# question

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## FlexSim Experimenter

power-unit-production-model.fsm

Hello all,

I had this model to build (attached below). I need to conduct experiments in order to ultimately optimise the whole system. However, I am a beginner in Flexsim and would like your help to first calculate the warm-up time: I tried plotting an output per hour graph for the sink but it keeps on oscillating. Then I need to calculate the simulation run length which I do not know how to derive.

Also, if you could please help me understand replications in a simulation context as I didn't get the hang of it.

Thank you very much in advance.

FlexSim 18.2.3
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57272233-483769785494548-4187799654282297344-n.jpg

Throughput per hour chart looks like this: it keeps oscillating so I can't really tell the warmup time.

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The warmup time is usually used to help your system get all the way set up. For example, do stations at the end of the line have to wait until parts start arriving? If that's the case (and you aren't interesting in measuring those start up time periods) then set your warmup time to be whenever the model is starting to run at the capacity you want to measure. There's no specific calculation for you, that's just something you'll have to determine. The same goes for end time, that's just for you to determine. What window of measurment are you looking to simulate? Do you want the data for how your system operates in a day, a week? a month? etc. That's something for you to determine, based on what you're looking for.

Replications just mean how many times the model repeats itself. (Because most models use random distributions for time, arrivals, etc.) The replications show multiple repeats of this so you can see what the numbers look like over many iterations.

I'll look at your model, but the oscillations in the data doesn't necessarily mean it's not working. You're looking at throughput over time. So this graph is showing that. At X time you had Y throughput which could fluctuate from moment to moment. As the time moves outward, you'll see that the line may have larger curves up or down depending on your system.

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Hi @Sam Stubbs, thanks a lot for your answer. Downstream stations wait for parts to arrive but only for the first part, after that flow is established. could you have a quick look at the model please ?Because I don't know if the oscillations mean that the model already runs at required capacity ? I looked at what a friend did for the warmup time and it looked like the chart below.screen-shot-2019-04-16-at-144343.png And for the running time he used the equation below with 960 minutes corresponding to two 8hour shifts and 15 is the number of scenarios. I don't know if that equation is valid in your opinion?

What is the warmup time you would advise given the situation.

Thank you very much again.

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Larbi F ·

Hi @Larbi F,

The idea behind warm-up time is to offset your statistics collection enough that you only capture the model's output while it's running at full capacity. Unfortunately, because we don't know the parameters of your model nor what you are trying to optimize in this system, any attempt we make at calculating your warm-up time will be ineffective. We have seen this model fairly frequently in the last couple months, however, so we are familiar with the general idea.

You can analyze your system and estimate your warm-up time by looking at when the output has climaxed if you're looking to calculate output at full capacity. For example, if it takes 300 minutes for the first power unit to roll off the assembly line, your warm-up time might be 300 so that it isn't collecting data while the factory is getting started and doesn't have output.

In your friend's data, you can see his warm-up time pretty clearly as shown in the image below:

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warmup-time.png (37.8 KiB)
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