[time-nuts] Capacitive temperature sensing

[Mike Monett]

  (I combined your two posts into one)

  > Mike

  > The RV  Jones  papers  were  published  in  either  the  Review of
  > Scientific instruments or the Journal of Sceintific  Instruments -
  > I forget which.

  > I'll look for my paper copies when I get back from  shopping later
  > today.

  > Sydenham also  wrote  at  least one  paper  on  various capacitive
  > displacement sensors.

  > Physik Instrumente   use   capacitive   displacement  sensors with
  > nanometer sensitivity in their piezostages and piezoactuators.

  > Bruce

  > You could also look at Queensgate Instruments.

  > Integral capacitive  sensors have been used in a feedback  loop to
  > maintain the  spacing  and parallelism of  a  pair  of Fabry-Perot
  > interferometer plates.

  > Bruce

  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