Dealing with uncertainty in DMM calibration
Whenever we take a measurement with any meter, we’re gambling that the instrument will give us the “real” reading. Fortunately, it’s a very safe bet that a quality multimeter will deliver readings that coincide with reality. Specifications quantify the confidence of getting accurate readings and the risk of seeing inaccurate readings. You should fully expect that the information you are getting is accurate and complete.
On the spec sheet, you should see measurement uncertainty specs, as well as modifiers that affect the uncertainty. You will also see operating limits that constrain the environment in which the uncertainty specifications will hold true.
Traceability and specifications
To ensure we’re talking about the same volt, ohm or amp, DMMs must trace their measurement performance back to national laboratory standards. DMMs are usually calibrated using multifunction calibrators. But there are usually a number of links between the DMM and national standards %%MDASSML%% including calibrators and transfer standards. As you move through the chain between your DMM and the national standards lab, the calibration standards become increasingly accurate. Each calibration standard must be traceable to national standards through an unbroken chain of comparisons, all having stated uncertainties.
The uncertainty of a DMM depends on the uncertainty of the calibrator used to calibrate it. Most DMM specs are written assuming two things:
The DMM has been calibrated using a particular model of calibrator, usually specified in the DMM service manual
The calibrator was within its operating limits and traceable to national standards.
This allows a DMM manufacturer to include the uncertainty of the calibrator in the DMM uncertainty specs. If you see an uncertainty listed as “relative,” this means the uncertainty in the calibrator output has not been considered and it must be added to the DMM uncertainty.
Adapted from Fluke Corp.