[volt-nuts] Best way to measure micro Ohms

[Brian Smith]

A DMM with good low resistance capability will have an "offset compensated
ohms" feature. A large current is used and then a small current is used.
The slope of the line formed by V-I points is the true resistance.

Measuring tiny DC voltages is easier than measuring tiny AC voltages. More
bandwidth is more cost for an amplifier.

Pass an amp or more through the device and use a KE 2182A to measure the
microvolts.

Since I do not own a 2182A, I use a KE 147. You will need a DMM on the
analog output of the 147 if you want digits of resolution.

When I do low resistance, I am just looking to bracket the resistance, not
measure to 6 1/2 digits.

The thermal offsets can require a lot of time to stabilize.

I built a Tetrajunction demo device that is in the tens of nano-ohms. It
takes all day to set up the measurement and have the offsets stabilize and
take a few measurements.


-Brian Smith

On Sun, Sep 17, 2017 at 1:23 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> I want to measure the resistance between two bits of aluminum. Each are 40
> x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering
> is surface oxides are on the faces, so despite being held together with
> bolts, the resistance is perhaps not as long as I would expect. There's
> also a layer of "copperslip" between these, to provide a waterproof joint.
> That might be adding unnecessary resistance.
>
> What sort of instrument is (if any) capable of measuring this? I have a 6.5
> digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% +
> 20315 counts. Those 20315 counts are a lot!
>
> I can't seem to see much in the way of commercial instruments for very low
> resistance measurements. I would have thought an AC source was needed, yet
> they all seem to use DC. Why?
>
> I've thought of hooking a signal generator up to an audio amplifier capable
> of driving a few amps, passing that through the joint, then using an EG&G
> 7260 lock-in amplifier to measure an AC voltage across the joint.
>
> Any better suggestions?
>
> Can anyone explain why commercial instruments use DC, despite that small DC
> voltages will be developed by unwanted thermocouples? I would have thought
> that using AC was a no-brainer no very low resistance measurements, but
> commercial instruments don't use to use AC.
>
>
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> drkirkby at kirkbymicrowave.co.uk
> http://www.kirkbymicrowave.co.uk/
> Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time)
>
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
> UK.
> Registered in England and Wales, company number 08914892.
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