strain gauge out of a mile long wire 1

[ultrasoundguy2]

hello
say I have a coax wire about a mile long that is subject to strains of 1% of the total length.
This particular wire has a resistivity of about 4 ohms per 200ft.

Is there a way to make a strain measurements out of this?.
if could do a bridge into a differential amplifier if needed.

what are the pros and cons of this system
I could put an amp though the wire to boost the generated voltages too.
I know it would be subject to temperature variations and joule heating which are not a problem in this case.

yes, I know there are better ways to measure strain over long distances.
this is a niche application and has to be done this way.

thanks
Jim

 

[rb1957]

so 1% strain is a guess ? well, it is a pretty important piece of the puzzle. is a mile long cable also a guess ??

again, what are you really trying to do ?

1) pull on a mile long cable ? why ?? i think most will tell you you shouldn't pull on a co-ax cable, and if the cable is liable to feel a tension load, then provide strain relief.

2) you're worried in case there is a displacement between the ends of the mile long cable. again, provide strain relief so the cable doesn't tension.

3) if you Have to tension a cable, then I'd look into overhead power line design (though i'd assume that the sheathing takes most of the catenary loads).

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[GregLocock]

Would time delay reflectometry (TDR) be a better approach than resistance?

Cheers

Greg Locock


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[tbuelna]

While I don't mean to be snarky, but in your OP post you asked how to measure strain in a coax cable "that is subject to strains of 1% of the total length". If you already know what the strain in the cable is why do you need to measure it?

It might also be helpful to measure resistance at several points along the length of the deployed cable.

 

[ultrasoundguy2]

I think the strains will be from 0 to 100 uE but maybe to 0.1 or 1%, I don't know.
it dosent really matter for the practical thoughts that go into it.

they key to making this work will be in the material properties.
such as the gage of the wire and resistivity and gage factor.
With a wire strong enough to hold its weight, say 20 gage copper wire, Im not sure that there is enough change in resistance per unit strain.
with a length of around 1600 meters, with this 20 gage copper, its only 33 ohms total resistance
so we would have to pick up im guessing around 100mohm change which is small.
there are actually other loads on the cable, with are very constant though.
so the total cable impedance is around 1Kohm
That's where this gets tricky, the 1Kohm total, with 100mohm variation is a small induced voltage even with 1Amp.
it can still be picked off with a differential amplifier but then, the changes induced by temperature can saturate it with the gain needed.

high frequency signals produce frequency dependent impedance would be more sensitive but cant do now.

 

[rb1957]

sorry, but what exactly are you trying to do ?

1) i think it's very bad practice to intentionally strain a conductor.

2) are you concerned about the change in electrical properties should some displacement happen ?

3) are you intentionally straining the conductor (to some unknown amount) and trying to detect this. In the strain aguage world they are quite comfortable measuring microstrains; how can this be applied to your application ?

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