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

[Neil Smith]

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

> On 17 Jan 2023, at 18:05, Bob Camp via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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