[time-nuts] Capacitive temperature sensing

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Aug 23 06:45:57 EDT 2008

Magnus Danielson wrote:
> Hej Bruce,
>> Hej Magnus
>> A capacitive sensing AC bridge can be very sensitive, one only has to 
>> look at the work of RV Jones at the university of Glasgow in the 50's, 
>> 60's and 70's. He and his collaborators used capacitive sensors to 
>> detect (among other things) the extrusion of a micrometer shaft as it 
>> was clamped to realise just how sensitive it can be.
>> They found it possible to detect length changes of less than 1E-11m with 
>> a suitably designed sensor.
> Indeed. However, I was trying to hint on a alternative approach or 
> temporary approach which would not need much design to achieve equal 
> performance if the electrodes could not be cleaned or replaced easilly.
> I trust the capacitive measurement would be sufficiently good even at 
> moderate design efforts.
> Cheers,
> Magnus
Hej Magnus

The biggest problem is that a stainless steel thermoregulator and not a 
mercury in glass one may have been used.
Retrofitting temporary capacitive sensing would be difficult to 
impossible in this case.
Metals other than platinum or iron are precluded from use as they tend 
to form amalgams with the mercury.

However one could probably temporarily substitute a mercury in glass 
thermometer with capacitive position sensing.
The only potential problem being that there are moves afoot to ban 
mercury thermometers.

A capacitance of around 1pF or so may be expected for a 1cm long 
cylindical  capacitive sensor electrode and an instability in the 
capacitance measurement of 100ppm or so would be required to achieve 
millidegree stability (provided the mechanical instability of the glass 
allows this).


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