Quote:
Originally Posted by terrykc
In other words, if you connected two identical heaters, one to a 200V DC supply and the other to a 200V (RMS) AC supply, both would would produce exactly the same heat output.
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Although there is a formal metrology definition of the DC Volt (in terms of a Josephson Junction, which is referred to frequency, also well defined in terms of the Caesium clock) there is no definition of the AC volt, certainly not in terms of fundamental constants.
So the way it is done is to use a very very precise and well insulated (usually in vacuum) thermocouple (or series connection of many thermocouples), and the heating effect of the AC is compared to the heating effect of DC. When they are the same value, the rms of the AC is equal the DC, which you know to high accuracy.
http://www.npl.co.uk/measurement-ser...rrent-transfer
The third panel down shows the old Fluke 540B thermal transfer standard, of which I have two. Basic accuracy of that is 0.01%from 5Hz to 50kHz, although it goes to 1MHz with reduced accuracy of 0.1%.
Craig