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From: emory!!gallant (Robert Gallant)
X-Source: The Hotrod Mailing list
Date: Apr 1993
Subject: Re: Scales
X-Sequence: 4977

> You jack the car up and assemble the four scales under it.  Since
>jacking with weight distribution is usually a lengthy task it's no
>trouble, particularly when you consider the price.  If you want drive-on
>scales, you'd better have a firm grip on your wallet.

I saw some digital readout bathroom scales that have a max capacity
of 325lbs. They use a strain gauge for the measurement.  They are $25
each.  I think i'll pick up one to see if it can be disassembled without
destroying it.  If so I'll get six, mount three on a steel plate for
each wheel.  Measure two wheels and block the other for the correct
height.  It will be a little inconvenient but I will be able to drive
onto them.

[I've ripped a bunch of digital scales apart (in the store and out :-) and
have never seen any that use strain gauges.  They use the same mechanism
as the conventional "analog" scales, only with a shaft encoder wheel
instead of a pointer.  A pair of optical interrupters feed the quadrature
signals (so direction can be dertermined) to a custom chip.  This chip, in
order to reduce costs, has a "feature" that makes the scales useless for our
needs.   The scale is turned on by operating a switch.  The chip assumes
there is no load on the scale and sets zero accordingly.  As weight is
applied, the scale tracks the rotation of the encoder wheel which oscillates
just like the dial on analog scales.  When the chip detects the wheel is
fairly stable, it displays the reading and starts timing out for an auto
shutdown.  It will not attempt to acquire another reading until the turn-on
switch is operated again.  Which, of course, tells the scale to again
assume zero weight and to zero the display.  Thus the chip enforces the
usual method of blubber evaluation of hit switch, stand on scales, read
weight, turn around and leave.  Not good when there's a car sitting on the
platform.  JGD]


X-Source: The Hotrod Mailing list
Date: Apr 1993
Subject: Re: scales
X-Sequence: 4998

      If you want to do it like the pros, get a load cell for a commercial
weighing system.  The load cells run $200-250 used from electronic surplus
houses like C & H and others (mail me if you need an address).  This gets you a
compression canister with a max load from 500 to 25,000 lbs depending on what
you order.  The canister is about  6 inches tall and 3.5 inches in diameter and
the load is applied to a 1-14 threaded hole in each end of the can or directly
to the end of the canister if you're not real fancy about your setup.  All you
have to do is dig a hole in the driveway, put a piece of steel plate in the
bottom to set the cell on, and bolt another plate on top.  That takes care of
the mechanicals of the setup.  For the electronics, you need one of the fancy
$2000 indicator boxes like we use in the lab or you can make do with a surplus
precision potentiometer box for about $30 and any semi-regulated power supply.
The load cells are simple strain gage units with a full wheatstone bridge
internally.  You put a voltage on two opposite corners of the bridge and read
the output in millivolts on the other corners.  If you want to know more, drop
me a line and I'll explain how the electronics work.

Steve Johnston

[How 'bout posting some contact information for these surplus outfits.
I have a NICE indicator box scavenged from a DOT weighstation but I don't
have any cells.  Sure would like to find a cheap one.  JGD]

From: "Chris Kent Kantarjiev" <emory!!cak>
X-Source: The Hotrod Mailing list
Date: Apr 1993
Subject: Re: Scales
X-Sequence: 5005

	Ah, but the scale mechanism that John describes might be useful for
	things like spring testing setups...

	[Probably.  But they're no more accurate than the analog scales so
	they're	probably not worth the hassles.  JGD]

Except that it seems that I can use them as the basis for a homebuilt
rig instead of shelling out the bucks for a prebuilt one. Unless you
know a source for a *cheap* valve spring and/or suspension spring tester?

[Oh, I'm all in favor of building such a device.  I was simply pointing
out that the analog scales are as accurate as the digital ones but without
the hassles of having to rezero.

A very cheap way to test springs is to simply install a pressure gauge
on a hydraulic jack, say on a press, and calibrate it in lbs.  JGD ]

X-Source: The Hotrod Mailing list
Date: Apr 1993
Subject: Re: Scales
X-Sequence: 5013

On the subject of scales, here's the company that sells used load cells,

	C & H Sales Co.
	P.O. Box 5356
	Pasadena, Ca 91117-9988

[I just called these dudes.  Free catalog is available.  In response to
my query about load cells, the guy said, and I quote, "We have a shitload
of them".  :-)  JGD]

I've dealt with this company before and their descriptions of equipment in the
catalog is accurate.  In addition to load cells, they have motor of all sizes
and RPM, tons 'o solenoids and solenoid valves, gauges, pumps, transformers,
relays up to huge, switches, SCRs, pressure transducers, laser parts, military
carrying cases, and other odd stuff.  They seem to have a mix of military
hardware, aircraft, and older electronic overstock, most of which is new.

Other surplus firms I know of are:

	Marlin P. Jones & Assoc.
	P.O. Box 12685
	Lake Park, FL 33403-0685

They have a more computer/radio orientation to their stock but carry lots of
individual components and a good selection of power supplies

	Herbach and Rademan
	P.O. Box 122
	Bristol, PA 19007-0122

This catalog is more oriented toward science fair/hobbyist types but has lot of
lasers, new multimeters and test equipment, rechargeable batteries, and lots of
other goodies.

If anyone gets a load cell and decides to hook it up, I have a listing of
wiring diagrams/color codes for almost all makes of load cells along with specs
of driving and output voltages which makes life much easier.  Do not attempt to
open a load cell to figure out how to wire it, this often results in permanent,
unrepairable damage, speaking from past experience.

Steve Johnston

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