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Author Topic: Extended test leads on Kelvin Bridge  (Read 1828 times)

Mike Sokol

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Extended test leads on Kelvin Bridge
« on: April 13, 2018, 12:03:47 pm »

OK guys, so this is an interesting question that just stumped engineering tech support at Greenlee. I need to measure the actual low-ohm resistance of a 100+ ft cable buried cable run. Greenlee makes a portable Kelvin Bridge that will measure resistance down in the milli-ohm numbers. And the cool thing about a Kelvin Bridge is that the test lead resistance is nulled out of the measurement. That's because there's two isolated paths through the resistor being measured. Once is a low impedance current loop, while the second one if a very high impedance voltage measurement. And these two loops are kept separated in the test jaws, with one side of the same clamp. See Kelvin Bridge theory here: https://en.wikipedia.org/wiki/Kelvin_bridge

Here's the Greenlee CLM-1000 which is just a Kelvin Bridge Meter. And you can see that it has 4 test leads terminating in a pair of test clamps, just as I described above.  https://www.greenleestore.com/products/greenlee-clm-1000-cable-length-meter-for-awg-kcmii-wire-and-cable?utm_medium=cpc&utm_source=googlepla&variant=15452440451&gclid=EAIaIQobChMIg_aVucW32gIVC4nICh39_w_6EAYYAiABEgK2hfD_BwE

What's to stop me from extending one of the double test leads by 100 feet or so? From all my time using Kelvin Bridges to measure super-low ohm resistors down to ppm accuracy back in the 80's, the test lead resistance isn't actually inside of the test loop at all. So from a practical standpoint I could make this several hundred feet long if I wanted and it should still measure 0.01 ohms on the test resistance, or whatever.

Their engineer is sending me a free sample to experiment with as long as I share my test results. What do you all think? Can I extend one of the test lead pairs by 100 ft without goofing up the accuracy of a sub-ohm resistance test?
« Last Edit: April 13, 2018, 12:13:30 pm by Mike Sokol »
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Chris Hindle

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Re: Extended test leads on Kelvin Bridge
« Reply #1 on: April 13, 2018, 12:29:24 pm »


Their engineer is sending me a free sample to experiment with as long as I share my test results. What do you all think? Can I extend one of the test lead pairs by 100 ft without goofing up the accuracy of a sub-ohm resistance test?

Mike, my guess is that's what the engineer wants YOU to tell him.
Uncharted territory, and all that.
Who knows, a future factory accessory may be the MS-100 Test Lead Extension Kit.
You sure do find interesting ways to push the limits of knowledge. - THANK YOU !!!
Chris.
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #2 on: April 13, 2018, 12:35:32 pm »

Mike, my guess is that's what the engineer wants YOU to tell him.
Uncharted territory, and all that.
Who knows, a future factory accessory may be the MS-100 Test Lead Extension Kit.
You sure do find interesting ways to push the limits of knowledge. - THANK YOU !!!
Chris.

Yes, that's exactly right. He said I only needed to promise to share my test data with him, which I think is a great deal. And of course I'll share it here as well.
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Jonathan Johnson

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Re: Extended test leads on Kelvin Bridge
« Reply #3 on: April 13, 2018, 03:06:21 pm »

Yes, that's exactly right. He said I only needed to promise to share my test data with him, which I think is a great deal. And of course I'll share it here as well.

Demonstrating with test data is one thing, but you also have to prove it mathematically.
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John Roberts {JR}

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Re: Extended test leads on Kelvin Bridge
« Reply #4 on: April 13, 2018, 04:02:37 pm »

Kelvin connections for measurement is a fairly mature technology so I suspect somebody has tried that.

In practice it may come down to the null range of bridge device.

You might be more symmetrical with two sets of 50' test leads with meter in the middle.

BTW subtracting out lead resistance is not a big deal, for a 100' buried conductor there may be larger stray ground currents and fields.

Have fun... but maybe check the library (IEEE et al).

JR
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #5 on: April 13, 2018, 05:11:46 pm »

Demonstrating with test data is one thing, but you also have to prove it mathematically.

Actually, I can prove it mathematically. Some 40 years ago I designed and built Kelvin Bridges on the bench from loose parts, and we used them on a production line for Laser trimming resistors while they were spinning in a chuck. So we did a lot of measurements with a Kelvin Bridge that would have been impossible any other way. I'm in the upper 99% certainty that this will work, but I haven't had to do it in battle for a very long time. I'll run some basic tests next week, then go in and show my math on paper. Unless I'm thinking about this completely wrong it should work exactly as I describe. Let's see, so 2+2=5 correct?  :o
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Frank DeWitt

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Re: Extended test leads on Kelvin Bridge
« Reply #6 on: April 13, 2018, 06:13:38 pm »

I agree with John.  On paper it works.  It will come down to the instrument having the range to deal with two very different probes.  My guess is that two 100 ft probes is better them one 2 ft and one 100 ft.
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #7 on: April 13, 2018, 07:20:14 pm »

I agree with John.  On paper it works.  It will come down to the instrument having the range to deal with two very different probes.  My guess is that two 100 ft probes is better them one 2 ft and one 100 ft.

My plan is to solder up 2 pairs of 100 ft extensions using 16 gauge twisted speaker wire. Now I know that won't have the uniform twist of a true twisted pair, but it should probably help reduce any stray hum pickup on the hi-z loop. I was even thinking of using mic cable and bonding the shield to the current/low-z side. But this tester is designed to work with up to 1,000 feet of unshielded wire, so I would think it has some pretty serious hum rejection from the get go. We shall see....
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Rob Spence

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Re: Extended test leads on Kelvin Bridge
« Reply #8 on: April 13, 2018, 10:23:00 pm »



We waved as we passed Shenandoah this morning heading south




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John Roberts {JR}

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Re: Extended test leads on Kelvin Bridge
« Reply #9 on: April 14, 2018, 10:00:15 am »

My plan is to solder up 2 pairs of 100 ft extensions using 16 gauge twisted speaker wire. Now I know that won't have the uniform twist of a true twisted pair, but it should probably help reduce any stray hum pickup on the hi-z loop. I was even thinking of using mic cable and bonding the shield to the current/low-z side. But this tester is designed to work with up to 1,000 feet of unshielded wire, so I would think it has some pretty serious hum rejection from the get go. We shall see....
My concern was also about stray fields/currents affecting the buried conductor. Typical VOM inject a reference current and measure voltage, so either could cause errors.

Maybe swapping leads and measuring both directions could reveal if my imaginary errors are present, or not.  ::)

JR
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