High Current High Accuracy AC Shunt Calibrations

Started by Hawaii596, 03-04-2013 -- 14:32:28

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Hawaii596

03-04-2013 -- 14:32:28
I have a Leeds & Northrup 4363 300A shunt rated from DC to 60 Hz and at +/-0.04%.  It seems no one can calibrate it at 60 Hz; just DC only.  Any wisdom on this one from anyone?
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

Bryan

#1
03-04-2013 -- 17:28:46
Well, if you have a 5520A you can juice it at 20A with an accuracy of 0.01% based on published specs (5520A absolute uncert/12 mo. tcal +/- 5 C at 0.012ppm of output + 2000uA) the 2000uA presumes you run it >30 sec.  Depending on what you have to deal with, the 5520A specs are 99% conf level so your 0.01% becomes around 0.0078% @ 95%.  This doesn't include calibration accuracy of the 5520A AND this is only the source accuracy, it would be more desirable to run it closer to 300 Amps.  The low output level at that input is probably going to muck it up pretty good.  I have no idea if the thing is linear in its response to lower levels.  On the other hand you did ask for wisdom, I don't claim that.

beadwork

#2
03-05-2013 -- 06:29:17
Process Instruments (wwwdotprocinstdotcom) bought the remaining L&N line some years ago and offers service on them.  They are ISO 17025 accredited by A2LA so they might be able to calibrate your shunt @ 60 Hz.

Hawaii596

#3
03-05-2013 -- 08:23:28
I've been emailing them, and they do DC only up to 100A.  I just conversed with NIST via email on the topic, and they don't do it either.  There is apparently not direct traceability about 20 Amps when it comes to AC.  According to a person from NIST, with the specs on the 4363 being +/-0.04%, he said self-heating effect is significantly more of a contributor than AC-DC difference at 60 Hz.  I could about keep that email from NIST as justification for doing it only at DC.  This is kind of a new one to me.  Even the GIDEP (can't remember if its T.O. or Navair) when I looked up a procedure for the L&N 4363, tests it only at DC. Interesting.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

Squidley

#4
03-05-2013 -- 09:01:41
Fluke is now has A2LA accreditation to 120Amps AC at 16Hz to 6 kHz... according to the docs posted on the Fluke Cal website...
Douglas J. Baird, USN(ret),

Hawaii596

#5
03-05-2013 -- 09:53:02
I had looked through that scope before and somehow missed it (maybe subconsciously expecting not to find it??).  Thanks for the good catch on that one.  I'm in email conversation with NIST on this topic.  I am presuming they must be somehow extrapolating, as it still looks like there is no direct NIST traceability.  Some of the words I got from NIST are that at least at 60 Hz, there is very little difference between DC shunt value, and that self-heating is much more a contributor to uncertainty.  I may need to look into that my self a little down the road.   I may contact them to see what they can do on this.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

John Treekiller

#6
03-05-2013 -- 10:16:49
The AFPSL calibrates the A40B Current Shunt Set to 100 A from DC to 100 kHZ.  The DC Accuracy is used for the AC-DC Current Transfer Difference Uncertainty for all frequencies <1 kHz.  AFPSL also has the capability to go to 300 ADC with the new 6625A system.  If you only require a 60 Hz and get it documented that it would be basically the same as DC specs it may open other sources for your calibrations.

scottbp

#7
03-05-2013 -- 11:17:12
Jay Klevins of Ohm-Labs Inc. put out a paper on calibrating shunts at one of the NCSL conference. In it, he states "Generally, at low frequency (50/60 Hz), the ac/dc difference of any shunt is below the uncertainty of state-of-the-art ac/dc difference measurements, so if correct lead wire routing is observed, accurate mains frequency measurements may be performed using dc calibrated values."
Kirk: "Scotty you're confined to quarters." Scotty: "Thank you, Captain! Now I have a chance to catch up on my technical journals!"

Hawaii596

#8
03-05-2013 -- 13:36:41
Good reinforcement of the email I received from NIST this morning.  I'll have to see if I can dig up that paper.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

dminesinger

#9
03-05-2013 -- 14:59:56
Calibrating DC Current Shunts: Techniques and Uncertainties

http://www.ohm-labs.com/pdfs/Shunt%20Calibration.pdf
PapaBear

*'It's the Veteran, not the reporter who has given us the freedom of the press.'
'It's the Veteran, not the poet, who has given us the freedom of speech.'
''It's the Military who salutes the flag, who serves beneath the flag, and whose coffin is draped by the flag, who allows the protester to burn the flag.'

measure

#10
03-09-2013 -- 14:45:37
Quote from: scottbp on 03-05-2013 -- 11:17:12
Jay Klevens of Ohm-Labs Inc. put out a paper on calibrating shunts at one of the NCSL conference. In it, he states "Generally, at low frequency (50/60 Hz), the ac/dc difference of any shunt is below the uncertainty of state-of-the-art ac/dc difference measurements, so if correct lead wire routing is observed, accurate mains frequency measurements may be performed using dc calibrated values."

I agree with Jay. Low-level AC measurements at the voltage levels typically associated with the shunts have relatively large uncertainties, as well as additional error sources that similar DC measurements are not afflicted by. However, if the measurements are to be accredited, I think accrediting bodies may take a more rigorous view on this statement, requiring actual measurement data to support the claim. Good Luck!
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