GR&R for Hardness Tester and Arc Spark Spectrometer

Ninja

Looking for Reality
Staff member
Super Moderator
#11
Again, I've never done Gage R&R for an Arc Spark Spectrometer, but my Spectro was one of the key instruments in my lab. 26 % measurement error sounds huge.
What I'm about to say is not strictly true...but would give decent perspective:

Not 26% measurement error...
26% of the 0.2% spec range is used up with measurement method variation (like measurement flicker of 0.05%).

In your experience, would you trust the output to not vary more than 0.05%? 0.02%?
 
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JeantheBigone

Quite Involved in Discussions
#12
Ah, thank you for that. That makes sense. I was reading it as if the measurement itself could be off by 26% due to measurement error alone, which even for impurity elements seems high.

Expressed as a percentage of the allowable range, it's a different story. From my experience you can usually keep sparking on the same spot on a uniform sample and most of the readings will barely change. The precision depends on the specific elements. However, at least in my instrument, there was drift, plus, adjusting the machine (which had to be done because of the drift) reduced the precision.

I found a resource on Arc Spark Spectrometers that has what appears to be a helpful discussion of this topic.

"Modern Spectrochemical Analysis of Metals: An Introduction for Users of Arc-Spark Instrumentation "

I've copied an excerpt.

I hope this helps.
 

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Bev D

Heretical Statistician
Staff member
Super Moderator
#13
.

Not 26% measurement error...
26% of the 0.2% spec range is used up with measurement method variation (like measurement flicker of 0.05%).

In your experience, would you trust the output to not vary more than 0.05%? 0.02%?
Yes sorry for my 'shorthand' statement...I would so add that using percentages instead of the original units of measure adds to the confusion. None the less, of course the AIAG method overstates the actual measurement error as it uses incorrect math. And in this case, it's fairly obvious how silly the approach is.
 

Ninja

Looking for Reality
Staff member
Super Moderator
#14
Yup, understood...that's why I couched it in "this is not strictly true...just to give perspective"...
I was just trying to get the 26% in the correct light for Jean...whatever the "26" actually means...
 

sinned

Registered Visitor
#15
Hi Miner,
HRC hardness testing is per my understanding affected by within part variation a lot. ie. homogenity of the tested sample. I have never done any GR&R study with real production samples that can meet the 10% requirement. May be possible with standard master block. I guess.

We got the 10 samples from different lots in the hope that the process variation can be captured with sample readings spread over the specification range.
I did also think of not using real parts from the line but standard master block to do the study. However, our company expert did not and never agree on this method.


Hi JeantheBigone,
Is it true that the precision of the tester vary with specific elements to be tested for ?
I assume that the measurement error of the entire system will be the same no matter what element we are testing for.
 

JeantheBigone

Quite Involved in Discussions
#16
Hi Sinned

Sorry for the late reply, I had to dig in my files for an example.

I would not have assumed that the measurement error of the entire system is applicable to each element. The OES has separate element "windows" which presumably are tailored to the individual elements.

Some elements are likely to be present in larger amounts (eg Cr, Ni) and some in smaller amounts (eg P, S).

The instrument is calibrated by measuring on several standard reference samples, measuring the actual number of photons obtained which can be attributed to each element (which in some cases can overlap), and applying a best fit regression for each element individually.

Based on that, even though I've never tried to quantify the individual or aggregate errors, I don't see why the precision would be the same for Ni as for S. Also, my Spectro even had two different calibrations for Ni, one for below 5 % and one for above 5 %. I presume that this is because the concentration is actually non-linear with counts, but the best fit was done with linear regression. More modern instruments probably can do polynomial or exponential fits as appropriate.

The best I can do is show you an example of what happened when I repeatedly sparked the same sample. I always sparked reference samples just before running the actual sample. A couple of things that are coming back to me now....the C-content would consistently drop with each "burn" and I never got to the bottom of it. Also the Si-calibration was remarkably stable.

I hope this helps...
 

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