[time-nuts] Re: MHM-A1 maser temperature stabilization

[Poul-Henning Kamp]

--------
Bob Camp writes:

> As I do this in my usual hand waving fashion, I come up with hundreds of liters of water for
> the thermal mass. It just goes up if I move from 1C and get closer to 0.1C. 

Liquids and gasses are trouble in this context, because you easily
end up with stationary or oscillating circulations which, if nothing
else, really ruins the predictability.

So let us stick to solids for the moment:

A thermal resistance is an insulating material with low thermal
mass: Sheep's wool, mineral wool, foam-boards, extruded foam and
ultimately aerogel.

A thermal capacitance, is a material with high thermal mass and
high thermal conductivity:  Ideally silver and copper, but in general
any metal.

We know of no materials which act as pure thermal inductances:  It
would be a material which conducts heat well, but resists change
to the heat-flow.  Certain crystaline semiconductors behave a little
bit like a thermal inductance under certain circumstances, but it
is not useful in practice.

So we are more or less limited to RC filters.

We can make a "lumped" RC with foamboard insulation and
a lot of metal inside.

This is what metrology-labs do for their resistance standards:

Typically a slap of aluminium roughly 1'x2'x2' with holes for the
individual resistors (+oil) insulated with 4" of foam/mineral wool
or similar.

But that concept, as your own calculation also showed you, scales
up badly:  A big box of foam-board and lots of metal (or water),
is both expensive and unpractical in so many ways.

Fortunately almost all geology is a distributed RC thermal filter:
Limited heat conductivity combined with some thermal mass.

A "box" built from 2" aerated concrete, cinderblocks or pretty much
any geology you might have at hand, is cheaper, much more practical,
and almost certain to give better results.

I mention 2" aerated concrete specifically, because if you cut the
slabs to size and paint them to bind the dust, they are very
handy building blocks:  You can stack them around your equipment
when you want to, and remove again when you need to access it.

-- 
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