I'm wanting to make a display for load cells, good looking ones on eBay seem
to start around $150. This is a bit hard to justify for tinkering around,
especially since the load cell I want also costs around $150. As I
understand it, most digital scales work with some sort of strain gauge load
cells. This leads to the idea of getting a cheap scale (bathroom, fishing
scale, food scale) for $20 or so and using the display and electronics to
read whatever load cell I want. In order to do this I would need to be able
to recalibrate the scale. I don't mind simple X10 or X100 display (example,
display shows 200 for 2000 or 20,000 lbs) but I don't want to need a
calculator to scale and offset the reading for units conversion.

Anyone here done anything like this? Know of any cheapie scales that can be
recalibrated to different weights with other load cells? If this isn't easy
to do I'll probably just get a load cell amplifier board and use the scale
and offset pots to get the reading on my voltmeter, maybe something like
1.000 Volts = 10,000 lbs or whatever depending on the load cell.

My most immediate use will be to tension the guy wires on the antenna tower
I'm preparing to put up and later hopefully to do tests on construction of a
homemade crane. I'd like to verify/test my force calculations to keep the
design factor correct.

RogerN

On Feb 21, 11:29*pm, "RogerN" wrote:
I'm wanting to make a display for load cells, good looking ones on eBay seem
to start around $150. *This is a bit hard to justify for tinkering around,
especially since the load cell I want also costs around $150. *As I
understand it, most digital scales work with some sort of strain gauge load
cells. *This leads to the idea of getting a cheap scale (bathroom, fishing
scale, food scale) for $20 or so and using the display and electronics to
read whatever load cell I want. *In order to do this I would need to be able
to recalibrate the scale. *I don't mind simple X10 or X100 display (example,
display shows 200 for 2000 or 20,000 lbs) but I don't want to need a
calculator to scale and offset the reading for units conversion.

Anyone here done anything like this? *Know of any cheapie scales that can be
recalibrated to different weights with other load cells? *If this isn't easy
to do I'll probably just get a load cell amplifier board and use the scale
and offset pots to get the reading on my voltmeter, maybe something like
1.000 Volts = 10,000 lbs or whatever depending on the load cell.

My most immediate use will be to tension the guy wires on the antenna tower
I'm preparing to put up and later hopefully to do tests on construction of a
homemade crane. *I'd like to verify/test my force calculations to keep the
design factor correct.

RogerN

Without a lot of messing about, you're kind of chasing your tail on
converting non-documented equipment. Me, I'd probably start with one
of the embedded microcontroller boards for dummies that uses Basic,
has A to D converters on board and display options. That's if you
want to get a job done. If you just want to fiddle-fart around with
something that'll be obsolete next week, hit the dollar store and pick
up a digital bathroom scale. I think you'll find the innards are
laser-trimmed and one giant blob with no pinouts. Since the model
you'll buy will never be seen again once the shipping container is
emptied, any work you put in on converting same won't ever apply to
anything else.

Raw strain gauges are what's used on a job like you want, epoxy them
on and measure the resistance change. Figure the strain from that.
They were doing that at John Deere 40 years back, before there were
any digital doodads. Cheap enough to be one-time use items.

If you just gotta have load cells, there are a number of surplus
outfits that have had various load ranges in the 10-20 buck range.

Stan

"RogerN" wrote...

I'm wanting to make a display for load cells, ...
Anyone here done anything like this? RogerN

What I had in mind was to make a 10:1 lever divider for a bathroom
scale mechanism so it would read to 3000 Lbs instead of 300. I haven't
dissected one to figure out the specifics.

http://en.wikipedia.org/wiki/Steelyard_balance

You could set up an unbutchered bathroom scale with a lever to confirm
a modified one or calibrate a homebrew cell.

The problem with a lever is frictional hysteresis. I would try needle
bearings and take the average of the reading with the load increasing
and decreasing. A strong spring deflection element like the Dillon
avoids it. On a stationary test setup you could let the pin of a chain
shackle roll on the lever bar.

The Dillon's gain can be adjusted by rotating the hexagonal plug with
the ramp, the zero by moving the indicator stem clamp or the dial
scale ring,
http://wagner.securesites.com/produc..._xc_dillon.jpg

Here's another geometry if you can find some thick-walled tubing,
perhaps a scrap hydraulic cylinder.
http://2.imimg.com/data2/WO/ST/IMFCP...pg-250x250.jpg
Make it too long and adjust by shortening it.

jsw

"Jim Wilkins" wrote in message
...

"RogerN" wrote...

I'm wanting to make a display for load cells, ...
Anyone here done anything like this? RogerN


The larger C clamps in my collection tighten to 1000 - 1500 lbs with
reasonable torque. There's an easy load cell frame if the range is
enough for you.

jsw

"Jim Wilkins" wrote in message ...

"RogerN" wrote...

I'm wanting to make a display for load cells, ...
Anyone here done anything like this? RogerN

What I had in mind was to make a 10:1 lever divider for a bathroom scale
mechanism so it would read to 3000 Lbs instead of 300. I haven't dissected
one to figure out the specifics.

I've been thinking about similar setups. There are lighter hanging scales
in the 300lb to 600lb range with a little mechanical advantage they could be
used for heavier weights. It could be a lever with a solid link at one end
and a scale at the other with various attachment points to get a leverage
from maybe 2:1 to 10:1. This would have friction inaccuracies like you said
but I'm mainly wanting to get ballpark figures to test and not overload.
Another idea is to use a snatch block or pulley (wire?)rope to expand the
scale of the lighter duty scale.

http://en.wikipedia.org/wiki/Steelyard_balance

You could set up an unbutchered bathroom scale with a lever to confirm a
modified one or calibrate a homebrew cell.

The problem with a lever is frictional hysteresis. I would try needle
bearings and take the average of the reading with the load increasing and
decreasing. A strong spring deflection element like the Dillon avoids it.
On a stationary test setup you could let the pin of a chain shackle roll on
the lever bar.

The Dillon's gain can be adjusted by rotating the hexagonal plug with the
ramp, the zero by moving the indicator stem clamp or the dial scale ring,
http://wagner.securesites.com/produc..._xc_dillon.jpg

Here's another geometry if you can find some thick-walled tubing, perhaps a
scrap hydraulic cylinder.
http://2.imimg.com/data2/WO/ST/IMFCP...pg-250x250.jpg
Make it too long and adjust by shortening it.

jsw

At work we have some very heavy duty tension gauges on our commercial
vehicle tire curing presses. There is a link like a connecting rod,
estimating 10 feet long, 2 feet wide by about 4 inches thick. There is a
dial indicator attached near the top of the rod and a free floating rod at
the other end. This measures how much the almost 100 square inches of steel
stretches over a distance. Expanding on that idea I could have a steel link
with a dial indicator to measure how much the link stretches, using
different sizes of links for different ranges.

RogerN

"Jim Wilkins" wrote in message ...


"Jim Wilkins" wrote in message
...

"RogerN" wrote...

I'm wanting to make a display for load cells, ...
Anyone here done anything like this? RogerN


The larger C clamps in my collection tighten to 1000 - 1500 lbs with
reasonable torque. There's an easy load cell frame if the range is enough
for you.

jsw

I'm thinking I can bolt a light duty load cell to a heavier bar and
calibrate. Most of the load cells I work with are rated to handle 150% load
before they are damaged, I think I can try using a 1.5 design factor and
hopefully the metal will deform the correct amount.

RogerN

"Stanley Schaefer" wrote in message
...

On Feb 21, 11:29 pm, "RogerN" wrote:
I'm wanting to make a display for load cells, good looking ones on eBay
seem
snip

Without a lot of messing about, you're kind of chasing your tail on
converting non-documented equipment. Me, I'd probably start with one
of the embedded microcontroller boards for dummies that uses Basic,
has A to D converters on board and display options. That's if you
want to get a job done. If you just want to fiddle-fart around with
something that'll be obsolete next week, hit the dollar store and pick
up a digital bathroom scale. I think you'll find the innards are
laser-trimmed and one giant blob with no pinouts. Since the model
you'll buy will never be seen again once the shipping container is
emptied, any work you put in on converting same won't ever apply to
anything else.

Raw strain gauges are what's used on a job like you want, epoxy them
on and measure the resistance change. Figure the strain from that.
They were doing that at John Deere 40 years back, before there were
any digital doodads. Cheap enough to be one-time use items.

If you just gotta have load cells, there are a number of surplus
outfits that have had various load ranges in the 10-20 buck range.

Stan

I have a couple of Arduino boards and an AD7730? chip for weigh scales, it
has 24 bit A/D converter IIRC. I like this idea because I can save
calibration for different load cells to eeprom. So I could use the same
Arduino scale display for a reloading scale, plug in a different load cell
and have a 10,000 lb crane scale. Arduino already has a built in function
to scale and offset readings. The down side of this is I need to get it all
mounted in a case and set up for battery operation for the crane scale duty,
not too bad though.

I found some big game scales online in the $30 range for 300lb digital or
440 pound dial, I will probably get one of those for tensioning my guy wires
for the antenna tower. According to the rules of thumb for my 1/8" wire
rope I need to tension the guy wires to 170lbs.

I saw some videos on youtube of calibrating some of the chinese scales, some
let you specify the weight you are calibrating at, not sure how well it
would work with a different load cell and a much larger full scale number.

RogerN

"RogerN" wrote in message
...
"Jim Wilkins" wrote in message ...

"RogerN" wrote...

I'm wanting to make a display for load cells, ...
Anyone here done anything like this? RogerN

...
Another idea is to use a snatch block or pulley (wire?)rope to
expand the scale of the lighter duty scale.

I made a 2x two-wheel block and tackle to raise my TV antenna, with
well-greased Oilite bushings. The ratio between the raising and
lowering rope tension is over two to one because of pulley bearing
friction. A pulley arrangement might be acceptable with needle
bearings but they are a lot of work to make, and waste if it isn't.

A dial scale is relatively safe if its hanging, but not when it's part
of a horizontal or angled pull unless you set up a table or such to
support the scale and the heavy pulling devices while you are hooking
up and taking in the catenary slack.

I wouldn't put anything that wasn't simple, rugged and foolproof in
line with a long cable under heavy tension and then stand next to it.
Once a cable eye clamp I was testing let go and dumped all my gear
hard on the ground, knocking the crystal off the Dillon's dial
indicator. For testing I now place the equipment on card tables and
connect it between two big trees. There are tow straps loosely
attached to the gear to prevent it from whipping in my direction if it
breaks.

jsw

"RogerN" wrote in message

I found some big game scales online in the $30 range for 300lb
digital or 440 pound dial, I will probably get one of those for
tensioning my guy wires for the antenna tower. According to the
rules of thumb for my 1/8" wire rope I need to tension the guy wires
to 170lbs.

RogerN

The elderly British radar boffin who taught the ham radio class at
MITRE 'invited' us to his house to see his station and work on his
antenna. I'm pretty sure he said that the guy lines didn't have to be
tensioned, just the slack taken up.

jsw

I had a $20 bathroom scale apart a few/5 years ago for the same reasons.
What is found is likely to be a nearly flat hump of epoxy covering a
leadless ADC IC on the module, and only about a dozen external components
(no board markings for components or module identity).

The trouble with throw-away products is that there is rarely any
documentation available.. no FCC approval or any other regulations wrt
standards or specs, so no need to submit any design specs before production.

I posted some questions in a newsgroup (maybe sci.electronics.repair) asking
about DIY adaptations of similar scale modules, and was lucky that a guy who
had some experience in designing and/or working background with similar
products, replied with the limited info I've included here wrt capacity and
span.

There are selected component values to meet the specific cell/sensor (also
unmarked), capacity of the scale, and span values, IIRC.
So as the Chinese are famously known for, they'll replace a 3 cent pot with
a .003 cent fixed resistor every time it's possible.

BTW, the 3 digit display wasn't even the usual 7-segment LED digits, but
instead, the actual tiny individual chips without encapsulization (you can
see 'em gazing into a clear acrylic LED), placed in an arrangement of 7 per
digit.. and the materials that made up the 7 "bars" of the 3 digits was
actually a colored cellophane-like film over a plastic box with partitions
in it to simulate a 3 digit LED display.
When seen thru the front panel red plastic display window, it's unlikely
that anyone would have guessed what the actual internal display components
were.

The individual LED chips had the miniscule gage gold bonding wires attached
to gold "plated" dots at the ends of the board traces.

So.. the the printed retail product box, molded battery compartment and 9V
clip lead were relatively expensive parts in the manufacture of that scale.

It looks as though at least several others would also be interested in what
you find out if you try experimenting with some inexpensive
off-the-retail-shelf items.

--
WB
..........


"RogerN" wrote in message
...

I'm wanting to make a display for load cells, good looking ones on eBay
seem to start around $150. This is a bit hard to justify for tinkering
around, especially since the load cell I want also costs around $150. As
I understand it, most digital scales work with some sort of strain gauge
load cells. This leads to the idea of getting a cheap scale (bathroom,
fishing scale, food scale) for $20 or so and using the display and
electronics to read whatever load cell I want. In order to do this I
would need to be able to recalibrate the scale. I don't mind simple X10
or X100 display (example, display shows 200 for 2000 or 20,000 lbs) but I
don't want to need a calculator to scale and offset the reading for units
conversion.

Anyone here done anything like this? Know of any cheapie scales that can
be recalibrated to different weights with other load cells? If this isn't
easy to do I'll probably just get a load cell amplifier board and use the
scale and offset pots to get the reading on my voltmeter, maybe something
like 1.000 Volts = 10,000 lbs or whatever depending on the load cell.

My most immediate use will be to tension the guy wires on the antenna
tower I'm preparing to put up and later hopefully to do tests on
construction of a homemade crane. I'd like to verify/test my force
calculations to keep the design factor correct.

RogerN

On Wed, 22 Feb 2012 21:13:42 -0500, "Jim Wilkins"
wrote:
"RogerN" wrote in message

I found some big game scales online in the $30 range for 300lb
digital or 440 pound dial, I will probably get one of those for
tensioning my guy wires for the antenna tower. According to the
rules of thumb for my 1/8" wire rope I need to tension the guy wires
to 170lbs.

The elderly British radar boffin who taught the ham radio class at
MITRE 'invited' us to his house to see his station and work on his
antenna. I'm pretty sure he said that the guy lines didn't have to be
tensioned, just the slack taken up.

Small towers it isn't all that critical, as long as they are tight and
the tower is straight vertical - Very Hard on the mast if the guys are
tensioned unevenly making it bend like a banana.

I would rig the digital scale in one of the three guys at the bottom
level to get 170 Pounds tension, then pluck the guy wire and note the
tone of the "Ping" that will correlate with length and tension. Get a
Guitar Tuner or microphone/pickup and a Frequency Counter if you want
to be pedantic.

Then take the tension scale out of the wire and reassemble the
thimbles and turnbuckle, tap with a wrench, and "Tune" all the guys at
that level to the same note. Done.

Repeat for the second, third, (fourth...) sets of guy wires attached
higher up on the mast.

-- Bruce --

"Bruce L. Bergman (munged human readable)"
wrote in message

Small towers it isn't all that critical, as long as they are tight
and
the tower is straight vertical - Very Hard on the mast if the guys
are
tensioned unevenly making it bend like a banana.

I would rig the digital scale in one of the three guys at the bottom
level to get 170 Pounds tension, then pluck the guy wire and note
the
tone of the "Ping" that will correlate with length and tension. Get
a
Guitar Tuner or microphone/pickup and a Frequency Counter if you
want
to be pedantic.

Then take the tension scale out of the wire and reassemble the
thimbles and turnbuckle, tap with a wrench, and "Tune" all the guys
at
that level to the same note. Done.

Repeat for the second, third, (fourth...) sets of guy wires attached
higher up on the mast.

-- Bruce --

Though this may not apply to you, the guy lines on my 50 foot TV
antenna mast run over pulleys on a suspended ring at the top and down
to rope cleat tie-offs at the base. I can sight up the mast to pull it
straight.

Since it telescopes the guy line tension is only a few pounds to
minimize the thrust on the base-mounted rotator and the lift.

jsw

On 2/22/2012 6:57 PM, Jim Wilkins wrote:
wrote in message
...
"Jim Wilkins" wrote in message ...

wrote...

I'm wanting to make a display for load cells, ...
Anyone here done anything like this? RogerN

...
Another idea is to use a snatch block or pulley (wire?)rope to
expand the scale of the lighter duty scale.

I made a 2x two-wheel block and tackle to raise my TV antenna, with
well-greased Oilite bushings. The ratio between the raising and
lowering rope tension is over two to one because of pulley bearing
friction. A pulley arrangement might be acceptable with needle
bearings but they are a lot of work to make, and waste if it isn't.

A dial scale is relatively safe if its hanging, but not when it's part
of a horizontal or angled pull unless you set up a table or such to
support the scale and the heavy pulling devices while you are hooking
up and taking in the catenary slack.

I wouldn't put anything that wasn't simple, rugged and foolproof in
line with a long cable under heavy tension and then stand next to it.
Once a cable eye clamp I was testing let go and dumped all my gear
hard on the ground, knocking the crystal off the Dillon's dial
indicator. For testing I now place the equipment on card tables and
connect it between two big trees. There are tow straps loosely
attached to the gear to prevent it from whipping in my direction if it
breaks.

jsw



they make a little tension guide for tuning sailboat rigging (so you can
get the mast in column straight). pretty cheap, sort of like a spring
scale on a simple piece of sheet metal.

On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the antenna tower
I'm preparing to put up and later hopefully to do tests on construction of a
homemade crane. I'd like to verify/test my force calculations to keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

"Kristian Ukkonen" wrote in message
...
On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the
antenna tower
I'm preparing to put up and later hopefully to do tests on
construction of a
homemade crane. I'd like to verify/test my force calculations to
keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

The elongation is two parts per thousand at the (arbitrarily) defined
yield point of steel.
http://www.keytometals.com/Article53.htm
Right above "Ductility".

This small amount is difficult to measure accurately in the field,
like on a steel cable guy line, being only 1mm per meter at a 2x
safety factor.

jsw

On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
wrote:


"Kristian Ukkonen" wrote in message
...
On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the
antenna tower
I'm preparing to put up and later hopefully to do tests on
construction of a
homemade crane. I'd like to verify/test my force calculations to
keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

The elongation is two parts per thousand at the (arbitrarily) defined
yield point of steel.
http://www.keytometals.com/Article53.htm
Right above "Ductility".

This small amount is difficult to measure accurately in the field,
like on a steel cable guy line, being only 1mm per meter at a 2x
safety factor.

jsw


That .2% is the amount of non-elastic stretch used to define the yield
point; it's not the amount of elastic deformation before yield. The
suggestion was to measure *elastic* strain (delta length/length) in
order to determine tension. The change in length before yield will be
approx = stress/Young's modulus x length, stress equal to
tension/area. Still a small number.

The bigger problem is that I'm quite sure the stress/strain behavior
of a spiral wound cable won't be nice and linear like a solid wire's.
The sailboat tuning devices someone else mentioned work by measuring
the force required to deflect the wire thru a given angle, and don't
rely on the elastic properties of the wire.

--
Ned Simmons

"Bruce L. Bergman (munged human readable)" wrote:

On Wed, 22 Feb 2012 21:13:42 -0500, "Jim Wilkins"
wrote:
"RogerN" wrote in message

I found some big game scales online in the $30 range for 300lb
digital or 440 pound dial, I will probably get one of those for
tensioning my guy wires for the antenna tower. According to the
rules of thumb for my 1/8" wire rope I need to tension the guy wires
to 170lbs.

The elderly British radar boffin who taught the ham radio class at
MITRE 'invited' us to his house to see his station and work on his
antenna. I'm pretty sure he said that the guy lines didn't have to be
tensioned, just the slack taken up.

Small towers it isn't all that critical, as long as they are tight and
the tower is straight vertical - Very Hard on the mast if the guys are
tensioned unevenly making it bend like a banana.

I would rig the digital scale in one of the three guys at the bottom
level to get 170 Pounds tension, then pluck the guy wire and note the
tone of the "Ping" that will correlate with length and tension. Get a
Guitar Tuner or microphone/pickup and a Frequency Counter if you want
to be pedantic.

Then take the tension scale out of the wire and reassemble the
thimbles and turnbuckle, tap with a wrench, and "Tune" all the guys at
that level to the same note. Done.

Repeat for the second, third, (fourth...) sets of guy wires attached
higher up on the mast.


And hope that an Airforce jet doesn't clip one of the guy wires.
It's happened at least once. A small UHF TV station on the east coast
of Florida lost the top of their tower that way and it 'went dark',
rather han rebuild. That was about 20 years ago.

--
You can't have a sense of humor, if you have no sense.

On 2/23/2012 3:03 PM, Ned Simmons wrote:
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
wrote:


"Kristian wrote in message
...
On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the
antenna tower
I'm preparing to put up and later hopefully to do tests on
construction of a
homemade crane. I'd like to verify/test my force calculations to
keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

The elongation is two parts per thousand at the (arbitrarily) defined
yield point of steel.
http://www.keytometals.com/Article53.htm
Right above "Ductility".

This small amount is difficult to measure accurately in the field,
like on a steel cable guy line, being only 1mm per meter at a 2x
safety factor.

jsw


That .2% is the amount of non-elastic stretch used to define the yield
point; it's not the amount of elastic deformation before yield. The
suggestion was to measure *elastic* strain (delta length/length) in
order to determine tension. The change in length before yield will be
approx = stress/Young's modulus x length, stress equal to
tension/area. Still a small number.

The bigger problem is that I'm quite sure the stress/strain behavior
of a spiral wound cable won't be nice and linear like a solid wire's.
The sailboat tuning devices someone else mentioned work by measuring
the force required to deflect the wire thru a given angle, and don't
rely on the elastic properties of the wire.


these things: http://loosnaples.com/how-tos/tension-gauges

"chaniarts" wrote in message
...
...
they make a little tension guide for tuning sailboat rigging (so you
can get the mast in column straight). pretty cheap, sort of like a
spring scale on a simple piece of sheet metal.

I felt an involuntary cold shudder at "sailboat", but they
aren't -that- expensive:
http://www.westmarine.com/webapp/wcs...1859&langId=-1

It looks like car serpentine belt tension gauges don't go high enough.

jsw

"Ned Simmons" wrote in message
...
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
wrote:

That .2% is the amount of non-elastic stretch used to define the
yield
point; it's not the amount of elastic deformation before yield. The
suggestion was to measure *elastic* strain (delta length/length) in
order to determine tension. The change in length before yield will
be
approx = stress/Young's modulus x length, stress equal to
tension/area. Still a small number.

--
Ned Simmons

If Young's Modulus is 30E6 and the load is 30E3, what's the
elongation?

jsw

On Thu, 23 Feb 2012 17:18:04 -0500, "Michael A. Terrell"
wrote:

And hope that an Airforce jet doesn't clip one of the guy wires.
It's happened at least once. A small UHF TV station on the east coast
of Florida lost the top of their tower that way and it 'went dark',
rather than rebuild. That was about 20 years ago.

That raises a lot of questions: What was the Air Force Jet doing
hugging the deck that tight (and not on a Military Base) without
knowing what was there first? And was the tower tall enough and/or
in an approach path to where it was required to have obstruction
lights? And did it have them installed and operational?

Unless there was negligence on the part of the Tower Owner, the Air
Force owes them a new tower. "You break it, you bought it." It's on
the charts...

-- Bruce --

On Thu, 23 Feb 2012 17:48:49 -0500, "Jim Wilkins"
wrote:


"Ned Simmons" wrote in message
.. .
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
wrote:

That .2% is the amount of non-elastic stretch used to define the
yield
point; it's not the amount of elastic deformation before yield. The
suggestion was to measure *elastic* strain (delta length/length) in
order to determine tension. The change in length before yield will
be
approx = stress/Young's modulus x length, stress equal to
tension/area. Still a small number.

--
Ned Simmons

If Young's Modulus is 30E6 and the load is 30E3, what's the
elongation?

jsw


Depends on the cross sectional area and length of the member. If, for
example, the load is 30E3 lb on 1 in^2 of steel (Y=30E6 lb/in^2):
stress = Force/area = 30E3 lb / in^2
strain = stress/Young's modulus = 30E3 lb/in^2 / 30E6 lb/in^2 = 1E-3
in/in

Over a 10 inch length the change would be 10 in x 1E-3 in/in = .01 in

In other words, strain is a ratio that represents the change in length
(length/length) of a body under strain.

--
Ned Simmons

On Thu, 23 Feb 2012 19:16:48 -0500, Ned Simmons
wrote:

On Thu, 23 Feb 2012 17:48:49 -0500, "Jim Wilkins"
wrote:


"Ned Simmons" wrote in message
. ..
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
wrote:

That .2% is the amount of non-elastic stretch used to define the
yield
point; it's not the amount of elastic deformation before yield. The
suggestion was to measure *elastic* strain (delta length/length) in
order to determine tension. The change in length before yield will
be
approx = stress/Young's modulus x length, stress equal to
tension/area. Still a small number.

--
Ned Simmons

If Young's Modulus is 30E6 and the load is 30E3, what's the
elongation?

jsw


Depends on the cross sectional area and length of the member. If, for
example, the load is 30E3 lb on 1 in^2 of steel (Y=30E6 lb/in^2):
stress = Force/area = 30E3 lb / in^2
strain = stress/Young's modulus = 30E3 lb/in^2 / 30E6 lb/in^2 = 1E-3
in/in

Over a 10 inch length the change would be 10 in x 1E-3 in/in = .01 in

In other words, strain is a ratio that represents the change in length
(length/length) of a body under strain.

Oops, make the last word "stress".

--
Ned Simmons

"Ned Simmons" wrote in message
...
On Thu, 23 Feb 2012 19:16:48 -0500, Ned Simmons
wrote:

On Thu, 23 Feb 2012 17:48:49 -0500, "Jim Wilkins"
wrote:...
If Young's Modulus is 30E6 and the load is 30E3, what's the
elongation?
jsw

Depends on the cross sectional area and length of the member. If,
for
example, the load is 30E3 lb on 1 in^2 of steel (Y=30E6 lb/in^2):
stress = Force/area = 30E3 lb / in^2
strain = stress/Young's modulus = 30E3 lb/in^2 / 30E6 lb/in^2 = 1E-3
in/in

Over a 10 inch length the change would be 10 in x 1E-3 in/in = .01
in

In other words, strain is a ratio that represents the change in
length
(length/length) of a body under stress.
Ned Simmons

And thus my millimeter per meter, which I think is too little to
measure accurately on a twisted wire cable with a catenary droop. It
might work with tie rods salvaged from a defunct hydraulic cylinder,
if you had a test rod that fit the gap between two collar clamps and
you measured the end clearance under load with feeler gauges.
http://image.made-in-china.com/2f0j0...ed-Eye-Nut.jpg

jsw

"Bruce L. Bergman (munged human readable)" wrote:

On Thu, 23 Feb 2012 17:18:04 -0500, "Michael A. Terrell"
wrote:

And hope that an Airforce jet doesn't clip one of the guy wires.
It's happened at least once. A small UHF TV station on the east coast
of Florida lost the top of their tower that way and it 'went dark',
rather than rebuild. That was about 20 years ago.

That raises a lot of questions: What was the Air Force Jet doing
hugging the deck that tight (and not on a Military Base) without
knowing what was there first? And was the tower tall enough and/or
in an approach path to where it was required to have obstruction
lights? And did it have them installed and operational?

Unless there was negligence on the part of the Tower Owner, the Air
Force owes them a new tower. "You break it, you bought it." It's on
the charts...


I heard claims that it was a careless pilot, but I also heard that
the jet had an instumentation failure. The results of the investigation
were never released. I beleve there were heavy clouds in spots, and the
tip of one wink snagged one of the top guy wires, snapping it and
buckling the top of the tower. The tower company was out of business,
so no new parts were availible and other companies wanted repalcment
parts signed off by a mechanical engineer. The station was low budget,
and already on the verge of failure, so the accident was the last straw.

The federal government finally paid for the damage, but by then the
station was bankrupt. The paid off some bills with the money and walked
away.


--
You can't have a sense of humor, if you have no sense.

"Jim Wilkins" wrote in message ...

snip

I wouldn't put anything that wasn't simple, rugged and foolproof in line
with a long cable under heavy tension and then stand next to it. Once a
cable eye clamp I was testing let go and dumped all my gear hard on the
ground, knocking the crystal off the Dillon's dial indicator. For testing I
now place the equipment on card tables and connect it between two big
trees. There are tow straps loosely attached to the gear to prevent it from
whipping in my direction if it breaks.

jsw

So far I'm leaning toward something like this:
http://www.amazon.com/gp/product/B00...A1DVPD9Q368DLM

I figure I can make a link with the same dimensions hole to hole as the
scale. This way I can use the scale to tension the lines, mark the
turnbuckles, loosen, replace scale with metal link, and adjust turnbuckles
to bring the tension back. I figure the scale will stretch a little under
the tension, if needed I can measure scale hole to hole with 170 lbs
tension. I like that design of scale because I should be able to use
couplers with pins that will fit the scale.

Some of the cheapie Chinese scales appear to be able to be calibrated at
different weights. I was going to paste a link to a video showing
calibration procedure but my internet connection is so slow I can't play a
video right now.

I have a Chinese scale that I can play with. If I figure out the milivolts
per volt of their load cells (assuming they use load cells) I can see what
values it lets me calibrate to. For example, my scale is 60 lbs capacity
IIRC, if I can get the correct millivolt input with perhaps a 1000 lb load
cell will it let me calibrate to 1000 lbs or is it programmed internally to
only work for a 60 lb scale? It would be an advantage for the manufacturer
to not have to change electronics for whatever range of scale they used it
for.

I will have ropes on the tower (small tower ~30' or so) for supporting while
I get the guy wires installed. Also I plan to use a plumb bob in the center
of the tower to get it adjusted vertical. I plan to tie my ropes in loops
so I can untie and remove from tower without climbing. I'm wanting to have
this tower hinged so I can lower it for service and raise it back up again
with a (gin?) pole and winch.

RogerN

Here's a video of calibrating a cheapie scale, shows how to change the
calibration weight.

http://www.youtube.com/watch?v=_O6n7hic9N4

RogerN

On Thu, 23 Feb 2012 09:49:47 -0800, "Bruce L. Bergman (munged human
readable)" wrote:

On Wed, 22 Feb 2012 21:13:42 -0500, "Jim Wilkins"
wrote:
"RogerN" wrote in message

I found some big game scales online in the $30 range for 300lb
digital or 440 pound dial, I will probably get one of those for
tensioning my guy wires for the antenna tower. According to the
rules of thumb for my 1/8" wire rope I need to tension the guy wires
to 170lbs.

The elderly British radar boffin who taught the ham radio class at
MITRE 'invited' us to his house to see his station and work on his
antenna. I'm pretty sure he said that the guy lines didn't have to be
tensioned, just the slack taken up.

Small towers it isn't all that critical, as long as they are tight and
the tower is straight vertical - Very Hard on the mast if the guys are
tensioned unevenly making it bend like a banana.

I would rig the digital scale in one of the three guys at the bottom
level to get 170 Pounds tension, then pluck the guy wire and note the
tone of the "Ping" that will correlate with length and tension. Get a
Guitar Tuner or microphone/pickup and a Frequency Counter if you want
to be pedantic.

Then take the tension scale out of the wire and reassemble the
thimbles and turnbuckle, tap with a wrench, and "Tune" all the guys at
that level to the same note. Done.

Repeat for the second, third, (fourth...) sets of guy wires attached
higher up on the mast.

-- Bruce --
IIRC, when you use three guys, they all be under the same tension so
long as they are spread at equal angles. With four guys they tension
in pairs.

On Thu, 23 Feb 2012 17:18:04 -0500, "Michael A. Terrell"
wrote:


"Bruce L. Bergman (munged human readable)" wrote:

On Wed, 22 Feb 2012 21:13:42 -0500, "Jim Wilkins"
wrote:
"RogerN" wrote in message

I found some big game scales online in the $30 range for 300lb
digital or 440 pound dial, I will probably get one of those for
tensioning my guy wires for the antenna tower. According to the
rules of thumb for my 1/8" wire rope I need to tension the guy wires
to 170lbs.

The elderly British radar boffin who taught the ham radio class at
MITRE 'invited' us to his house to see his station and work on his
antenna. I'm pretty sure he said that the guy lines didn't have to be
tensioned, just the slack taken up.

Small towers it isn't all that critical, as long as they are tight and
the tower is straight vertical - Very Hard on the mast if the guys are
tensioned unevenly making it bend like a banana.

I would rig the digital scale in one of the three guys at the bottom
level to get 170 Pounds tension, then pluck the guy wire and note the
tone of the "Ping" that will correlate with length and tension. Get a
Guitar Tuner or microphone/pickup and a Frequency Counter if you want
to be pedantic.

Then take the tension scale out of the wire and reassemble the
thimbles and turnbuckle, tap with a wrench, and "Tune" all the guys at
that level to the same note. Done.

Repeat for the second, third, (fourth...) sets of guy wires attached
higher up on the mast.


And hope that an Airforce jet doesn't clip one of the guy wires.
It's happened at least once. A small UHF TV station on the east coast
of Florida lost the top of their tower that way and it 'went dark',
rather han rebuild. That was about 20 years ago.
After Transport Canada spent MEGA-bucks on tree trimming/clearing for
the secondary approach at our local airport, I made the comment on how
much we had improved the reception on a certain whip antenna dead on
centerline and about 4000 feet from the end of the runway. A lot of
people got exited very quickly - no one had even noticed it, they had
been so busy looking at trees.

wrote:

After Transport Canada spent MEGA-bucks on tree trimming/clearing for
the secondary approach at our local airport, I made the comment on how
much we had improved the reception on a certain whip antenna dead on
centerline and about 4000 feet from the end of the runway. A lot of
people got exited very quickly - no one had even noticed it, they had
been so busy looking at trees.


Sounds like what they used to say at Ft. Greely, AK: "There is a
beautiful woman behind every tree, but you can't find the trees for the
forest..."


--
You can't have a sense of humor, if you have no sense.

On Thu, 23 Feb 2012 20:11:33 -0500, "Jim Wilkins"
wrote:


"Ned Simmons" wrote in message
.. .
On Thu, 23 Feb 2012 19:16:48 -0500, Ned Simmons
wrote:

On Thu, 23 Feb 2012 17:48:49 -0500, "Jim Wilkins"
wrote:...
If Young's Modulus is 30E6 and the load is 30E3, what's the
elongation?
jsw

Depends on the cross sectional area and length of the member. If,
for
example, the load is 30E3 lb on 1 in^2 of steel (Y=30E6 lb/in^2):
stress = Force/area = 30E3 lb / in^2
strain = stress/Young's modulus = 30E3 lb/in^2 / 30E6 lb/in^2 = 1E-3
in/in

Over a 10 inch length the change would be 10 in x 1E-3 in/in = .01
in

In other words, strain is a ratio that represents the change in
length
(length/length) of a body under stress.
Ned Simmons

And thus my millimeter per meter, which I think is too little to
measure accurately on a twisted wire cable with a catenary droop. It
might work with tie rods salvaged from a defunct hydraulic cylinder,
if you had a test rod that fit the gap between two collar clamps and
you measured the end clearance under load with feeler gauges.
http://image.made-in-china.com/2f0j0...ed-Eye-Nut.jpg

jsw


I agree with your conclusion, but the .2% number you used to get there
has nothing to do with elastic deformation, and it's only a
coincidence that our numbers are close. I could just as easily have
used .1 in^2 as the area in my example, and the resulting strain would
be 10mm per meter.

I suspect the practical limitations of tensile testing machines 100
years ago is why the transition from elastic to plastic was designated
at .2% elongation. In other words, that's what could be reliably
detected.

--
Ned Simmons

"RogerN" wrote in message

So far I'm leaning toward something like this:
http://www.amazon.com/gp/product/B00...A1DVPD9Q368DLM

I figure I can make a link with the same dimensions hole to hole as
the scale. This way I can use the scale to tension the lines, mark
the turnbuckles, loosen, replace scale with metal link, and adjust
turnbuckles to bring the tension back. I figure the scale will
stretch a little under the tension, if needed I can measure scale
hole to hole with 170 lbs tension. I like that design of scale
because I should be able to use couplers with pins that will fit the
scale....
RogerN

Good catch! It looks considerably more compact and durable than the
discontinued HF 440 Lb Big Game Scale.

You could make a simple comparison gauge that measures cable
deflection between pins with a fishing scale and tighten the
turnbuckle to the same reading. It would be like checking engine belt
tension by thumb pressure deflection or how far it will twist, unable
to measure the force but adequate to set it close enough to the
correct value.

In practice I've found that the forces I want to measure around the
house range from 500 Lbs to a max of ~3000 Lbs, for logging and
pulling stumps. It's difficult to apply more than 3000 Lbs with
equipment made from lumber yard and hardware store materials and I can
buy higher-rated chains and tow straps or call a wrecker for stuck
vehicles.

3000 to 3500 lbs seems to be enough to move the base of a large (24")
oak that caught in another one as it fell. I've pulled down two that
blew down during storms behind neighbor's houses. Both were straight
enough to saw into lumber so I pulled them down intact.

jsw

"Ned Simmons" wrote in message
...
On Thu, 23 Feb 2012 20:11:33 -0500, "Jim Wilkins"
wrote:

I agree with your conclusion, but the .2% number you used to get
there
has nothing to do with elastic deformation, and it's only a
coincidence that our numbers are close. I could just as easily have
used .1 in^2 as the area in my example, and the resulting strain
would
be 10mm per meter.

I suspect the practical limitations of tensile testing machines 100
years ago is why the transition from elastic to plastic was
designated
at .2% elongation. In other words, that's what could be reliably
detected.
Ned Simmons

I should have written 30E3 PSI, not total stress. 300KSI material is
beyond my machining and heat-treating ability so its superior
elongation doesn't help for homebrew load cells.

I operated one of those old Tinius Olsen tensile strength machines in
a test lab and agree with you. It was kind of tricky to get good
repeatable results from it.

jsw

On 2/24/2012 0:03, Ned Simmons wrote:
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
"Kristian wrote in message
On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the
antenna tower
I'm preparing to put up and later hopefully to do tests on
construction of a
homemade crane. I'd like to verify/test my force calculations to
keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

The elongation is two parts per thousand at the (arbitrarily) defined
yield point of steel.
http://www.keytometals.com/Article53.htm
Right above "Ductility".

This small amount is difficult to measure accurately in the field,
like on a steel cable guy line, being only 1mm per meter at a 2x
safety factor.

Here are two patents describing such devices:

http://www.google.com/patents/US5333...page&q&f=false

http://www.docstoc.com/docs/32588662...Patent-4562743

For bandsaw blades:

http://www.toolcenter.com/62126.html
http://bbs.homeshopmachinist.net/showthread.php?t=21171

Although I have to admit that it makes sense to fix two wheels against
the cable and deflect the cable at the middlepoint between wheels
a known amount, and measure force needed for deflection. Elegant.

http://www.checkline.com/cable_tensiometer/CTM

BUT it again requires for force measurement.. The ones above only
require measurement of elongation with dial indicator, which is cheap
compared to load cell and electronics..

On Fri, 24 Feb 2012 20:24:25 +0200, Kristian Ukkonen
wrote:

On 2/24/2012 0:03, Ned Simmons wrote:
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
"Kristian wrote in message
On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the
antenna tower
I'm preparing to put up and later hopefully to do tests on
construction of a
homemade crane. I'd like to verify/test my force calculations to
keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

The elongation is two parts per thousand at the (arbitrarily) defined
yield point of steel.
http://www.keytometals.com/Article53.htm
Right above "Ductility".

This small amount is difficult to measure accurately in the field,
like on a steel cable guy line, being only 1mm per meter at a 2x
safety factor.

Here are two patents describing such devices:

http://www.google.com/patents/US5333...page&q&f=false

http://www.docstoc.com/docs/32588662...Patent-4562743

For bandsaw blades:

http://www.toolcenter.com/62126.html
http://bbs.homeshopmachinist.net/showthread.php?t=21171

Although I have to admit that it makes sense to fix two wheels against
the cable and deflect the cable at the middlepoint between wheels
a known amount, and measure force needed for deflection. Elegant.

http://www.checkline.com/cable_tensiometer/CTM

BUT it again requires for force measurement.. The ones above only
require measurement of elongation with dial indicator, which is cheap
compared to load cell and electronics..

In practice, I think that most people who use the deflection-type
gages use them more for relative measures. From experience, you gain a
good idea of what values work for you, and at what point you start
breaking blades.

--
Ed Huntress

On Fri, 24 Feb 2012 12:29:22 -0500, "Jim Wilkins"
wrote:


"Ned Simmons" wrote in message
.. .
On Thu, 23 Feb 2012 20:11:33 -0500, "Jim Wilkins"
wrote:

I agree with your conclusion, but the .2% number you used to get
there
has nothing to do with elastic deformation, and it's only a
coincidence that our numbers are close. I could just as easily have
used .1 in^2 as the area in my example, and the resulting strain
would
be 10mm per meter.

I suspect the practical limitations of tensile testing machines 100
years ago is why the transition from elastic to plastic was
designated
at .2% elongation. In other words, that's what could be reliably
detected.
Ned Simmons

I should have written 30E3 PSI, not total stress. 300KSI material is
beyond my machining and heat-treating ability so its superior
elongation doesn't help for homebrew load cells.

300ksi is close to the ultimate tensile of the highest strength steel
wire used in cable, so clearly you wouldn't tension it anywhere near
that level. I was just trying to clarify the distinction between the
..2% elongation (that's used to mark the elastic/plastic boundary) and
the elastic elongation that occurs between 0 PSI and yield.


I operated one of those old Tinius Olsen tensile strength machines in
a test lab and agree with you. It was kind of tricky to get good
repeatable results from it.

jsw


I've never operated one, but can imagine there are lots of variables
that are difficult to control.

--
Ned Simmons

On 2/24/2012 20:24, Kristian Ukkonen wrote:
Although I have to admit that it makes sense to fix two wheels against
the cable and deflect the cable at the middlepoint between wheels
a known amount, and measure force needed for deflection. Elegant.

http://www.checkline.com/cable_tensiometer/CTM

BUT it again requires for force measurement.. The ones above only
require measurement of elongation with dial indicator, which is cheap
compared to load cell and electronics..

Actually! The easy way is to use PULL instead of push there..
A digital fish scale or such ready-made-cheap-load-cell could
then be used directly to pull the cable a known amount (angle).
Then it becames a vector sum of the forces, I think..

Of course, it is not an on-line measurement, but one-shot..

"Kristian Ukkonen" wrote in message
...
...
Actually! The easy way is to use PULL instead of push there..
A digital fish scale or such ready-made-cheap-load-cell could
then be used directly to pull the cable a known amount (angle).
Then it becames a vector sum of the forces, I think..

Of course, it is not an on-line measurement, but one-shot..

That is an advantage since it doesn't have to be waterproof.

It will have to account for the bending resistance of the cable as
well as the geometry, unless you use webbing.

The last time this subject came up, it quieted after I suggested a
pressure gauge on a pullback ram.
http://www.chinahydraulicjacks.com/b...ram_201056.jpg

I put the gauge on a tee so it can apply the tension as well as
measure it. It's long and heavy and awkward to use, and the short
stroke won't take up the droop and stretch on a long steel cable, but
it's enough to test fittings.

The weight of this gear is very significant when you try to hold both
cable ends together with one hand and screw in the final shackle pin
with the other. I use a 2" ratchet cargo strap hooked to the fixed
anchor and attached to the cable with a prusik loop to pull in the
slack.
http://www2.worksafebc.com/i/guideli.../guide11-2.gif
Pull the strap through the split pin to tighten the line hand-tight,
then secure it with the ratchet, a one-hand operation. On mine the
ratchet lever can pull up to 1000 lbs for a short distance.

jsw

The American Weigh Digital scale models appear to be very practical for a
number of different uses, and not especially expensive for their stated
accuracies.

For a different approach, 3-point surface-contact tension gages are used in
a wide variety of applications, one particularly sensitive one is a tension
gage for VRC tape during Play mode as a way to see if the brake bands/pads
on the reels were set properly.
The running tape would be slipped between slender fingers on the gage.. how
far the center finger was moved/deflected indicated the tape tension on a
dial indicator or similar gage face.
Tentel was one brand of tape tension gages for VHS/Beta 1/2" machines, Sony
and other manufacturers had similar devices for professional 3/4" tape
machines.

I have a 3-point contact device for measuring lens surface curvature which
works similarly to a tension gage, but the 3 points are all on one side of
the lens and indicates the focal length on a small dial indicator-type gage
face (zero is checked on an optical flat).
There is a conversion chart included for 2 different types of lens
materials.. forget what they are now, haven't used it for so long.

Perhaps a similar 3-point jig could be custom made to utilize a digital dial
indicator with an output signal connector (SPI?) for attaching a signal
cable to a remote DRO unit.

I'm guessing that the (maybe bar of steel) "tension element" could have
eyelets or other hardware attachment features to allow it to be used for
numerous applications.. (something like using a current shunt with a DMM
discussed about a week ago).

I'm fairly certain that some of the engineering-types here could figure out
an adjustable or adaptable 3-point model which would utilize separate sets
of pads/blocks to be installed for different applications (band saw blade,
1/4" wire rope, etc) for one tester, a sorta universal tension tester.

--
WB
..........


"Jim Wilkins" wrote in message
...

"RogerN" wrote in message

So far I'm leaning toward something like this:
http://www.amazon.com/gp/product/B00...A1DVPD9Q368DLM

I figure I can make a link with the same dimensions hole to hole as the
scale. This way I can use the scale to tension the lines, mark the
turnbuckles, loosen, replace scale with metal link, and adjust
turnbuckles to bring the tension back. I figure the scale will stretch a
little under the tension, if needed I can measure scale hole to hole with
170 lbs tension. I like that design of scale because I should be able to
use couplers with pins that will fit the scale....
RogerN

Good catch! It looks considerably more compact and durable than the
discontinued HF 440 Lb Big Game Scale.

You could make a simple comparison gauge that measures cable deflection
between pins with a fishing scale and tighten the turnbuckle to the same
reading. It would be like checking engine belt tension by thumb pressure
deflection or how far it will twist, unable to measure the force but
adequate to set it close enough to the correct value.

In practice I've found that the forces I want to measure around the house
range from 500 Lbs to a max of ~3000 Lbs, for logging and pulling stumps.
It's difficult to apply more than 3000 Lbs with equipment made from lumber
yard and hardware store materials and I can buy higher-rated chains and
tow straps or call a wrecker for stuck vehicles.

3000 to 3500 lbs seems to be enough to move the base of a large (24") oak
that caught in another one as it fell. I've pulled down two that blew down
during storms behind neighbor's houses. Both were straight enough to saw
into lumber so I pulled them down intact.

jsw

A little info on load cells. This could be useful for us poor folk here
that may not want to buy a $300 or more scale but have occasional need to
weigh something that we don't have the proper scale for.

Load cell output is normally expressed in millivolts per volt. Most of the
industrial load cells I run across at work are rated at 2 millivolts per
volt, that means if you use a 10 Volt excitation then at the load cell
capacity the output would be 10 X 2 mV or 20 millivolts. So, if you had a
20,000 lb. load cell, and used a 10V excitation, you would get 1 mv output
for every 1000 lbs.
But what if you have a different range load cell, like maybe 2,500 lb.? You
can adjust the excitation voltage to 12.5V and get 25 mV out for 2,500 lbs.,
1 mV represents 100 lbs..

If you want better resolution you could get a meter that reads microvolts or
use a load cell amplifier.
http://www.ebay.com/itm/Load-Cell-Am...em519d34 1b6f

With the load cell amplifier you can adjust out the tare weight, such as
tare out the weighing platform. Then you can put your calibration weight on
your scale and adjust the gain for a reading that makes sense, you don't
want to adjust anything like 1 volt per pound if you intend to weigh 100lbs
unless your load cell amp can output 100 volts...

Or, you can do something like buy a cheap eBay 100 gram load cell and adjust
the amp for a voltmeter reading of 1.000 V with a 100 GRAIN calibration
weight. Of course you want to test closer to your load weight if you would
be using it for reloading.

You can use that info to hack a cheap Chinese scale, read the excitation
voltage and read the sense voltage with the calibration weight applied. If
the calibration weight isn't 100% of the scales capacity you'd have to
calculate the mV/V at full load. Then you can replace the load cell with
one in the range you want and calibrate with voltages close to the original
load cell calibration voltages.

RogerN