Low ohm precision resistor standard and testing

[The Soulman]

Is using single binding posts sufficient for measuring these low values? Well apparently it is. 

Edit: I missed the second set of terminals, why use 6 wires?

[HighVoltage]

There are 4 binding posts on the 1 mOhm resistor.

For the 2 high current posts I am using spade connectors and 2.5 mm^2 cable
All Banana plugs are gold plated from MultiContact

The extra spade to 4 mm connectors I used to measure the voltage drop separately.
But this was not hooked up for the measurements.



 

[CalMachine]

There are 4 binding posts on the 1 mOhm resistor.

For the 2 high current posts I am using spade connectors and 2.5 mm^2 cable
All Banana plugs are gold plated from MultiContact

The extra spade to 4 mm connectors I used to measure the voltage drop separately.
But this was not hooked up for the measurements.

Is that a Keithley supplied cable you're using?

[HighVoltage]

Is that a Keithley supplied cable you're using?

No, I did not like the cable that was supplied by Keithley SMU and made this one myself.
The cables are all lab grade LiFY, ultra flexible, ultra fine strain copper.
The banana plugs MultiContact gold plated.

For higher current measurements, I have not found a better cable setup.
 

[HighVoltage]

I calibrated the zero Ohm of the 34470A again new, to a short across the inputs.
And when the calibration is finished, it shows only zeros, as it should.

Also, this calibration is confirmed with the 100 mOhm and 10 mOhm reference resistors.
But the 1 mOhm resistor is not showing correctly on the 34470A at all. It moves between -10 uOhm and +6 mOhm.

May be the 34470A was not meant to be for low resistance value measurements.

[CalMachine]

Is that a Keithley supplied cable you're using?

No, I did not like the cable that was supplied by Keithley SMU and made this one myself.
The cables are all lab grade LiFY, ultra flexible, ultra fine strain copper.
The banana plugs MultiContact gold plated.

For higher current measurements, I have not found a better cable setup.

I love it!  Especially the braided sheath touch.  Very nice craftsmanship.      

[HighVoltage]

I love it!  Especially the braided sheath touch.  Very nice craftsmanship.   

Thanks.

Indeed, this cable works very well.
I see no difference in Low Ohm measurements, compared to a PTFE cable with only crimped gold plated copper spade connectors.
These MultiContact banana plugs are very smooth to insert in to the instrument.

[HighVoltage]

Now, this is funny...

I hooked up the 1 mOhm resistor to an older 34401A instrument and what a surprise, it sits at 0.0010 Ohm
The last digit is moving up and down by +/- 1 in short test.

So, one of my 34401A is better to use for the 1 mOhm range than the 34470A?

[CalMachine]

Now, this is funny...

I hooked up the 1 mOhm resistor to an older 34401A instrument and what a surprise, it sits at 0.0010 Ohm
The last digit is moving up and down by +/- 1 in short test.

So, one of my 34401A is better to use for the 1 mOhm range than the 34470A?

Have you tried with the offset comp on, on the 34470A?

[gamalot]

Now, this is funny...

I hooked up the 1 mOhm resistor to an older 34401A instrument and what a surprise, it sits at 0.0010 Ohm
The last digit is moving up and down by +/- 1 in short test.

So, one of my 34401A is better to use for the 1 mOhm range than the 34470A?

Have you tried with the offset comp on, on the 34470A?

I guess he didn't because I got much better result on my 34465A with the offset comp turned on. 

[CalMachine]

Now, this is funny...

I hooked up the 1 mOhm resistor to an older 34401A instrument and what a surprise, it sits at 0.0010 Ohm
The last digit is moving up and down by +/- 1 in short test.

So, one of my 34401A is better to use for the 1 mOhm range than the 34470A?

Offset comp always makes low ohms measurements more accurate! 

Have you tried with the offset comp on, on the 34470A?

I guess he didn't because I got much better result on my 34465A with the offset comp turned on. 

[onemilimeter]

May be the 34470A was not meant to be for low resistance value measurements.

Hi...

I wonder if Keysight states about this "low resistance value measurement" limitation in the datasheet... If it's not stated, can we assume that the 34470A, which shows 100.0000 at its 100-ohm range, must be able to measure a low resistance like 000.0010 ohm?

Thanks

[onemilimeter]

So, one of my 34401A is better to use for the 1 mOhm range than the 34470A?

That's really funny... 34401A can measure 1 mOhm but not the 34470A... 

[onemilimeter]

I guess he didn't because I got much better result on my 34465A with the offset comp turned on. 

Hi.. how much difference in the results with and without the offset comp turned on? Thanks.

[HighVoltage]

I guess he didn't because I got much better result on my 34465A with the offset comp turned on. 

Hi.. how much difference in the results with and without the offset comp turned on? Thanks.

Ok, it is true, Offset Comp was turned OFF yesterday.
So just now I made another test and here is the result: No difference!

[onemilimeter]

Ok, it is true, Offset Comp was turned OFF yesterday.
So just now I made another test and here is the result: No difference!

Aiii... 

Not sure Keysight knows about this...

[HighVoltage]

Aiii... 

Not sure Keysight knows about this...

I think something is wrong with my 34470A and I will send it to Keysight for repair and/or calibration.
Yes, the old 6 1/2 digit 34401A being more accurate for this measurement than the new 7 1/2 digit 34470A is kind of embarrassing.

[2N3055]

If I remember correctly member here had problems with noisy ohm measurements ( In kohms range ) I belive his DMM was replaced by Keysight (it was brand new)...

[onemilimeter]

Yes, the old 6 1/2 digit 34401A being more accurate for this measurement than the new 7 1/2 digit 34470A is kind of embarrassing.

100% agree...

[Alex Nikitin]

If I remember correctly member here had problems with noisy ohm measurements ( In kohms range ) I belive his DMM was replaced by Keysight (it was brand new)...

That was me  . I've checked the low ohm performance (4W) on the 34465A right now - with a short copper wire ~ 0.7 mOhm the span is about 0.5mOhm, either on NPLC 10 or NPLC 100. With the same Kelvin clips and the same bit of wire the Agilent 34401 I have here temporarily does show about 0.7-0.8 mOhm span.

Cheers

Alex

[Dr. Frank]

HighVoltage,

at NPLC10, the stddev of about 0.12 mOhm seems reasonable, my 34465A shows the same for a 10 mOHM resistor.
Both instruments resolve the same at those low values, i.e. 0.1 mOhm on the display, and 0.01 mOhm on the statistics.

At NPLC100, which you also might test, stddev is about 0.08 mOhm, span 0.4 mOhm, Min = 9.8 mOhm, Max = 10.1mOhm, Mean = 9.96 mOhm. So for a 1mOhm resistor, I wouldn't expect negative Min values as well. That makes no sense, as the Ohm circuit obviously is linear down to zero.


On the other hand, you shouldn't expect too good an accurcay at 1mOhm, these instruments are really not made for that, as the test current of 10mA is much too low.
The 100 Ohm range is specified worst, i.e. 30ppm of range (24h). That is +/- 3mOhm !!

Maybe your zero calibration did not result in a perfectly centered result, around 1mOhm, but in face of the specification, that's no wonder at all.

In the end I think, that your instrument is fine, if you look at the stddev values.


Anyhow, if you let KS calibrate and adjust your 34470A, for the 2nd time in its life, maybe, then the 100V and 1kV DC calibration then should be precise,  for the first time.
When I checked your 34470A, it also had this 15ppm bug in the HV ranges.
Frank

[gamalot]

I have done the test again 

[HighVoltage]

On the other hand, you shouldn't expect too good an accurcay at 1mOhm, these instruments are really not made for that, as the test current of 10mA is much too low.
The 100 Ohm range is specified worst, i.e. 30ppm of range (24h). That is +/- 3mOhm !!

Frank

Hello Dr.Frank
Thanks for this great explanation, this does make perfect sense what you are writing and I would totally agree.
But If I keep the 1mOhm resistor hooked up to the 34470A, the shown values drift significantly larger than the +/- 3mOhm
Sometimes I see even negative values.

And on the other side, ... the 34401A, 34410A and 34411A seem to be more stable for this measurement than the 34470A
And then you are also right, I should not expect the 34470A to perform so well in the low resistance values because that is not what it is made for.

Well, in a few weeks I will have my 34470A calibrated at Keysight Germany and then I will do another test with the 1 mOhm resistor.

[HighVoltage]

I have done the test again 

Hello gamalot,
Interesting, it seems you confirmed my finding, that there is no difference in Offset Comp ON or OFF

What is the real nominal value of your 1 mOhm resistor?
It seems you are also confirming with your 34465A, that this instrument really is not up for the 1 mOhm task.

[gamalot]

I have done the test again 

Hello gamalot,
Interesting, it seems you confirmed my finding, that there is no difference in Offset Comp ON or OFF

What is the real nominal value of your 1 mOhm resistor?
It seems you are also confirming with your 34465A, that this instrument really is not up for the 1 mOhm task.

No, don't get me wrong! 

Look at the two pictures carefully, you will find the difference between the two averages:

1.02 mOhm with Offset Comp turned on

0.73 mOhm with Offset Comp turned off

The resistor under test is 1 mOhm