Using "Clock Time" and "Time From Start" Controls

  Applies To 
WaterGEMS, SewerCAD, WaterCAD
  Version(s): V8i, CONNECT Edition
  Original Author: Mark Pachlhofer, Bentley Technical Support Group


What is the difference between a "Clock Time" control and a "Time from Start" control?


A Clock Time control will create a condition based on the clock time during an extended period simulation. If the extended period simulation is for a period longer than 24 hours, this condition will be triggered every day at the specified time.

Example of clock time control are as follows:

"IF Clock Time  < 8:30 PM  THEN PMP-2 Pump Status = Off "

"IF Clock Time  >= 8:30 PM  THEN PMP-2 Pump Status = On"

When using clock time, midnight is the earliest possible time when using the less than or greater than operators. For example using the above two statements, the pump will turn turn off from midnight to 8:30 PM and turn on between 8:30 PM and 11:59 PM.

A Time From Start control will create a condition based on the amount of time that has passed since the beginning of an extended period simulation. Controls using this method do not repeat (like with the 24 hour cycle of Clock Time) and the exact elapsed time must be specified. So, if you want to have a control that does something at a specific time each day, you should use clock time instead. If you need a control to trigger after a specific elapsed time (which could be more than 24 hours from the start of the simulation), Time From Start controls can be used.
For example if you want a valve to close only between 36 hours and 72 hours after the start of the simulation, a composite condition could be used along with an ELSE:

"IF Time from Start  >=  36 hours AND Time from Start < 72 hours THEN TCV-1 Status = Closed ELSE TCV-1 Status = Inactive"

Use of the Equal Operator

When using time based controls, care should be taken when using the "=" operator, as the time needs to be exactly equal in order for the condition to be true. Depending on the calculation timestep you select, the simulation could potentially skip over the desired time. For example if the condition is "Time from Start = 6.5 hr" and the timestep is 1.0 hours.

Specific Time-Based Pattern of Operation

If you know the exact times when your controls will trigger and there are multiple time ranges, consider using a pattern instead of controls, for simplicity. See more here: Using patterns to control pumps or reservoir HGL over time