Hi

The “typical life” numbers on the tubes in the various Cesium standards
are fairly accurate. Most units that are well cared for “die” when the tube
goes out and come back to life when it’s replaced. The tube life dominates
the MTBF in this case.

Rb’s are a device that by it’s (possibly unfortunate) physics needs to run
hot. The vast majority in service are “miniature” in comparison to a 5071 or
a 5065. That drives the temperatures of all the parts up. MTBF on them
is very “temperature of use” dependent. They fail for a range of reasons
as the parts in them die of “hot old age”.

Efratom had some pretty good data on MTBF vs temperature in the LPRO
data sheets. The internals of all the designs are similar enough that it likely
applies over more than  just one design. Temex has a similar data snapshot.
I have a sample of the Temex units in front of me as I type this. They died
as one would expect - from something other than the bulb. Capacitors, IC’s,
crystal drift, board corrosion, being crusted (physical damage), each pop up
in the sample.

Bob

I do not have it but I stumbled into it on the internet. There was one
paper it was military, naval observatory or NIST and it did indeed show
failure rates of cesiums of the reference that were owned and it must have
been 30-50 of them.
I remember it showed failures of units over years.
Since it did not at all addres my need I did not keep it.
Regards
Paul
WB8TSL

On Sun, Mar 27, 2016 at 7:53 AM, Attila Kinali attila@kinali.ch wrote:

Hi Attila, I am out of the business now, well retired, so my opinion carries
little weight,
:-)) but for whatever it does, my thought is that MTBF is a pretty useless
parameter in general. This is a relatively low volume unit manufactured by a
variety of different firms with each their opinion on the best optimum.

The statistical base to MTBF is faulty and in my opinion its only use is to
indicate where a design might be improved by changing the component mix. The
actual value that falls out of the end of the calculation for a desgn is
completely meaningless, but the non-tech bean-counters wanted a way to
justify more expensive designs, and the purchase of expensive kit.

I would doubt that anyone collected the data on completed units, though
there may have been spec values quoted. I guess in this usage area most used
expected them to fail and guarded against it by duplication, the exception
may be the space environment but I have no experience there.

Good Luck with it
Alan
G3NYK

----- Original Message -----
From: "Attila Kinali" attila@kinali.ch
To: time-nuts@febo.com
Sent: Sunday, March 27, 2016 12:53 PM
Subject: [time-nuts] Reliability of atomic clocks

My Efratom M-100 has been running for about 15 years 24/7.
I have no idea if that is typical.
It was purchased as NOS for $300.
Rob
NC0B

Sent from my iPad

Dear Paul,

You are probably thinking of one of these:

Chadsey et al “Maintenance of HP5071A frequency standards at
USNO” in Proc. 29th PTTI, p49-60 (1997)

Chadsey “An automated alarm program for HP5071A frequency
standards” in Proc. 31st PTTI, p649-655 (1999)

Brock et al “End-of-life indicators for NIMA’s high-peformance
cesium frequency standards” in Proc. 34th PTTI, p117-125 (2002)

Cheers
Michael

On Mon, Mar 28, 2016 at 1:50 AM, paul swed paulswedb@gmail.com wrote:

On Sun, 27 Mar 2016 10:13:32 -0400
Bob Camp kb8tq@n1k.org wrote:

Lifetime is only one variable I am interested in. I suspect that most
of the standards have a much longer lifetime than their MTBF between
intermittent faults. E.g. Chirstopher Ekstrom reported at FSM8 that their
newly build Rb fountains show a frequency jump once in a while. After a
restart of the fountain it's back to normal.

		Attila Kinali

--
Reading can seriously damage your ignorance.
-- unknown

On Sun, 27 Mar 2016 10:50:13 -0400
paul swed paulswedb@gmail.com wrote:

Do you remember anything more about the paper? I couldn't find anything.

The only remark i found, is a small sentence in an introduction
of an paper[1]:

The USNO has 60 high performance cesium frequency standards (clocks).
These clocks are combined to form a timescale or mean that serves as the
long-term frequency reference for USNO. At any given time, approximately 75%
of these clocks are weighted in that mean, the remainder unweighted due
to hardware failures or behavior deemed uncharacteristic for that clock.

But no details...

[1] "Analysis Of Clock Modeling Techniques For Usno Cesium Mean",
by Skinner, Koppang, 2007
http://www.dtic.mil/get-tr-doc/pdf?AD=ADA474175

--
Reading can seriously damage your ignorance.
-- unknown

Salut Alan,

On Sun, 27 Mar 2016 15:52:00 +0100
"Alan Melia" alan.melia@btinternet.com wrote:

Yes, the MTBF is a very simplicistic measure and there are a couple
of assumptions in its calculation which do not hold generally (or
rather, it's rather seldom that they hold). Yet it gives a number to
something that is otherwise relatively hard to measure and the number,
even though flawed, makes it possible to compare different devices
on their reliability. As this is more a rule of thumb comparison,
you shouldn't read too much into a 10% difference. Yet a 100% difference
is significant, no matter which of the assumptions do not hold.

I my case here, I use the MTBF as a stand in for a more general reliability
probability density function, a term which might confuse more than clarify
in the question asked.

			Attila Kinali

		Attila Kinali

--
Reading can seriously damage your ignorance.
-- unknown