Add a new correlation

Follow these steps to add a new correlation:

1. Open the Correlation Manager

   Click the Correlations button in the Managers group of the Reference Data ribbon tab to open the correlation manager.

   

   The correlation manager shows the list of currently configured correlations on the left-hand side, and details about the currently selected correlation on the right.

   

2. Add a correlation

   Add a correlation by clicking the Add Correlation button . The New correlation dialog box opens.

   

   Enter a name for the new correlation and a unit category for evaluating the correlation.

   The Unit Category field describes the measurement unit type of the result of the correlation. For example, a 'Flash Point Correlation' would return a result in temperature units, and hence the appropriate unit category would be Temperature. If the unit category you require is not present in the list, you can add one by using the units manger. See Configure your units of measurement.

   The new correlation will now be in the correlation manager. At this stage, you can optionally enter a comment for the correlation.

   

   Tip: Note that the Unit Category field is read only. If you want to change the unit category for your correlation, you have to delete the existing correlation and recreate it. To delete a correlation, select it in the left-hand side list and click the delete button.

   At the bottom of the dialog, you can change the calculation method for your correlation. The following table shows an example of each calculation method:

   Option	Description	Example: square of density for 300-400°C cut
   Calculate on Cut	Evaluation of the correlation takes place using cut values.	The density is calculated for the 300-400°C cut by integrating the density profile, with respect to the corresponding yield profile. This density value is then squared to give the final result of the correlation.
   Integrate over Cut	Evaluation of the correlation takes place using instantaneous values read from the property profile, prior to integration over the cut range.	The density profile itself is transformed by squaring it, to produce a new profile which is the square of the density. This profile is then integrated over the range 300-400ºC, with respect to a specified yield profile (as specified using the property editor).

   In depth: If you choose to calculate on cut, the correlation is evaluated after the cut values have been computed for each of the input property values. In contrast, if you elect to integrate on cut, the correlation is evaluated before the cut values have been computed for each of the input property values.
   By default, Assay calculates on cut. Sometimes, however, the option Integrate over cut is more appropriate. This is especially so for graphable properties. For example, suppose you want to graph the property Basic Nitrogen against a narrow fraction boiling point, where Basic Nitrogen is calculated using some formula based on Total Nitrogen and API. Assuming you chose the option Integrate over cut, the order of evaluation of the correlation would be: read the total nitrogen at every point along its boiling point profile; then read the API at every point along its profile; and then apply the correlation to compute a profile for Basic Nitrogen. Finally, evaluate Basic Nitrogen for the cut by integrating with respect to yield, as usual.
   In many cases, the choice of calculating or integrating over cut may not be critical. For example, if a correlation defines Basic Nitrogen to be simply 0.3 * Total Nitrogen, then both methods should yield the same results, since the transformations are linear.

3. Define the property

   Click on the definition of the property, on the left hand side of the Correlation Manager.

   In the Input Properties section:

   1. Either double click the whitespace or click the button.

   2. Select the property and unit you require, and enter a variable name in the Tag column. This tag will be used as a variable name for the property when writing formulae.

   3. Repeat this second step to set up all the input properties in the correlation.

      It can be useful to split your correlation into parts. For example, you might choose to write the formula SI = SUL/DEN *0.995 as: SI = DS*0.995, where DS = SUL/DEN. In this case, you can use the Intermediate Formula section to define DS:

   4. Double click in the whitespace, or press the button, to create a new row.

   5. Choose a variable name and enter the formula which defines it, using any of the variables you have defined in the Input Properties section (or higher up in the list of Intermediate values). See Writing formulae.

   6. To delete a row, select it and press the Delete key, or the button.

      In the Definition section:

   7. 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. See Writing formulae.

   8. Choose the unit of measurement from the unit combo box.

      

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.