Naming
Model diagrams should be clearly presented to facilitate building, analysis, and presentation. Most of the models in this manual follow certain naming conventions that we recommend, though you may choose otherwise if you wish.
Levels have initial letters capitalized; e.g., Population
Rates, auxiliaries, constants, lookups, data variables, and other variable types are all lower case; e.g., average lifetime
Sketch
Levels or stocks are entered with the Level tool. When using the Level tool, the variable is designated as a Level. When you open the Equation Editor you will see that variables added with the Level tool have type Level. You can change type in the Equation Editor, or make variables without boxes have type Level in the Equation Editor but this can cause confusion and is not recommended except for special purposes.
Rates are usually entered with the Rate tool. By default rates are added with a name and one arrowhead to indicate a flow in the direction you drew the rate. You can leave a rate unnamed by pressing the Esc key when the edit box for the rate name appears. You can add an arrowhead to the other end of the Rate by right-clicking on the handle (with the Move/Size tool selected) and checking the arrowhead box to indicate a two-way flow. Except in PLE and PLE Plus you can also change the default behavior by Right-Clicking on the Rate tool to open the options and unchecking Query Valve Name or One Way Flow.
IMPORTANT NOTEThe presence or absence of an arrowhead on a Rate has no effect on the equation for that Rate in a simulation model. The equation for a Rate could allow it to decrease a Level, even though a single arrowhead indicates on the sketch that the Rate increases that Level. It is the equation that determines whether the Rate behave properly.
Constants, Auxiliaries (and Lookups, Data, and other variables) are usually entered with the Variable tool as words in a clear box or with shape None. Some diagramming conventions give Auxiliaries and Constants a Circle shape (usually with the name appearing below), but this adds clutter to a diagram without increasing the information conveyed. Having different shapes for each variable type displayed does increase the information conveyed, but no standard conventions exist for doing this and the visual clutter can be extensive. For building simulation models we recommend, and will use in this documentation, diagrams that contain Levels in boxes, Rates with bowtie valves and all other variables unadorned.
That said, except for PLE and PLE Plus you can add more tools to the Sketch toolset and change the behavior of the tools so that the Variable Sketch tool will generate shapes such as Circles or Diamonds.
