Countries that use English units, such as degrees Fahrenheit °F for temperature and cfm (cubic feet per minute) for volumetric flow, must convert from metric to English values. It is important to understand the available unit conversion options.

Display unit conversions

In Workbench running on a PC, you may globally set unit conversion to automatically display Out values in English units (where the default is no conversion for display). Do this from the menu selection Tools > Options > General.

This setting is local to your Workbench display usage only If you provide other users browser access to Lon data (or any data using SI units), and need the same automatic display conversion, you must specify English unit conversion in each station user account (Facets, Unit Conversion).

This does not take the place of adjusting Facets for Lon proxy points.

Facet conversion for Lon proxy points

When you create Lon proxy points under a Lon device, by default each parent point’s facets are the same as the device facets in its Lon ProxyExt (SI units). This is true regardless of your Workbench option settings for unit conversion display. For example, if you add Lon proxy points to a device’s nci nciTempSetPts, for each data element (using SNVT_temp_setpt), each of the corresponding Lon proxy points defaults to facets of °C.

Figure 4.   Default point facets reflect device facets

In U.S. installations, you edit each point’s facets to English equivalents. Otherwise, without any further change (with your Workbench option set to English unit conversion), point Out values display in English units (such °F) in the Lon Point Manager. However, the actual (numeric) Out values remain in SI units (such as °C). If you link point outputs to other components, and/or add history extensions to these proxy points, the SI unit values are used.

Therefore, in a U.S. installation you should set the facets of Lon proxy points to the needed English equivalents, to avoid any control issues or other confusion. To do this, gang edit multiple (related) proxy points from the Lon Point Manager, or even in the initial Add window when you first create the proxy points.

Differential temperature

In cases where you add proxy points for nvs or ncis known to have a temperature differential (delta) application, yet the point is using absolute temperature facets/units (for example, Fahrenheit), change the proxy point’s facets/units to the differential type, that is to degrees Fahrenheit. This prevents issues with unit conversions performed by the system. This also modifies the proxy units, device units, and the element qualifiers in the LonPrimitives in the associated Lon component.

You can also edit the Lon Xml (.lnml) file for a device, if needed, to accomplish the same conversion.