[volt-nuts] Best way to measure micro Ohms
cfharris at erols.com
Wed Sep 20 08:28:49 EDT 2017
An aluminum electrical connection needs a few things to
1) a "springy" fastener
2) mechanical precleaning
3) an oxygen blocking coating.
In the US, aluminum conductors are allowed for certain
usages. We used to allow 14 and 12AWG receptacle wiring,
but too many houses burned down. The receptacles were
redesigned for Cu or Al, but the codes remained stubbornly
against the practice. A few more times where copper prices
go through the roof, and the codes will change.
For larger conductors, the wire, or bar, is brightened up
with Emory paper, or a stainless steel (important!) brush,
and then is covered with "Gorilla Snot", or some sort of
NoAlOx grease. NoAlOx is a grease made of an oxygen
resistant heavy oil, and a coarse emory grit. I like to
again rough things up after the NoAlOx is liberally applied.
Finally, the conductors are tightened to specified torque
using a springy fastener... The springy fastener is often
simply an ordinary fastener with a "Bellview Washer" stack
to give it compliance.
The big thing that makes high current aluminum joints
fail is thermal expansion. If the fastener isn't springy,
the aluminum expands from the heat, finds it cannot go
in the direction of the tightened fastener, and flows
elsewhere. When the joint cools, and the aluminum under
the fastener shrinks, the joint is now loose, and will
arc when current is once again applied, evaporating more
aluminum out of the joint. Soon the fire department will
be coming... if you are lucky.
NoAlOx prevents this issue, if you use a springy fastener.
Mitch Van Ochten wrote:
> I once repaired a Valhalla 2555A Current Calibrator <https://valhallascientific.com/DataSheets/2555A_Data_Sheet.pdf> (good for up to 100A output). It uses aluminum bus bars inside to route the current. The junctions between the bars had become higher than normal impedance and it could no longer deliver 100A. I disassembled all joints, cleaned them with emery cloth, then applied a drop of Caig Deoxit and re-assembled. That was over four years ago and no complaints from the customer so far.
> Best regards,
> -----Original Message-----
> From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Andre
> Sent: Wednesday, September 20, 2017 3:54 AM
> To: Discussion of precise voltage measurement
> Subject: Re: [volt-nuts] Best way to measure micro Ohms
> Hi, just my $0.02 worth.
> I have some instrumentation amplifiers here also looked into low resistance connections for my other projects.
> If I recall correctly you need to look at the electrochemical series. For interconnects on Al you want a metal similar on the ES.
> The oxide is a problem but if you connect it properly eg with an oil droplet and clamp connnector using compatible vernier it should be fine.
> Looking at how wiring in the US is done might give you some ideas.
> Kind regards, -Andre
More information about the volt-nuts