Dave,

Another meter is the Cambridge LOM-510A. I am not sure if it is in your
budget but there has been a review made on EEVBlog. There is one currently
on eBay with a current amplifier that I have never seen before today. It
may be worth reviewing if it meets your needs.

Todd

On Mon, Sep 18, 2017 at 12:31 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:

On 18 September 2017 at 18:36, Todd Micallef tmicallef@gmail.com wrote:

Hi Todd,
Something struck me as odd about the model number LOM-510A.

When I google the Keithley 580, I see a paid advert for an IET Labs
LOM-510A.

http://www.ietlabs.com/lom-510.html?gclid=CjwKEAjw3f3NBRDP_NHS9fq53n4SJACKIfEYZT_jyIk_TKn0gqEkzvfxwKASIpIZTcDFJfJlMwvnRRoCZYHw_wcB

But it looks nothing like the Cambridge LOM-510A on eBay

http://www.ebay.com/itm/Cambridge-510A-Micro-Ohmmeter-DLRO-Includes-NEW-leads-NIST-Calibrated-LOM-510A-/291956733548

It seems a bit odd for two manufacturers (IET Labs and Cambridge
Instruments) to both have the exact same model number (LOM-510A) for an
instrument with identical functionality (micro ohms). Yet photographs of
the two instruments show they look similar, but are not identical. The IET
Labs one appears to have a couple of extra switches.

Just to make something appear even more odd, the IET Labs has a link to a
review on EEVblog.

http://www.eevblog.com/forum/testgear/ie/30/

but the picture shows Cambridge Instruments, but the title shows IET Labs.

Dave

Dave,

It looks like IET updated the design of the instrument with better
switching. If you download the manual from the IET website, they still show
the older model.
I wonder if the newer model incorporates a front switch that enables the
pulsed mode that was added by the owner on EEVBlog? I can't tell by the
photo.

Todd

On Mon, Sep 18, 2017 at 5:44 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:

David wrote:

Looking at the pix, there appear to be lots of aluminum joints due to
the "built-up" construction, maximizing the potential for the sort of
troubles you are having.  I would re-make the piece in brass, doing
everything possible to use as few pieces as possible (for example, by
milling recesses into one piece rather than building up a compound piece
from more easily machined sub-parts).

I would also plate everything in silver.

Best regards,

Charles

On 19 September 2017 at 20:32, Charles Steinmetz csteinmetz@yandex.com
wrote:

Looking at the pix, there appear to be lots of aluminum joints due to the

A practical problem is the tools available to me. The U-channel was
machined by someone in my radio club, and the rest I made myself using
nothing more than a drill and hand tools. It would be nice to make more out
of one piece, but it would require better tools than I have readily
available. There are certainly engineering companies that could do a better
job, but it would be quite costly.

Yes, I was thinking that.

I contacted one local plating company, and asked them about silver plating.
The lady said they did not do brass, but did aluminum. This struck me as
odd, as they had plated brass for me before -  but that was gold, not
silver. From what I read, aluminum is a lot more tricky to plate than
brass. I sent them a drawing some time ago, but got no response. I will
have to chase them up.

The conductivity of aluminum is better than brass, and since I am mainly
interested in low frequencies (1.8 to 28 MHz), the skin depth will be
deeper than it is possible to silver plate. So the "RF resistance" might
well be set by the material its constructed from, rather than any plating.
Of course, if the plating stops an oxide building up, that should cut the
losses.

But certainly something is not quite right, as the RF losses are higher
than expected. I was keen to see if any of those could be explained by DC /
low frequency losses.

Best regards,

Charles

Dave, G8WRB

David wrote:

If you can make a decent drawing of what you need (it doesn't have to be
a draughtsmanlike job, as long as the form and measurements are clearly
shown), I bet you can find someone in your radio club, or a personal
friend, or someone on a list you frequent to make it for a nominal cost
(you would probably need to pay for the raw material, although I don't
charge for anything that comes out of my scrap pile and this is true of
many other home shop machinists).  HSMs are behind every third or fourth
garage door in the UK.

Another option would be to build up the parts as you have done, then
have someone TIG weld the pieces together.

Best regards,

Charles

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

From: volt-nuts volt-nuts-bounces@febo.com on behalf of Charles Steinmetz csteinmetz@yandex.com
Sent: 19 September 2017 21:30
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Best way to measure micro Ohms

David wrote:

If you can make a decent drawing of what you need (it doesn't have to be
a draughtsmanlike job, as long as the form and measurements are clearly
shown), I bet you can find someone in your radio club, or a personal
friend, or someone on a list you frequent to make it for a nominal cost
(you would probably need to pay for the raw material, although I don't
charge for anything that comes out of my scrap pile and this is true of
many other home shop machinists).  HSMs are behind every third or fourth
garage door in the UK.

Another option would be to build up the parts as you have done, then
have someone TIG weld the pieces together.

Best regards,

Charles

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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

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,

mitch

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@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

From: volt-nuts volt-nuts-bounces@febo.com on behalf of Charles Steinmetz csteinmetz@yandex.com
Sent: 19 September 2017 21:30
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Best way to measure micro Ohms

David wrote:

If you can make a decent drawing of what you need (it doesn't have to be a draughtsmanlike job, as long as the form and measurements are clearly shown), I bet you can find someone in your radio club, or a personal friend, or someone on a list you frequent to make it for a nominal cost (you would probably need to pay for the raw material, although I don't charge for anything that comes out of my scrap pile and this is true of many other home shop machinists).  HSMs are behind every third or fourth garage door in the UK.

Another option would be to build up the parts as you have done, then have someone TIG weld the pieces together.

Best regards,

Charles

volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

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and follow the instructions there.

An aluminum electrical connection needs a few things to
be reliable:

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.

-Chuck Harris

Mitch Van Ochten wrote: