An ISO17025 Question About Derivative Measurements from CMC

[USMC kalibrater]

Okay, I got hammered on an audit for this: We did an accredited calibration for a customer on a power meter that measures watts at 60 Hz (not a watt-hour meter, just a direct reading watt meter). We used a Fluke 5500A to source voltage to the voltage input terminals, and current to the current input terminals. Our AB auditor said we were not supposed to give out an accredited calibration to the customer because we did not have power listed on our scope. I tried to explain that we have voltage and current listed on our scope, and we were mathematically calculating the power and the uncertainty of the voltage and current from the calibrator was well within our scope, and the combined uncertainty was well below the tolerance of the watt meter, but the auditor would not take that as an answer. So now I have to compile an uncertainty budget for power and submit it to the AB to get power put on our scope of accreditation.

Scott,
So lets break this into a few pieces.
1) They had resistance on their scope already, which tells the auditor they have been proficeincy tested to make resistance measurements and that they understand the measurement.
2) They are not directly measuring the resistor they are measuring values they are accredited to measure.
Do you have a voltmeter and ammeter in circuit? Using a 5500 to cal a watt meter  means to me that you have a direct connection to the Wattmeter and in such a case you are not measuring Volts and current and calculating power.

The biggest difference is that they have resistance listed on their scope already. 

[flew-da-coup]

You can absolutely do this.  You should state how the value was derived(which is redundant being that measuring resistance with a meter is doing basically the same thing), you obviously need to combine uncertainties and there is some additional error to consider.

I completely agree. This should be a non-issue.
If you think about Ohms is a derived SI unit anyhow so the math portion is inherent.

[Hawaii596]

Let me re-iterate that I don't want to be the one who stirs the pot here.  I am the one seeking truth.  So I honestly feel your pain, ScottBP.  And I will certainly be taking those details into account.  So again, as the learner here, I'm wondering if on the certificate where a value was certified as what I'm calling a "Derivative" value, I wonder if (as in my case), if I stated the two accredited measurements as those accredited in my scope, included their expanded uncertainties, then stated the other was a derived value, stated the combined RSS calculated expanded uncertainty, would the auditor have accepted it.  This would be a great detail to get maybe even ILAC to come out with a position statement.  I am keeping the AB person who responded anonymous out of respect for their privacy.  But I believe the person is former PMEL and may even read this thread.  I wouldn't expect that person to be able to participate as an AB person.  But maybe the topic of derivative measurements should be brought up with ILAC (and/or other MRA(s)).

[RFCAL]

listen to Scottbp---you cannot do as is stated. You cannot use ohm's law to get to a point and call that accredited. If your AB is telling you that you can, I would get another Ab's opinion. I would really get another AB. Do you want to tell us who the AB is? I would really like to know.

[John Treekiller]

listen to Scottbp---you cannot do as is stated. You cannot use ohm's law to get to a point and call that accredited. If your AB is telling you that you can, I would get another Ab's opinion. I would really get another AB. Do you want to tell us who the AB is? I would really like to know.

I've been following this from the start.  I guess it's time to throw my two cents in.  Every reply talks about larger resistances and the scope covered, and a few people have responded that you can't exceed the scope using Ohm's Law.  Just to ask a question; how do you go about calibrating a current shunt without using Ohm's Law?  Very few Meters measure resistances accurately below 0.1 Ohms, there are too many floor adders.  You apply an amount of current, measure the voltage and calculate the actual resistance versus the specified.  I would think that's a certifiable parameter and I would imagine that there are people that are accredited to calibrate current shunts.  I would believe that doing the same thing for larger resistances SHOULD be accredited, but I'm not an AD so I do not make those judgments, but it would be a way I would explain to an AD why I performed a measurement using that method.  Hope this helps, just an old retired tech's opinion.

[USMC kalibrater]

listen to Scottbp---you cannot do as is stated. You cannot use ohm's law to get to a point and call that accredited. If your AB is telling you that you can, I would get another Ab's opinion. I would really get another AB. Do you want to tell us who the AB is? I would really like to know.

You are correct in the fact that I just cannot just report a result in ohms, call it accredited but that is not what we are saying. 
What we are saying is to use your accredited current source, measure and your voltage measure capabilities to setup a measurement.  You apply a current and measure the voltage drop across a resistor.  We measure and report both on the report of calibration.   Then we mathmatically extrapolate the unit ohms and report the correct uncertainty on the report of calibration.  The accredited measurements voltage, current and a derived unit in ohms is supplied to the customer. 

Additionally their AB added that I cannot use this method and report a tighter uncertainty than is already listed on their CMC.   In other words if I reported .1% +/- x  for some values as my uncertainty on my scope I cannot list any uncertainty better than this even if the uncertainty would be better using the alternative method.  Its kind of a "duh" statement in my opinion.