DC zeroing methods

Based on the opposition (or zero, or equilibrium) method, the procedures that will now be described were at the origin of electrical metrology (bridge by C. H. Christie and C. Wheatstone; potentiometer by J.-C. Poggendorf's potentiometer): they represent the fundamental methods that can be used to set up a laboratory for precise DC measurements.

Despite the development of electronic measuring equipment that enables us to achieve ever-lower uncertainties, and to combine speed of reading with the convenience of a digital display, zeroing methods continue to be used in all areas of precision electrical metrology.

Zeroing methods involve a ratio of two passive quantities (resistances, number of turns, etc.), which is often either "self-calibrating" or "self-adjusting". This means that, without the aid of an external standard, we can either know the exact value of this ratio, or adjust it to its nominal value. These methods therefore make it possible to determine very precisely the ratio between the quantity to be measured and a quantity of the same type, but of known value, known as the standard quantity. The element representing the standard quantity ensures, on the one hand, that the measurement laboratory is connected to the national standard and, on the other hand, that the unit is maintained in the laboratory between two successive connections.

The connection is made via the calibration chain set up by the Bureau national de métrologie (BNM ) in the 1970s, now known as the "chaîne nationale d'étalonnage BNM-COFRAC". COFRAC (Comité français d'accréditation) provides users with a list of laboratories and companies accredited to issue calibration certificates.

The zeroing methods described here enable measured quantities to be traced to national standards, via the connection and conservation standard. They have been classified into two main categories: those for comparing resistances, and those for comparing DC voltages. At the head of each of these two sections, we remind you of the characteristics, metrological definitions, influence quantities and precautions for use of the standard elements used (i.e., respectively, standard resistors on the one hand, and standard batteries and zener diode reference on the other).

For more details on the definition and influence quantities of standard resistors, please refer to the article "Definition of metrological characteristics of passive elements" [R 1075] .

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