2009/11/13 Mike S mi...@flatsurface.com:
I'm sure someone with more statistics background can add to this, but useful
(or expected) lifetime cannot be determined from an MTBF number.
Here's an example I found, demonstrating this:
There are 500,000 25-year-old humans in the sample population.
The failure rate of a human is not constant over the lifetime and just
taking a figure at the age of 25 will get you nowhere.
Steve, I think you'll find that's a total red herring.
That's because if you measure failure rates of almost anything, you will
find that the failure rate varies over
Dave,
The point I should have made is that most quoted MTBF figures have a
reasonable bearing on the lifetime of the item, this example was well
off given the inbuilt expiry dates of humans. In addition, a lot of us
are using equipment that is well past it's use by date and it keeps on
going,
At 05:22 AM 11/18/2009, Steve Rooke wrote...
The point I should have made is that most quoted MTBF figures have a
reasonable bearing on the lifetime of the item,
But your point would then be almost perfectly incorrect. MTBFs are not
meant to, nor do they, predict product lifetimes. They are
Steve Rooke escribió:
The MTBF is the inverse of failure rate or 1 / 0.00125 = 800 years.
The meaning is that if you a representative sample of 25 year-old
humans, and their ages remains at 25 years constantly during 800 years,
you should expect that half of the humans would have failed ater
The big problem with MTBF is that it doesnt really mean ANYTHING if you
invoke the proper statistical properties of the calculation! It is a process
dreamed up out of thin air by Military and other users who felt they needed
an index of quality and at least some life testing on the product they
If it weren't so horribly true I would be ROTFLMAO!
Dave
-Original Message-
From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On
Behalf Of Alan Melia
Sent: 18 November 2009 14:27
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Rubidium
From my experience as a Design Validation Lab manager, MTBF and its
equivalents were never more than a method for comparing the design
revisions. The relationship to reality is quite bogus unless your tests are
an exact replication of the world the item will experience. Cost and
available time,
It used to be, that if you knew the right people at IBM, you could
get a printout of the actually in-field observed MTBF of all their
components.
That database is why they managed to respond to the infamous
legionnaires disease in one of their DASD units, where pretty
much all shipped drives
I have two Vectron Labratories CO-255A17-R 400 MHz Crystal Oscillators that
I would like to use but have no information on. They are 2X3 inches 3/4
high SMA output and have five pins on the bottom, two are ground. Does any
one have information. Thank you
Bert Kehren Miami
For the CO-255 Series:
Holding the oscillator with the SMA connector up and looking at the bottom,
reading left to righ;
1. Supply (+)
2. Case
3. N/C (No connection)
gap
4. 0V, Case
and below:
5. Case.
By the way, the A17 series has a stability rating for =15C t0 +35C of +/- 1
part in
As I have learned in school from a department head, mean time between
failures (MTBF) means anything only if you are being mean. If you are
not being mean, it means nothing.
MS
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At 01:14 PM 11/18/2009, Michael Sokolov wrote...
As I have learned in school from a department head, mean time between
failures (MTBF) means anything only if you are being mean. If you are
not being mean, it means nothing.
There is at least one practical use for MTBF, at least the real-world
Hi Alan,
I am reading a book about the Apollo computer, they bet their life on it
not failing (everything related to spacecraft maneuvering went through the
computer, there were no mechanical or other backups whatsoever). They only
had a single computer per spacecraft!
The book states
In message cb8.5c4f5c26.3835d...@aol.com, saidj...@aol.com writes:
A single transistor, ROM bit, solder-joint, or resistor failure could have
killed them.
Actually there were a perfectly good spare in the lunar lander module
and most single points of failure would not kill them, but merely
At 04:21 PM 11/18/2009, Alan Melia wrote...
Sorry Mike , unless, as someone else said, the figures are derived
from
field failures over at least a good porton of the expected like the
MTBF
tells you absolutely nothing!!
That is exactly what I meant by the real-world statistical form -
data
Aside from the spare in the LM, they had a backup computer called the abort
guidance system developed by TRW. I think it was bolted up under a seat
somewhere.
-Bob
On Wed, Nov 18, 2009 at 4:01 PM, saidj...@aol.com wrote:
Hi Alan,
I am reading a book about the Apollo computer, they bet their
Hi Poul-Henning,
from what I read, they a) had no tools to replace the units or even open
the computer, b) the software was different between the units (LEM had 1/2
the ROM to save weight), and c) the spacecraft attitude thrusters and the
main engines were fully computer controlled (their
Hi Bob,
I read this was done only on Block-1 systems. In Block-2 (the units that
actually carried humans) they had all attitude thrusters, and the main engine
control done by the computer itself. No spares or backup control systems
for the thrusters!
No CM computer, no return to earth. I
Hello Said
What is the title / details of the book that you refer too ?
Thanks
Roy
- Original Message -
From: saidj...@aol.com
To: time-nuts@febo.com
Sent: Wednesday, November 18, 2009 11:01 PM
Subject: Re: [time-nuts] MTBF (was Rubidium standard)
Hi Alan,
I am reading a book about
In message d03.691da87f.3835d...@aol.com, saidj...@aol.com writes:
No CM computer, no return to earth. I think this is also discussed in the
movie Apollo 13.
http://history.nasa.gov/ says otherwise.
Lot of good stuff there.
In particular:
http://history.nasa.gov/computers/Part1.html
Hi Roy,
two of them:
Digital Apollo, Mindell, David A., MIT 2008
and:
Journey to the Moon, History of the Apollo Guidance Computer, Hall, Eldon
C., 1996
The Mindell book is really excellent, reads like a novel, no EE degree
required.
Mindell discusses the trade-offs they made such
Hi Poul,
that AGS system existed only on the LEM, and only to abort a lunar
landing. It couldn't even dock the two craft.
Mindell discusses in great detail how they went to a single-computer
digital Autopilot on pages 138 to 143, because they considered a failure so
remote as to being
Yeah, but in the one-off spaceflight world, MTBF calculations don't get used
much, except perhaps to compare designs. (e.g. A design with an MTBF of
200khrs is probably better than one with 2000 hrs)
The problem is that it's a statistical sort of life measure: out of 1000
units with an MTBF of
Hi All,
If you have a parts HP 5335A I am looking for option 010 (high stability osc)
option 030 (C channel input)
Please let me know if you might have any of the parts I am looking for.
contact me off list: tamara.wisd...@_wiztech.biz (remove _'s)
Thanks
--Tammy
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