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To use flow rates properly, you must define all rates in terms of units (i.e., gallons, pounds) per time unit defined in the General Information dialog. Whenever you call one of the empty, fill, or transfer subroutines, you must specify the rate of flow. The units of flow, however, may change when you move material from one tank to another (e.g., pounds of dry material may transfer into a tank containing gallons of liquid).
To specify a variable rate of flow that changes dynamically with each time step, pass a value of 0. This signals the subroutine to call the Tank_Rate subroutine with each time step. To return the desired rate value for each time step when you use a variable rate, you must modify the Tank_Rate subroutine appropriately.