Re: Cpacitance or Dielectric Measuring

From: John Fields (jfields_at_austininstruments.com)
Date: 03/03/05 

Date: Thu, 03 Mar 2005 06:40:02 -0600

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>
>
>> >> I suggest that instead of a capacitance meter, per se, you
>fabricate
>> >> the capacitor with the view of using it as the tuning element of
>an
>> >> oscillator, and then use a gated counter to count (integrate) the
>> >> oscillator cycles over a fixed time. Then, by selecting the
>nominal
>> >> oscillator frequency you can get whatever resolution you need over
>> >> that period for the expected change in dielectric constant.
>> >>
>> >> Probably the thing which will introduce the most error will be the
>> >> dimensional change of the capacitor with temperature, so that will
>> >> need to be dealt with. One option would be to make the capacitor
>and
>> >> its supporting structure out of materials with a very small
>thermal
>> >> coefficient of expansion, and another would be to measure the
>change
>> >> in capacitance with temperature using a dielectric with known
>> >> characteristics over that temperature range, then to generate a
>> >lookup
>> >> table which would be used to compensate for the measured in situ
>> >> temperature when the device was in use.
>> >
>> >A third option is to design your capacitance sensor to include a
>> >second, closed reference capacitor, whose expansion with temperature
>> >will track that of the sensing capacitor - this makes even more
>sense
>> >if you measure the capacitance of the sensing capacitor against the
>> >capacitance of the reference capacitor with a capacitance bridge,
>> >rather than the using a oscillator circuit to measure the product of
>> >the sensing capacitance with some arbitrary tuning inductance which
>is
>> >likely to be just as temperature dependent.
>>
>> ---
>> One of the problems with using a capacitance bridge is that the OP
>> would like to make 5000 measurements per second, another is that the
>> two caps _must_ be isothermal. Using option 1, which is a capacitor
>> with zero or close to zero tempco sidesteps the entire issue and
>> allows a simple counter to make the measurement.
>
>There's no obvious reason why a bridge can't be used to make 5000
>measurements a second 

---
Perhaps not, but for this application it sounds kind of iffy to me.
If we had more information about the application it would help, but in
lieu of that I'd prefer to stick with the oscillator.
---
>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? 
---
>There's nothing difficult about demodulating and digitising the
>out-of-balance signal fast enough to get 5k independent samples per
>second.
---
Schematic???
---
>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...
---
>>  Of course some
>> consideration must be given to the inductor, but since it's not
>> required to be in the bath with the cap, it could easily be ovenized
>> with the rest of the oscillator, away (but not _too_ far away!) from
>> the cap.
>
>It's not a bath, but a fluidised bed.
---
Oh, then, the capacitor _won't_ be surrounded by a fluid?
---
> I do like the glib way you say
>"ovenised" - maintaining a stable and uniform temperature inside an
>oven isn't exactly a sinecure, and gets trickier as your oven gets
>bigger (crystal ovens are tiny).
---
And that isothermal capacitor pair is going to be duck soup? 
You may not have noticed, Bill, but electronic components lately have
gotten to be way smaller than those 12AT7's you're used to. ;)
---
 
>> Of course, the whole thing may be moot depending on the range and
>> accuracy the OP needs but which, unfortunately, along with the
>> temperature range the thing would be expected to work under, he
>didn't
>> state.
>
>He certainly didn't. Some fluidised beds run quite hot.
---
OK...
-- 
John Fields

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