[time-nuts] cesium clocks..
bruce.griffiths at xtra.co.nz
Fri Jun 27 23:14:36 EDT 2008
Neville Michie wrote:
> Hi Bruce,
> 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
> thermocouple problems.
> cheers, Neville Michie
Yes, not a great problem if used in matching pairs.
The room temperature junctions will be most problematic as their
temperature swing is much wider than those at the oven temperature
(provided that the oven controller is working properly)
Stainless is trickier to solder than constantan.
Welding may be preferable.
Using an AC bridge (preferably a transformer bridge) or equivalently a
dc bridge using periodic bridge supply reversal eliminates the effect of
slowly varying thermojunction voltages.
Some of the sigma delta instrumentation ADC's allow this, however one
has to be careful to ensure that the sensor self heating doesn't vary
too much when reversing the bridge excitation polarity.
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