[time-nuts] Practical considerations making a lab standard with an LTE lite
Charles Steinmetz
csteinmetz at yandex.com
Sun Nov 23 09:46:22 EST 2014
Poul-Henning wrote:
>Charles' design works great from the outside, but doesn't do anything
>with respect to the thermal energy expended by the encapsulated
>device themselves, which will cause convection in the inner box.
I have been using the technique for 30+ years, including with many
OCXOs (which, obviously, generate significant heat) and have never
observed any problems of that nature at the 1e-13 level. I did
consider the possibility when I first started doing it, and tested
two potential fixes: (i) putting a fan inside the box to homogenize
the internal temperature, and (ii) filling the air space inside the
box with irregular solid shapes to break up the convection
pattern. I tested both methods extensively with instrumented
sources, in many variations (fan speeds and orientations, mass and
porosity of passive internal shapes), and did not find any difference
at the 1e-13 level. I have occasionally used an internal fan just on
theoretical grounds, but I have never measured any practical difference.
Thinking about it, this does not seem too surprising -- one would
expect any convection to settle into a stable pattern and thus not to
cause any temperature changes over time (once it is warm and
settled). Whether this explains my results or some other effect
predominates (for example, convection may move enough air in the
limited space to achieve substantial isothermy), I have confirmed to
my satisfaction that it is simply not a factor in practice at the
levels we are concerned with.
If you test the "cast aluminum box" method and find that your results
do not accord with mine, please publish them and we can discuss what
might account for the observed differences and how the method could
be improved. Until then, you are just posting speculative musings on
the subject based on no data, which does not seem helpful.
And good luck fitting a cubic foot box with a surround of bricks into
a 3U rack cabinet, or any other relocatable (much less,
semi-portable) enclosure.
Best regards,
Charles
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