Delay Conveyors with Leakage

[Liz]

I am using a lot of delay conveyors in my model and am basing them on the simple example supplied with Vensim help, particularly Convey3.mdl.

The leakage equation in that model shows leakage = (conveyor contents + (inflows - outflows)*TIME STEP)*decay rate.

My problem is that I want my conveyor to have two different leaks, each of which is controled by different factors. In order to do this, I tried counting Leak 2 as an outflow in the leakage equation for Leak 1 and visa versa. Not surprisingly (on subsequent thought!), this returned an error since neither leakage can be calculated until the other one has been.

Is there a way of having two independant leaks from a single delay conveyor?

My delay conveyor has 2 inflows, a delayed outflow and 2 leakage outflows.

[Wilson]

Hi Liz,

I haven't used leaky conveyors much, but it strikes me you could combine the 2 outflow leak proportions into just one prior to applying to the conveyor. Then apply the combined leakage proportion to the conveyor.

If you need to continue to trace the leakages by source within your model, create the individual leakage outflow rates by adjusting by the ratio of the two proportions.

For example, if Leak1=5% and Leak2=7% then the actual leakage proportion used in the conveyor would be 12%.

To covert the single leakage outflow, split it by the ratio of LEAK1 to LEAK2.

LEAK1 RATE = TOTAL ON CONVEYOR* LEAK1 Prop/(LEAK1 prop+LEAK2 prop)

LEAK2 RATE = TOTAL ON CONVEYOR* LEAK2 prop/(LEAK1 prop+LEAK2 prop)

So long as "LEAK1 prop" + "LEAK2 prop" don't add up to more than 1 which they shouldn't.

[Wilson]

what I really meant to type was :

LEAK1 RATE = TOTAL ON CONVEYOR*
LEAK prop *
LEAK1/(LEAK1+LEAK2)

where LEAK prop = LEAK1 + LEAK2

[Liz]

Thank you! That works fine.