[volt-nuts] Best way to measure micro Ohms

[Dr. David Kirkby (Kirkby Microwave Ltd)]

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)

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