Re: [volt-nuts] Best way to measure micro Ohms

[ed breya]

Coincidental to all this micro-ohms and nanovolt talk, I've been doing 
some severe large scale garage cleaning to thin stuff out. I found that 
audio amplifier that I mentioned earlier, that is good for some LIA 
reference driver applications. I also found my low-level measurement 
notebooks, including the datasheet and my notes about its operation and 
modifications. I found that the exact same datasheet I got online years 
ago is still available, so here it is, if anyone is interested.


http://www.toacanada.com/assets/files/BG-10_IM.pdf

I also dug out the old Keithley 148 nanovoltmeters, etc, and couldn't 
resist fooling around with them for a bit. I'll have more to say later, 
but in new threads.


Ed

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Re: [volt-nuts] Best way to measure micro Ohms

[ed breya]

Hmm. Alternating the direction of the current repeatedly and processing 
the results - sure seems like that is fundamentally an AC measurement 
too, despite using DC measurement equipment.


Ed

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Re: [volt-nuts] Best way to measure micro Ohms

[John Phillips]

Any technology that does not DC must have some other overriding redeeming
factors given the AC measurements are not nearly as accurate DC and AC
currents are just as inaccurate. Best bridges I have used take several
reading with half of them opposite polarity and average them together.
Sometimes called the reversing DC method. Given what you are trying to
measure your temperature and skin effects  (not RF skin) induce more error
than a good instrumentation system. The current duty cycle can make a big
difference when you are looking this deep into the mud. The current source
should be a fast settling DC supply of about 10 amps. More than 10 amps and
stability and accurate usually degrades. The DC voltage measurement system
should look for a stable DC level before taking a usable current is then
turned off then reversed for the next reading. Several reading are filtered
and averaged together.

Use something like a Fluke 5520A. I believe the 488 bus control can reverse
the polarity, if not use an external relay to reverse the DC current.
Different current levels may be used to see where you get the most stable
readings. Use a standard resistor in the same range as the UUT.  2 HP
3458As can simultaneously read the standard restore and the UUT. 1 3458 may
work in ratio mode but probably will not give good readings at these
voltage levels. In any case, each input should be switched between the
standard and UUT. All of the hardware would be controlled by a basic
program running in excel to load the data into a spreadsheet for filtering
and analysis. In the end, the ratio between the standard resistor and the
UUT is all the counts. Well not quite. The stability of each instrument
will make a difference. The absolute current and voltage calibration kindof
cancel out as long as the meters are close to calibration stable and
linear. Make sure the voltage measurements are all taken in the same range.

If you do not want to get this elaborate there are several 10
amp micro-ohm metes to be had. The good ones do the reversing DC with math
internally and are quick enough to do minimal sample heating. Any that put
out a constant 10 amps while measuring are inferior.

On Wed, Sep 27, 2017 at 3:51 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkir...@kirkbymicrowave.co.uk> wrote:

> On 27 September 2017 at 22:22, ed breya  wrote:
>
> > I just noticed this discussion recently, so I'm late to the party, but
> > that never stops me from adding my one-cent's worth.
> >
> > David, regardless of the aluminum and other material issues, I think your
> > initial idea of using a lock-in analyzer is definitely the way to go. I'm
> > very fond of LIAs, although I seldom need or use them, so my opinion is
> > somewhat biased. I have five - two Ithaco 391A orange-band, a PAR 5204,
> an
> > SR830 and SR850.
> >
>
>
> Yes, me too. I have used a number over the years, the nicest of which was
> the Stanford Research SR830. The EG 7260 I own has rather user-hostile
> interface. There's not even a power switch on the front, and I'm not sure
> if there's even on on the rear.
>
> The following link might interest others who don't know about these
> instruments
>
> http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf
>
>
>
> >
> > If you use an audio power amplifier for driving the experiment, you can
> > rig it up so that the LIA can be used to measure the drive current as
> well
> > as the resulting voltage drop. Let's say the amplifier is for 8 ohms, so
> > you put a few ohms in series with the output, then from there into a
> > precision one-ohm sampling resistor, then into the RUT, forming a voltage
> > divider. The RUT is expected to be in the micro-ohm region, which is many
> > thousands of times smaller than the sampling R, so its tiny voltage drop
> > will be negligible, allowing the sample voltage to be a good
> representation
> > of the test current. You could also just treat the whole thing as a
> voltage
> > divider and calculate the "exact" results.
> >
>
> I did purchase a Radio Shack 100 W Public address  I will set this up
> later.
>
>
> >
> > This could be fun and interesting. There are plenty of pieces and
> > variables involved to experiment with to optimize the measurement, and
> lots
> > of other tricks available to enhance it if necessary.
> >
> > Ed
>
>
> Yes, with lock-in amplifiers there are an almost infinite set of
> possibilities of how to do the actual measurement. I don't know if the
> reference output is a sine wave or square wave on this unit. I have the
> option of using a Stanford Research DS345 30 MHz function generator to
> generate a sine wave if needed. There are almost a million things that one
> can change.
>
> Dave
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-- 

*John Phillips*

Re: [volt-nuts] Best way to measure micro Ohms

[ed breya]

I just noticed this discussion recently, so I'm late to the party, but 
that never stops me from adding my one-cent's worth.


David, regardless of the aluminum and other material issues, I think 
your initial idea of using a lock-in analyzer is definitely the way to 
go. I'm very fond of LIAs, although I seldom need or use them, so my 
opinion is somewhat biased. I have five - two Ithaco 391A orange-band, a 
PAR 5204, an SR830 and SR850.


If you use an audio power amplifier for driving the experiment, you can 
rig it up so that the LIA can be used to measure the drive current as 
well as the resulting voltage drop. Let's say the amplifier is for 8 
ohms, so you put a few ohms in series with the output, then from there 
into a precision one-ohm sampling resistor, then into the RUT, forming a 
voltage divider. The RUT is expected to be in the micro-ohm region, 
which is many thousands of times smaller than the sampling R, so its 
tiny voltage drop will be negligible, allowing the sample voltage to be 
a good representation of the test current. You could also just treat the 
whole thing as a voltage divider and calculate the "exact" results.


The voltage on the RUT is measured at whatever gain is needed. The 
voltage on the sample resistor will be plentiful at 1V/A, and both 
signals will have very low source R, and minimal noise. Since both 
signals can be measured by the LIA, the uncertainties in assessing each 
part with different equipment are much reduced.


The reference frequency should be as low as possible, limited by the 
amplifier's low-end capability, and selected so it and its harmonics 
land as far as possible from the power line frequency and its harmonics, 
for say up to n=15, or whatever is practical. This will help to reduce 
line interference from nearby sources, and ground loops, and from the 
amplifier. Especially at low frequencies, the amplifier may show a lot 
of line harmonics when driven to high levels - the filter capacitors in 
its power supply can only do so much, and audio PAs are likely not all 
that great in terms of PSRR. Turning on the LIA's line notch filter will 
also help, at least with the fundamental.


The frequency needs to be very low in order to minimize the parasitic 
currents that will cause errors, especially considering that this setup 
is dividing on the order of a million in a single stage. If this appears 
to cause problems, you can reduce the large division ratio by using a 
much smaller sample resistor, and treating it as a divider for 
calculation purposes. Alternatively, adding some appropriate shielding, 
or splitting the division into isolated sections can greatly reduce the 
effects. To avoid signal ground loops, measuring the drive current and 
the RUT voltage should be separate operations, each carried on its own 
BNC cable to the LIA, while the other is completely disconnected and out 
of the way - having no common-grounding or cable bundling or fancy 
signal routing/switching is best. The weakest link ground-loop-wise may 
be the necessity of carrying the reference drive between the LIA and 
power amplifier input, likely sharing the same ground as the output. 
This could force you to set it up for differential measurement of the 
RUT signal. The special audio PA that I have for such purposes has its 
ins and outs transformer-coupled, which helps a lot.


This could be fun and interesting. There are plenty of pieces and 
variables involved to experiment with to optimize the measurement, and 
lots of other tricks available to enhance it if necessary.


Ed

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Re: [volt-nuts] Best way to measure micro Ohms

[Chuck Harris]

I guess I would have to ask, "How do you know everything
was done 'right'?"  The evidence you have stated makes a
pretty good case that it wasn't.

The neutral entering a sub panel at most has the same current
going through it as one of the line input leads, though in a
240V sub panel, load balancing makes it typically much lower.

If the neutral was installed correctly, and yet still cold
flowed, then one or both of the line inputs should have cold
flowed too.  If they didn't, then I don't believe the neutral
was installed correctly.

The grades and finishes of the aluminum used in the electric
supply systems are well specified and regulated through UL,
and the NFPA.  If the wrong grade, temperature, or alloy of
aluminum was used; or the wrong service equipment, fastener,
or torquing was used; or NoAlOx was omitted; then it wasn't
done "right".

As to aluminum oxide, unless it is removed, and prevented, a
good electrical connection to the aluminum is impossible.

NoAlOx does both.  It prevents the oxygen from reaching the
joint, and the grit embedded in it breaks through any oxide
layer, and mechanically raises fresh aluminum to the surface
to make a gas tight joint.

Even though the manufacturer says it isn't necessary to
mechanically remove the oxide layer, I do it anyway.  I have
never had an aluminum connection (made in this way) fail.

-Chuck Harris



Dan Kemppainen wrote:
> Aluminum...
> 
> Even with everything done right, with approved hardware, aluminum wiring will 
> tend to
> 'creep' (Aluminum mushing out of the way of the lug screw or clamp.
> 
> Just recently I checked the main lugs on my 150A garage sub panel, and sure 
> enough
> the neutral lug was loose. This was about 8 years after installation, with 
> virtually
> no use (only one circuit used for lights until now.)
> 
> However I believe this is only a problem with the softer grades of aluminum. 
> If your
> enclosure were out of 7075, or 2024 my guess is you wouldn't see aluminum 
> creep at
> all. 6061-T6 or similar would probably be OK also. It's the soft stuff that 
> moves
> easily. They all form the oxide layer, so that's still a problem...
> 
> In a pinch, I may be able to machine something, but shipping to there from 
> here is
> probably expensive...
> 
> Dan
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Re: [volt-nuts] Best way to measure micro Ohms

[Mitch Van Ochten]

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-boun...@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-boun...@febo.com> on behalf of Charles Steinmetz 
<csteinm...@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:

> 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.

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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Re: [volt-nuts] Best way to measure micro Ohms

[Andre]

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-boun...@febo.com> on behalf of Charles Steinmetz 
<csteinm...@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:

> 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.

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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Re: [volt-nuts] Best way to measure micro Ohms

[Charles Steinmetz]

David wrote:


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.


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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Re: [volt-nuts] Best way to measure micro Ohms

[Charles Steinmetz]

David wrote:


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


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


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Re: [volt-nuts] Best way to measure micro Ohms

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 18 September 2017 at 23:28, george  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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Re: [volt-nuts] Best way to measure micro Ohms

[Chuck Harris]

If this stuff is what I think it is, it does contain
a sand of sharp, probably aluminum oxide, abrasive.

The idea is when you bolt the connection together, the
abrasive will break through the aluminum oxide layer
on the conductors, and will mushroom out the aluminum as
the abrasive burrows in, making lots of pure aluminum
metal contacts.

Usually, it also contains a dense oil to keep oxygen
away, and prevent further oxidization of the aluminum.

-Chuck Harris

george 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.
> 
> 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.
> 
> 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.
> 
> 73 George G6HIG ___ volt-nuts 
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Re: [volt-nuts] Best way to measure micro Ohms

[Todd Micallef]

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) <
drkir...@kirkbymicrowave.co.uk> wrote:

> On 18 September 2017 at 18:36, Todd Micallef  wrote:
>
> > 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
> >
>
>
> 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
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Re: [volt-nuts] Best way to measure micro Ohms

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 18 September 2017 at 18:36, Todd Micallef  wrote:

> 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
>


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
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Re: [volt-nuts] Best way to measure micro Ohms

[Todd Micallef]

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) <
drkir...@kirkbymicrowave.co.uk> wrote:

> On 18 Sep 2017 00:43, "Charles Steinmetz"  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.
>
> Dave
>
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Re: [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


Sent from Yahoo Mail for iPad


On Sunday, September 17, 2017, 4:41 PM, Charles Steinmetz 
 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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Re: [volt-nuts] Best way to measure micro Ohms

[Brian Smith]

A DMM with good low resistance capability will have an "offset compensated
ohms" feature. A large current is used and then a small current is used.
The slope of the line formed by V-I points is the true resistance.

Measuring tiny DC voltages is easier than measuring tiny AC voltages. More
bandwidth is more cost for an amplifier.

Pass an amp or more through the device and use a KE 2182A to measure the
microvolts.

Since I do not own a 2182A, I use a KE 147. You will need a DMM on the
analog output of the 147 if you want digits of resolution.

When I do low resistance, I am just looking to bracket the resistance, not
measure to 6 1/2 digits.

The thermal offsets can require a lot of time to stabilize.

I built a Tetrajunction demo device that is in the tens of nano-ohms. It
takes all day to set up the measurement and have the offsets stabilize and
take a few measurements.


-Brian Smith

On Sun, Sep 17, 2017 at 1:23 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkir...@kirkbymicrowave.co.uk> wrote:

> I want to measure the resistance between two bits of aluminum. Each are 40
> x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering
> is surface oxides are on the faces, so despite being held together with
> bolts, the resistance is perhaps not as long as I would expect. There's
> also a layer of "copperslip" between these, to provide a waterproof joint.
> That might be adding unnecessary resistance.
>
> What sort of instrument is (if any) capable of measuring this? I have a 6.5
> digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% +
> 20315 counts. Those 20315 counts are a lot!
>
> I can't seem to see much in the way of commercial instruments for very low
> resistance measurements. I would have thought an AC source was needed, yet
> they all seem to use DC. Why?
>
> I've thought of hooking a signal generator up to an audio amplifier capable
> of driving a few amps, passing that through the joint, then using an EG
> 7260 lock-in amplifier to measure an AC voltage across the joint.
>
> Any better suggestions?
>
> 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.
>
>
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> drkir...@kirkbymicrowave.co.uk
> http://www.kirkbymicrowave.co.uk/
> Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time)
>
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
> UK.
> Registered in England and Wales, company number 08914892.
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Re: [volt-nuts] Best way to measure micro Ohms

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 17 September 2017 at 21:58, Mitch Van Ochten <
mi...@vincentelectronics.com> wrote:

> The Keithley 2002 uses DC but automatically takes a reading of any offset
> voltage and subtracts it (offset compensation).  Rated accuracy on the 20
> ohm range (2 years) is +/- 26 ppm, and with 10 averages it has a resolution
> of 0.1 microohms.
>
>
> Best regards,
>
> mitch
>

Also very pricy!

I've made an offer on a Keithley 580 micro ohm meter, and ordered a couple
of $3 audio amplifier boards. I will try one with the lock-in amplifier. It
it does not work, or works poorly, it will not have broken the bank.

Dave
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Re: [volt-nuts] Best way to measure micro Ohms

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 17 September 2017 at 20:12,  wrote:

> The question is what accuracy you need.
>

No a lot. I just want to find out if there's any voltage drops that are
significantly higher than I would expect. The unit makes an RF transmission
line, and the loss at RF is significantly higher than predicted by a
computer model, which takes into account the skin depth of the materials.
I'm wondering if there's something odd going on. I suspect the problem is
the current in the aluminum is not being computed properly due to the oxide
on the surface. But I just wanted to make sure there was no unexpected DC
resistance. I don't think there will be, but I want to climate that
possibility.


> The classical way to do that (achieving high accuracy) is to apply a known
> accurate current (say 10A) and measure the voltage drop accross the rod
> with a nanovoltmeter.
> As the piece of aluminum is isothermal you should not expect a big
> thermovoltage. You could also compensate for this by reversing the current
> and take the average, also by nulling the voltage reading prior to applying
> any current. Generating precisely known AC currents (low uncertainty) is
> difficult (i.e. measuring it precisely), therefore DC currents are ususaly
> used also in metrology for this.
> If you do some internet search you will find metrology reports about this.
> If you do not have a nanovoltmeter you could build a measurement amplifier
> with not that much of an effort (based on chopper amp or low drif precision
> opamp)
>

The only nV meter I have is the lock-in amplifier, which has a full-scale
sensitivity of 2 nV to 1 V in a 1-2-5 sequence.

The only instrument I have able to measure > 3 A of current is a handheld
multimeter. One of my power supplies can supply 35 A, and has an ammeter in
it. I don't have any particularly accurate means of measuring DC current
outside the limited of the 3457A.

In terms of simplicity, getting a $10 audio amplifier from China and using
the lock-in amplifier is the way to go, but I accept a metrologist would
not like that idea!

Dave
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Re: [volt-nuts] Best way to measure micro Ohms

[Mitch Van Ochten]

The Keithley 2002 uses DC but automatically takes a reading of any offset 
voltage and subtracts it (offset compensation).  Rated accuracy on the 20 ohm 
range (2 years) is +/- 26 ppm, and with 10 averages it has a resolution of 0.1 
microohms.


Best regards,

mitch

-Original Message-
From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Dr. David 
Kirkby (Kirkby Microwave Ltd)
Sent: Sunday, September 17, 2017 1:23 PM
To: Discussion of precise voltage measurement
Subject: [volt-nuts] Best way to measure micro Ohms

I want to measure the resistance between two bits of aluminum. Each are 40 x 30 
mm across. One is 250 mm long, the other is 8 mm long. I'm wondering is surface 
oxides are on the faces, so despite being held together with bolts, the 
resistance is perhaps not as long as I would expect. There's also a layer of 
"copperslip" between these, to provide a waterproof joint.
That might be adding unnecessary resistance.

What sort of instrument is (if any) capable of measuring this? I have a 6.5 
digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% +
20315 counts. Those 20315 counts are a lot!

I can't seem to see much in the way of commercial instruments for very low 
resistance measurements. I would have thought an AC source was needed, yet they 
all seem to use DC. Why?

I've thought of hooking a signal generator up to an audio amplifier capable of 
driving a few amps, passing that through the joint, then using an EG
7260 lock-in amplifier to measure an AC voltage across the joint.

Any better suggestions?

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.


Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
drkir...@kirkbymicrowave.co.uk
http://www.kirkbymicrowave.co.uk/
Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time)

Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT, UK.
Registered in England and Wales, company number 08914892.
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Re: [volt-nuts] Best way to measure micro Ohms

[acbern]

The question is what accuracy you need.
The classical way to do that (achieving high accuracy) is to apply a known 
accurate current (say 10A) and measure the voltage drop accross the rod with a 
nanovoltmeter.
As the piece of aluminum is isothermal you should not expect a big 
thermovoltage. You could also compensate for this by reversing the current and 
take the average, also by nulling the voltage reading prior to applying any 
current. Generating precisely known AC currents (low uncertainty) is difficult 
(i.e. measuring it precisely), therefore DC currents are ususaly used also in 
metrology for this.
If you do some internet search you will find metrology reports about this. If 
you do not have a nanovoltmeter you could build a measurement amplifier with 
not that much of an effort (based on chopper amp or low drif precision opamp)


> Gesendet: Sonntag, 17. September 2017 um 19:23 Uhr
> Von: "Dr. David Kirkby (Kirkby Microwave Ltd)" 
> 
> An: "Discussion of precise voltage measurement" 
> Betreff: [volt-nuts] Best way to measure micro Ohms
>
> I want to measure the resistance between two bits of aluminum. Each are 40
> x 30 mm across. One is 250 mm long, the other is 8 mm long. I'm wondering
> is surface oxides are on the faces, so despite being held together with
> bolts, the resistance is perhaps not as long as I would expect. There's
> also a layer of "copperslip" between these, to provide a waterproof joint.
> That might be adding unnecessary resistance.
> 
> What sort of instrument is (if any) capable of measuring this? I have a 6.5
> digits HP 3457A with a 30 Ohm 4-wire mode, but the uncertainty is 0.0065% +
> 20315 counts. Those 20315 counts are a lot!
> 
> I can't seem to see much in the way of commercial instruments for very low
> resistance measurements. I would have thought an AC source was needed, yet
> they all seem to use DC. Why?
> 
> I've thought of hooking a signal generator up to an audio amplifier capable
> of driving a few amps, passing that through the joint, then using an EG
> 7260 lock-in amplifier to measure an AC voltage across the joint.
> 
> Any better suggestions?
> 
> 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.
> 
> 
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> drkir...@kirkbymicrowave.co.uk
> http://www.kirkbymicrowave.co.uk/
> Tel: 01621-680100 / +44 1621-680100 (0900 to 2100 UK time)
> 
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
> UK.
> Registered in England and Wales, company number 08914892.
> ___
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Re: [volt-nuts] Best way to measure micro Ohms

[Poul-Henning Kamp]

In message