[volt-nuts] Best way to measure micro Ohms

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Sun Sep 17 17:13:38 EDT 2017

On 17 September 2017 at 20:12, <acbern at gmx.de> wrote:

> The question is what accuracy you need.

No a lot. I just want to find out if there's any voltage drops that are
significantly higher than I would expect. The unit makes an RF transmission
line, and the loss at RF is significantly higher than predicted by a
computer model, which takes into account the skin depth of the materials.
I'm wondering if there's something odd going on. I suspect the problem is
the current in the aluminum is not being computed properly due to the oxide
on the surface. But I just wanted to make sure there was no unexpected DC
resistance. I don't think there will be, but I want to climate that

> 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)

The only nV meter I have is the lock-in amplifier, which has a full-scale
sensitivity of 2 nV to 1 V in a 1-2-5 sequence.

The only instrument I have able to measure > 3 A of current is a handheld
multimeter. One of my power supplies can supply 35 A, and has an ammeter in
it. I don't have any particularly accurate means of measuring DC current
outside the limited of the 3457A.

In terms of simplicity, getting a $10 audio amplifier from China and using
the lock-in amplifier is the way to go, but I accept a metrologist would
not like that idea!


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