Re: Cpacitance or Dielectric Measuring

bill.sloman_at_ieee.org
Date: 03/04/05 

Date: 4 Mar 2005 09:36:34 -0800

John Fields wrote:
> On 3 Mar 2005 10:05:45 -0800, bill.sloman@ieee.org wrote:
>
> >
> >John Fields wrote:
> >> On 2 Mar 2005 15:08:17 -0800, bill.sloman@ieee.org wrote:
> >>
> >> >
> >> >John Fields wrote:
> >> >> On 2 Mar 2005 02:12:25 -0800, bill.sloman@ieee.org wrote:
> >> >>
> >> >> >
> >> >> >John Fields wrote:
> >> >> >> On 26 Feb 2005 14:02:08 -0800, "shayan"
<shayan_che@yahoo.com>
> >> >wrote:

<snip>

> >Okay - you've only used bridges, as opposed to designing them.
>
> ---
> On the contrary, I've designed RTD bridges which were temperature
> compensated and worked from the surface of the ocean to the bottom of
> the sea of Japan, differential capacitive proximity sensors capable
> of determining parallelism to within a couple of arc-minutes, as I
> recall, and an interesting self-heated thermistor mass flowmeter,
not
> to mention the garden-variety stuff that comes up every day.
> ---

So you should know that if your excitation frequency is high, a fast
demodulator is trivial.

> >> >it is lot easier if you keep the bridge
> >> >excitation frequency high, which you want to anyway with most
> >> >capacitance sensors to keep the impedance of the sensor
manageably
> >low.
> >>
> >> ---
> >> "Manageably low " seems to be the catch phrase here, but what do
you
> >> mean quantitatively?
> >> ---
> >
> >10pF of capacitance at 15kHz is about 1Mohm.
>
> ---
> OK. ???
> ---
>
> >> >There's nothing difficult about demodulating and digitising the
> >> >out-of-balance signal fast enough to get 5k independent samples
per
> >> >second.
> >>
> >> ---
> >> Schematic???
> >> ---
> >
> >You are offering to do this for money - I'm not doing your job for
you.
>
> ---
> Really? Sounds to me like you're telling me how _not_ to do it.
> Thanks, but no thanks, and I didn't think the schematic would be
> forthcoming anyway, since I've never seen you post anything but lip
> service.
> ---

There are a couple of published papers in the U.K. journal "Measurement
Science and Technology" which do include circuit diagrams - sadly, no
demodulators, so you'll have to crib from someplace else.

Since you are American, you won't ever have heard of it - it is the
British equivalent of the Review of Scientific Instruments, which you
ought to have heard of. So far I've only published comments there -
their referees on't seem to know much about electronics.

> >> >Making the balancing capacitor and the sensing capacitor
isothermal
> >> >ought not to be that difficult if you make them part of the same
> >> >mechanical structure - probably easier than making a capacitor
with
> >> >a near zero temperature coefficient.
> >>
> >> ---
> >> Yes, it's always easy when you don't have to do it...
> >> ---
> >
> >I've had to hunt for funny low-coefficient-of-expansion alloys in my
> >time, which is what your scheme seems to call for. The sensing cell
> >isn't going to be an off-the-shelf item in any event, and the amount
of
> >extra thinking required to design in a closed reference capacitance
> >doesn't strike me as being in the same league.
>
> ---
> I agree; I think having to design in two isothermal caps with the
> same thermal time constants would be _much_ more difficult than
> finding the right combination of materials to make a single zero TC
> cap would be.
> ---

In fact I've been thinking about that problem, and I suspect that the
OP is going to have use a planar capacitor, measuring the capacitance
between inter-digitated metal fingers printed onto a insulating
surface. This leaves you stuck with a large-ish fixed capacitance
between the fingers, associated with the field lines in the insulating
support, but the field lines above the surface should buy you a
component which will vary with the particle loading in the fluidised
bed.

The obvious way of building such a capacitor is as a thick- or
thin-film hybrid circuit on an alumina substrate. You'd want to make
the other side of the substrate a solid ground plane, which would
minimise the capacitance associated with the field in the alumina.

You balancing capacitor is then a second identical capacitor soldered
back-to-back onto the first, with a dust-tight box to keep the
fluidised particles out of range - you'd probably put a porous plug of
sintered metal or ceramic in the wall of the box to allow slowish
equalisation of gas pressure.

Making all that iso-thermal isn't exactly monsterously difficult - the
alumina has a much higher thermal conductivity than the fluidising gas.

<snip>

> >> And that isothermal capacitor pair is going to be duck soup?
> >
> >Even without the closed reference capacitance, the sensing capacitor
> >isn't going to be duck soup - the guy is going to go through a
couple
> >of prototypes to get rid of all the problems that weren't obvious at
> >the brain-storming session.
>
> ---
> But... ISTM that with two caps you've got at least twice the headache
> to try and make them play nice together.
> ---

See above.

----------
Bill Sloman, Nijmegen

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