[volt-nuts] Best way to measure micro Ohms

[Howard Davidson]

I use a Keithley 2182 and 6221 nano-ohm setup at work. It is a combination of a reversing precision current source and a nanovoltmeter with embedded software to manage the process. I can reliably measure into the 50 nano-ohm regime.The surface chemistry of the metal joint is very important. Both nickel and aluminum have thin tenacious oxides. It takes a compliance setting of >65 V to punch through nickel oxide films on pressure contacts.If a bolted contact has sufficient contact pressure to crack the film yo can obtain dry circuit conduction. My experience is that a silver plated contact surface will have a decade lower contact resistance than an otherwise identical nickel plated one.Keithley has an excellent Low Level Measurement Handbook and appnotes for download. 
Howard Davidson


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On Sunday, September 17, 2017, 4:41 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:

David wrote:

>  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.

Difficulty of measuring AC, compared to DC, is one reason, as has been 
mentioned.  But the main reason is that skin effect (and usually to a 
much lesser extent, inductance) is a significant factor at surprisingly 
low frequencies, *particularly* when the expected value is in the micro- 
to deci-ohm range.

Also, since you said the waveguide is aluminum (and didn't say anything 
about plating), be aware that aluminum exposed to air is covered by a 
thin aluminum oxide layer (Al2O3), which forms within seconds after a 
new surface is exposed.  This layer is thin -- generally about 4 nm -- 
but the bulk resistivity of Al2O3 is very high, so there is a finite and 
variable resistance across the interface between two joined pieces of 
aluminum (depending on the area of the joint, the joining pressure, and 
the extent to which the joining method produces a clean [oxide-free], 
gas-tight interface between the joined surfaces).

Best regards,

Charles


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