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# Operation time vs percentage of duty time - how to calculate MTBF?

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 Bichon (Civil/Environmental) 6 May 12 15:11
 I need some help to figure out how to do some calculations correctly. I have made a RBD and then I want to calculate the MTBF and the availability using exponential failure rate.Ok, so far so good but what I don't get is the difference when changing operation time or using percentage of duty time. Let's say the operation time is 700 h. I make a calculation and then let's say the MTBF for the system is 200 000 h. If I then let this system be active 50 % during the operation time instead of 100% how should this be done correctly?I can then use percentage of duty time and specify that out of these 700 h my system is active 50 % of the time (every single part of the rbd is set to 50%). The result is  that the MTBF is 400000 h. The calculations are made for 700 h.I also thought I could use the fact that if I know the operation time is 700 h and if the system then is active 50% of this time this would be the same that to say that system is active 100% in 350 h. However if I do that then the result is MTBF 200000 h. So I guess I'm doing something wrong here... Can someone please help me out?Thank you in advance.
 IRstuff (Aerospace) 6 May 12 18:35
 Huh?  Generally, MTBF refers to power on time.  Duty cycle only matters if there is still some sort of reliability failure process that happens with the power off.  You appear to be confusing power on and power off time.
 Bichon (Civil/Environmental) 7 May 12 0:53
 Thank you for your reply.Well, I see your point but I'm still a bit confused... I don't fully understand what is wrong. Can you please clarify? I still think that operational time in both cases should be the same... But the answers are from the same... So I'm doing this completely wrong! Please help me out! Thank you.
 IRstuff (Aerospace) 7 May 12 11:41
 Let's say the operation time is 700 h. I make a calculation and then let's say the MTBF for the system is 200 000 h. If I then let this system be active 50 % during the operation time instead of 100% how should this be done correctly?MTBF remains the sameI can then use percentage of duty time and specify that out of these 700 h my system is active 50 % of the time (every single part of the rbd is set to 50%). The result is  that the MTBF is 400000 h. The calculations are made for 700 h.This is erroneous.  Failure rate does not change.I also thought I could use the fact that if I know the operation time is 700 h and if the system then is active 50% of this time this would be the same that to say that system is active 100% in 350 h. However if I do that then the result is MTBF 200000 h.And why is this wrong?  Again, MTBF is for operational hours; 200000 OPERATIONAL hrs, regardless of how you distribute them, at least for this example.
 BigInch (Petroleum) 7 May 12 13:29
 Proof that if you don't turn it on, it will run be available forever. What would you be doing, if you knew that you could not fail?
 IRstuff (Aerospace) 7 May 12 16:58
 There are scenarios where failure mechanisms still continue on during power-off times.  Thermal shock, mechanical shock, vibration, very high temperatures all potentially can continue to nibble on the MTBF when power is off.  I don't see anything that indicates those scenarios.
 Bichon (Civil/Environmental) 9 May 12 2:09
 IRstuff (Aerospace) 9 May 12 10:24
 Never used Relex, so no useful help from me in that arena.However, duty cycle, in general, can be used to provide a nuanced approach to temperature, vibration etc.  Often, the environment includes multiple levels, like 20% Ground Benign + 80% Ground Mobile, so some sort of duty cycle is applied to allow the program to come up with an answer that's between a solely GB and solely GM prediction.

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