Resistivity of carbon steel 7-wire prestressing strand

[Ingenuity]

According to a Google source, carbon steel (type AISI 1010 steel) has a resistivity: ρ = 14.3 x 10[sup]-8[/sup] Ωm at 20 °C.

Does the resistivity of carbon steel vary significantly depending on its carbon content, heat treatment, relaxation properties etc?

I am trying to determine the electrical resistivity properties of 1/2" diameter, 7-wire, grade 270 ksi prestressing strand used in prestressed concrete. Cross sectional area = 0.153 in[sup]2[/sup] (98.7 mm[sup]2[/sup])

I tried using a multimeter with a known length of strand to calculate the resistance but the results did not seem accurate - I think the multimeter range is not sensitive enough.

 

[waross]

You need a "Ductor" to measure that.
Roll your own:
Pass a current through the wire under test.
Monitor the value of the current.
Measure the voltage drop with a voltmeter.
The voltmeter probes should contact the wire under test, at or between the points at which the current is injected.
Use the measured Volts and Amps to calculate the resistance between the points at which the voltage probes make contact.
How much current?
Enough to get a reliable voltage reading.
If the voltage reading is too low to give you confidence, increase either the length of the sample wire or increase the current.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Ingenuity]

Thanks Bill.

Those micro-Ohmmeters (ductors) are pricey @ $5k. Definitely going to "roll my own".

I think I will need a variable DC power supply to "play" with the required current to get a reliable value. Is that correct?

 

[waross]

I would use a truck battery, maybe two.
I would look for a heating element out of a discarded appliance such as an electric clothes drier or a stove or oven element.
The current has to be stable enough for you to read the current and then read the voltage before the current drifts.
Even with a stable power supply, I suggest some ballast resistance in the circuit.
Your power supply may see a short circuit without some added resistance.
A Ductor may output 100 Amps or more. I would start with 10 or 20 Amps and see what results I get.
By the way, test as long a piece of wire as possible.
The more resistance (length) your test piece has, the better results you will see.
50 feet or 100 feet if possible would be great.
I would read the current, read the voltage, read the current.
Then compare the two current readings.
If the current hasn't drifted, then you are good to go.
If the current has drifted, look at the amount of drift.
The change in current divided by the average current will give an indication of the limits of accuracy.
For a moderate drift, use the average of the before and after current readings.
If the current drift is too much for confidence, try more voltage and less current, or better batteries.
If you are seeing a lot of voltage drop across the length of the cable, you may try increasing the ballast resistance for less current, after you recharge the batteries.
You are on your own for cut and try, but it is doable.
For a one time test, you may not need the stability that you pay for with a Ductor set.
Let us know your findings, please.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[xnuke]

You don't need to buy a ductor, you can rent one or find a local company that does electrical testing that owns one and pay them to test your wire. I think that either of those solutions is better and safer than trying to make your own. It will be calibrated, able to deal with drift, and UL listed.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

how much of a length are you measuring? If you can get enough for, say, 500 ohms, the probe contact resistance, being around 0.5 ohm, should be less relevant.

Otherwise, the Kelvin measurement described by waross is the way to go. A couple of DMMs wouldn't be that expensive.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[Ingenuity]

Thanks, all.

I have a 2,861 ft (872 m) long spool of this strand material, so I used my DMM (no external power supply) and the resistance measured only 2.2 Ω (very consistent measurements with multiple readings), so dividing by length and multiple by sectional area, that gives resistivity of ρ = 2.5 x 10[sup]-7[/sup] Ωm

This compares to AISI 1010 steel published resistivity of 1.43 X 10[sup]-7[/sup] Ωm

So 7-wire carbon strand has about 75% more resistivity compared to AISI 1010 carbon steel. I have no idea if that is correct.

I think my measured resistance of 2.2 ohms is not accurate.

 

[TugboatEng]

The strands are quite a bit longer than the length of the cable because they aren't straight, they twist around each other. I can't tell you how much longer they are.

 

[3DDave]

Divide by the sine of the pitch angle to the perpendicular to the axis; straight wire is at 90 degrees.

 

[waross]

Why do you need to know?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[IRstuff]

A DMM might have around 0.5 ohm of contact resistance, so if you're not doing a 4-point measurement as earlier suggested by waross, your measurement error might be quite large. A higher end meter might have a zeroing function that could get you closer.

How is the cable insulated, if at all? How did you make contact to all 7 strands at the same time?



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[IRstuff]

I think I will need a variable DC power supply to "play" with the required current to get a reliable value. Is that correct?

You know the maximum resistance is around 1 to 2 ohms, and a typical DMM can measure 200 millivolts, so you need around 200 mA, which should get you very close, assuming you're not looking for ultra-accurate measurements.

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[Compositepro]

Steel cable is not electrically insulated so you cannot measure spooled cable and get a meaningful result.

 

[IRstuff]

Steel cable is not electrically insulated so you cannot measure spooled cable and get a meaningful result.

That's sort of what I thought, but OP supposedly got a plausible measurement. Any chance there's some sort of lubricant that provides sufficient insulation?

TTFN (ta ta for now)
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[TugboatEng]

I should have clarified that is why I brought up the strand length as well. There is no control over lubricants and surface oxidation before spinning the cable in the case of non-electrical rated cables. Many of the crossings may not be conducive at low voltage/current levels (I recently learned about whetting current in relays). For irregular situations it's probably best to assume the worst case.