[volt-nuts] Best way to measure micro Ohms

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Tue Sep 19 08:16:36 EDT 2017

On 18 September 2017 at 23:28, george <g_einst at hotmail.com> wrote:

> The reason that DC is used commercially to measure resistance is simple,
> if you use AC you may well get the reactive component as well as the
> resistance coming into play.

That may not be an an issue with a dual-phase lock-in amplifier, as the
phase angle of the voltage can be resolved. In fact, it hints at something
I have long thought about - using a lock-in amplifier as an LCR meter. I do
however have a decent HP 4284A LCR meter, but it can't read very low
impedances in the micro ohm range.

I've put a couple of offers in on micro-ohm meters, but also bought a 150 W
public-address (PA) audio amplifier for £25 (around $35). With that, and
the lock-in amplifier, I should be able to make measurements, although I
accept the uncertainty will be higher than a dedicated micro ohm meter.

> Such low resistance measurements commercially are normally only made on
> high current power distribution networks as part of a periodic test regime
> where you need to determine the quality/resistance of such things as bus
> bar joints/connections and loop resistance.
> It is not a good idea to use copperslip around aluminium, there is an
> aluminium based version that should be used, but, be warned, just like
> copperslip it is an insulator, try putting your meter probes, set for
> resistance, into a tub of both, I have.  I do not know just why but the
> aluminium version is just like sand, it gets everywhere when you use it.

So does Coperslip! It sure is messy, but I was given it free, and it
stopped a leak.

> To check your joint I would use a four wire Kelvin set up using say 10
> Amps from my constant current bench supply and then use my Keithly 616
> digital  electrometer to measure the voltage/s present across the joint, a
> simple application of Ohms law will then give the resistance.

I don't have such an instrument, whereas the audio amplifier cost me very
little, and I already have the lock-in amplifier. I did buy a lower powered
(15 W, $5) amplifier from China, but thought by the time I box it up with a
PSU, it would cost more than a PA amplifier with a built in mains supply.
The PA amp, being in the UK, should also arrive a lot quicker than the
units from China.

> 73 George G6HIG

Dave, G8WRB

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