# Measuring tension in a nylon loop (springs?)

1. Nov 17, 2009

### Dunnymel

Warning: I have a minor grasp of physics.....
I am devising a study looking at the tension required in a surgical prosthesis (which is essentially a length of nylon with 80LB breaking strain) to produce stability between 2 bones.
The nylon will be placed in a simple loop between 2 structures and I want to tighten the loop until I have produced the desired effect. In practice this means placing the loop, then forming a single knot and applying tension to each end of the loop (ie the first part of tying a shoelace) before securing the loop with additional 'throws' .
Importantly I want to measure the tension that I have produced in the loop.
The loop will be quite small (say 3cm diam) and sitting against tissue which means I will not be able to place a gauge or other device within the loop unless the device is very small.
I was thinking of using a spring with a known constant attached to the ends of the nylon which could be measured once I have the desired physical effect.
Is this the simplest way of measuring the tension in my loop? If so, do I need a spring attached to both ends of the loop as I tension it?

Thanks

Dunnymel

2. Nov 17, 2009

### Q_Goest

Hi Dunnymel. Welcome to the board. I'm not a strain gauge expert, in fact I've never used one. But the idea is simple enough. Strain of a copper conductor changes the resistance, and the resistance can be measured. Here's an overview:
http://en.wikipedia.org/wiki/Strain_gauge

If you had an extremely thin wire wrapped inside the nylon (string?) you might be able to measure any change in resistance as the nylon stretched. Obviously, the longer the copper, the more accurate you can get, so there's going to be a tradeoff in how complicated you get with the geometry of the wire (ie: how long you can make it) and how accurate you want it. Figure out how accurate you need it, then have someone expert in the field figure out how long it needs to be and how you might be able to mount it.

3. Nov 17, 2009

### Dunnymel

Thanks, you've just profoundly increased my knowledge on the subject of strain gauges!

..however the nylon I am using is akin to heavy fishing line, about 1.5mm diam, so I cant place anything 'inside'. I would have to use something on the outside surface.

Any other ideas would be great.

Dunnymel.

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