The Flow Time Step

To model continuous flow, ProModel uses a Tank_TimeStep macro. This macro is the time step used when filling/emptying tanks and is an RTI (run-time interface) parameter. Initially, ProModel sets this value to .2 minutes. If you wish to use a different value for the time step, you may change it temporarily (for a particular model) through the Simulation/Parameters menu option, or permanently by changing the macro itself. The larger the time step, the longer the interval between filling and emptying (which speeds up the simulation). For example, suppose you set the time step to .1 minutes. If a tank empties at a rate of 60 gpm, the simulation would actually empty the tank by a discrete amount of 6 gallons every .1 minutes. When filling or emptying a tank, if the remaining quantity doesn't require the full time step, ProModel reduces the time step using a linear interpolation.

Please note

The only adverse effect of using a large time step is that any WAIT UNTIL statement or other test based on the Tank_Level array may be off by as much as the flow amount for the time step. For example, if the time step is .5 minutes and the rate of flow is 60 gpm, the level will change in 30 gallon increments. This means that the tank will not satisfy the statement "WAIT UNTIL Tank_Level[TankA]>=31" until the level reaches 60.