shayan said:
Tank you all for you kindness and answering to my request. Indeed
I'm
a chemical engineer and I want to use a special capacitor for
measuring
the variation of dielectric constant in a fluidized bed. A fluidized
bed is column in which air or another fluid is entering from bottom
and
fluidized the fine particles (5 to 500 micrometer) exist in the
column.
By variation in the particle concentration in the capacitor volume
its
dielectric also changed. If we could measure this changing in
dielectric with a high sampling rate, we could calculate the particle
concentration using convenient equations. But the traditional LCR's
have not enough measurement speed (minimum 2 ms) and we want the
sampling frequencies above 5 kHz (measurement speed below 0.2 ms). by
which type of capacitance meters can I do this?
If you need high speed sampling, then you may need to rig up a
relatively simple circuit, but beware - there's more to capacitance
than meets the eye. One of the reasons the standard meters are slow is
they are designed to be to make sure their readings are accurate. Long
story and way beyond the scope of the question.
First the basics.
The capacitance of a plate style capacitor is directly proportional to
the dielectric constant.
That being so, it's quite simple to feed a signal (we'll get to
frequency) through a series resitor and measure the phase shift. As the
phase is predictable [given by arctan X/R, or alternatatively, arctan
V(c)/V(r)]. Phase shift can be measured in any number of ways. The
simplest I know of is simply to measure the voltages (suitably
amplified so they are reasonable measurable, becaues you don't want to
impose a big signal for your measurement - I'll get to that) with an
add-in A-D card using a dual half-flash converter, which should be
adequate for your needs. (Full flash converters are expensive).
Alternatively, you could use a card with a successive approximation
converter, but it would have to be fast.
Having done the measurement in a PC of some description, the software
to do the calculation is trivial.
Before you get there, you should do an rms conversion so you have a
stable signal for conversion (getting an idea why LCR meters are
slow?).
To do that, with a sampling frequency of about 5kHz, you'll need to
excite the circuit with about 50kHz or so, which gets us into other
practical problems with capacitors. To work around them, choose a
resistor that has about the same resistance as the reactance of your
capacitance at 50kHz at some midpoint condition. (The phase at this
point is 45%). This implies, of course, that you have some idea of the
capacitance at this point. Another important point is the range of
capacitance you are speaking of.
[Two major issues that arise at frequencies well below this are leakage
and effective series resistance/inductance - so it could impact you.]
RMS conversions are fairly straightforward, especially if you have a
pure sinusoid driving the circuit.
The reason I chose 50kHz is so the RMS converter has the bandwidth to
respond to changes at a 5kHz rate. 10:1 is a good rule of thumb for
such things.
As you might note, it's not that simple - the fastest 'standard' meter
I know of converts at a rate of 800 samples / sec and it's very
expensive.
Another possible circuit is much simpler, but depending on the
capacitance in question may be slow.
Make a circuit that has a single capacitor in series with your
effective capacitance in the tank. Apply a DC voltage. The voltage
across each will vary as the capacitance of the tank varies
(specifically, it acts as a voltage divider). The charge stored in each
capacitor is equal (and equal to total circuit charge), and each
capacitor develops a voltage given by V=Q/C. Note that as the
capacitance of your tank varies, the total circuit charge will vary,
but the ratio of voltages will accurately reflect the ratio of
capacitance.
The big problem here is that to do a measurement, you need some current
from the measured object, which can destroy the measurement (because it
is taking the charge from the circuit). There are ways of dealing with
this, using ultra high impedance inputs and input current cancellation
(both easily available and fairly straightforward - input current
cancellation is also straightforward to design but does require certain
skills in feedback loops).
So if you have someone who knows how to do that, then you can use the
second method.
So your solution is either to use a very expensive (about $5k - $10k)
lab meter, or get one of the electrical guys to design and build you a
circuit that you can then feed to a high speed measurement system.
Cheers
PeteS