[time-nuts] Capacitive temperature sensing
bruce.griffiths at xtra.co.nz
Sat Aug 23 06:09:43 EDT 2008
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
> Best Regards,
> Mike Monett
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
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.
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