In some cases, LonComponents (nvs, ncis, cps) in Lonworks devices have temperature values that are intended as differential (“delta,” or “offset”) temperatures. This can be an issue when Niagara performs unit conversions, as “absolute” values use a conversion offset, and differential values do not.

Engineering unit support in Niagara distinguishes between absolute temperatures (e.g. “celsius” or “fahrenheit”) and differential temperatures (e.g. “degrees celsius” or “degrees fahrenheit”).

There are special SNVT types for differential temperatures (e.g. SNVT_temp_diff_p), but some controllers were designed before these types were introduced. As a consequence, these devices may have certain nvs defined as absolute temperatures, when in fact they are intended to be differential.

For unit conversion purposes, if you know which items (nvi, nvo, nci) in a Lonworks device should be treated as a differential temperature, you can edit its corresponding device lnml file in a text editor. Using this edited lnml file in Workbench to define Lonworks devices ensures that unit conversions are performed correctly.

Do this for any affected nv or nci by adding “+diff” to the snvtType attribute, as shown:

As an alternative to editing device lnml files, you can also change the facets/units in associated Lon proxy points under a device. See Differential temperature notes.