Ohm Measurement Problem
I came across a very uncommon error on ohm measurements, when I checked a brand new 34465A against a 3458A, using my old precision ‘reference’ resistors.
Setting on both DMMs was: 4W Ohm for values <= 10k, Offset Compensation for values <= 100k, and NPLC 100. So the highest accuracy of < 1ppm could be expected.
For the 100k resistor, there was a gross reading difference of about 15ppm between both instruments. This is no ppm nit-picking, and no volt-nuttery any more, that is a severe measurement problem, obviously.
In contrast to that, a 10.0000k Vishay Bulk Metal Foil resistor, which had been accurately measured to 1ppm independently by Third Party, gives identical readings on both DMMs to 1ppm.
This resistor serves as a baseline calibration standard for the 3458A, which conducts by its AUTOCAL function all necessary internal precision ratio transfers, calibrating all ranges from 10.00 Ohm to 10MOhm, to about 2ppm uncertainty.
As the 34465A was brand new, I concluded, that the 3458A has a failure in its Ohm measurement circuitry, maybe a defect of one of the FETs. This would also discredit the complete AUTOCAL functionality for Ohm, and for the current ranges.
At that point, my confidence in the 3458A was completely shattered.
From a metrological point of view, that’s the classical ‘man-with-two-clocks’ problem: As the absolute value of the 100k resistor had not been independently measured, it is not possible to make a decision about, which of both instrument is failing.
Even worse: As I before had relied on only one precision instrument, i.e. the 3458A with its AUTOCAL function, I had absolutely been ‘blind’ for any possible failures.
As a conclusion, an independent verification of the functionality and calibration of any instrument is always needed.