Thought Technology makes a very accurate one, which uses a hall effect
sensor moving towards and away form a permanent magnet. It works very well.

http://www.thoughttechnology.com/
514-489-8251

C. Webb

>At 01:39 PM 1/24/98 +0100, you wrote:
>>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 ...