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  Hi all,

  I am looking for a way to measure how much something has stretched.  I am probably looking for something like a strain gauge, but one which will allow for motion over a relatively long variation in length (something like 50-100mm, ideally with about 1mm accuracy). 
  Does anyone have any experience with something like this?  More generally, does anyone have any advice on a good source for inexpensive "hobby" grade strain gauges?
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I've seen these things in hardware stores called retractable tape measures ...

Since I imagine you want the arduino to read this tho, perhaps a wheel that turns as your something moves, and you read the amount of wheel turning.
Could even have the wheel be on a potentiometer shaft, with the pot set up as a voltage divider, and read the voltage - then have a  lookup table to see what distance goes with a voltage.
Need a wheel that is 100mm over ~270 degrees of rotation, so ~135+mm total circumference.
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A strain gauge measures force not travel, plus strain gauges don't have any travel or give at all to speak of.
To use a strain gauge you would have to couple it with a suitable spring that can handle the physical travel distance and transfer the stress force to the strain gauge.

 Lefty
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Deep within....

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1276355492

.. an excellent discussion of various possibilites developed.

There are devices called "linear potentiometers" which would be fine, if the mechanical aspects are suitable to your needs, e.g...

http://www.celesco.com/linearpot/

(The term is also used to speak of a pot with a linear change in resistance, though, so you'll get some "false positives" out of Google.)
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Possibly some of this:
http://www.inventables.com/technologies/stretch-sensing-rubber
Can do 100% elongation and is probably relatively precise. The issue would be measuring the resistance. Using a voltage divider would work but you might want to use an external ADC (as the 10-bit one on the atmega chips isn't the greatest for precision) and an accurately measured resistor for the other side of the voltage divider.
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  Thanks for the good tips.  For my use, the "Stretch Sensing Rubber" seems to be exactly the right thing.  At $13 a strip, that is a little steep, but I may be able to get by with it.  The option of using a strain gauge with some kind of elastic was the more obvious choice, maybe I should have some kind of bake off between the two.  Thanks for the tips though, I think I will order some of this rubber stuff and see how it works out.
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Well the prices on that site are a little steep but they are generally 'sample' materials rather than something you'd buy a lot of from them. If you wanted a few hundred of something you'd contact the people who make it and probably get something custom a bit cheaper (as it'd be in bulk).

They have some interesting materials and it's one of the few places you can get little bits of things to play with.
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So you will be measuring the stretch sensing rubber as you stretch it in parallel with the "something" you began with?
Seems plausible. Perhaps even repeatable.
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Quote
Seems plausible. Perhaps even repeatable.
Sounds easy enough to me.
Resistance change presumably is linear so just measure the limits, pick a resistor, do some minor calibration and you're away.
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