[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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