Skip Withrow via time-nuts writes:

Apologies for harping about this again:

Please think in terms of thermal impedance!

Look at it as electronics for a second:

You have put a (very big!) resistor between your AC noise source
(the heating) and the sensitive kit (the maser).

Because your resistor is so large, you now have an over-temperature
problem inside your enclosure, which you have tried to mitigate with
a fan.

What you actually need is a low-pass filter with a cut-off lower than 1/24h.

That means /some/ insulation, but not so much that your maser cannot
get rid of the heat it produces, and /a lot/ of thermal mass on the
inside to "short the AC to ground".

This is not magic, and the math is trivial when you already know electronics.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

Hi

I may well be missing something obvious in the electronics to heat conversion process
( it certainly would not be the first time … :). That said:

I set up a box with foam and the 100W from the device inside heats it up by about
4 C. You immediately get into questions about “4C where?”.  Just accept the number and
move on for now.

The room wanders a bit. How much depends on a lot of things. 2C is not a bad guess on
most days. At the wrong time of year it could be 2X that. There is a day to night component
that usually dominates.

Target is to damp out the 24 hour swing. The R/C should be longer than that. How much
will depend a bit on the temp stability target. 1C/24 hrs is an improvement. The folks who
made the gizmo suggest that 0.1C is a better target. Yes, the “C” in the R/C just moved 10:1
as you changed that target. (There are other issues as well, for now, let’s ignore them)

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.

This assumes that everything else is zero mass. Things do “follow” the room temp with an
lag of a couple hours. Even to get to 1C, significant thermal mass needs to be added.  ( or
lots more insulation, stick with mass for now ).

Am I missing something or is the “hundreds of liters” guess more or less in the right range?
There are some practical implications to playing with groups of ten  “jerry cans” full of water.
(even as a “try it and see" experiment).

No, this isn’t an attempt to come up with a full up answer. It’s just a question about what
the rough order of magnitude is in this approximate case. There are enough holes in the
data above that any sort of precise answer is data limited.

Bob

Bob Camp writes:

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@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.

One thing I've tried to reduce diurnal and seasonal thermal variation is putting the sensitive components in a hyper-insulated enclosure with high thermal mass mounted inside another stabilised oven, also with plenty of thermal mass, but then connecting a well insulated copper bar from the inner enclosure to a stabilised Peltier cooling plate mounted outside. That gives me a very stable thermal flow with a large temperature gradient so the inside oven thermal control loop can stay in a nice linear regime and doesn't have to deal with large changes in required heating power.
It's overkill for my application, which is a only a 10MHz Morion MV-89a OCXO.

I also have a 120 litre water tank with a circulating pump that can keep a cooling plate at a very stable temperature over periods of an hour or so, but its main use-case is for cooling high powered RF amplifiers.

Neil
https://youtube.com/MachiningandMicrowaves

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

Hi

I’ve looked at Peltier gizmos. If you are pumping 100W of heat they seem to use
quite a bit of energy to get the job done. A compressor based setup of some sort
might do a better job. With any sort of cooler, my fear is a loss of cooling. If the
device is very well insulated, damage could occur.

The 120 L bucket of water is about half the size of my “rough guess” thermal mass.
I fear that the guess may be way low ….

Bob

Bob Camp writes:

As I said: Forget the water and look into 2" aerated concrete:  It's
cheap, they probably have it at the local home depot, and it won't
throw your back out or leak.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@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.

Hi

Ok, but do I need  > 800 L of the stuff ?

I assume it’s called Autoclaved Aerated Concrete over here. If so, it has about half the thermal
capacity per L of “normal” concrete. That puts it in the range of 700 to 800L being the same as
my 250L water guess.

Yes, having something that is self supporting and not going to leak is a plus. I’m not debating
that point in any way. I’m still trying to scope the project.

So what would 800L look like?

Because I’m still lazy, say this is a 3/4 cubic meter right now without any added mass.. I could
go measure it all, but that’s pretty close. Roughly speaking 700 to 800L of added “stuff” doubles
the volume of the enclosure. That would take some fairly detailed planning. Thus the need for a
bit of checking on the order of magnitude ……

Also, just for the record, 250L of water isn’t a very attractive answer either. Oopss … crash ….
splash … sparks … sparks … smoke ….

Bob

Hi Bob, luckily I'm only pulling about 6 watts of heat from my OCXO. The
Peltier takes about 25 watts once it is stable.  It would be impractical
for anything needing to lose more heat.

If I lose the cooling, the system still works, but the Morion heater
backs off and starts to cycle a little more.  There's just enough
passive cooling via the copper bar to make sure the OCXO heater doesn't
turn right off, at least so long as the room temperature is below 30C. I
keep the Peltier cold-side temperature above the worst-case dew point to
avoid condensation.

I think I could swap the Peltier for a new one without turning the
system off, but I don't need long-term stability over weeks, just over a
few hours. It has two gel cells which are float-charged, so I'm pretty
much immune to mains power variation or outages, but this ain't a maser.

Amazingly good for an old quartz lump, so long as the vibration
isolation works properly. It gets weird if the gravity vector doesn't
point the right way or if a heavy tractor drives past the house and
rattles the equipment racks.

At that point I get plaster falling out of the cracks in the ceiling. 
Clever new-fangled things like foundations hadn't been invented back
when this house was built, so I have to compromise a bit.

--
Neil
https://youtube.com/MachiningandMicrowaves

On 17/01/2023 20:09, Bob Camp wrote: