheit but the display UOM is Celsius, AF converts on the fly.

Best Practices

Always set UOMs on AF attributes — it prevents unit confusion
Use AF UOMs rather than relying on EngUnits strings
Standardize on SI units as canonical where possible
Document any custom UOMs and their conversion factors
Include UOM in attribute naming conventions for clarity

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Related Questions

How do I configure PI AF templates?How do I write PI AF Analytics expressions?

Dive Deeper

Managing Units in PI System: EngUnits vs. AF Units of Measure Dynamically Set Source UOMs in AF Templates with PI Builder Accessing UOM Abbreviations in PI AF with AF SDK

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