[time-nuts] Cheap Rubidium

Joe Gwinn joegwinn at comcast.net
Wed Dec 23 20:31:47 UTC 2009


>Date: Thu, 24 Dec 2009 08:57:42 +1300
>From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>Subject: Re: [time-nuts] Cheap Rubidium
>To: Discussion of precise time and frequency measurement
>	<time-nuts at febo.com>
>
>Bob Camp wrote:
>>  Hi
>>
>>  So if I want to set up 4 uncorrelated systems, that would require 20 tons of
>>  water split into 4 tubs. Each tub would be roughly 3' x 4' x 15'. Of course
>  > if they are all in the same basement, I still have a correlation 
>problem. My
>>  guess is that no matter what I do, any system that controls all the systems
>>  the same way will run into correlation.
>>
>>  Oils, silicon fluids, and the like mostly hold less heat than water so the
>>  tubs would get bigger. Maybe a few tons of mercury...
>>   
>Try about 145 tons of mercury per rubidium source as the specific heat
>of mercury is about 1/29 that of water.
>The redeeeming feature is that it will only occupy about 2.14x the volume.
>The specific of some oils may be as large as 1/2 that of water however
>the density is around 10-20% lower.
>
>>  Active heat control and a rational heat sink is sounding like a better
>>  approach...
>>
>>   
>Distributed heating using wire wound or printed heaters perhaps, but to
>reduce the associated magnetic field bifilar winding should be considered.

Non-inductive power resistors, which are commercially available, have 
very low magnetic fields.

The low-inductance resistors have Ayrton-Perry windings, which are bifilar.

<http://www.token.com.tw/resistor-pd/power-resistor-ah.htm>


>The major limitation is that the 25W or so dissipated by the rubidium
>source has to be transferred to ambient without raising the rubidium
>temperature too much.
>This limits the maximum thermal resistance between the baseplate and
>ambient that can be safely used.

I would be tempted to regulate temperature by actively controlling 
the speed of the fan (or pump) driving air (or oil) through the heat 
sink, as has been suggested.

Joe Gwinn




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