Question for: Cg & Cgk calculation

#1
Hi, everyone

I have some questions on the Cg and Cgk. Could you explain this to me, please?
I got a certificate from the supplier.
Please see the picture below;
Cg and Cgk calculation.GIF

So I tried calculating those based on my experience to find out where the Cg and Cgk come from.

However, there is only Cgk which I cannot get the result the same as mentioned in the certificate.

For example Torque calibration at 2 Nm

My result from calculation is -> (Cg = 2,11) & (Cgk = 2,10) that is not the same at 2,07

Could you help me and explain this to me on how to get the correct Cgk, please?

Thank you so much for your support, everyone.
 
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Welshwizard

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#2
Hi TLim_AC,

The differences you find will be due to rounding I'm sure. For the 2Nm column the process mean is the same as the nominal therefore the Cg and Cgk value should be the same and this column represents data which is stable so it can be characterised with some confidence.

Just for your reference, a quick calculation on this column reveals a probable error of 0.002 Nm. This means that half of the time your recorded values will be greater or less than this value, it's the demonstrable resolution of the process. If we know we have this grain of variation affecting our spread, by the time we multiply it by number which better represents a confidence band it's hardly surprising that the denominator changes and therefore the outcome.

Of course, whether Cg and Cgk represent anything that's useful is another topic!

Hope this helps you!
 
#3
Hi, Welshwizard

Thank you so much for your advice.
However, other nominal torques (6 and 10 Nm), there are some errors as well which is significantly different from the result in the certificate.
I will do the calculation with the correct formula again.

This, Cg and Cgk, are the new one I just jumped into this topic.
Normally, I get used to working with others like Cm, Cp, Cmk, and Cpk.

For the Cgk is the new one, and I believed that the calculation is different from those as well.
That is why I checked the result I got from our supplier to see my result is the same as theirs or not.
 

Welshwizard

Involved In Discussions
#4
TLim_AC,

The nominator can only really change due to some difference with calculating the mean, the other information being your chosen percentage and the tolerance. The denominator is dependent upon the computation of the standard deviation.

Both 2nM and and 6 nM are as near as makes no difference to their ref (if the whole numbers are meant as reference) whilst 10nM is 0.009 nM off its ref. This should meant that only 10nM should slightly differ for its Cgk to its respective Cg.

Cheers
 
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