[time-nuts] 10811 foam enclosure

Chris Albertson albertson.chris at gmail.com
Sun Jan 9 14:58:27 EST 2011


When I was working with a group that was buillding astronomical
cameras using CCD chips we wanted to keep the CCD chip at a very
constant temperature that was much lower than ambient.  This is
actually a harder problem then building a crystal oven but still not
out of amateur's range.    We ued a Peltier device, aka
"thermoelectric".  These act like two why heat pumps and can either
cool or heat the CCD depending on what way the current flows.    The
Pelier, had a large heat sink on one side and a block of aluminum on
the other.  The CCD was cemented to the large aluminim block.  Also
holles were drilled and thermisters embedded in the block.   A
computer read the thermister temperatue and controlled current tothe
Pelier via a 16-bit DAC.   The entrire assemble was inside a thick
wall aluminum can and we pumpped the can down to a rough vacuum
because vacuum is a good insulator.  Others might fill the can with
argon a few times as it is a better insulator than air.  Sounds
complex but not really, only about a half dozen parts., unless you
count the heatsink.  We used water cooling.  My system uased a 60
galon aquarium tank as a stable resivor for the water, others used
large trash cans.    OK long story but we could keep the CCD inside a
0.01C range.     The trick is (1) to find some one who understands
servo control loops and (2) use a large enough aluminum block and keep
is insulated from the environment.  Vacuum insulation works well

On Sun, Jan 9, 2011 at 6:07 AM,  <EWKehren at aol.com> wrote:
>
> Being a Shera Fan I finally broke down and bought a Tbolt. I experimented
> with a foam enclosure with roughly 1 centimeter on all sides. What I found
> that  it only increased the board temperature by 19 C. It was still as
> sensitive to  ambient. Thanks to Lady Heater it even shows an increase of 40 mC
> when I walk up  to it with one minute delay.. Changing to an aluminum
> extrusion the increase is  only 7 C and the response is much slower. In its final
> assembly it will be  inside a larger metal mass, I do not think that the
> higher temperature of  56 C is conducive to longer life. Opinion: putting foam
> around a 10811 will only  give you a warm feeling unless you make it much
> larger than shown in the picture  of of KH6GRT.
> Based on the above test results I feel mass is more important. So I took a
> aluminum enclosed dewar weighing 943 gr. and did first put a resistor in it
> and  heated it up with 2 W. Temp. did rise to 70 C. Next I disassembled my
> worse  10811, which is very easy, since it is a nice compact unit and
> inserted it in  the dewar. Monitoring temp with my YSI it shows 65.2 C with 24 C
> ambient. Power  dissipation is 1.575 W. Will track it over time before I
> replace the 10811 with  one of my better ones for frequency tests.
> Questions to our experts:
> A) will Removing the foam  mess with the temp. control loop
> B) How low can total power into the 10811 be before temp control can become
>  a problem, I think I am ok now,  but once in a chassis with Shera and
> power  and battery the temp. surrounding the dewar may become a problem.
> C) What is the preferred orientation of the unit.
>
> I asked already once before if any one has data on the 1 1 PPS output of
> the Tbolt over time, since I am considering it as an alternative to a stand
> alone GPS receiver, and if there are GPS receivers that outperform a Tbolt
> what  is the recommendation. It will be used to control a Rb.
> Bert Kehren
>
>
> In a message dated 1/7/2011 8:22:51 P.M. Eastern Standard Time,
> tvb at LeapSecond.com writes:
>
> Has  anyone experimented with the amount of insulation on a
> 10544 or 10811  oscillator? They are meant to run hot by
> design and I worry that adding any  insulation, or too much
> insulation, will either cause over-heating or limit  the ability
> of the oven control to maintain the set point.
>
> None of  the hp/Agilent bench test equipment that uses these
> oscillators uses  insulation. Perhaps that's a  clue.
>
> /tvb
>
>
>
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-- 
=====
Chris Albertson
Redondo Beach, California




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