[volt-nuts] Best way to measure micro Ohms

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Mon Sep 18 12:31:10 EDT 2017

On 18 Sep 2017 00:43, "Charles Steinmetz" <csteinmetz at yandex.com> wrote:

> 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

Thanks.  You have confirmed what I was thinking - it is *probably* the
oxide causing the problem.

It's not a waveguide in the normal sense of the word, transmitting a TE or
TM wave down a hollow tube,  but more like a coaxial line transmitting
something close(ish) to a TEM wave. The outer conductor is uncoated
aluminum and rectangular in cross section.  The inner conductor is brass.
See pictures attached (I made them small, so quality his not great, but it
should not too use much bandwidth)

Attached are a couple of pictures, and also S11 measured on a VNA, with one
connector shorted Since this is a reflection measurement, the EM wave
travellels along this twice, so about half the loss would be in each
direction. It is only a rough measurement, but a transmission measurement
showed similar results, but half as much attenuation, as it is only being
attenuated one way.

Maybe I need to use brass, or silver plate the aluminum.

The purpose of this was to measure the loss of a very low loss liquid
dielectric, but from discussions I had with someone at NPL, such a
structure is not suitable if the loss is very low.

Anyway, I have put it an offer on a Keithley microohm meter. I notice there
are a lot of Chinese ones at quite low priced. I've no idea how good/bad
they are. But they are much more modern and cheaper than an *affordable*
Kiethley meter. A Keithley 2002 is well outside my budget.

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