Maybe a little ot ...
I'm trying to measure the expansion of chest during
breathing using a pic (there You are ;-) )
How would You measure a span of about 10cm ?
Usually flexible rubber tubes with quick silver are used,
the change of lenght is measured by taking the resistance
with a wheatstone bridge. only, Hg on the floor can be hard
to hunt ;-) (and sooner or later it gets there ...)

The device should be rather small and to be attached
to a person easily. And it should be as "electronical"
as can be, that is: preferably no mechanical parts ...

Any ideas?

Christoph