[time-nuts] Temp/Humidity control systems?

Attila Kinali attila at kinali.ch
Wed Oct 26 15:05:17 EDT 2016

On Wed, 26 Oct 2016 13:58:10 -0400
John Ackermann N8UR <jra at febo.com> wrote:

> This room would be a large closet in my basement where two racks of 
> various OCXO, Rb, Cs live. There wouldn't be a lot of in-and-out 
> traffic. I'm not looking for 0.01 degree regulation -- <1 degree C and a 
> few percent humidity throughout the year seems a reasonable goal.
> What I envisioned was a very small heat pump or other heating/air-con 
> unit coupled with some sort of proportional control. I just don't know 
> where to start looking for that, or what other issues to be thinking about.

Probably the easiest is to get some glass/mineral wool insulation and
put it over all the walls, including ceiling and floor. I do not recommend
any foam or styropor based insulation as almost all of them are inflamable.
This should get you into the area of 10-100W/K thermal resistance for your
closet (assuming something like 4cm thick insulation gets about 40W/K).
Yous should put some vapor barrier on the inside of the insulation, as
otherwise you will get mold between the insulation and wall.

Assuming you have gear that generates 1-2kW of heat, that means you are
in the region of 100K to 10K above outside temperature. Now all you have
to do is getting the heat out. This can be easily done with an PWM fan
controller which you get for a couple of bucks on ebay/aliexpress. 

In case you don't get enough heat inside, just add some heating element
that you run with constant power (eg a hair dryer run from a constant
current source).

I wouldn't go for A/C systems, as these need quite a bit of hysteresis
to work properly (2-3°C) and the switch on/switch off spikes will cause
all kind of headaches.

For added stability, fill the empty space in the closet with half-liter
PET water bottles and place a small fan blowing at them.

As for humidity control.. Guessing from where I think you live, you will have
a wide range of ambient humidity, which means you need to be able to both
control it up and down. The easiest solution there is to have an humidifier
constantly running (please use one that evaporates instead of sprays,
the latter is a good way to spread bacteria and mold everywhere. ultrasonic
devices are spraying) and use a controllable dehumidifier (they are available
for 100€) take the excess humidity out again. 

As I mentioned, bacteria and mold are a problem with those. Either you
need to make sure that the water used in the humidifier is always fresh
and the water extracted from the dehumidifier gets discarded somewhere,
or you need to sterilize the water if you let it sit or, even worse,
circulate. Probably the easiest way to sterilize is UV light, but it
degrades all kind of materials, especially plastic.

All of this should be doable with 200-500€ and whithin a long weekend.


			Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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