shayan said:
Hi Win. Tank you for your 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?
Hi, Shayan. Glad to hear from you again. Looking back on my previous
post, I think I was a little impatient with your very brief problem
description, and wasn't sure if it was a legitimate post. Sorry.
But I believe I got the drift of where you're headed. I'm hearing you
saying that you've got two plates suspended in a fluidized bed, and
you're trying to find particle density by measuring capacitance to find
dielectric constant of the fluidized powder between the plates. A
couple of questions come to mind:
* I'm going to assume you've got a fluidized bed with a powder being
fluidized by air (again, your description of your problem leaves
something to be desired -- what's the powder, what's the medium?). If
so, you're depending on the difference between the dielectric constant
of the air and that of the powder to provide you with something you can
measure to infer powder density. Note that unless there's a lot of
difference in the dielectric constant of the air and the powder
('tain't necessarily so), it might be that you are trying to infer more
accuracy than you can get with that type of measurement. If the
dielectric constants are close, the most accurate capacitance
measurement isn't going to help, except in a very rough way. If
there's a more dramatic difference in masses per given volume for
powder and medium than their respective dielectric densities, might it
be better to use a stirrer and measure torque?
* Fluidization usually occurs with mechanical vibration of the bed.
In a system with air bubbles, the mechanical shaking makes the air
bubbles progressively smaller, until the powder/air mix acts like a
liquid. I'm not sure why, in a mechanical system like a fluidized bed,
5,000 measurements per second are desirable. Usually it takes from
tenths of seconds to seconds for the powder and the meduim to
homogenize. Again, a better description might help. This might be an
esoteric application we don't know about.
* This sounds like a fairly small (low pF range) capacitance. From a
production/lab standpoint, if "real-time control" required such speeds,
I might be tempted to cobble together a fairly high speed oscillator
using the capacitance of the plates in the bed (possibly even
suspending the rest of the oscillator in the bed itself), divide it
down to an appropriate range with a comparator and some logic ICs, and
after optocoupling, use a high speed counter board in a PC to collect
and process period data. Either that, or you can use the divided down
optocoupled signal to gate a counter, which would drive a D-to-A
converter. You would infer capacitance from period or analog voltage,
and then infer powder density from the inferred dielectric constant of
the fluidized powder -- after all, your plate size remains the same,
right? This has several advantages. And you'll have your 5KHz data
stream.
I believe that, with reasonable care, you can get much better than 1%
accuracy with this type of system, which might be enough if the
differences in dielectric constants are great enough. It would require
some cobbling, but might cost significantly less than your instrument,
and might be a lot more reliable in a production/lab environment.
By the way, you should watch for the development of static charge in
the fluidized bed, and see if that's going to interfere with your
measurements. You might be able to detect this by putting a high
impedance voltmeter on your measurement plates with nothing attached,
and monitoring to see if any charge is developed. Also, if you're
using an air bed, you want to get a good handle on humidity issues,
because that will seriously affect your measurements, as well as
possibly affecting your process. Fluidized beds are a battle to get
running well, and usually something of a mess. By the way, I'm sure it
really isn't necessary to mention this, but from my experience, make
sure everyone's read the MSDS and uses adequate breathing protection
where necessary. Lab/machine operators sometimes don't follow even the
best instructions.
Thanks for posting again, and putting up with some unjustified
impatience on my earlier post. I'd like to hear any additional details
or project requirements.
Chris
