He was thinking of something like this. The only experience I have with
strain gauges is with big load cells. I didn't immediately see how the
strain guage could be coupled to a spring wire type of thing. (I was
thinking about using the type of steel wire you see as actuators on some
microswitches. These are commonly used as coin detectors in vending machines
etc.) It is possibly still an option for him if the mechanical design is
right, but I have no idea if he will be able to measure to 0.1mm accuracy
with this method......

Joe

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Brent Brown
Sent: 17 December 2004 00:11
To: Microcontroller discussion list - Public.
Subject: Re: [EE]:Interesting measurement problem.


Another idea, hard to describe so use your imagination :-)

Light spring wire a bend in it and strain gauge at base. Force on strain
gauge 
varies with deflection of wire, very light force so no damage. Drag wire 
through the tube to be measured and record distance/force profile. Could 
measure the start and end diameters with vernier calipers or other 
measuring device to calibrate the results. Note that force/distance 
relationship will not be linear, but can be established during calibration
and 
converted with lookup table or formula.

Fun.

-- 
Brent Brown, Electronic Design Solutions
16 English Street, Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
eMail:  brent.brown@clear.net.nz


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