Implicit unit conversion
- Last UpdatedMar 19, 2025
- 2 minute read
The following table describes the relationships between engineering unit dimensions when they are multiplied or divided by each other.
|
Factor1 |
Factor2 |
Product |
|
Data Rate |
Time |
Data |
|
Density |
Volume |
Mass |
|
Energy Density |
Volume |
Energy |
|
Length |
Area |
Volume |
|
Length |
Length |
Area |
|
Mass Flow |
Time |
Mass |
|
Pressure |
Area |
Force |
|
Pressure Rate |
Time |
Pressure |
|
Resistance |
Current |
Electric Potential |
|
Speed |
Time |
Length |
|
Temperature Rate |
Time |
Temperature |
|
Unknown Rate |
Time |
Unknown |
|
Volumetric Flow |
Time |
Volume |
|
Radiant Exposure |
Time |
Irradiance |
|
Rate |
Time |
Ratio |
|
Rotation Speed |
Time |
Rotation |
|
Electric Potential |
Power |
Current |
|
Acceleration |
Mass |
Force |
|
Force |
Speed |
Power |
|
Mass |
Specific Energy |
Energy |
Each row in the table corresponds to four equivalent dimension relationships:
-
Dimension 1 * Dimension 2 = Result Dimension
-
Dimension 2 * Dimension 1 = Result Dimension
-
Result Dimension / Dimension 1 = Dimension 2
-
Result Dimension / Dimension 2 = Dimension 1.
When working with expressions involving tags or streams with different engineering units, the engineering units of the results are determined as follows:
-
When adding or subtracting streams with units in the same dimension, all streams are first converted to the same unit as the first stream, and then the operations are performed. The results are in the same unit as the first stream. For example, using the expression SysTimeHour + SysTimeMin, both tags use units in the Time dimension. When the expression is evaluated, the units from the first tag (hours) are used, so SysTimeMin is first converted to hours, and the results are in hours.
-
When adding or subtracting streams with one or more streams having "undefined" units, the results are in the same unit as the first stream.
-
When multiplying or dividing a dimensionless stream or numeric constant with a stream that has an associated unit, the results are in that unit.
-
When multiplying or dividing streams where both units are related by one of the entries in the DimensionMath table, the result is in the associated result dimension's unit.
-
When multiplying streams where both units are in the same dimension, the scale factors of the two units are multiplied together. If the scale factor matches a unit in the result dimension, the results are in this unit. Otherwise, the original units are first converted to their dimension's base units, and the results are in the base units of the result dimension.
-
Any other combinations cannot be implicitly converted, so the results are in "undefined" units.
Note: AVEVA Historian 2023 updates the base units for some dimensions to enable implicit unit conversion. If you have additional Historian nodes configured for replication, the engineering unit catalog will be out of sync on tier 2 nodes until they are upgraded to the 2023 version.