[time-nuts] Cheap Rubidium

[Bruce Griffiths]

Joe Gwinn wrote:
>> 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.
No, Ayrton-Perry windings arent bifilar.
Classically a flattened helical winding was made on a insulating card.
An identical winding was then wound in the opposite direction on top of 
the first winding and the 2 were connected in parallel.
The idea being that the small magnetic field produced by one flattened 
helix is cancelled by that of the other flattened helix.
>
> <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
Bruce