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'PIC Scale -'
1997\08\25@025135 by Jacques Vrey

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Hi to all.

I was wondering if anyone out there has attempted to make an
electronic scale using PIC's. I have a few ideas for the PIC side
of things but 'What to use for a SENSOR??' I was thinking of
strain guages but over here in sunny South Africa, these are either
hard to come by or very costly. I'm considering a LVDT (Linear
Variable Differential Transformer) option but I wan't to keep
mechanics to a minimum. I was wondering if any of you out there have
some idea's that you would like to share on the topic.

Thanks.

P.S. Thanks to Mal Goris, Andrew Errington and Don McKenzie for
their help, my programmer is now up and running!

Jacques Vrey
Iscor Profile Products Newcastle
Tel:27-(0)3431-48759
spam_OUTjvreyTakeThisOuTspamit.new.iscorltd.co.za

1997\08\25@041532 by Lee McLaren

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Gday Jacques,
I have seen a long time ago a design based on a printed cct board track.
It consisted of a fine track that traced a s pattern up and down a
"finger" that was used as a strain gauge.
It worked on the principle that the resistance varied with the amount of
bend on the finger.
The track went up and down the finger about 10 times.

regards

Lee McLaren
Tasmania Australia



Jacques Vrey wrote:
{Quote hidden}

1997\08\25@052343 by blunn

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Bob Lunn
08/25/97 07:22 PM


> I was wondering if anyone out there has attempted
> to make an electronic scale using PIC's.

    I have been building electronic scales using
    PIC's for a dozen years.  All of these scales
    have been built using strain gauge sensors.

    There are a few points you should clarify:

      - what is the maximum capacity of the scale.
      - what is the dead load of the scale.
      - how many graduations are required.
      - how is the load to be applied.
      - what is the nature of the load.

    Please don't underestimate the difficulty of
    building an accurate and robust load sensor!

___Bob

1997\08\25@055024 by Jacques Vrey

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{Quote hidden}

In answer, The following:

- 5Kg
- no more than 500g(probably less)
- 1 (0-5Kg)
- vertical mass measurement
- anything

Exceptional accuracy is not critical (Glorified Kitchen Scale)

Jacques Vrey
Iscor Profile Products Newcastle
Tel:27-(0)3431-48759
jvreyspamKILLspamit.new.iscorltd.co.za

1997\08\25@072701 by Octavio Nogueira

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>Hi to all.
>
>I was wondering if anyone out there has attempted to make an
>electronic scale using PIC's. I have a few ideas for the PIC side
>of things but 'What to use for a SENSOR??' I was thinking of
>strain guages but over here in sunny South Africa, these are either
>hard to come by or very costly. I'm considering a LVDT (Linear
>Variable Differential Transformer) option but I wan't to keep
>mechanics to a minimum. I was wondering if any of you out there have
>some idea's that you would like to share on the topic.

I'm working a lot with scales using PIC's. The best way, and the only way
I'm working, is with strain-gauge load cells. You have to pay special
atention on the amplification, or use no amplification at all if you use
some special A/D.

Regards,

Octavio
======================================================
Octavio Nogueira  - e-mail:   .....nogueiraKILLspamspam.....mandic.com.br
http://www.geocities.com/~oct_nogueira
"ProPic" Production PIC Programmer Windows under US$20
======================================================

1997\08\25@082338 by Russ Steinbach

picon face
At 08:07 AM 8/25/97 -0300, you wrote:
>>Hi to all.
>>
>>I was wondering if anyone out there has attempted to make an
>>electronic scale using PIC's. I have a few ideas for the PIC side
>>of things but 'What to use for a SENSOR??' I was thinking of
>>strain guages but over here in sunny South Africa, these are either
>>hard to come by or very costly. I'm considering a LVDT (Linear
>>Variable Differential Transformer) option but I wan't to keep
>>mechanics to a minimum. I was wondering if any of you out there have
>>some idea's that you would like to share on the topic.

You may want to look at the new AD7730 chips.  They are specifically
designed for use with strain gage load cells.  They interface to a PIC via
an SPI bus.

These chip are really a specialized DSP with a programmable gain
instrumentation amplifier front end.  They have lots of capability for
filtering, tare, and self calibration and test.  They are also about $12 in
small quantities.

As you can tell, I like the chip.  But, beware, there is a steep learning
curve to use them.  Its the price you pay for all the functionality.


Russ

1997\08\25@084551 by rlunn

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Jacques Vrey wrote:

> In answer, The following:
>
> - 5Kg
> - no more than 500g(probably less)
> - 1 (0-5Kg)
> - vertical mass measurement
> - anything
>
> Exceptional accuracy is not critical (Glorified Kitchen Scale)

       Jacques, If I'm reading you right you want to
       weigh 5000 grammes by 1 gramme.

       That's 5000 grads!

       This is not a glorified kitchen scale.

       Can we maybe reduce this by an order of magnitude?

___Bob

1997\08\25@095807 by Glenn Johansson

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part 0 106 bytes
How much do those cells cost? What about springs and some simple motion control
sensor?

Regards,
Glenn

1997\08\25@113456 by Tom Mariner

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On Monday, August 25, 1997 2:46 AM, pic microcontroller discussion list  On
Behalf Of Jacques Vrey wrote:
{Quote hidden}

Hello Jacques,

Years ago we did a talking scale that used a Pic. The technology was an
optical up/down counter that was spun by a spring mounted mechanism. The unit
measured in pounds, kiligrams, ounces and stone ans spoke in all of the
radixes. The trick was to keep the count going while the multiplies and
divides for radix conversion was being accomplished and displayed in real
time.

Therefore, not strain guage at all and it was "inexpensive".

Tom

1997\08\25@122828 by mikesmith_oz

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On 25 Aug 97 at 15:20, Glenn Johansson wrote:

> >I'm working a lot with scales using PIC's. The best way, and the only way
> >I'm working, is with strain-gauge load cells. You have to pay special
> >atention on the amplification, or use no amplification at all if you use
> >some special A/D.
>
> How much do those cells cost? What about springs and some simple
> motion control
>  sensor?

I wouldn't like to predict accuracy / long term repeatability - this
is how the early digital bathroom scales worked - springs and a
slotted wheel / counter system - and they seemed to give a different
result every time you stepped on them.  (within seconds - not days,
guys <g>)
If you wanted to try the idea, you could get a cheap set of spring
kitchen scales and a mouse, hook the thing up so one mouse axis spun
when you put a weight on scales and do the quadrature trick, or
you could even just read it using the standard serial output from
mouse.
MikeS
<mikesmith_ozspamspam_OUTrelaymail.net>

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1997\08\25@124835 by mikesmith_oz

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On 25 Aug 97 at 22:45, Robert Lunn wrote:

{Quote hidden}

I'd be inclined to reduce it to PIC A/D proportions - probably the
14000, since the 16c7x is only 8bits resolution, much too coarse to
be any use, unless you stuffed around a lot with ranging.
This would give the 5000 graduations easily - in fact, data sheet
indicates it could go up to 65536 ...  The harder part would be the
analog circuit - I seem to recall they are somewhat prone to drift
with temperature.
A scale that could measure down to fractions of a gram would be
somewhat useful - counting things - measuring correct ratio of
sulphur / charcoal / potassium chlorate etc (no, I don't make this in
5k lots!) <g>
MikeS
<KILLspammikesmith_ozKILLspamspamrelaymail.net>

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1997\08\25@130420 by Octavio Nogueira

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The 5kg load cells costs me about US$60 and I don't thing the spring/motion
sensor
could have the same precision as load cells, but if you don't need
precision,
it's just ok, but then you have mor mechanics to deal with.

Regards,

Octavio
P.S. if anyone interested in buing load cells in large quantity, send me
private e-mail
======================================================
Octavio Nogueira  - e-mail:   spamBeGonenogueiraspamBeGonespammandic.com.br
http://www.geocities.com/~oct_nogueira
"ProPic" Production PIC Programmer Windows under US$20
======================================================
----
De: Glenn Johansson <TakeThisOuTglennEraseMEspamspam_OUTWRITEME.COM>
Para: RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU
Data: Segunda-feira, Agosto 25, 1997 11:07
Assunto: Re: PIC Scale -

>>I'm working a lot with scales using PIC's. The best way, and the only
way
>>I'm working, is with strain-gauge load cells. You have to pay special
>>atention on the amplification, or use no amplification at all if you use
>>some special A/D.
>
>How much do those cells cost? What about springs and some simple motion
control
> sensor?
>
>Regards,
>Glenn
>

1997\08\25@134937 by Tim Kerby

picon face
Hi
Most scales use a device called a load cell which is like a Z shaped chunk
of metal that gives an output proportional to load.  These use a strain
gauge principal.  Try the South Africa branch of Maplin Electronics.  They
should get you strain gauges or try RS Components.  Remember to use at
least two sensors.  It gives the best readings.

The CPR practice doll I was using during a heartstart course used a large
spring and a slotted opto switch with a very fine scale that went through
it vertically.  This gave amazing accuracy and was cheap and easy.  As below

___________Surface____________
///             -               ///
///             -               ///
///             -               ///
///             -               ///
///             -               ///
///             - _____ ///
///           |-|_____| ///Spring
///                     |       ///
///                     |       ///
///                     |       ///
///                     |       ///
///                     |       ///
///                     |       ///
///                     |Post   ///
_____________________|_________


  _____
|-|_____| Represents slotted opto switch

As pressure is put on spring, a pulse train is output.  The post limits
maximum weight and should come to half total height.


Tim


At 08:46 25/08/97 +0200, you wrote:
{Quote hidden}

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Personal Web Pages: http://web.ukonline.co.uk/members/tim.kerby/
Email: EraseMEtim.kerbyspamukonline.co.uk
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1997\08\25@145625 by Scott Newell

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I've been thinking about a capacitive sensor.  You might use a couple of
conductive parallel plates, with the mass changing the spacing.  I'd probably
use the capacitor in an oscillator, and then count pulses with a PIC.

Then you'd have to correct the non-linearities, either in software or in the
oscillator.  (Maybe a 1/n transfer-function V-F converter?)  It appears the
resolution would be a lot better at one end than the other.

I imagine it would be pretty drifty with temperature.  Mechanically, it could
be simple and rugged.


Then again, all the good scales I've ever seen were based on either load
cells or quartz crystals.


later,
newell

1997\08\25@160652 by Harold Hallikainen

picon face
On Mon, 25 Aug 1997 13:24:55 -0300 Octavio Nogueira
<RemoveMEnogueiraEraseMEspamEraseMEMANDIC.COM.BR> writes:
>The 5kg load cells costs me about US$60 and I don't thing the
>spring/motion
>sensor
>could have the same precision as load cells, but if you don't need
>precision,
>it's just ok, but then you have mor mechanics to deal with.
>

       Wow!  I'm surprised they're that much, though I never tried to
buy one.  I have, however, bought some pressure transducers from
Motorola.  As I recall, they were pretty cheap (I think under $10 US) and
included conditioning circuitry so something like 0 to 5 volts came out.
So...  maybe make a scale by having a piston and cylinder drive a
pressure transducer?  There's gotta be an easier way!

Harold

1997\08\25@171009 by John Shreffler

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part 0 946 bytes
-----Original Message-----
From:   Harold Hallikainen [SMTP:RemoveMEharoldhallikainenspam_OUTspamKILLspamJUNO.COM]
Sent:   Monday, August 25, 1997 4:04 PM
To:     RemoveMEPICLISTTakeThisOuTspamspamMITVMA.MIT.EDU
Subject:        Re: PIC Scale -

On Mon, 25 Aug 1997 13:24:55 -0300 Octavio Nogueira
<EraseMEnogueiraspamspamspamBeGoneMANDIC.COM.BR> writes:
>The 5kg load cells costs me about US$60 and I don't thing the
>spring/motion
>sensor
>could have the same precision as load cells, but if you don't need
>precision,
>it's just ok, but then you have mor mechanics to deal with.
>

       Wow!  I'm surprised they're that much, though I never tried to
buy one.  I have, however, bought some pressure transducers from
Motorola.  As I recall, they were pretty cheap (I think under $10 US) and
included conditioning circuitry so something like 0 to 5 volts came out.
So...  maybe make a scale by having a piston and cylinder drive a
pressure transducer?  There's gotta be an easier way!

Harold

1997\08\25@173219 by John Payson

picon face
> I have seen electronic scales use current to electromagnetically counter
> gravity, with the equilbrium point being determined by light that was
> obstructed by a blade on the moving part.  The current servos until the
> light is dimmed to about half.  Over narrow ranges the
> response was quite linear.  But I would think that force transducers
> would be a more rugged approach.

Interesting notion.  How about a mechanical balance with a weight that slides
along a lever under software control via either stepper or motor+quad encoder?
Should be simple enough to program, and the mechanics shouldn't be too bad
either.

1997\08\25@192211 by Mike Keitz

picon face
On Mon, 25 Aug 1997 16:03:57 EDT Harold Hallikainen
<RemoveMEharoldhallikainenKILLspamspamJUNO.COM> writes:
>On Mon, 25 Aug 1997 13:24:55 -0300 Octavio Nogueira
><nogueiraSTOPspamspamspam_OUTMANDIC.COM.BR> writes: >The 5kg load cells costs me about
>US$60 and I don't thing the
>>spring/motion
>>sensor
>>could have the same precision as load cells, but if you don't need
>>precision,
>>it's just ok, but then you have mor mechanics to deal with.
>>
>
>        Wow!  I'm surprised they're that much, though I never tried to
>buy one.  I have, however, bought some pressure transducers from
>Motorola.  As I recall, they were pretty cheap (I think under $10 US)
>and included conditioning circuitry so something like 0 to 5 volts
>came out. So...  maybe make a scale by having a piston and cylinder
>drive a pressure transducer?  There's gotta be an easier way!

A piston and cylinder would be hard to make as well as suffer from
leakage and friction.  Something like a bellows or air bag holding the
load platform up might be better for a low-capacity scale.  It would be
very resistant to overloading, since those sensors can withstand
something like 10x rated full-scale pressure.  The pressure in the air
system with no load on the platform would need to be positive to keep the
sides of the air bag from collapsing.

1997\08\25@211821 by blunn

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Bob Lunn
08/26/97 11:18 AM


> A scale that could measure down to fractions of
> a gram would be somewhat useful - counting things
> - measuring correct ratio of sulphur / charcoal /
> potassium chlorate etc (no, I don't make this in
> 5k lots!) <g>

    Laboratory scales and commercial counting scales
    are most certainly useful things.

    You might care to price these items.  This will
    give you a first order estimate of the difficulty
    of building one.

    In considering mechanisms to use for load or mass
    determination please keep in mind:

         span drift
         zero drift
         hysteresis
         creep

    ...just for starters.

    Jacques requires a mechanism to measure a load
    from 0 to 5,000 grammes with a resolution of 5
    grammes, with a dead load not exceeding 500
    grammes.

    We should still clarify the temperature range over
    which the scale is to operate, the repeatability
    of readings, long term drift of readings, need for
    zero and tare mechanisms, and requirements (if any)
    for standards certification.

    These days, the conversion of an electronic signal
    to a digital value is easily solved for reasonable
    cost.  The greatest problems usually arise in the
    physical load sensing mechanism.

    There's no point in having a 16-bit A/D converter
    if an elastomeric mounting imposes 5% hysteresis,
    or a knife edge continually binds.

    Anyway, the most common sensing techniques for
    electronic load measurement are:

         metal foil and silicon strain gauges

           (configured to respond to shear, bending,
            torsion, compression, or tension)

         piezoelectric bending beams

         capacitive measurement of displacement

         optical measurement of displacement

    Strain gauges are most commonly used for loads in
    the range of 50kg to 25,000kg.  The most common
    implementation would be a shear beam, with a nominal
    accuracy of 2000 to 3000 grads.

    Physical constraints on the load sensing element to
    which the strain gauge is bonded makes the use of
    strain gauges for very small, and very large, loads
    more difficult.

    Piezoelectric systems have probably had most success
    with very large loads requiring modest accuracy, such
    as gross tonnage measurement of trucks.

    The difficulty with piezoelectric elements is that
    they only respond to changes in load.  A static load
    produces no output.  They can, however, be used to
    build very robust (and thus safe) weighing systems.
    Hence their success with high capacity weighing.

    Capacitive and optical measurement of displacement
    are the most common mechanisms for small loads.  For
    a 5kg scale these are certainly the approaches I
    would be considering.  I would not use a strain
    gauge based load cell for this purpose.

    Scale design and manufacture is a _very_ competitive
    industry, and has been so for many decades.  There
    are a large number of patents covering scale design
    and manufacture.  An excellent education in these
    issues can be drawn from reading patent documents.

    Given that many of these patents have expired, the
    techniques they describe are available for use.

___Bob

1997\08\25@212608 by Pioneer Microsystems

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At 08:46 AM 8/25/97 +0200, you wrote:
{Quote hidden}

I was at a customer's place one day and he explained to me how his Metz__r
scale worked.  The principle is pretty slick.  A balance is attached to the
plate.  A sensor ( i forget, magnetic or optical) sensed when the balance
was centered.  The other end of the balance has a low tech solenoid
attached.  The circuit adjusts the current through the solenoid until the
balance balances, and voila, the current being sent to the solenoid is
proportional to the weight.  I am sure there are issues, such as
temperature compsation, but Metzler (oops) solved it, that scale was super
accurate and not cheap.  But on that principle, you could develop a
reasonably cheap version.  I was tipped off to ask him about the darned
thing when I noticed that the platter did not apear to deflect at all.
Word of warning, I would check and see if they have a patent on the
principle.  It is too neat not to.

Chris Eddy
Pittsburgh, PA

1997\08\26@020405 by William Chops Westfield

face picon face
   > A scale that could measure down to fractions of
   > a gram would be somewhat useful - counting things
   > - measuring correct ratio of sulphur / charcoal /
   > potassium chlorate etc

        Laboratory scales and commercial counting scales
        are most certainly useful things.

        You might care to price these items.  This will
        give you a first order estimate of the difficulty
        of building one.

There's an excellant source of information on electronic (and mechanical
balances (from a consumer point of view) at http://www.balances.com.  Aside from
a wide selection and good prices, their web-page setup is WONDERFULLY
cross-connected (a page for a given scale will have buttons "need more
capacity?", "need better accuracy?", "check another vendor", etc.

Just bought two scales from them, in fact (one for weighing that potassium
chlorate.)  Had an interesting and informative discussion with the
proprietor as well...

BillW

1997\08\26@022635 by Pasi T Mustalahti

picon face
On Mon, 25 Aug 1997, Tim Kerby wrote:

> Hi
> Most scales use a device called a load cell which is like a Z shaped chunk
> of metal that gives an output proportional to load.

PTM: Most cheap letter scales use counter, spring, coil and a magnet. A
coil and a magnet from a bass loudspeaker makes a good scale 0..200g.
Of course you can alleways use lewers as they use in the bathroom scales
to make this 0..200g -> 0..200kg.

Another way is to use this thing that I used as a pressure meter:
I had a moving foil and a HEDS1000. I read the the light that reflected
from the foil. It was _sensitive_ when you selected an EU-normal condom as
a foil

--------------------------------------------------------------------------
PTM, KILLspampasi.mustalahtispamBeGonespamutu.fi, EraseMEptmustaspamEraseMEutu.fi, http://www.utu.fi/~ptmusta
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1997\08\26@024953 by Jacques Vrey
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{Quote hidden}

Hi there again
I checked through the Maplin catalogue and couldn't find anything - I
do remember that RS does have something like that in their catalogue
- I'll check.
Dare I say, I like the Idea from Chris Eddy - using a solenoid to
balance the load and then measure the current needed to achieve this.
The mechanics shouldn't be too difficult to implement, or the
electronics for that matter.

Jacques Vrey
Iscor Profile Products Newcastle
Tel:27-(0)3431-48759
@spam@jvrey@spam@spamspam_OUTit.new.iscorltd.co.za

1997\08\26@065227 by nvdw

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John Shreffler wrote:
>
> I have seen electronic scales use current to electromagnetically counter
> gravity, with the equilbrium point being determined by light that was
> obstructed by a blade on the moving part.  The current servos until the

Electronic lab scales use an electromagnetic repulsion type
system. I had a good look at a Sagatorus scale a while back.
It seemed to simply increase the current through a coil until
the weight was balanced by the magnetic repulsion. Measure
current and you can calculate weight. VERY complex mechancics
though......

Nic

1997\08\28@010612 by Josef Hanzal

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face
I remember an article in Czech electronic magazine describing
electromagnetical scale using a big speaker, a tube glued with epoxy to the
cone holded a board on which you put the load. Also had some
optoelectronical sensor. Then you apply current to the speaker, until the
opto is some half way covered and measure the voltage on the speker
proportional to the current. This system had a phase shift due to the
mechanical part even at low frequencies and dependable on the load, so the
feedback part was a little bit tricky. Another drawback - it uses quite a
lot of current.

I am just not sure, if you can find that big speaker to load it directly
with 5 kg. Otherwise it is mechanicly very simple and cheap if you use
speaker with little damage, not suitable for HiFi applications.

Josef Hanzal

>From:    John Shreffler <spamBeGonejohnsspamKILLspamAVENUETECH.COM>

>I have seen electronic scales use current to electromagnetically counter
>gravity, with the equilbrium point being determined by light that was
>obstructed by a blade on the moving part.  The current servos until the
>light is dimmed to about half.  Over narrow ranges the
>response was quite linear.  But I would think that force transducers
>would be a more rugged approach.

1997\08\28@162350 by Octavio Nogueira

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face
Hi Russ,

I'm using the AD7714 and I can not find the AD7730, could you tell me
where You buy it?

Regards,

Octavio
======================================================
Octavio Nogueira  - e-mail:   .....nogueiraspam_OUTspammandic.com.br
http://www.geocities.com/~oct_nogueira
"ProPic" Production PIC Programmer Windows under US$20
======================================================
----
De: Russ Steinbach <TakeThisOuTRussell.E.Steinbach.....spamTakeThisOuTLERC.NASA.GOV>
Para: TakeThisOuTPICLISTKILLspamspamspamMITVMA.MIT.EDU
Data: Segunda-feira, Agosto 25, 1997 09:32
Assunto: Re: PIC Scale -

{Quote hidden}

via
>an SPI bus.
>
>These chip are really a specialized DSP with a programmable gain
>instrumentation amplifier front end.  They have lots of capability for
>filtering, tare, and self calibration and test.  They are also about $12
in
>small quantities.
>
>As you can tell, I like the chip.  But, beware, there is a steep learning
>curve to use them.  Its the price you pay for all the functionality.
>
>
>Russ
>

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