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

[Bob Camp]

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

> On Jan 17, 2023, at 4:07 AM, Poul-Henning Kamp via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> --------
> Skip Withrow via time-nuts writes:
> 
>> One issue that I have been up against is the temperature variations of the
>> lab location.  The diurnal excursions of the oven heaters were clearly
>> visible.  My solution was to build an environmental box (1.5" foam
>> insulation board) around the unit 
> 
> 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 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.
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