REPLY Age of material in a stock (SD6400)

[SDMAIL Jean-Jacques Laublé]

Posted by  Jean-Jacques Laublé <jean-jacques.lauble at wanadoo.fr>

Hi Richard

As soon as the inputs and the outputs are varying you must consider if the
material inside the stock is undifferentiated or not.
It can follow a FIFO law and you can then use discreet functions as you will
find in Vensim.

If the material is undifferentiated and you use a first order exponential
delay, I do not see any analytical solution (I am not sure that the Vensim
functions do not work numerically).

You must calculate the time in stock using Jay's formulation and integrating
it step by step.

This average time delivery will vary each time period, depending on the
future inputs and outputs still to come.

I have a model that calculates the average time in stock with varying inputs
and outputs for the material in the stock at the start of the simulation.
It must be reinitialised every day, to recalculate the new average.
Of course you must predict the inputs and outputs.

The average is becoming more and more precise as the time passes.
It becomes sufficiently precise after 100 days.   

The model can be dowloaded from:

http://www.ventanasystems.co.uk/forum/

under the Sytem Dynamics Discussion forum and can be used with Vensim PLE.

I will build as soon as I have the time, a model that calculates the average
time in stock not only for the initial stock, but for each new inputs to
come.

It will probably use subscripts.

Regards.
Jean-Jacques Laublé
Posted by  Jean-Jacques Laublé <jean-jacques.lauble at wanadoo.fr>
posting date  Sat, 14 Apr 2007 18:16:32 +0200