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Author Topic: Extended test leads on Kelvin Bridge  (Read 2274 times)

Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #10 on: April 15, 2018, 02:17:27 pm »

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

Yes, you're 100% correct. I don't know all the variable that can affect the readings, but I'm going to find out pretty quick if this will work. Please Stand By....
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Mike Sokol
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #11 on: April 23, 2018, 07:51:33 am »

Yes, you're 100% correct. I don't know all the variable that can affect the readings, but I'm going to find out pretty quick if this will work. Please Stand By....

First test with a single 75 ft run of 16-gauge extension on one of the Kelvin clamps seems to work perfectly. The meter has a calibrate function and test bar which nulls out to 0 ohms correctly, and I've used it to read several different lengths of wire correctly. I'll build a second 75 ft run for the other clamp today and take both of them to the test site on Wednesday. I'll report back how well it works under dirty conditions in a swamp. No kidding, I'm doing this in a swamp...
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Mike Sokol
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Chris Hindle

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Re: Extended test leads on Kelvin Bridge
« Reply #12 on: April 23, 2018, 09:04:38 am »

First test with a single 75 ft run of 16-gauge extension on one of the Kelvin clamps seems to work perfectly. The meter has a calibrate function and test bar which nulls out to 0 ohms correctly, and I've used it to read several different lengths of wire correctly. I'll build a second 75 ft run for the other clamp today and take both of them to the test site on Wednesday. I'll report back how well it works under dirty conditions in a swamp. No kidding, I'm doing this in a swamp...
Don't let the alligators bite............
Chris.
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Ya, Whatever. Just throw a '57 on it, and get off my stage.

Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #13 on: April 23, 2018, 01:51:28 pm »

Don't let the alligators bite............
Chris.

Roger that.... And I just tried the second 75-ft extension and it calibrates and seems to work correctly. This is getting interesting.
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Mike Sokol
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Chris Hindle

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Re: Extended test leads on Kelvin Bridge
« Reply #14 on: April 23, 2018, 06:42:21 pm »

Roger that.... And I just tried the second 75-ft extension and it calibrates and seems to work correctly. This is getting interesting.
Kinda looks like that as long as you can get calibration, limits are up to you.
Now, here's a question. Is the bridge "good enough" to make a valid measurement in, say, a 20 foot D-U-T line, using 300 feet of leads ?
I would imagine that as long as the leads don't drastically outsize the D-U-T, you should be good.
Chris.
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Stephen Kirby

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Re: Extended test leads on Kelvin Bridge
« Reply #15 on: April 24, 2018, 12:28:30 am »

If you can manage to stretch things out, anchoring one end and twisting with a drill can result in a pretty even wind.  A shop I was at made cables for the Bradleys.  We'd lay out the wires across several work benches, clamp one end and use a cordless drill to twist it.  You have to hold a pretty good tension on it so make sure the anchored end is solid.  Then we'd have one person shooting an air gun up one end of shrink sleeving while someone shoved the cables up the other end.  Then run them through turned down SMT reflow ovens to shrink the sleeving.  Fortunately we got to charge Mil-Spec prices for all this labor.
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Mike Sokol

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Re: Extended test leads on Kelvin Bridge
« Reply #16 on: April 24, 2018, 09:36:00 am »

Kinda looks like that as long as you can get calibration, limits are up to you.
Now, here's a question. Is the bridge "good enough" to make a valid measurement in, say, a 20 foot D-U-T line, using 300 feet of leads ?
I would imagine that as long as the leads don't drastically outsize the D-U-T, you should be good.
Chris.

Back when I did this seriously in the 80's we used a battery source for the current and a DC null meter. The comparison source was a precision decade resistance box calibrated annually at the National Bureau of Standards. With that simple setup we could match resistors down to 5ppm and sometimes as low as 2ppm. When I get some time I'll have to write a paper on the theory of Kelvin bridges, but in theory you can add hundreds of feet of cable to the clamps and still read sub-ohm resistances. It doesn't work at all like a standard 2-wire meter.   
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John Roberts {JR}

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Re: Extended test leads on Kelvin Bridge
« Reply #17 on: April 24, 2018, 09:49:54 am »

Back when I did this seriously in the 80's we used a battery source for the current and a DC null meter. The comparison source was a precision decade resistance box calibrated annually at the National Bureau of Standards. With that simple setup we could match resistors down to 5ppm and sometimes as low as 2ppm. When I get some time I'll have to write a paper on the theory of Kelvin bridges, but in theory you can add hundreds of feet of cable to the clamps and still read sub-ohm resistances. It doesn't work at all like a standard 2-wire meter.
The Kelvin connection for precision testing is known to engineers, maybe not so well known around sound reinforcement forums.

This technique has other applications audio, like using sense leads on speakers to ignore wire losses. Again not common for sound reinforcement.

JR
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Chris Hindle

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Re: Extended test leads on Kelvin Bridge
« Reply #18 on: April 24, 2018, 12:29:46 pm »

Back when I did this seriously in the 80's we used a battery source for the current and a DC null meter. The comparison source was a precision decade resistance box calibrated annually at the National Bureau of Standards. With that simple setup we could match resistors down to 5ppm and sometimes as low as 2ppm. When I get some time I'll have to write a paper on the theory of Kelvin bridges, but in theory you can add hundreds of feet of cable to the clamps and still read sub-ohm resistances. It doesn't work at all like a standard 2-wire meter.
Thanks Mike and J.R.
Very enlightening.....
I am well versed in "standard" instrumentation, but this is new to me.
I've never had a need for this kind of tool, at this kind of precision.
Chris.
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Stephen Kirby

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Re: Extended test leads on Kelvin Bridge
« Reply #19 on: April 24, 2018, 04:25:52 pm »

The Kelvin connection for precision testing is known to engineers, maybe not so well known around sound reinforcement forums.

This technique has other applications audio, like using sense leads on speakers to ignore wire losses. Again not common for sound reinforcement.

JR
Going around with some folks at work about how close the sense leads have to be to the resistive element.  There's this bright idea that if you partition the board pad for an SMT resistor the current sense is more accurate than if you just run two traces off the one pad.  I tried explaining that the component's internal parasitics from the resistive element down to the board pad would swamp any differences.  Much as the leads on a conventional axial resistor does.  Maybe not as bad, but there are comparatively high resistance metalizations going from the element on top around the sides of the ceramic body to where it's connected to the board.
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