Add a new correlation

As an example of the use of the Correlation Manager to add a new correlation, consider the following example property: the Example Characterisation Factor (ECF), which is calculated by dividing the specific gravity (SG), at 20°C, by the square of the mean boiling point (MBP) expressed in Kelvin.

Add a correlation

Add a correlation by pressing the Add Correlation button . You will see a dialog box asking you to enter a name for the new correlation and a Unit Category for evaluating the correlation. If the unit category you require is not present in the list, you can add one by using the Units editor (Unit Manager).

For our example we choose the name Example Characterisation Factor. Our example correlation will have a dimensionless result, so we choose Dimensionless from the Unit Category menu.

We now see our new correlation in the Correlation Manager. At this stage you can optionally enter a comment for the correlation. For the example we enter This is an example correlation which characterises products.

At the bottom of the dialog we can change the calculation method for this correlation: either the correlation is calculated after cut values have been evaluated (Calculate on Cut), or evaluated for integration over the cut (Integrate over Cut).

In this example, the correlation should be calculated on cut.

Define the property

Now click on the Definition of the property, on the left hand side of the Correlation Manager. The right hand side will be reconfigured to show the input dialogs for defining the numerical values for the correlation.

In the Input Properties section, choose the input property values required for the correlation. You can do this by double clicking in the whitespace under the column header Property, or pressing the button. This will launch a Property Control, allowing you to select the property you require. Also enter the unit required, from the unit combo box, and choose variable names for these properties (in the Tag columns), by which to refer to them in subsequent formula definitions. Repeat this to set up all the input properties in the correlation. In our example, we enter the two properties:

• Specific gravity at 20°C unit: none tag: SG

• Mean boiling point (vol) unit: K tag: MBP

Assess whether it is useful to split your correlation, if it is complex, into many parts. For example, in this case, the complete formula is ECF = SG/MBP^2. We may choose to rewrite this, for ease of reading, as: ECF = SG/M2, where M2 = MBP^2. The allowed mathematical operations in formulae are indicated in the Appendix (see "Write Formulae" on page 203).

In the Intermediate Formula section: enter any intermediate variables for your correlation. For each intermediate formula, double click in the white space, or press the button, to create a new row.

Choose a variable name and enter the formula which defines it, using standard mathematical operators, and any of the variables you have defined amongst the input properties, or higher up in the list of intermediate values. To delete a row, select it and press the Delete key, or the button. In our example, we enter the one intermediate variable:

• Tag: M2 and Formula: MBP^2

Finally enter the definition of the correlation you want, using standard mathematical operators, and any of the variables you have defined amongst the input properties, or in the intermediate values. Also, choose the unit that this calculated value is expressed in from the unit combo box in the definition section of the output. In our example:

• Formula: SG/M2 and Unit: none

The bottom of the definition pane on the right hand will show a warning, together with an explanation of the formula error, when it cannot be parsed.