Mean time between critical failures (MTBCF) is the mean time until the function defined by the RBD diagram fails. RAM Commander uses numeric integration of the reliability function R(t):

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where Max Time is an upper limit of integration.

After computation, RAM Commander displays a graph of the function R(t) which describes the system’s reliability over time:

 

With the help of changing the Max. Time you reach the result – when you reach a certain Max. time when the function almost reaches 0 and  MTBCF stops changing. This means that you can ignore the “tail” of the graph.

 

The general rule is to start with the Max. time which is about 10 times of the expected MTBF and the Step 100 times less than the Max .time. You just match the Max. time and Step. When after some tries you get necessary Max.Time, you may reduce the Step to get more accuracy.

 

If you choose Epsilon option, set Epsilon to the minimal MTBCF value where you want to stop the R(t) integration – on the graph the Epsilon is a distance between T axe and graph, where you are satisfied with the preciseness of the result and want to stop calculations.

 

Larger Max Time, smaller epsilon, and smaller integration steps lead to better results yet longer computation times:

if you choose short Max Time and large step, the function integration may be presented as squares area calculation:

if you choose large Max Time and small steps, the function integration may be presented as squares area calculation:

So you may see how the parameters you enter influence the calculation process.