Low ohm precision resistor standard and testing

[tszaboo]

I commonly run it more the "old way" with the DC current resitance, but this is more historical based.

Can you explain that a bit further?

I took some pretty photo while ago too.
Short resistance of Fluke 5700A in 4-wire mode, using 5440B-700x cable.

Nice    The 8508A showed 35µ Ohms on my Fluke 5450A.

OK, I looked up the cal certificate, they wrote 12 PPM uncertainty on it forr a 50 Ohm resistor, with EA 4/02
http://www.european-accreditation.org/publication/ea-4-02-m-rev01--september-2013

Das anyone know how they calculated the 12ppm? Is it the 99% Tcal +/-5K spec? That would give 11.2ppm

"The measurements have been calibrated using measurement standards traceable towards international and national standards, and meets the requirements of ISO/IEC 17025:2005."

"Based on a standard uncertainty multiplied by a coverage factor of K=2, which provides a confidence level of
approximately 95%. The standard uncertainty has been determined in accordance with EA-4/02."

[Zermalmer]

I commonly run it more the "old way" with the DC current resitance, but this is more historical based.

Can you explain that a bit further?

Hi Phil,
will try it.

You supply the resistor with a constant current and measure the voltage drop over the resistor and simply calculate the resistance.
R=U/I

And according to the fact that a resistance in DC usage isn't equal in both directions you have to do the same with switched polarity.
(If you use the resistor always in the same polarity that isn't really needed, but who can guarantee that?)

The MU of this configuration is for sure higher than the accuracy of the 8508A, but the results of this method give normally very trustfully results.


ADD:plesa already mentioned this method in the 2nd post of the thread ... looks like I have to do some reading in the thread 

[e61_phil]

Hi Zermalmer,

thanks for your explaination.

Normally it is much harder to measure current very accurate than comparing resistors.

I also doubt that a resitor behave in one direction other than in the other direction at DC. The reason why you change the direction of the current is to eleminate offset voltages (thermal EMF). Most of these voltages stay constant no matter in which direction the current flows. Therefore, they cancel each other out if you measure in both directions.


@NANDBlog: I wonder why they stated 12ppm. The 95% uncertainty 1 year specification of the 8508A for 50 Ohm is better than that.

[Zermalmer]

Normally it is much harder to measure current very accurate than comparing resistors.

That's the reason why you use accurate current source and measure voltage 

@NANDBlog: I wonder why they stated 12ppm. The 95% uncertainty 1 year specification of the 8508A for 50 Ohm is better than that.

You need a closer look for what the numbers are and what they mean.
The specified uncertainity in the manual is the accuracy of the device itself under referenced conditions.
The number from the calibration lab is the measurement uncertainty.

For the measurment uncertainty you have to calculate based on all known accuracies,  conditions and specifications (and sometimes to assume from experiences).

For that the „Guide to the Expression of Uncertainty in Measurement“ ... in short form GUM is the key.

If you want to know more about GUM
german wikipedia : https://de.wikipedia.org/wiki/GUM_(Norm)
english wikipedia : https://en.wikipedia.org/wiki/Measurement_uncertainty

You can find a lot of info about GUM in the web and direct at the source... https://www.bipm.org/en/publications/guides/
(if link doesn't work correct, go to main page > publications > Guides in Metrology )

But attention... lot of math is involved  and very very dry 

[e61_phil]

Normally it is much harder to measure current very accurate than comparing resistors.

That's the reason why you use accurate current source and measure voltage 

It doesn't matter much if you create a current or measure it very accurate. The source also needs a feedback which ist in principale the same as a measurement. I don't know any current source outside a NMI which gives you such an accurate absolute current to measure resistors better than the 8508A does. Even the best Fluke 5720A 24h current specification is 25+x ppm.

You need a closer look for what the numbers are and what they mean.
The specified uncertainity in the manual is the accuracy of the device itself under referenced conditions.
The number from the calibration lab is the measurement uncertainty.

For the measurment uncertainty you have to calculate based on all known accuracies,  conditions and specifications (and sometimes to assume from experiences).

And what is there to add? The Fluke 8508A specification already includes the instrument drift, TC, noise and so on?

[CalMachine]

Normally it is much harder to measure current very accurate than comparing resistors.

That's the reason why you use accurate current source and measure voltage 

It doesn't matter much if you create a current or measure it very accurate. The source also needs a feedback which ist in principale the same as a measurement. I don't know any current source outside a NMI which gives you such an accurate absolute current to measure resistors better than the 8508A does. Even the best Fluke 5720A 24h current specification is 25+x ppm.

The idea is to use a stable current source and 'characterize' your nominal current you plan on using by measuring the voltage drop across a known resistance.

Put your known resistance in series with your DUT resistance, connected to your current source (IE: 5700/5720).  Measure the voltage drop across your known resistance and DUT resistance.  Swap polarity of your sense connections and measure again.  Average your +/- results to null out any thermal emf errors.  Compare your results of your known resistance to your DUT resistance.

This procedure is further documented in 5700/5720A performance verification.

[e61_phil]

Normally it is much harder to measure current very accurate than comparing resistors.

That's the reason why you use accurate current source and measure voltage 

It doesn't matter much if you create a current or measure it very accurate. The source also needs a feedback which ist in principale the same as a measurement. I don't know any current source outside a NMI which gives you such an accurate absolute current to measure resistors better than the 8508A does. Even the best Fluke 5720A 24h current specification is 25+x ppm.

The idea is to use a stable current source and 'characterize' your nominal current you plan on using by measuring the voltage drop across a known resistance.

Put your known resistance in series with your DUT resistance, connected to your current source (IE: 5700/5720).  Measure the voltage drop across your known resistance and DUT resistance.  Swap polarity of your sense connections and measure again.  Average your +/- results to null out any thermal emf errors.  Compare your results of your known resistance to your DUT resistance.

This procedure is further documented in 5700/5720A performance verification.

Yes, but that is a direct resistor comparison like the 8508A does it.
The absolute values of voltage and current doesn't matter. The current has to be stable only and the voltage measurement have to be linear. No need for correct gain.

I think that wasn't meant by Zermalmer. Or was that the way you meant by R=U/I?

[Zermalmer]

Normally it is much harder to measure current very accurate than comparing resistors.

That's the reason why you use accurate current source and measure voltage 

It doesn't matter much if you create a current or measure it very accurate. The source also needs a feedback which ist in principale the same as a measurement. I don't know any current source outside a NMI which gives you such an accurate absolute current to measure resistors better than the 8508A does. Even the best Fluke 5720A 24h current specification is 25+x ppm.

The idea is to use a stable current source and 'characterize' your nominal current you plan on using by measuring the voltage drop across a known resistance.

Put your known resistance in series with your DUT resistance, connected to your current source (IE: 5700/5720).  Measure the voltage drop across your known resistance and DUT resistance.  Swap polarity of your sense connections and measure again.  Average your +/- results to null out any thermal emf errors.  Compare your results of your known resistance to your DUT resistance.

This procedure is further documented in 5700/5720A performance verification.

Yes, but that is a direct resistor comparison like the 8508A does it.
The absolute values of voltage and current doesn't matter. The current has to be stable only and the voltage measurement have to be linear. No need for correct gain.

I think that wasn't meant by Zermalmer. Or was that the way you meant by R=U/I?

Ah sorry... yes...totally forgot the reference resistance

[e61_phil]

Ahh, ok. That will work of course. But that is exactly what the 8508A does in True Ohms mode with the build in reference resistors.

[Zermalmer]

The important thing to understand is, that the specification of a device is only part of the measurement uncertainity.
In this case of such high quality devices the manufacturer give us a lot more informations, which makes it more easy to decide and calculate the measurement uncertainity.

[e61_phil]

Do you have an example what is to add to the manufacturer specification?

[Zermalmer]

Hi Phil,
that depends on all involved components and if their uncertainity affects the whole calculation or if you can neglect the value.

One simple thing is for example the aircondition of the lab.
Commonly this one will be within the needed range, but this measuring of this also have an uncertainity.
If this one is small enough you can neglect this value... if you have sometimes more worse conditions you have to include this into the calculation.

You can breakdown a measurment uncertainity to the used test leads... if it is necessary or usefull...this have to be decided.

As told before... measurement uncertainity is build on math, experiences and theories and is a very dry stuff.

greetings
Andreas

[e61_phil]

If Fluke stated a specification for 1 year with a temperature range of 23°C +/- 5°C I would expect this specification cover the described enviroment. They also say what the maximum lead resistance in your 4-Wire measurement should be. Therefore, I would expect even that is included.

[CalMachine]

If Fluke stated a specification for 1 year with a temperature range of 23°C +/- 5°C I would expect this specification cover the described enviroment. They also say what the maximum lead resistance in your 4-Wire measurement should be. Therefore, I would expect even that is included.

You still have to take into account all of the uncertainties that are inherent to the DUT and measurement system, as a whole.  TCR of resistor, uncertainty of ambient temp, stability of ambient, uncertainty of temp measurement of resistor

[e61_phil]

Thanks CalMachine!
But how do you know these parameters?

Lets say I send you a DIY Box (a 10k reference for example like many people here have build). How do you know the TC and so on?

The certificate can only include short term influences anyhow. Therefore, I thought the uncertanty only describes the DUT as found during the measurement and no DUT properties. Another story might be a noisy measurement due to noise of the DUT. But this will affect the measurement itself.

[Zermalmer]

That's the reason why in the certificates always is stated that the result can't make states about the long term stability.

Manufacturers invest a lot of time and money in alaysis of their products and work with aging, stress test aso.

Otherwise there where no possibility of such detailed data which for example is given with high end devices.

For your DIY resistance box you have basically the specifications of the manufacturer of the resistor.
All additional parts, like plugs, wire, case aso. you have to calculate for yourself.
For the case you can't test it for your own you send it for example over a time of 5 or 10 years to a calibration lab.
After this time you can make, based on the uncertainity of the lab and the results, a long term probability calculation... but only based on this time
... but be aware... even good and well handled reference resistors can change their drift direction 

That's why a lot of people at the german PTB have a whole lot of stuff to explore.
And have a look to the new VIM... after a long period the definition of some SI elemantary components are defined in another way, based on new experiences over the last years.

Actually I have no english link to latest changes...  but sure there is some in the internet
here is short description from PTB in german: https://www.ptb.de/cms/fileadmin/internet/presse_aktuelles/broschueren/intern_einheitensystem/Das_neue_Internationale_Einheitensystem_V2.pdf
Some of the used constants are known here... some for light for example... and for Ampere and others.
But does this change everything for us? Commonly not, except you work in a very high level of research or production level... and if you work their you already know about the changes for sure 

[TiN]

I wonder where these come from..





 

[Vgkid]

I own some of those(30 now). I should have bought the rest  
Looking forward to your test.

[babysitter]

These are so cool you should have some to... well, have some.

[doktor pyta]

Can anyone tell me the value of Rth (bulk foil- case) for above mentioned Vishay resistors ?
In the datasheet of VHP-3, VHP-4 and VPR247 I couldn't find such crucial info.
Some of my older resistors from Sfernice times had Rth 7K/W.

[HighVoltage]

I wonder where these come from..

TiN, you really do find great deals!

I bought a few Vishay VHP-4 100 Ohm resistors, but have not taken detailed measurements.

[Vgkid]

Gravedigging this with a current shunt
JRL CS-1R-100 . Yes it is bolted to a very heavy fan cooled AL heatsink
Last measured value 5.00017mOhm
rough measured value using a 34420A calibrated most likely after it was made. only has a cal count in the low 30's
4.9994354m avg with a std dev of 71,665nOhm measured across 303 readings.

[HighVoltage]

That resistor has some serious wattage behind it with such heat sink.
Any idea what the power rating is?

I have never seen such resistor.
 

[Vgkid]

Taking a guess since the cs-100-1 is rated for 100w, maybe 50, but could be more. I emailed ohm labs to see if they have any old docs.

[HighVoltage]

This is my newest edition to the precision low value resistor current shunts.
After years of searching, finally one showed up on the used market.

This one is a Burster, Made in Germany

- Type: 1281-Q
- Built: 1990
- R: 100 mOhm
- Tol. : 0.02 %
- Tk : <10ppm/K
- Pmax: 10 Watt

The resistor seems to be spot on, when measured with the Keithley 2450 SMU.
Here are some pictures and the Datasheet (German from 1990)