Electronics > Metrology

how to calibrate 10A range with 2A reference ?

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elianto:
I have a calibrated KEITHLEY 2001 which I use periodically to benchmark the accuracy of various other instruments, in conjunction with stable voltage and current sources.
Now I need to check the 10A current range of a multimeter but can only go up to the 2001 2A range.
Is there any approach to extend the range of the current that I can check? And what accuracy can I maintain?

alm:
I'd use a Hamon type arrangement. Take five good quality, low tempco resistors of roughly equal value that can easily handle 2A without excessive heating. Calibrate each of them at 2A by measuring the voltage over them and the current through them (or all of them in series?). This gives you the resistance at 2A. Then put them all in parallel, and put ~10A through this combination. Each resistor is now carrying ~2A, and you can calculate the resistance of this parallel combination, and measure the voltage across this compound shunt to get the actual current.

I'm sure there will be a devil in details like wiring resistance and thermal EMF if the burden voltage is low.

mzzj:

--- Quote from: alm on March 20, 2023, 09:24:12 am ---I'd use a Hamon type arrangement. Take five good quality, low tempco resistors of roughly equal value that can easily handle 2A without excessive heating. Calibrate each of them at 2A by measuring the voltage over them and the current through them (or all of them in series?). This gives you the resistance at 2A. Then put them all in parallel, and put ~10A through this combination. Each resistor is now carrying ~2A, and you can calculate the resistance of this parallel combination, and measure the voltage across this compound shunt to get the actual current.

I'm sure there will be a devil in details like wiring resistance and thermal EMF if the burden voltage is low.

--- End quote ---
Paralleing low-value resistors seems tricky, current distribution and unexpected voltage losses vs. original 4-wire measurement setup seem to spoil the day in paradise.
On the other hand Keithley 2001 is only about 0.1% accurate on 2A range so paralleing bunch of resistors might work.

Another alternative would be total overkill of shunt resistor that is known or can be assumed to be linear to 0.05% or so between 2A and 10A. Just calibrate it at 2A and use the resistance value to calculate current at 10 amps.
Example of suitable resistors would be metrology-grade standard resistors or large low value resistor that doesn't heat up noticeably during use. To minimize shunt heating select lowest value resistor that still gives enough voltage drop at 2 Amps to measure properly.

Keithley 2001 is good to about 1uV on voltage measurement in short-term, from there we can work out needed minimum resistance. We can measure 4 mV drop on the shunt with approx 0,025% precision or or 4 times better than the 2001 current measurement itself.
20mV @ 10A shunt resistor would minimize heating and still provide enough accuracy at 2A calibration. Now we just have to find 50W 0.002 ohm resistor  ::)
100mV 50A shunt like this https://www.newark.com/empro/ha-50-100/base-mounted-dc-shunt-100mv-50a/dp/08R4301#anchorTechnicalDOCS might be pretty good guess and enough linear between 2A and 10A.

bdunham7:

--- Quote from: elianto on March 20, 2023, 09:05:59 am ---I have a calibrated KEITHLEY 2001 which I use periodically to benchmark the accuracy of various other instruments, in conjunction with stable voltage and current sources.
Now I need to check the 10A current range of a multimeter but can only go up to the 2001 2A range.
Is there any approach to extend the range of the current that I can check? And what accuracy can I maintain?

--- End quote ---

You don't say which multimeter you are checking, but in most cases if you simply check it at 0 and 2A and it is accurate, it should be pretty accurate over the entire scale.  That's not an ideal method, but it is more than likely good enough and also likely better than any ad-hoc methods.  If you need an exact 10A stimulus to set the calibration constant, then that is a different matter.  If you really need 10A and have a suitable PSU, you need a precision shunt with a specified tolerance and tempco, those will determine how accurately you can measure the 10A. 

donlisms:
I agree that Hamon would be helpful, but I think you can do it even more... Hamon-y.  Keep in mind that his approach was the ratio of N resistors in series to those same resistors in parallel, giving an accurate ratio of N^2.  There are ways to get accurate ratios of N, but they seem a little harder. 

The error in the ratio is based on the square of the sum of the individual errors wrt the average, but you don't need to measure all that.  If you know the resistors are within 1% of their average, you know the ratio is within 0.01%. And 0.1% resistors, if you can manage or select them, gives you a ratio at 1ppm.  That's the magic.

So a very accurate resistive ratio of 4 or 9, with some good high-res voltage measurements across them, giving current ratios, seems helpful somehow.

IOW, I think you can avoid having to know the individual resistor values, other than their relative values to get some idea of the ratio accuracy.  Use them in series, use them in parallel, and use the ratio.  This also avoids having to know the absolute voltages, because you're only interested in their ratio.  Everything pretty much lands on the accuracy of the reference current measurement. 

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