Units Error
Applicability: Cranium, Synapse (core versions 0315+)

Sometimes, when working with properties that have a composition variable, the application will display the term '- units err -' instead of the composition value.

For example, the MKS Sample Knowledge Base (on 2021-06-16) would display the following values for cyclohexane's minimum boiling azeotropes. (See here for documentation on the Phase Invariants Section.)

Units Error: the X,2 column of the section's table control displays the term '- units err -' for 1-propanethiol (row 1), 2-butanethiol (row 2), and 2-methyl-1-propanethiol (row 6).

Clicking the left mouse button on the table's data pane activates the data edit dialog.

Clicking the left mouse button on the row displaying values for 2-butanethiol. Then press the dialog's Edit button. The application activates the datum edit dialog.

The dialog shows that the X,2 composition value is a valid value. However, note that the datum's original units for composition are 'wt %'. Looking back at the Phase Invariants Section's table control, we see it is displaying composition in 'mol frac' composition units.

Cranium and Synapse always store values in their original units. When we assign different units for display, Cranium and Synapse will automatically convert values from their original units to the requested display units. To perform certain composition units conversions, e.g., mole fraction to weight fraction or mole fraction to weight percent, the application must know the chemical's molecular weight. Thus, during such a units conversion, the application also retrieves the chemical's molecular weight from the source document. If the molecular weight is not known, i.e., no datum or estimated value is available, or if the chemical is not present in the source knowledge base, the units conversion cannot be performed and the application displays '- units err -'.

In the case of this example, the MKS Sample Knowledge Base did not contain 2-butanethiol (when accessed on 2021-06-16). Thus, without knowledge of this chemical, a molecular weight could not be retrieved and the units conversion could not be performed.

To resolve a units error, you simply need to add a molecular datum or estimate to the chemical or add the chemical to the source knowledge base. However, as the example above shows, Cranium and Synapse will always store the original units of each datum in their documents. So even though a units error is displayed, you can always edit the datum to view its value.

Related Documentation
Topic Description
Getting Started using Cranium provides a quick tour of Cranium's capabilities including a discussion of structure editing.
Getting Started using Synapse provides a quick tour of Synapse's capabilities including examples of chemical product design.