[time-nuts] Capacitive temperature sensing

[Mike Monett]

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

  > Magnus Danielson wrote:

  >> Hej Bruce,

  [...]

  >> Indeed. However,  I was trying to hint on a  alternative approach
  >> or temporary approach which would not need much design to achieve
  >> equal performance  if  the  electrodes could  not  be  cleaned or
  >> replaced easilly.

  >> I trust  the  capacitive measurement would  be  sufficiently good
  >> even at moderate design efforts.

  >> Cheers,
  >> Magnus

  >Hej Magnus

  > The biggest problem is that a stainless steel  thermoregulator and
  > not a mercury in glass one may have been used.

  > Retrofitting temporary  capacitive sensing would  be  difficult to
  > impossible in this case.

  > Metals other than platinum or iron are precluded from use  as they
  > tend to form amalgams with the mercury.

  > However one  could  probably temporarily substitute  a  mercury in
  > glass thermometer with capacitive position sensing.

  > The only potential problem being that there are moves afoot to ban
  > mercury thermometers.

  > A capacitance  of around 1pF or so may be expected for a  1cm long
  > cylindical capacitive  sensor electrode and an instability  in the
  > capacitance measurement  of  100ppm  or so  would  be  required to
  > achieve millidegree stability (provided the mechanical instability
  > of the glass allows this).

  >Bruce

  Gentlemen,

  The problem  is  very simple. If the sensor lead  that  contacts the
  mercury has  a diameter of 0.010 inch, with a flat end,  and  is 100
  microinches away  from the mercury, the capacitance is  0.35  pf. An
  Analog Devices AD7747 24-bit capacitance-to-digital  converter could
  be used to measure this. It sells for $4.60 in quantity.

  It has a resolution of 20 aF (1e-18F), and an accuracy of 10 fF.

  This would  allow  it  to detect a  change  in  distance  of 0.00566
  microinch or  0.144  nanometers.  This  is  on  par  with commercial
  capacitance sensors  such  as Physik Instrumente.  But  since  it is
  completely integrated,  the drift specifications are about  an order
  of magnitude better.

  If the  mercury contacts the sensor wire and shorts it to  ground, a
  7.5pF cap  could be placed in series. This would  limit  the maximum
  capacitance to within the range of the AD7747.

  The AD7747 can accept up to 17pF of stray capacitance.  The existing
  sensor wire  should be much less than this, especially if  the leads
  were as short as possible.

  The problem would be a bit more complicated if the sensor lead had a
  v-shaped end  caused by cutting with pliers instead of  the shearing
  action of scissors. But the resolution of the AD7747 should allow it
  to measure the capacitance just before making contact.

  It also  has  an on-board temperature sensor with an  accuracy  of 2
  degrees C.  This  would allow the system  to  do  coarse temperature
  measurements to confirm it was working.

  Regards,

  Mike Monett