[time-nuts] Capacitive temperature sensing

Mike Monett 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:



  > Bruce

  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

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