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

[Bob Camp]

Hi

Yes, a big swimming pool of mercury or water is going to do a lot of things that
make it really fun to analyze. Folks seem to run circulation pumps to reduce the 
impact. Not on my list right now ….

Water is attractive because it holds a lot of heat per unit mass. One KG of concrete 
soaks up 880 J/(Kg-K). One L of water comes in at  4,181. More fun numbers at:

https://www.omnicalculator.com/physics/specific-heat
Specific Heat Calculator
omnicalculator.com


Solid concrete weighs about 3X what water does by volume, but water still wins 
the race. If the concrete has air spaces in it, it falls further behind on a volume basis. 

In terms of a practical answer for a fluid, put it in small(er) containers (or baffle it).
Then you don’t have it doing weird flow stuff. Yes, another rabbit hole to wander down.
Right now a bunch of 2 L glass jars are sitting over there on the shelf …..

Coming back to the basic question: 

	Concrete *or* water, how much do I need?

Not looking for anything past a rough order of magnitude to see if my magic
math number of  > 250 liters of water is anywhere close. ( In concrete numbers, 
is > 400 liters close? )

Just trying to get an idea of the scope of the project. If the answer is 25 and not 250, 
it’s trivial to do. If it’s 250 … gulp … If its 2,500 … yikes ….. As you might guess from
the reference to the glass jars, empirical data suggests that 25L is not the answer. 

At some point adding this or that makes the enclosure bigger and the insulation needs
to get thicker to stay at the same net loss value. There’s only so much room over there :) 

Bob

> On Jan 17, 2023, at 2:03 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> 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.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.