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
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
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
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
RE: strain gauge out of a mile long wire
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
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
RE: strain gauge out of a mile long wire
RE: strain gauge out of a mile long wire
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
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
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
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
Quando Omni Flunkus Moritati
RE: strain gauge out of a mile long wire
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
RE: strain gauge out of a mile long wire
This will apply to the solid core of a wire, not the braided shield.
RE: strain gauge out of a mile long wire
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
RE: strain gauge out of a mile long wire
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
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
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
Cheers
Greg Locock
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RE: strain gauge out of a mile long wire
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
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
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