I'm trying to do an expanded uncertainty calculation for my Agilent 3458 Digital Multimeter. I noticed when looking at the specifications that there are many errors including RMS noise, linearity, and a temperature coefficient as well as other sources of error. When doing an overall expanded uncertainty calculation, do I include all these errors for an accurate representation of measurement uncertainty? If so, do I combine all these errors in a linear combination, or do a root-sum-squares analysis treating them each as a separate error source? I ask this mainly because in one of the examples on the first page of Appendix A the RMS error is added directly to the accuracy specification.

The 3458A is a strange one indeed, unfortunately the answer isn’t as straightforward. It is a multifaceted answer.

- if I were to calculate the expanded MU for this product I would RSS(Root Sum Square) the accuracy spec with the rms noise, although from the specifications you can see that the RMS noise contributor will be very small compared to the accuracy spec.

- The TC specification needs to be added directly to the accuracy specification. If you are in a stable environment ±5degC of cal temperature, and performed ACAL 24 hours and ±1degC of ACAL temp then the TC specification is not added

- The linearity measurements are not a factor for accuracy measurements unless you are doing relative measurements in the same range. In this case, you would only use linearity specs and not the other components that we have addressed. For example, if I’m on the 10V range and I want to determine the measurement uncertainty of a 9V measurement relative to a 10V measurement. I would then just use the linearity specifications and ignore the accuracy specs. On the other hand, if I want the absolute MU I would only use the accuracy and noise specifications from above.

- One more factor that most people forget about is the absolute accuracy of the traceability error. In this case, I believe that you add this to the accuracy specifications.

I have also attached an old white paper that discusses where the test limits came from and also contains a statistics section.WHERE 3458A TEST LIMITS COME FROM.zip