[time-nuts] Capacitive temperature sensing

[Bruce Griffiths]

Mike Monett wrote:
>   Hi Bruce,
>
>   Thanks very much for the url. That is a very interesting  article on
>   Jones. Nice  to have an award named after you, and to  be  the first
>   one to receive it:)
>
>   The abstract for his article states:
>
>   "A general  account  is  given   of   both  the  electrical  and the
>   mechanical aspects of the design of capacitive transducers and their
>   associated electronic circuitry suitable for observing displacements
>   of the  order  10-2 to 10-11 mm. The lower figure  is  the  order of
>   magnitude of noise and drift averaged over a second, the  drift over
>   a day of the order 10-8 to 10-9 mm. Their application is illustrated
>   by descriptions  of   an   apparatus   to   explore   the  limits of
>   performance, a moderately sensitive micrometer, and  two geophysical
>   instruments, a tiltmeter and a gravimeter. Full details of a general
>   purpose electronic system are given."
>
>   That article,  and "Microdisplacement Transducers" by  Sydenham both
>   cost $80.00 USD. That's a bit high. It's a shame they have to charge
>   so much,  when  the internet has reduced  the  distribution  cost to
>   nearly zero.
>
>   It's not  very useful to claim a resolution of 10-11mm when  that is
>   the magnitude  of noise and drift averaged over a second.  The drift
>   of 10-9mm per day is more realistic, but I'd really like to  see the
>   drift expected per year.
>
>   Probably the capacitive sensor has the best application in measuring
>   dynamic issues such as the roundness of rotating shafts, tiltmeters,
>   and other areas where interferometry is difficult or cannot be used.
>
>   But for short-term measurements, it looks very good. Thanks for the
>   info!
>
>   Best Regards,
>
>   Mike Monett
>   
Mike

The drifts experienced were all due to mechanical instabilities which 
would affect an interferometer sensing the position of the sensor mass 
in exactly the same way.
Jones used mica insulators, phosphor bronze and brass to construct his 
instruments.
He also took great care to isolate the sensitive parts of the instrument 
from undesired variable external forces to minimise distortion.
With more stable materials and better mechanical design such as using 
fused silica parts or etching the entire instrument from single crystal 
silicon  much higher stability is possible.

I obtained my copies the old way from the university library well before 
the advent of the Internet.

Bruce