[time-nuts] Capacitive temperature sensing

Mike Monett XDE-L2G3 at myamail.com
Sat Aug 23 11:09:32 EDT 2008


  Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

  >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.

  [...]

  >Bruce

  Bruce, that might be a bit optimistic. There is electronic  drift as
  well. Physik  Instrumente talks about tolerances of  several hundred
  ppm, and  laser  interferometers   are   a   tenth   of  a  ppm. The
  interferometer is three orders of magnitude more stable.

  This stands to reason since the interferometer is  counting fringes.
  These are determined by the frequency of the light source, which can
  be measured and controlled better than any other physical parameter.

  Also, it  is significant that Physik Instrumente uses  Zygo ZMI-2000
  and ZMI-1000  laser interferometers in their  calibration  labs, and
  not their own capacitive sensors.

 
http://www.physikinstrumente.com/en/products/nanopositioning/test_calibration.php

  If there  sensors were as good as an  interferometer,  they wouldn't
  need Zygo.

  Regards,

  Mike Monett



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