[time-nuts] cesium clocks..
namichie at gmail.com
Fri Jun 27 22:37:19 EDT 2008
You would have to be careful with your constantin wire as there is a
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
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
>> itself. The cable only passes power and the already-processed
>> 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
>> Bill Ezell
> 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
> 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
> 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
> its electrical resistance and dividing it by the product of the
> 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
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