Measurement Systems

IPSE supports working with multiple measurement systems. This section explains how to work with different measurement systems and change units of measurement.

Functionality

Consider a model library that is used in Europe and the US. The following table lists some units of measurement that are typically used.

Item	Unit of Measurement
	ISO	US
temperature	°C	°F
mass	kg	lb
pressure	bar	psi
heat	kW	BTU
power	kW	kW

Item

Unit of Measurement

ISO

temperature

°C

°F

mass

pressure

bar

psi

heat

BTU

power

As a user of this library, you want to use your preferred units of measurement. You may also want to switch between two different units of measurement while working on a project because some of the component data in your project is only available in certain units.

IPSE defines a mechanism that allows you to choose freely from the units of measurement that are defined in a model library. It depends on the model library that you are using which units are available.

In a library that supports multiple measurement systems, categories for the units of measurement are defined. The variables are assigned to the appropriate categories. For example, all variables and parameters representing temperatures are assigned to the measurement category temperature, all pressure drops are assigned to a category pressure_drop, etc. For each category, the possible unit of measurement and the according conversions are defined in the model library.

In IPSE you can choose for each category your preferred unit of measurement. In the example, you would change the units of measurement for all temperatures or for all pressure drops. While this is less flexible than switching the unit of measurement for each individual component, it eliminates the risk of errors due to wrong interpretation of the values.

You can not only switch individual categories but also select an entirely different measurement system. For example, you may want to switch from ISO units to US units. IPSE allows also to define such different measurement systems. In each measurement system a unit of measurement for each category is defined. By selecting a new measurement system, you switch all categories at the same time.

Figure 1. Selection of measurement systems

Changing the Unit of Measurement for a Single Category

To change the unit of measurement for a single category:

Select Project Measurement Systems. The system displays the window shown in Figure 1.
Find the category that for which the unit shall be changed.
Click on the Display Unit column to open and a menu showing all alternative units should be displayed.
Select the required unit of measurement.
Click on Save to submit the changes.

You can also reset all units of measurement to the default definitions of the underlying measurement system.

To reset all measurement units:

Select Project Measurement Systems. The measurement systems window shown in Figure 1 appears.
Click on Reset.

Selecting a Different Measurement System

A measurement system lets you change the displayed units for all defined categories simultaneously. Many model libraries support multiple measurement systems such as ISO or US.

Not all model libraries have different measurement systems. In such cases it may still be possible to switch units for a single category but settings described in this section may not be available.

To select a different measurement system:

Select Project Measurement Systems. The measurement systems window shown in Figure 1 appears.
Select a different measurement system from the selection box.
Click on Save to submit the change.

Units of Measurement in Data Frames

For data frames some additional considerations are required: Data frames contain expressions, not only single items. This expressions may contain conversion factors which make it impossible to determine the conversion automatically. IPSE implements a mechanism for the conversion of units of measurement as follows:

By default unit of measurement conversion is not enabled.

For a data frame you enable the conversion by selecting one or more cells and open the Edit Data dialog, either via the menu or by double clicking on the selection. Enable conversion by selecting the Apply conversion check box.

Enabling unit of measurement conversion has the following effect:

Converting Numerical Expressions in Data Frames

By default, expressions are evaluated on the basis of the respective selected units of measurement. If you insert a $(category-name) directive right after the "=", the expression is evaluated on the basis of generic units of measurement and then converted to the selected units of measurement for the respective category. If conversion is not enabled, the expression is evaluated on the basis of generic units of measurement, and the $(category-name) directive is ignored.

To insert a $(category-name) directive into a field proceed the following:

In the field for editing the expression, place the caret in the position where you want to insert the directive.
Click on the Insert Category button and select the category that you want to insert.

Converting Textual Expressions in Data Frames

Each occurrence of a $(category-name) directive is replaced by the selected unit of measurement. If conversion is not enabled, each concurrence of a $(category-name) directive is replaced by the generic unit of measurement.

This satisfies all conversion requirement, as long as the result is either a combination of consistent generic units of measurement or a conversion for the result unit is defined.