Comparisons Of Two Measurement Methods For Mutual Inductance In Joule Balance

In International system of7basic units, mass unit Kg is currentlythe only unit using physical basis to repet, in order to realize its quantization,many kinds of solutions are under research, such as “watt balance”,“siliconball” and “Joule Balance” proposed by National Institute of Metrology （NIM）.Mutual inductance measurement is one of key technologies in “JouleBalance”, precise measurement methods such as “Low Frequency Extrapolationmethod” and “Standard Square Wave Compensation method” are proposed andboth of the measurement uncertainties are10^-7. So as to furtherly verify theuncertainty of standard square wave compensation method measurementsystem can reach to10^-7and the accuracy can reach to10^-6（by comparingconsistency between the two techniques）,“comparisons of two measurementmethods for mutual inductance in Joule Balance” is proposed.The significant achievements of techniques for mutual inductance precisionmeasurement are introduced and analyzed. The latest two mutual inductancemeasurement techniques proposed in "Joule Balance" and systems areresearched furtherly. The approximate calculation for voltage proportionaland the degree of influence by non ideal analog switch are not improved orassessed exactly in the previous “Standard Square Wave Compensation method”system. Similarly, the inconsistent current in sampling resistance and mutualinductance coil is not improved or assessed exactly in the previous “LowFrequency Extrapolation method” mutual inductance measurement system.To avoid bringing error of approximate calculation, data processingprogram, embedded in the mutual inductace measurement program, is used tocalculate voltage ratio directly in the “Standard Square Wave Compensationmethod” analog switch mutual inductance measurement system developed inthis paper. The analog switch circuit of above system is researched by nodevoltage method and a theory model of impact is concluded; a whole DACmutual inductace measurement system is developed to assess the influence experimentally based on “Standard Square Wave Compensation method”. Newmeasurement circuit, using two sampling resistance, is designed to reduce theerror introduced by current inconsistent between mutual inductance coil andsampling resistor in “Low Frequency Extrapolation method” mutualinductance measurement system, and amplitude adjustable voltage source isdeveloped to regulate voltage.The340mH mutual inductor in NIM is used to measure, the measurementuncertainty of "standard square wave compensation method" analog switchmutual inductace measurement system is4×10^-7and the one of “whole DACmutual inductace measurement system” is5.1×10^-7, the effect of analogswitch to mutual inductance measurement is4×10^-7assessed by symmetricalobservation method for above two sets of systems. The measurementuncertainty of “low frequency extrapolation mutual inductace measurementsystem” is3.7×10^-6; the consistency of two methods is1.1×10^-6assessed bysymmetrical observation method for this system and the analog switch mutualinductace measurement system.In conclusion, the measurement uncertainty of "standard square wavecompensation method" can reach to10^-7which is verified again in this paper,the measurement accuracy is10^-6, which is verified by the consistencybetween "standard square wave compensation method" and “low frequencyextrapolation method”.