[time-nuts] cesium clocks..

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jun 27 22:26:25 EDT 2008

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
> ----------

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.


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