[time-nuts] Capacitive temperature sensing
XDE-L2G3 at myamail.com
Sat Aug 23 05:44:42 EDT 2008
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> One good general reference for capacitance micrometers is:
> /Microdisplacement Transducers/ by P.H. Sydenham, Journal of
> Physics E, Scientific Instruments, Vol 5, p721-33, 1972.
> It seems that capacitance micrometers can be 3 orders of magnitude
> more sensitive than I stated i.e. they can detect displacements of
> about 1E-14 m.
> This compares well with interferometry which can detect
> displacements of around 1E-10 m.
> RV Jones was at the University of Aberdeen not Glasgow.
> RV Jones was the first recipient of the CIA's RV Jones
> Intelligence award:
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
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
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