# Cycle-Time Rules

This section uses graphics to present the rules of cycle-time, in an easily accessible form. Figure 10-7 shows how the software handles multiple time-stamps within a conserved-flow chain. Figure 10-8 shows how the software deals with cycle-time calculations when a Sub-model is involved.

Figure 10-7 Rules of Multiple Time-Stamping

Figure 10-8 Rules of Time-Stamping with Sub-models

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Internal Considerations

Whenever a flow is time-stamped in a model, the software will automatically shift from the "Normal" to the "Cycle-time" Run mode. The change in the Run mode is required for the software to collect cycle-time information. The Run mode radio buttons can be found within the Time Specs dialog, under the Run menu.

Internally, the following changes occur as a result of shifting the Run mode from Normal to Cycle-time.

- The model will be set to run using Euler's integration method. The Runge-Kutta 2 and Runge-Kutta 4 methods will be disabled.

- Inflows to reservoirs will be "lined up" within the reservoirs, and will exit via outflows on a first in, first out basis. In this respect, reservoirs will act like queues.

- Non-negativity will be turned "on" for all reservoirs, and will not be accessible within the reservoir dialog.

Practically speaking, you should notice only two behavioral changes when the Run mode shifts to Cycle-time. First, it is possible that the execution of a simulation run will be somewhat slower than might otherwise be the case. This is because of the additional calculational overload that is brought about by the generation of cycle-time metrics. Second, any reservoirs that have taken on negative values in the Normal mode (because Non-negativity has been turned "off" in their dialogs), will be kept from taking on negative values.

It's always possible for you to override the shift from Normal to Cycle-time Run mode. To do so, simply enter the Time Specs dialog and click on the "Normal" radio button. If you do shift to the Normal Run mode, cycle-time calculations will be disabled. As a result, cycle-time Builtins will return a value of zero. All time-stamped flows will lose the clock face on their icons. Their check boxes will be turned "off." Similarly, all cycle-time converters will lose the clock face on their icons.

You can shift back to the Cycle-time mode by clicking on the Cycle-time radio button within the Time Specs dialog. Or, you can time-stamp any flow in your model. When you shift back to the Cycle-time mode, the software will remember all of your previous work with cycle-time. The clock face will return on the icons for all flows which had previously been time-stamped, as well as for all converters which contain cycle-time Builtins.