04-10-2016, 09:15 PM
The problem with AC measurement is that there is no fundamental definition of an AC waveform. "Say What??" I hear you say.
But in terms of measurement standards, the DC volt is closely defined in terms of fundamental constants and is measured using an array of Josephson Junctions.
Resistance is likewise defined via the Quantum Hall Effect Resistance Standard
DC current is still a work in progress, the aim being to count the number of electrons per second to define a fundamental comparison standard.
But - you see that they are all DC quantities. There is no such definition of AC quantities in terms of fundamental constants.
So given, say an AC calibration instrument with which you are going to calibrate your Fluke DMM - how can you be certain that the AC calibrator is in calibration?
The only way that this can be done is to compare the heating effect of the AC quantity with a DC quantity. That is usually done using a highly refined thermocouple. Fluke themselves made one called the 540B Thermal Transfer Standard, although this is now obsolete. One of several papers from NIST on this topic is here http://nist.gov/calibrations/upload/sp250-27.pdf
But in terms of measurement standards, the DC volt is closely defined in terms of fundamental constants and is measured using an array of Josephson Junctions.
Resistance is likewise defined via the Quantum Hall Effect Resistance Standard
DC current is still a work in progress, the aim being to count the number of electrons per second to define a fundamental comparison standard.
But - you see that they are all DC quantities. There is no such definition of AC quantities in terms of fundamental constants.
So given, say an AC calibration instrument with which you are going to calibrate your Fluke DMM - how can you be certain that the AC calibrator is in calibration?
The only way that this can be done is to compare the heating effect of the AC quantity with a DC quantity. That is usually done using a highly refined thermocouple. Fluke themselves made one called the 540B Thermal Transfer Standard, although this is now obsolete. One of several papers from NIST on this topic is here http://nist.gov/calibrations/upload/sp250-27.pdf
