[time-nuts] cesium clocks..

[Neville Michie]

Hi Bruce,
You would have to be careful with your constantin wire as there is a  
thermocouple
junction of 40 microvolts/K at each copper/constantin connection.
If the pairs of junctions are kept together thermally all cancels out.
Stainless steel is also very non-conductive of heat, but would have  
similar
thermocouple problems.
cheers, Neville Michie

On 28/06/2008, at 12:26 PM, Bruce Griffiths wrote:

> wje wrote:
>>    In this case, the temp thermistor  bridge is outside the oven  
>> cavity
>>    itself. The cable only passes power and the already-processed  
>> bridge
>>    delta to the heater power amp. So, there's no particular  
>> benefit from
>>    having the cable stuck to the heater wrap. (at least, I think  
>> so; my
>>    basic failure was because the cable fried and shorted power to  
>> ground)
>> Bill Ezell
>> ----------
>>
> Bill
>
> If the temperature bridge is outside the oven cavity then its critical
> that the temperature sensor leads are thermally shunted to the oven.
> Substituting constantan wire for copper wire also helps as the thermal
> conductivity of constantant is significantly lower than that of  
> copper.
>
> For example if the temperature sensor has a thermal resistance of 1K/W
> to the oven and the leads have a thermal resistance of 100K/W to  
> ambient
> then ambient temperature fluctuations of 10 K will induce temperature
> sensor temperature variations of 0.1K which the oven controller will
> correct by varying the oven temperature by 0.1K.
> The lead themal resistance would have to be > 1E4K/W to maintain oven
> temperature fluctuations below 1mK when the ambient temperature varies
> by 10K.
> Such a high thermal resistance is difficult to achieve.
>
> The thermal resistance of a length of wire can be estimated by  
> measuring
> its electrical resistance and dividing it by the product of the  
> thermal
> conductivity and electrical resistivity.
>
> For copper wire thermal resistance ~ 1.6E5  x Electrical resistance
>
> For 10 cm of 20swg Cu wire the electrical resistance (at 20C) is about
> 4.4 milliohms and the corresponding thermal resistance is about 700  
> W/K.
>
> Bruce
>
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