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strain gauge out of a mile long wire

strain gauge out of a mile long wire

strain gauge out of a mile long wire

(OP)
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



RE: strain gauge out of a mile long wire

are you really getting 5' movement over 1 mile, or is this a spec thing ?

sure you could measure the change in resistance, but first you'd need a slack cable (which i'm guessing is hard to do) to give you a zero.

what are you trying to prove ? how the co-ax performs (electrically) with 1% strain ? or how the co-ax performs structurally with 1% strain ? or signal degradation ?? sounds to me like you could do this is a lab ??

Quando Omni Flunkus Moritati

RE: strain gauge out of a mile long wire

(OP)



cant think of a way to strain a wire a mile in the lab.
really looking for references or ideas so I can do some calculations on preliminary work.

even if I can record a strain, I need to know what it means as I have to know how to interpret the strain..


RE: strain gauge out of a mile long wire

(OP)
woops... need to know how to interpret the recorded voltages into strain..

RE: strain gauge out of a mile long wire

"cant think of a way to strain a wire a mile in the lab."
1) you could roll it up.
2) you could test 10' with 1% strain.

"how to interpret the recorded voltages into strain?"
1) google "strain gauge", go to the wiki article
2) buy a text book

Quando Omni Flunkus Moritati

RE: strain gauge out of a mile long wire

(OP)


don't think im going to see an effect with 10' unless I have a super accurate ohmmete3r.
rolling it wont product enough change to get the gage factor..

the terms I needed were "elastic wire strain meter", a type of "unbounded strain meter"
these are giving me the background I need such as the best wire types..

yea, Wikipedia has all the answers :)
thanks anyway buddy

RE: strain gauge out of a mile long wire

There's got to be better ways to measure displacement over a mile.

RE: strain gauge out of a mile long wire

Don't forget to run a second wire in parallel that is not under strain to compensate for temperature.

RE: strain gauge out of a mile long wire

"yea, Wikipedia has all the answers :)
thanks anyway buddy"

hence option 2),
and you're welcome.

btw, you seemed to pick on one comment in my post, so ...

1) are you really getting 50' movement over 1 mile, or is this a spec thing ?

2) sure you could measure the change in resistance, but first you'd need a slack cable (which i'm guessing is hard to do) to give you a zero.

3) what are you trying to prove ? how the co-ax performs (electrically) with 1% strain ? or how the co-ax performs structurally with 1% strain ? or signal degradation ??

4) 1% strain sounds like an enormous amount, a typical yield strain is 0.02%. i doubt the co-ax will remain intact.

Quando Omni Flunkus Moritati

RE: strain gauge out of a mile long wire

Ultrasoundguy2:
I think you should do your study and testing on a 50 or 100' length of cable, in a lab, and then factor the results by 5280ft./mile. In the lab you have a chance of controlling the tension on the cable and the ambient temperature around it. In the field, on a mile long cable, you will have variations in initial tension, and in the temps. over various unit lengths of the cable. Also, unlike a single fine wire, cables have funny manufacturing strains, and relaxation and tightening and bending of wires over each other, as a function of the way the cable is manufactured. You should study how this affects your results. Maybe you should pre-stretch the cable to take some of this manufacturing slack out of the cable.

RE: strain gauge out of a mile long wire

(OP)

yes, I would have to do about 100ft to get a measurable resistance. (2-4ohms per 200ft)
I will probably have to do this, but not in the lab, more outside in the road..
its the only way to get the gage factor

I can check temperature effects in a controlled environment and then remove the effects after data is recorded.

ive now looked in the literature.
this method can measure the strain typically in the range of 4- 20" and is accurate to 1.5 to 6 microstrain
so this would be stretching the knowing distance it works.


I need to figure out what type of wire to use though.

Im not sure what kind of stain we are going to get.

RE: strain gauge out of a mile long wire

i don't know of any material that can carry 1% strain.

eg Al (which i know) ... E = 1E7psi, 1% strain = 100ksi (> ftu)
steel E = 30E7psi, 1% strain = 300ksi (> ftu)
etc

Quando Omni Flunkus Moritati

RE: strain gauge out of a mile long wire

if you two end points are moving 50' relative to each, i'd recommend having strain relief loops (ie 50' extra length so you don't strain the cable).

Quando Omni Flunkus Moritati

RE: strain gauge out of a mile long wire

(OP)


I don't know the strain we are going to get.
Im guessing 1% might be high, more like 2-3 u strain


but im still not sure this can work.
all the references out there on unbounded strain gages only cite certain types of metals with high gage factors.
such as nickel or constantan
http://soliton.ae.gatech.edu/people/jcraig/classes...


I want to know if this can work with standard coax and steel or copper cable.
cant find the specs on gage factor for these metals.

RE: strain gauge out of a mile long wire

(OP)
I guess copper is in the table if I look carefully

RE: strain gauge out of a mile long wire

Look at the bulk resistivity for the material and the Poisson's ratio. Poisson's ratio will tell you what the expected decrease in area is for a given increase in length. Then use the bulk resistivity in an equation that relates section area and length to produce the resistance.

This will apply to the solid core of a wire, not the braided shield.

RE: strain gauge out of a mile long wire

(OP)

thanks you dave and everyone..

there is no potential on the braided shielding so it shouldn't matter if it and the jacket strain too

really not sure if this will work, its a very niche thing.
there has to be a reason it wont work, something non ideal like the wire thickness not being uniform?

RE: strain gauge out of a mile long wire

How do know what amount of the resistance change measured can be attributed to strain solely created by displacement in the conductor length due to a change in the distance between the cable end points, and how much resistance change is missed by effects like elimination of slack in the cable? Over the distance of 1 mile, just the weight of the cable itself resting on the ground will create a substantial amount of friction drag when the cable is tensioned. The strain produced along the length of the cable will not be uniform, it will be greater towards the end the force is applied to.

RE: strain gauge out of a mile long wire

If your coax has a solid core, you might be able to get a useful strain signal out of it, using the shield as your reference material. If the resistance per unit length of the shield is much different from that of the core, as i suspect, you will have an unbalanced bridge, so you may need funny business to correct for that. I'm pretty sure that you will need a bridge; no ordinary ohmmeter will have the resolution.

If your coax has a stranded core, I don't think you'll get a stable and usable signal that correlates to strain out of it.

You'll also need to take several wraps around something at each end; the usual crimped connections won't survive much pull force.


Mike Halloran
Pembroke Pines, FL, USA

RE: strain gauge out of a mile long wire

(OP)
tbulena - excellent! of course, im counting on this, I cant say why though, sorry
mike, somewhat good.. The shield wont matter much I don't think as there is no potential on it... I can make a bridge and differential amplifer if I need it from another cable of the same length without strain on it.

RE: strain gauge out of a mile long wire

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).

Quando Omni Flunkus Moritati

RE: strain gauge out of a mile long wire

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.

RE: strain gauge out of a mile long wire

(OP)

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.


RE: strain gauge out of a mile long wire

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 ?

Quando Omni Flunkus Moritati

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