By default, tanks are treated as having a circular shape with a constant cross section described by its diameter. If the tank has a constant cross section that is not circular, the user can select Non-circular and specify the cross sectional area. If the user selects Variable Area, it is necessary to provide a depth to volume table.

In a variable area tank, the cross-sectional geometry varies between the minimum and maximum operating elevations. A depth-to-volume ratio table is used to define the cross sectional geometry of the tank.

To Define the Cross Section of a Variable Area Tank

1. Select the tank in the Drawing View.
2. In the Properties window, click the Section menu and select the Variable Area section type.
3. Click the ellipsis button (...) in the Cross-Section Curve field.
4. In the Cross-Section Curve dialog that appears, enter a series of points describing the storage characteristics of the tank. For example, at 0.1 of the total depth (depth ratio = 0.1) the tank stores 0.014 of the total active volume (volume ratio = 0.014). At 0.2 of the total depth the tank stores 0. 028 of the total active volume (0.2, 0.028), and so on.