[time-nuts] Capacitive temperature sensing

[Bruce Griffiths]

Mike Monett wrote:
>   Hi Bruce,
>
>   Thanks for  the  info.  I looked at  Physik  Instrumente,  and their
>   performance is   truly   impressive.   For   example,  the D-510.020
>   single-electrode capacitive sensor has a nominal range of 20 um, and
>   a sensor active area of 11.2 mm^2:
>
>   http://www.physikinstrumente.com/en/pdf/D510_Datasheet.pdf
>
>   According to  my  calculations, that works out to  a  capacitance of
>   about 4.95 pf.
>
>   They show a resolution for this sensor of <0.001%, which is  20e-6 *
>   0.001 * 0.01 * 1e9 = 0.2 nm.
>
>   The change  in  capacitance is 4.95 * 0.001  *  0.01  = 0.0000495pf,
>   which is quite amazing. That is a very small change.
>
>   If the  nominal sensor voltage was 1 volt, this represents  a change
>   of 1  *  0.001  *  0.01 = 0.00001 Volt, or  10  uV,  which  is quite
>   acceptable for good S/N.
>
>   One problem  might be long-term drift. The  temperature coefficients
>   and other  errors are in the hundreds of ppm, whereas  the tolerance
>   in interferometry are down to 0.1 ppm.
>
>   It is interesting to note they use Zygo ZMI-2000 and  ZMI-1000 laser
>   interferometers in their calibration labs, presumably  for long-term
>   accuracy:
>
>  
> http://www.physikinstrumente.com/en/products/nanopositioning/test_calibration.php
>
>   Also, they  use flat plate capacitors in the sensors.  I  don't know
>   how well this would work on a thin column of mercury surrounded by a
>   glass dielectric.
>
>   But this is new information to me, and I am quite impressed with the
>   performance. Thank you for posting the information.
>
>   Regards,
>
>   Mike Monett
>
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>   
Mike

One technique would be to use the mercury column as the central plate of 
a differential capacitor.
Alternatively use a large cylindrical sleeve to make a high capacitance 
connection to the mercury column and use a position sensing cylindrical 
sleeve surrounding the set point.

Temperature compensation can be achieved by using a reference capacitor 
of similar construction by metal coating the bore of a similar glass 
capillary.
Since this is a fixed setpoint device it need not have a large dynamic 
range.
Thus a tempco of 100ppm need not be an issue if the range is small 
enough and the reference capacitor shares the same tightly regulated 
thermal environment.

Cylindrical geometry variable capacitors can be made to work quite well, 
Syndenham covers this as well as the parallel plate variety.



Bruce