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Scaling issue
Posted: Fri Jun 03, 2005 6:58 pm
by PhilippeG
I am using Vensim PLE Plus for Windows (xp) version 5.4 d with a very simple model where I repeatedly input 30 mg of a substance in a "body". I made no scale change. At the first iteration, where I should have 30 mg in the body, I only get 0.3 on the y axis of the resulting graph. I looked in this forum, I checked my model a number of times, redrew it completely and get the same result. Can anybody help me with this? It (I) appear(s) so stupid!
Many thanks for your help.
Scaling problem
Posted: Sat Jun 04, 2005 9:33 am
by LAUJJL
Hi
Can you just send your model or a model simplified. Il will be easier to spot the problem.
Regards.
J.J. Laublé
Posted: Sat Jun 04, 2005 1:20 pm
by bob@vensim.com
At a guess you are assuming difference equations, but have set TIME STEP to a number less than 1. If you want to do difference equations set time step to 1 - but it is better to conceptualize as a differential equation in which case rates are derivatives of the levels - so a rate of 30 mg/second causes the level to incrase with a slope of 30 mg/second.
If you put in units and do units checking things may become clearer.
Posted: Tue Jun 07, 2005 8:43 am
by PhilippeG
Many thanks to both Bob and Jean-Jacques for their very usefull answers both on- and off-line