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

[Bob Camp]

Hi

The 4C rise with the box in place is a measure using a real box. Simply put, the stuff inside
runs at about 4C higher with the box in place relative to the room temperature than it does 
with the box missing. That’s after letting things sit for many hours …. (and checking after 
a few days). Given all the variables, yes, it could be 5 or 6C. It’s certainly not anything less 
than 4C. The room cycles so picking a target is a bit wonky.

The box deliberately has a somewhat open top, to keep some level of convection cooling
what’s inside. Yes, that’s another un-mentioned detail and it certainly has an impact. Lots
of details like that would get involved in a precise answer to the question. One could get 
all sorts of cool modeling software involved. 

Right now, all I’m really looking for is an order of magnitude sort of answer. Put it in terms
of whatever your favorite material is:

How many liters or kg of this or that will it take to do the job of knocking the swing down to 
under 1C over 24 hours? 

That’s with the room swinging 2 to 4C over the same period. 

The limit on insulation is pretty obvious: Put to much on and the device(s) inside the box
overheat and may be damaged. Some significant margin between max ambient on the
device(s) and box temp needs to be maintained. 

The limitation on the thermal mass really is however much room you happen to have.
Heading down the mass route without a rough idea of what’s going to be needed does
not sound like a lot of fun. In my case a redesign of several things comes in each time 
you adjust dimensions. 

Obvious alternatives get you right back to some sort of servo setup running this or that 
as a control element. (unless there is something else ….). 

Most of the “big guys” seem to have found a convenient cave (or built one). Then then
get to publish papers on the shortcomings of thermal control in their custom built 
cavern ….. All of the things they complain about apply here. The power load in the
area is variable. People (and dogs) wander through on a random basis. There’s a door
to an unheated space here and another one over there. They get opened from time 
to time …. (none of that goes into the 4C number, but probably should ….. )

Bob

> On Jan 17, 2023, at 3:37 PM, ed breya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi Bob,
> 
> I think what may be missing is the unknown thermal resistance of the outermost insulation surface to the ambient air. This can be very messy, but could be estimated experimentally on an actual thing, including the conduction, radiation, and convection effects. If there's a draft in the room, then it's a different deal.
> 
> Deleting the foam, say, takes out its 0.04 deg C/W of resistance, but doesn't short it to ambient temperature. If it were in a water bath or heavy metal outer box held at Ta, it would be a different story, but here the air interface is the big part.
> 
> Ed
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