Availability
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 This topic has 8 replies, 6 voices, and was last updated 14 years, 11 months ago by Ron.

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October 25, 2007 at 5:16 pm #48507
I am reading a formula for availability in the six sigma handbook by Thomas Pyzdek that states on page 572 the formul for calculating availability as:
MTBF
availability = ——————
MTBF + MTTR
If I assume a 30 day month for MTBF 43200 minutes and have an outage with a MTTR of 220 minutes I believe it would calculate out to 99.49% availability. but if I had an outage of 43000 minutes it calculates only to 50% when I have been down all but 200 minutes in the month. What don’t I understand.
0October 25, 2007 at 5:36 pm #163861These are not supposed to be snapshots but instead indicate how the system performs over time.
0October 25, 2007 at 6:26 pm #163865
AldermanParticipant@Capt.Kaizen Include @Capt.Kaizen in your post and this person will
be notified via email.Hello, if you redo the calculation it should show that your availability will be only .0047 or 99.53% downtime.
0October 25, 2007 at 6:34 pm #163867I think my point is being missed, if my downtime is over 50% percent of the available time in a given month, this formula stops at 50% avaiability, what am I doing wrong, or what am I missing?
0October 25, 2007 at 6:53 pm #163870
TaylorParticipant@ChadVader Include @ChadVader in your post and this person will
be notified via email.GD
MTBF is mean time between failures so if you have an outage of 43000 mins then the MTBF was 200 mins. Time from one failure to the next failure in this case was 200 mins.
200/200+43000=.004629 Availability
Hope this helps
Chad Vader
0October 25, 2007 at 7:13 pm #163873The confusion is coming about because the total time for the two events to occur, MTTF and MTTR, is longer than the total available time in a month (86200 minutes). Suppose in Month 1 you are down for repairs first, and then run the remaining 200 minutes. Your available time would be approx. 0.5% for that particular month. But since the MTTF is 43200, the next failure won’t occur until 43000 minutes into Month 2, leaving only 200 minutes in Month 2 for downtime (99.5% availability). So the availability over time is 50% which is what the formula is telling you.
0October 25, 2007 at 7:37 pm #163878Pyzdek… no wonder you are confused.
0October 25, 2007 at 7:42 pm #163880Availability % = actual running time / planned cell production time
0October 25, 2007 at 7:48 pm #163882The problem with the logic in these arguments is that the equation is incorrect. MTBF and MTBR are probability calcualtions.
Availability is defined and the percentage of uptime when compared to total time a machine is scheduled for operation. No more or no less should be implied.
For the record when MTBF is quoted there is only a 37% chance of actually meeting the stated MTBF – Do the calcualtion ans you will see it is true)0 
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