[time-nuts] Cheap Rubidium

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Dec 23 22:46:23 UTC 2009


The stability of the temperature distribution within the baseplate can 
be improved with respect to changes in the speed of the cooling fluid 
(gas, or liquid) by connecting the heat exchanger (finned heatsink or 
equivalent) to the rubidium base using alternating layers of good 
thermal conductor and poor thermal conductor).
As the number of layers increases the inner high thermal conductivity 
layers will tend to become isothermal even if the heat exchanger or the 
base of the rubidium source is not.
This stabilises the temperature gradients seen at the base of the 
rubidium source.

Bob Camp wrote:
> Hi
>
> The idea of locating the fan far removed from the heat sink and then using some kind of a duct might work pretty well.
>
> Bob
>
>
> On Dec 23, 2009, at 3:31 PM, 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.
>>
>> <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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>>      
>
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