About Magnitudes and Units handling

This topic aims at providing you with complementary information on magnitudes and units handling in the Knowledge workbench.

Magnitudes

A magnitude is a set of customizable units that you can use to value a parameter, i.e. length, angle, time...

Magnitude customization is available in the Units tab in the Options dialog box (Tools > Options, General  > Parameters and Measure).

For more information on the available Magnitudes, refer to the the Units chapter in the Infrastructure User's Guide.

Units

Units are a set of standard measures available for a given magnitude. Catia V5 allows you to set a default unit for a given magnitude.

For example, you can set a Meter (m), Centimeter (cm), Kilometer (km), Millimeter (mm), Inch (in), Foot (ft)... unit for a Length magnitude.

For more information on the available Units, refer to the the Units chapter in the Infrastructure User's Guide.

Numbers

Display

Displayed numbers take into account the Magnitude / Units settings available through Tools > Options, General  > Parameters and Measure.

Storage

Numbers that involve magnitudes are stored in MKS values, length and angle excepted. Length magnitude is stored in Millimeters (mm) and Angle is stored in Celsius degrees (Cdeg).

Whatever the storage type, the displayed values take into account the Tools > Options, General  > Parameters and Measure user customized settings.

Example

  1. Go to Tools > Options > General  > Parameters and Measure.

  2. Change the length magnitude default unit to Centimeter (cm).

  3. Click to create a length magnitude parameter.

  4. Set it to 1mm. The displayed value is 0.1cm.

The stored unit for the length magnitude prevails over the change made to the parameter unit itself.

All Knowledgeware formulas, rules, checks, set of equations, constraint satisfactions calculi are done in MKS units.

Given M1=1g and M2=2g M3=M1+M2w

The computation is made as follows : 0.001+0.002=0.003 (0.003kg) and then displayed as 3g.

Example 1

This way of processing numbers has a drawback that appears when dealing with temperatures.

  1. Go to Tools > Options > General  > Parameters and Measure.

  2. Change the Temperature magnitude default unit to Celsius degree (Cdeg).

  3. Click to create a T1=0Cdeg 1 Temperature magnitude parameter

  4. Click src="../../../ICONS/icons.doc/src/images/I_FormulaP2.gif" width="24" height="24" to create a T2=1Cdeg Temperature magnitude parameter.

  5. Click to create a T3=T1+T2. T3 is displayed as 274.15Cdeg.

In MKS the units used to express temperatures are Kelvin degrees (0 Celsius degree = -273.15 Kelvin degrees). Hence, the calculus is made as follows:

T3 = 274.15degK + 273.15degK (result is 547.30DegK).

Then displayed back into Celsius degrees (547.30 - 273.15 =) 274.15Cdeg

Example 2

Create a formula of type

T4 = 1Cdeg + 1Cdeg + 1Cdeg
the displayed result is as follows:

549.3Cdeg

The internal calculus is done as follows:

273.15degK + 273.15degK  + 273.15degK = 819.45degK. The result is displayed in degC 549.3degC

This phenomenon appears for all magnitudes that have a conversion behavior between their units. This behavior involves either an addition or a multiplication or an addition transformation.

The magnitudes listed here bellow are impacted by this phenomenon:

Magnitude

Unit Conversion type
  • Temperature
  • Celsius degrees to Kelvin degrees or to Fahrenheit
  • Pressure
  • N/m2 and barg
When dealing with such magnitudes inside calculus (formulas, rules, checks, set of equations, constraints satisfactions), it is strongly advised to use either a MKS unit or real numbers for doing calculation and then multiply the final result by the unit itself.