Re: [volt-nuts] Best way to measure micro Ohms
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
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 mailing
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On 18 September 2017 at 23:28, george g_einst@hotmail.com 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
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
On 27 September 2017 at 22:22, ed breya eb@telight.com 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&G 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
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) <
drkirkby@kirkbymicrowave.co.uk> wrote:
On 27 September 2017 at 22:22, ed breya eb@telight.com 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&G 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
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
On 28 September 2017 at 19:17, ed breya eb@telight.com wrote:
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
I guess the point is current/voltage can be measured accurately before the
direction is changed, whereas that's less so with AC. But I don't need
great accuracy in trying to sort out if there are contact resistance
problems caused by things such as copper slip, or aluminum oxides. I need
to measure very low resistances, but not with great accuracy.
For me at least, I have no Fluke 5520A, or low resistance meter. The
Agilent programmable DC power supplies I have are not designed for
precision measurements.
But the lock-in amplifier I already had, and the audio amplifier was cheap.
Sometimes you have to make do with what you have, or can justify buying.
Dave
Ed,
It is not sinusoidal AC. It is a DC voltage which is allowed to settle
before it is measured, It is then measured a few times to get an average.
The excitation current is then turned off to allow the resistors to cool
before the DC voltage is reversed and the process is continued. The key
factor is that all measurements are taken in a stable state.
There is a difference between reversing DC and AC.
Years ago I ask a guy how he got resistance values plotted to as many
digits as he did on a set of bridge resistors.
He told me he used an HP 3457A in statistics mode and measured the voltage
drop on the sent then revised the current exciting the bridge and took the
same measurements again.
He then averaged the values together to get the ratios of the 2 bridge
arms. He called it reversing DC. The first time I remember hearing the
term.
AC current sources are inherently less stable than DC current sources and
AC measurement system are inherently less accurate than DC system. If you
use AC rather than DC you are going to give up at least one and maybe 2
least significant digits. You would not be having this discussion if you
were willing to throw that much accuracy away. The standard resistor used
in the test typically has a DC documented value as well as a documented
temperature coefficient and an undocumented AC value with an undocumented
frequency coefficient and undocumented AC temperature coefficient.
True AC measurements are not nearly as accurate as DC measurements. HP
3458A has a mode for measuring AC where it takes a bunch of DC measurements
at high speed and calculates the root mean square. It works best on
repetitive low noise sinusoidal signals. For the correct signal, it is the
most accurate AC mode. For other signals, there is an AC to DC converter
that feeds a DC measurement system. In that case, the vast majority of
error comes from the converter. If the converter method gives you a tighter
range of readings than the DC method you know you have dirty
signal/distorted when compared to a sine wave or not very repetitive. It
works a lot like a sampling O-scope. Both of which depend on a solid
trigger signal.
On Thu, Sep 28, 2017 at 11:17 AM, ed breya eb@telight.com wrote:
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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John Phillips
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