[time-nuts] Re: Envornmental control

Brooke Clarke brooke at pacific.net
Wed Mar 9 18:06:38 EST 2005

Hi Poul:

I'm doing a similar thing only on a much smaller scale.  I have a PIC 
whose clock comes from a 1/2 size 20 MHz unit oscillator.  An interrupt 
routine in the PIC divides this down to a 1 PPS output.  The PIC is 
inside a standard two gang (about 4"x4") metal electrical box that has a 
solid metal cover (about $ 1.00 for the box and cover).  The electrical 
box is sandwiched between a couple of aluminum plates that have flexible 
heaters on them and a thermistor temp sensor.  The whole thing is inside 
what's called a "lunch box" that's cylinder about 5" diameter and 6" 
high.  The "lunch box" is really a stainless steel thermos bottle with 
an opening about 4.4" in diameter (much larger opening than most thermos 
bottles).  This provides a nice thermal environment that allows the 
aging of the unit oscillator to easily be seen independent of room 
temperature.  I need the heater to get the temperature at the turning 
point of the unit oscillator to further reduce the effects of room temp.

The cal lab at a prior employer's had the AC set for the same 
temperature.  The system always had the chillier operating, that way the 
outside air had the moisture rung out, and then it added back heat to 
control the temp.  So Tom's method is in line with what they were doing.

The HP 117 is sensitive to drafts in the equipment rack.  By placing a 
piece of paper outside the 117 near the 100 kHz crystal the sensitivity 
was noticeably reduced.  In a similar way equipment that's rack mounted 
with other powered equipment directly above and below is in a thermal 
environment not too far from the non operating refrigerator.

Have Fun,


Poul-Henning Kamp wrote:

>In message <006901c524f6$6d6c5c60$4715f204 at computer>, "Tom Van Baak" writes:
>>I'm cc'ing the list since I'd be interested in other
>>stories about temperature control.
>I visited the Danish Fundamental Metrology lab some time ago, and
>had a chat with them about a lot of stuff, including temperature
>control (they need it for calibration of standards resistors
>and all sorts of other stuff).
>It is important to realize that it not enough to control the
>temperature of the air, you also need to control the flow and finally
>the humidity is very important because it seriously affects the
>airs ability to absorb heat.
>They also gave me the following cheap hint about environmental
>control: Find and old fridge and use it as an insulated chamber.
>(The compressor doesn't have to work: it is not used).
>Provided the equipment you want to have constant temperature does
>not produce too much heat (ie: OCXOs etc), putting it 1/3 from
>the bottom of an old fridge with a slow moving fan 1/3 from the
>top to keep the air moving (slowly!) and putting the fridge
>in a room with reasonable thermal stability, you should be able
>to get milli-kelvin temperature control.
>For equipment which generates more heat it is harder because
>you will need active temperature control, and the task becomes
>close to impossible as the heating goes up.
>For moderate heat output (a Cs ?)  the fridge method can still
>be used but with a twist:  Take the compressor out and fill the
>circuit with glycerol (water would make it corrode) put a small
>pump on it (aquarium stuff) and use fans/peltier elements to control
>the temperature at the inlet to the fridge.  (I'm wondering if not
>some of all the "overclocker watercooling" hardware out there would
>be usable).
>Some experimentation is necessary to find which fluid temperature
>gives the desired inside temperture.  Again, remember to have a
>slow moving fan on the inside to keep the air moving in a steady
>If the power dissipation of the equipment is (close to) constant,
>like for a HP5061, then nothing should be needed.  If the power
>dissipation varies, it may be necessary with a temperature sensor
>and active feedback.
>I havn't gotten around to try this myself, but I would probably put
>a couple of Dallas 1-wire temperature sensors and maybe a webcam
>in there as well, so I wouldn't have to open the door to check the
>green light...

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