Conductivity measurement of copper rods/cables

[isurunalaka]

hi, i am looking for device which can measure copper rod/cable conductivity. is there any method other than measuring resistance and calculating conductivity?

 

[KlausST]

Hi,

it seems to be low ohmic, so suggest a four wire measurement system.

For us, who want help, it is always good to get values. Like estimated resistance, current rating ...

Klaus

 

[D.A.(Tony)Stewart]

Use any power supply that can deliver a measured large current into a resistive load.
Measure voltage drop across cable and length and then compute V/I=resistance per unit length.

This will be more accurate than an ohmmeter that uses ~1mA CC to measure resistance because you are driving with a larger measured current. The voltmeter leads should be directly on the wire and not the jumpers applying power to cable, so contact or fringe resistance near contact is excluded. ( aka 4 wire method)

 

[BradtheRad]

You can use a transistor in common-base operation, to amplify a voltage generated across your conductor.

Sample simulation:



Response is not necessarily linear. You may need to draw calibration marks on a custom meter face, or make a chart of readings versus ohm value, etc.

Connections should be clean as possible. You don't want to measure the resistance of tarnish!

 

[isurunalaka]

can you suggest me a good 4 wire commercial product to buy?

 

[KlausST]

Hi,

can you suggest me a good 4 wire commercial product to buy?

For what resistance range? What resolution? What precision?

Klaus

 

[D.A.(Tony)Stewart]

can you suggest me a good 4 wire commercial product to buy?

I have an HP3478A 6 digit DMM which you can find used for a low price. It has 2 and 4 wire options as well as measures V,I,R with very high accuracy.

 

[isurunalaka]

Hi,



For what resistance range? What resolution? What precision?

Klaus

range is 1 Ohm/km - 200 Ohm/km
resolution - 0.1 Ohm/km
precision - +/-1%

 

[KlausST]

Hi,

you specify Ohm/km

do you need a measurement device to show Ohm/km?

If not: Specify what you need.

Klaus

 

[isurunalaka]

I have an HP3478A 6 digit DMM which you can find used for a low price. It has 2 and 4 wire options as well as measures V,I,R with very high accuracy.

can this be used for conductor resistance of copper cables?

- - - Updated - - -

Hi,

you specify Ohm/km

do you need a measurement device to show Ohm/km?

If not: Specify what you need.

Klaus

basically i need to measure resistance either Ohm or Ohm/km. is there any specific devices to measure Ohm/km?

 

[KlausST]

Hi,

To measure a resistance of 100 Ohms is relatively simple and precise.
To measure a resistance of 0.1 Ohms is more difficult and the ammount of errors is getting bigger.
To measure a resistance of 1uOhm with a DC source is almost impossible.

To measure Ohm/km you need to know the resistance and the length. Both in (lower than) the desired tolerance.
For sure it is possible to develop a measurement device for Ohm/km. I don´t think there is one to buy. And if you find one it may be very expensive.
We don´t know if you have a 10 cm piece of wire or a cable drum with 500m.

I don´t know wether you have temperature controlled rooms where you measure the copper resistance.
The resistance of copper depends on temperature with about 3.9E-3/K. This means a change in temperature of 2.5° gives a resistance error of 1%....10°C give 4% error.

If all this is for quality management, you will need a protocol with: date, temperature, cable length, measurement device and inspector name. At least..

**

Till now you didn´t give us the specification of Ohms measurement.
So you have to look for the HP3478A datasheet and find yourself if it fits your needs.


Klaus

 

[D.A.(Tony)Stewart]

can this be used for conductor resistance of copper cables?

- - - Updated - - -

basically i need to measure resistance either Ohm or Ohm/km. is there any specific devices to measure Ohm/km?

Yes it will work. HP3478A has 100 µΩ resolution and 5 ½ digit range
$150 used. High accuracy.

Accuracy depends greatly on your purpose and how you intend to calculate resistance.

If you want to determine which distribution cable will have the lowest drop voltage per km, then skin effects and inductance must also be measured at operating frequency and compared with DC.

At 60 Hz in copper, the skin depth is about 8.5 mm.

What is your purpose?

But to measure installed cable resistance in Ohm/km do you also need to measure distance electronically? then you need an expensive TDR.

 

[isurunalaka]

1-1000 micro ohm range resistance measurement

hi, i need to measure resistance of a 1m(minimum) length 0.2mm(minimum) diameter copper cable resistance with 1 micro ohm resistance least count. is there any commercial product available for this task.

 

[d123]

Re: 1-1000 micro ohm range resistance measurement

Hi, search for micro- or milli-ohmeter, microohmeter/milliohmeter, kelvin connection, four-terminal sensing, etc. Not sure, but from what I saw, maybe commercial offerings' prices reflect their quality.
The RAR file has a few professional device advertising and descriptive pamphlets/pdfs - they were all great to drool over but well beyond my means, hint, hint.

If you only need it for something homemade/a prototype, and legally-binding 0.0000001% precision is not an issue I recommend making the micro-ohmeter yourself, in many ways it's actually a very simple circuit, but worth reading up on beforehand - for example - Keithley's recommendations for such measurements.

- - - Updated - - -

Hi again, I imagine you need real measurement of a specific piece of metal, not just a calculation based on standard values (which as a result could be quite misleading in a real application), but if you don't mind maybe we can add these links here in case they're of use to anybody else who just wants to calculate resistance or conductivity of conductors and insulators:

http://chemandy.com/calculators/round-wire-resistance-calculator.htm

http://www.endmemo.com/physics/resistance.php

http://www.engineeringtoolbox.com/resistance-resisitivity-d_1382.html

http://www.easycalculation.com/physics/electromagnetism/wire-resistivity-calculator.php

**broken link removed**

http://www.phys.uri.edu/gerhard/PHY204/tsl136.pdf

**broken link removed**

Thanks.

 

[KlausST]

Hi,

did you do some calculations for this?
I doubt it.

1m length of copper wire with 0.2mm diameter makes about 570m ohms.
This is a resolution of 1:570000, or 0.00017%

--> With a resolution of 1 microohm you need at least 19 bits of resolution (6 digits).
--> this resolution equals to a length of 1.7 um (@1000mm). Can you set your measurement points with a precision of 1.7um?
--> this resolution equals to change in diameter of 0.18nm (@0.2mm). Can you measure the diameter with a resolution of 0.18nm? You should consider, that the copper is corrosive at ambient air. But copper oxide is about non conductive. So you need to subtract the (increasing) thickness of the coper oxide.. I find this is a difficult task...
--> resistance temperature coefficient of copper is about 0.4% /K. Your resolution equals to about 0.0004K. Can you ensure a temperature stability of 0.0004°C?
--> the resistance of copper depens a lot on impurities. I read that 0.006% Fe in pure Cu increasies resistance in the range of 20%. But 20% means about 100000 times your resolution. You need very, very pure copper, not to influesnce your measurement.

****

Maybe you find a ready to buy micro-ohm meter..
But it makes only sense if you can answer ALL of my above questions with YES.

****
Are you sure you need 1 uOhms resolution at a nominal value of 570mOhms?

If so, then it could only used in high tech laboratories for research on copper. But then a price of 10000$ is nothing against all other necessary equipment in the laboratory.

Klaus

 

[chemelec]

Re: 1-1000 micro ohm range resistance measurement

hi, i need to measure resistance of a 1m(minimum) length 0.2mm(minimum) diameter copper cable resistance with 1 micro ohm resistance least count. is there any commercial product available for this task.

I have a 4 Wire Low Ohm meter using a 3 1/2 digit display, so it will only measure to .001 of an ohm.
If a 6 1/2 digit display were available it could measure what you want.
And I could Build you one!

Just for curiosity, I cut a 1 meter length of 32 AWG wire. This wire is Close to your .2mm diameter.
It measured 0.541 Ohms.
Calculating this based on my Wire Chart, it should be 0.5488845 Ohms.

But I may have been out a Tiny Bit in my 1 Meter Length?
Or Maybe the wire has a Slight difference in Copper Purity?
Or a Possible Temperature Difference?

Even a 1 Degree in Temperature Change Will Change the Resistance with that many Decimal Places.

Why do you need such an Accuracy?

 

[chuckey]

This is a measurement that has been done before. Just measure the wire and look up the resistance in a table, you will not be able to measure it any more accurately. If on the other hand you work in a factory that makes wire and you intend to use the resistance of the wire as an indication of copper purity, then a proper jig must be made up, so your voltage measuring points are fixed on some sort of temperature compensated baseplate. Like wise you need to stabilise the temperature and do a series of diameter measurements to find the mean diameter. Also stabilise the tension in the wire because as you tension it it gets thinner.
Frank