[volt-nuts] Best way to measure micro Ohms

acbern at gmx.de acbern at gmx.de
Sun Sep 17 15:12:45 EDT 2017

The question is what accuracy you need.
The classical way to do that (achieving high accuracy) is to apply a known accurate current (say 10A) and measure the voltage drop accross the rod with a nanovoltmeter.
As the piece of aluminum is isothermal you should not expect a big thermovoltage. You could also compensate for this by reversing the current and take the average, also by nulling the voltage reading prior to applying any current. Generating precisely known AC currents (low uncertainty) is difficult (i.e. measuring it precisely), therefore DC currents are ususaly used also in metrology for this.
If you do some internet search you will find metrology reports about this. If you do not have a nanovoltmeter you could build a measurement amplifier with not that much of an effort (based on chopper amp or low drif precision opamp)

> Gesendet: Sonntag, 17. September 2017 um 19:23 Uhr
> Von: "Dr. David Kirkby (Kirkby Microwave Ltd)" <drkirkby at kirkbymicrowave.co.uk>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: [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
> 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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