Cpacitance or Dielectric Measuring

[shayan]

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?

 

shayan wrote:

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?

What you want is a capacitance bridge - these are used in
stretched-diaphragm capacitance pressure gauges, amongst many other
application.

I'd look at a Blumlein tranformer bridge myself, which would use a
bifilar-wound centre-tapped winding on a ferrite core transformer. One
end of the winding would drive one side of your sensing capacitor (the
other side would be grounded) and the other end would drive one end of
a stable reference capacitor of the same nominal capacitance (whose
other end would also be grounded, ideally to the same point as the
grounded side of your sensing capacitor).

The centre-tap is then your unbalance source - hook it up to the input
of a high-input impedance amplifier, and demodulate at the excitation
frequency.

Tony Williams wound a nice transformer for a fast resistance bridge
(15kHz excitation frequency IIRR) which you might find interesting.

Higher frequencies are certainly practical, but you start having to
worry about phase shifts in the bridge-driving circuit and the
amplifier.

Analog Devices now make synchronisable DDS circuits with quadrature
outputs which make it very simple to produce an array of excitation and
in-phase and quadrature demodulation waveforms. It would be a fun
project to put such a system together, but what you probably need are
the electronics from an MKS Baratron capacitance manometer

http://www.mksinst.com/barainfo2c.html

I don't know if the exitation frequency is high enough for what you
want, but the system goes back to a time when most such gauges used a
455kHz excitation frequency (the IF frequency used in AM radios of that
period), so you should be okay.

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

 

[PeteS]

shayan wrote:

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

 

[Fred Bloggs]

PeteS wrote:

shayan wrote:

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

That would be well and good if it were not for the fact that particle
density to dielectric constant relationship is for static steady state.
This may not hold against serious deviation with unknown displacement
current circulating due to dD/dt, and probably makes the OP's entire
approach fundamentally flawed.

 

[John Fields]

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

 

[Tony Williams]

In article <ebpc2190eb570ejdegp0ittfmriidtvs9f@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:

On 2 Mar 2005 15:08:17 -0800, bill.sloman@ieee.org wrote:
There's nothing difficult about demodulating and digitising the
out-of-balance signal fast enough to get 5k independent samples
per second.

---
Schematic???
---

No schematic, but in the fast resistance bridge that
Bill referred to earlier in the thread we managed an
analogue freq response of about 2.4KHz, ADC'd at
10000 conversions per second.

15KHz bridge excitation frequency. V-bridge and
V-unbalance picked off and AC amplified in similar channels.
V-bridge used as the clock for the synchronous demodulation
of both V-bridge and V-unbalance.
250KHz clock for a pair of Maxim digital low pass filters.
Final dc signals into an ADC, where the demodulated V-bridge
was used as the Vref. The demodulated V-bridge was also
used to servo the amplitude of the excitation oscillator.
The 250KHz clock also generated the (synchronised) 10KHz
CONVERT timings to the ADC.

I suspect the bum-biter with a capacitive bridge would be
that the excitation frequency *has* to be chosen to get
a reasonable current in the bridge without requiring
unreasonable voltages. Can't move anywhere else in the
design until those numbers are sorted.... and that depends
on the range of capacitance values to be measured.

--
Tony Williams.

 

[Fred Bartoli]

"Tony Williams" <tonyw@ledelec.demon.co.uk> a écrit dans le message de
news:4d464dd17dtonyw@ledelec.demon.co.uk...

In article <ebpc2190eb570ejdegp0ittfmriidtvs9f@4ax.com>,

No schematic, but in the fast resistance bridge that
Bill referred to earlier in the thread we managed an
analogue freq response of about 2.4KHz, ADC'd at
10000 conversions per second.

15KHz bridge excitation frequency. V-bridge and
V-unbalance picked off and AC amplified in similar channels.
V-bridge used as the clock for the synchronous demodulation
of both V-bridge and V-unbalance.
250KHz clock for a pair of Maxim digital low pass filters.
Final dc signals into an ADC, where the demodulated V-bridge
was used as the Vref. The demodulated V-bridge was also
used to servo the amplitude of the excitation oscillator.
The 250KHz clock also generated the (synchronised) 10KHz
CONVERT timings to the ADC.

I suspect the bum-biter with a capacitive bridge would be
that the excitation frequency *has* to be chosen to get
a reasonable current in the bridge without requiring
unreasonable voltages. Can't move anywhere else in the
design until those numbers are sorted.... and that depends
on the range of capacitance values to be measured.

On a question I asked previously, the OP answered with a totally unrealistic
range:

Measurement frequency: minimum 5kHz

Estimated capacitance: .001pF - 1mF

Budget: maximum 5k$

Seems he doesn't know what he's after.


--
Thanks,
Fred.

 

[Winfield Hill]

Fred Bartoli wrote...

On a question I asked previously, the OP answered with a
totally unrealistic range:

Measurement frequency: minimum 5kHz
Estimated capacitance: .001pF - 1mF
Budget: maximum 5k$

Seems he doesn't know what he's after.

Yes.

BTW, my hp4280A 1MHz C-V, C-t meter measures down to 0.001pF
and can take bursts of high-resolution measurements in 10us.
This capability is for Zerbst analysis, used to determine
minority-carrier lifetime and surface-generation velocity.
Perhaps the O.P. can purchase one of these instruments.


--
Thanks,
- Win

 

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

 

[John Fields]

On 4 Mar 2005 09:36:34 -0800, bill.sloman@ieee.org wrote:

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.

---
Well, certainly not from you if I want something that will work...
---

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.

---
It always gets down to the America-bashing thing with you, doesn't it,
Sloman? Seems you can't even get into the most light-hearted
technical discussion without turning it into political nastiness.

I no longer have any desire to communicate with you, and I'll be glad
when you're dead.

--
John Fields

 

John Fields wrote:

On 4 Mar 2005 09:36:34 -0800, bill.sloman@ieee.org wrote:

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.

---
Well, certainly not from you if I want something that will work...

None of the papers includes a 555, so you'd be out of your depth.

---

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.

---
It always gets down to the America-bashing thing with you, doesn't
it,
Sloman? Seems you can't even get into the most light-hearted
technical discussion without turning it into political nastiness.

Nothing political about that - American scientists and engineers can
get away with ignoring the rest of the world, and often do. A couple of
my published comments in the Review of Scientific Instruments point to
papers in non-American journals which authors and referees should have
been aware of.

I no longer have any desire to communicate with you, and I'll be glad
when you're dead.

An understandable attitude - but one that won't do anything to fill the
lamentable gaps in your knowledge of electronics.

-------
Bill Sloman, Nijmegen

 

[Fred Bloggs]

bill.sloman@ieee.org wrote:

Nothing political about that - American scientists and engineers can
get away with ignoring the rest of the world, and often do.

Those days are rapidly coming to an end. The US share of published
scientific papers peaked in 1992 and has been declining by 10% per annum
since. Looks like the heyday of this grossly mismanaged mess is nearly
over with a recent survey showing an inflection point has finally
occurred in the traditional measures of rate of technological
breakthrough. I notice a lot of selfish people seem to be taking
personal comfort in some sort of belief in economic inertia attenuating
the severity of impact, but that type of moron is just not paying
attention to the established rapidity of high technology industry growth
and decline among winners and losers.

 

[Rich Grise]

On Sun, 06 Mar 2005 02:10:13 -0800, Winfield Hill wrote:

http://www.technologyreview.com/articles/05/03/wo/wo_delio030405.asp
http://it.slashdot.org/it/05/03/05/2015250.shtml?tid=173&tid=218

The story has little detail, but it does trigger one's curiosity.

Well, the bottom line isn't all bad - my HP4600 see-through scanner is
pretty spiffy, and a hundred and fifty bucks is a pretty amazing price for
something like that.

Cheers!
Rich

 

[Rich Grise]

On Sun, 06 Mar 2005 10:35:26 +0000, Fred Bloggs wrote:

See: http://www.alternet.org/story/21400/

I wouldn't worry too much - the only reason them furriners are catching
up is that they're all learning English. ;-)

Cheers!
Rich

 

[Chris]

Fred Bloggs wrote:

See: http://www.alternet.org/story/21400/

Thanks. Great link, and well worth reading.

Chris