[volt-nuts] Best way to measure micro Ohms

Mitch Van Ochten Mitch at vincentelectronics.com
Sun Sep 17 16:58:05 EDT 2017

The Keithley 2002 uses DC but automatically takes a reading of any offset voltage and subtracts it (offset compensation).  Rated accuracy on the 20 ohm range (2 years) is +/- 26 ppm, and with 10 averages it has a resolution of 0.1 microohms.

Best regards,


-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Dr. David Kirkby (Kirkby Microwave Ltd)
Sent: Sunday, September 17, 2017 1:23 PM
To: Discussion of precise voltage measurement
Subject: [volt-nuts] Best way to measure micro Ohms

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
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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