Units

ETBX provides integrated support for unit conversions. Input units can be uniquely specified for each input field that defines a physical quantity. Input values are converted to one of the following user-specified Reference Unit Systems to calculate results:

• Imperial FPS (Foot-Pound-Second)
• Imperial IPS (Inch-Pound-Second)
• SI MKS (Meter-Kilogram-Second)
• SI CGS (Centimeter-Gram-Second)

Results are displayed in a user-defined set of units, called the Results Unit System.

Input Units

The units associated with an input quantity are displayed to the right of the input label as shown in Figure 1. If no units are shown, the input will be treated as a dimensionless parameter.

Figure 1. Units associated with input fields

To change input units, right-click on the input field and select the type of units that you want to change from the context menu (shown in Figure 2).

The units listed in the context menu represent ETBX base unit types; for example Distance, Mass, or Time. If the input quantity is a derived unit type, any of the constituent base types can be selected from the context menu and changed individually. For example, Figure 2 shows the context menu that appears when the user right-clicks in the Density input field. In this case, density is defined as the derived type mass/volume. The units for the base types mass and volume can be individually specified by selecting the appropriate menu item.

Figure 2. Right-click to change input units

Select a base unit type from the context menu to open the unit list shown in Figure 3. Choose the new units from the list. If you want to convert the existing input value to the new units, click the checkbox labeled "Convert input value to selected units". Click the OK button to update the units associated with the input field.

Figure 3. Select the new units from the list.

Reference Units

Prior to calculating results, all input quantities are converted to a consistent system of units called the Reference Unit System. All internal computations in ETBX are conducted in reference units. To specify a reference unit system for the active module, open the module units dialog box shown in Figure 4 by choosing the Module -> Units menu item. Select the desired reference system from the list at the top. To store your selection as the default reference system, check the Make Default Units box before closing the dialog.

ETBX defines four reference unit systems:

• Imperial FPS (Foot-Pound-Second)
• Imperial IPS (Inch-Pound-Second)
• SI MKS (Meter-Kilogram-Second)
• SI CGS (Centimeter-Gram-Second)

ETBX modules are designed to be compatible with any consistent system of units and thus the choice of reference system should have a negligible effect on calculated results. Note however that the conversion of input quantities to the reference system, and the conversion of calculated quantities to the results system, may result in slight round-off errors if the input and/or results units are not in the reference system.

Figure 4. Module units dialog box.

Results Units

Calculated results are converted to a user-defined set of results units to be output, for example, in a text report or two-dimensional plot. Results units do not need to form a consistent system of units.

To specify results units for the active module, open the module units dialog box shown in Figure 4 by choosing the Module -> Units menu item. Select the desired units from the the drop-down lists in the Results Units input group. There is one list for each base unit type defined by ETBX.

To use reference units for output, click on the Reset to Reference System button. All results unit lists will be updated to display the appropriate units from the reference system.

Click the OK button to accept the unit settings. If the Apply To Module Input Fields box is checked, all input fields will be updated to use the selected results units. Note that existing numeric input values are not converted to the new units.

To store your unit settings as the ETBX default, check the Make Default Units box before closing the dialog.