Negative Levels

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brianbirgen
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Vensim version: PLE

Negative Levels

Post by brianbirgen » Wed Oct 09, 2019 6:36 pm

I have been using VenSim PLE for about ten years now in teaching a Modeling Calculus course. This is a course in which we teach Euler's method and Runge-Kutta and modeling with differential equations. Every now and then a new feature is added to the software that causes some small problems I need to account for. (The last time this happened was when you dropped Euler's method as an option for computations.)

In the latest version (8.0) it appears that it is impossible for levels (AKA box variables) to take on negative values. I run models for the equations of motion, and I like to demonstrate the need for using inequalities for testing values. (i.e. you can't test when height = 0, you must test for height <= 0) Additionally, I give abstract differential second order differential equations, just to demonstrate stability of solutions and other analysis of solutions. Now there is some change in the latest version where if the level becomes negative, a value of zero is given.

It is my hope that there is a setting I can change to allow for negative levels.

Brian Birgen
Wartburg College

brianbirgen
Junior Member
Posts: 5
Joined: Wed Oct 09, 2019 5:02 pm
Vensim version: PLE

Re: Negative Levels

Post by brianbirgen » Wed Oct 09, 2019 6:47 pm

I downloaded the latest version of this to test it on my own machine (rather than the machine in the classroom) and the problem has gone away. When the classroom is available I will check to see if an older different version is installed on those computers.

Brian Birgen
Wartburg College

brianbirgen
Junior Member
Posts: 5
Joined: Wed Oct 09, 2019 5:02 pm
Vensim version: PLE

Re: Negative Levels

Post by brianbirgen » Wed Oct 09, 2019 6:57 pm

Apparently this was a problem with version 8.0.3, but it has gone away, so all I need to do is update my software.

Wow! You solved my problem before I even asked it. That is awesome!

Brian Birgen
Wartburg College

tomfid
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Re: Negative Levels

Post by tomfid » Wed Oct 09, 2019 7:23 pm

This is puzzling ... we've never dropped Euler's method, and there haven't been any changes to the sim engine in v8. If you have a model demonstrating the problem, or a screenshot, we'd be very interested to explore the issue.

Administrator
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Re: Negative Levels

Post by Administrator » Wed Oct 09, 2019 9:02 pm

There was an issue with 8.0.3 where the results were not shown correctly (but we fixed it the same day it became apparent). If you download today, the problems are fixed.

Euler should still be available as Tom says.
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559

brianbirgen
Junior Member
Posts: 5
Joined: Wed Oct 09, 2019 5:02 pm
Vensim version: PLE

Re: Negative Levels

Post by brianbirgen » Wed Oct 09, 2019 9:16 pm

Euler Method was dropped about seven years ago, but quickly brought back. That was a long time ago.

Yes, I figured out the problems I was having were removed in version 8.0.4. Sadly the 8.0.3 was the version installed on all the campus computers.

Brian Birgen
Wartburg College

brianbirgen
Junior Member
Posts: 5
Joined: Wed Oct 09, 2019 5:02 pm
Vensim version: PLE

Re: Negative Levels

Post by brianbirgen » Wed Oct 09, 2019 9:18 pm

Oops! No my mistake. It was RK4 that was dropped back in 2011, but quickly brought back.

Brian Birgen

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