[time-nuts] Capacitive temperature sensing
David
t_list_1_only at braw.co.uk
Sat Aug 23 10:21:26 UTC 2008
Bruce,
A totally trivial correction for you :
"the work of RV Jones at the university of Glasgow in the 50's, 60's and
70's. "
Jones was at Aberdeen university post war. A curious character, his
recollections of his wartime research are fascinating and worth reading but
I fear are best read with care alongside some other histories to maintain
balance.
Regards
David
> -----Original Message-----
> Hej Magnus
>
> A capacitive sensing AC bridge can be very sensitive, one only has to
> look at the work of RV Jones at the university of Glasgow in
> the 50's,
> 60's and 70's. He and his collaborators used capacitive sensors to
> detect (among other things) the extrusion of a micrometer shaft as it
> was clamped to realise just how sensitive it can be.
> They found it possible to detect length changes of less than
> 1E-11m with
> a suitably designed sensor.
>
> Even more sensitive capacitance bridge displacement sensors
> have since
> been constructed. Off course the critical bridge components
> have to be
> maintained at a reasonably constant temperature. Since the
> most critical
> component the balancing capacitor in the other bridge arm can be very
> small it can easily be located in the same controlled temperature
> environment as the sensor itself. One of the major contributors to
> instability will be creep in the glass capillary and bulb dimensions.
> Even with a relatively crude guard ringed coaxial sensor electrode
> surrounding a capillary tube is capable of submicron
> sensitivity without
> undue effort.
>
> If one uses a mercury in glass thermometer with say 0.1C
> resolution with
> the 0.1C graduation say 0.5mm apart then 1micron change in mercury
> column length is equivalent to a temperature change of 200uK. The
> performance will be determined largely by mechanical
> instabilities not
> the bridge sensitivity.
>
> Bruce
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