Critical Path breaks when linking two tasks with different calendars

[oconnorkj]

I am using two calendars in a project: one is a 12-hour calendar
(8:00A-8:00P) and the other is a 24-hour calendar. I have three
successive tasks named Paint Walls, Wait for Paint to Dry, and Paint
Trim. Paint Walls and Paint Trim are on the 12-hour calendar and Wait
for Paint to Dry is on the 24-hour calendar. The rationale is that
painters work on a 12-hour clock, but paint drying happens
around-the-clock. Paint Trim must lag Paint Walls in order for the
paint to dry.

Paint Walls (Task # 50) finishes at 5:30PM. I cannot link Paint Trim
to Paint Walls as 50FS+8h, because Paint Trim will start at 2:30PM the
following day--Project calculates and schedules the 2.5 hours from
5:30PM to 8:00PM, then the remaining 5.5 hours start at 8:00AM the next
day, ending at 2:30PM (when the working day includes a 1-hour break
from 12:00PM-1:00PM). Therefore, I include the Wait for Paint to Dry
task on the 24-hour calendar.

The Start and Finish times calculate properly using the Wait for Paint
to Dry task. Paint Walls (#50) finishes at 5:30PM. Wait for Paint to
Dry (#51) succeeds #50, starts at 5:30PM and finishes at 1:30AM. Paint
Trim (#52) succeeds #51 and starts at 8:00AM (since that is the start
of the 12-hour calendar day).

However, the problem is this: When using this approach, the critical
path blows up! As soon as I enter the Paint Trim task and link it to
the Wait for Paint to Dry task, the critical path is no longer there
(where it had been theretofore). All the tasks that follow Paint Trim
start a new path (in this case).

If I enter a dummy task (#52) after Wait for Paint to Dry, assign the
24-hour calendar, and schedule it for 6.5 hours, then link Paint Trim
(now #53) to the dummy task, my critical path comes back; that is, all
the tasks that were in the critical path before Paint Trim reappear.

Certainly this seems like an undesirable "feature" of MS Project. Does
anyone know what I can do differently to correct this scheduling
problem. Thanks,

Keith

 

[Gérard Ducouret]

Hello Keith,
Link Paint Trim to Paint Walls as 50FS+8eh
"e" stands for "ellapsed"

Gérard Ducouret

 

[oconnorkj]

Thanks for the help, Gerard, but the same result occurs when specifying
elapsed time. After Paint Walls completes, eight hours elapses--which
would be 1:30AM. Since that time is not on my 12-hour calendar, the
Paint Trim task is scheduled to start at 8:00AM. That is correct--the
schedule is accurate--but the critical path won't show tasks prior to
and including the Paint Trim task.

Any other thoughts?

 

[Gérard Ducouret]

If the lag time (+8eh) you introduced between Paint Wall and Paint Trim ends
at 1:30AM, and if obviously the Paint Trim task can't begin before 8:00; the
Paint Wall task could drift on several hours without postponing the finish
date of the project : that's the definition of slack.
An idea : Tools / Options / Calculation / Tasks are critical if slack <= : 1
day

Gérard Ducouret

 

[Steve House [Project MVP]]

It "breaks" the critical path because now the tasks up to and including
"Paint Walls" could be delayed by some finite amount without delaying the
start of "Paint Trim." Since the slack in those predecessor tasks is >0
they are no longer citical, by definition. That's not a bug in Project -
that's the ANSI definition of a critical task.

Paint Walls ends at 1730, 8 hours drying time ends at 0130, Paint Trim
starts at 0800.
but it could also be ...
Paint Walls ends at 1830, 8 hours drying time ends at 0230, Paint Trim
starts at 0800
or
Paint Walls ends at 1930, 8 hours drying time ends at 0330, Paint Trim
starts at 0800
or
Paint Walls ends at 2030, 8 hours drying time ends at 0430, Paint Trim
starts at 0800
etc etc
It's only if the Paint Walls task is delayed to the point it doesn't finish
until 0000 midnight so the drying time doesn't finish until 0800 or later
does it (and its predecessors) become critical again because you've got that
much "wiggle room" before the Painting Trim task will have to be delayed.

 

[oconnorkj]

Steve and Gerard,

Thanks for the help. I understand that the Paint Walls task now has
slack and, therefore, is no longer on the critical path. I intented my
comment about the critical path "breaking" to mean that the CP that
Project calculates is no longer valid--it starts after the Paint Trim
task--because it does not reflect the path with the longest project
duration. It appears as a segment of a longer path.

Perhaps my comments about adding the dummy task was misleading. I
don't want to force a specific critical path by including the dummy
task. I used it only to troubleshoot the problem.

The problem I am asking for help is...If all paths in my project
originate and terminate with Start and Finish tasks, respectively, I
should have a critical path beginning at Start and ending at Finish,
which indicates the path of longest duration. However, I have a
three-task path, hanging like an orphan on the end of an existing path.
Why does this happen? My initial post asked if this was the result of
using the different calendars.

One more thing: I certainly did not suggest that this was a bug in
Project. I simply wanted to know how to accomplish something according
to the way Project is designed.

Regards,
Keith O'Connor